André Ebel

Olivier Marsan

Jacques Lacaze
email: jacqueslacaze@ensiacet.fr

Benoit Malard

Cyclic oxidation of high-silicon spheroidal graphite iron

Keywords: iron, B: thermal cycling, B: Raman spectroscopy, B: X-ray diffraction, C: oxidation 1

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Introduction

Due to a combination of good casting ability, mechanical properties, corrosion resistance and low cost, spheroidal graphite cast irons (SGI) are commonly used for complex parts requiring high material performances [START_REF] Labrecque | Ductile iron: Fifty years of continuous development[END_REF]. Within this family of materials, high-silicon SGI with 3.5-4.5 wt.% silicon (Hi-Si SGI) have been developed for service at intermediate to high temperature [START_REF] Fairhurst | High-silicon nodular irons[END_REF]. The primary effect of silicon addition is to significantly increase the eutectoid temperature and thus to extend the temperature range without phase change of the cast iron matrix [START_REF] Lacaze | The austenite to pearlite/ferrite transformation[END_REF]. However, the maximum service temperature may be limited by additional phenomena such as surface oxidation, subsurface decarburization and graphite coarsening that can take place below the eutectoid transformation temperature.

In Fe-Si alloys and Hi-Si SGI, isothermal oxidation proceeds with the formation of a thin, protective layer of silicon-rich oxide, followed by rapidly growing iron oxide nodules. Over time, these nodules expand laterally and eventually cover the entire sample surface [START_REF] Ekström | Influence of Cr and Ni on high-temperature corrosion behavior of ferritic ductile cast iron in air and exhaust gases[END_REF]. The resulting scale consists of an outward growing layer composed of magnetite ( ) and hematite ( ) and an inward growing layer of magnetite, wüstite ( ) and fayalite ( ), as well as a Si-rich oxide at the metal-oxide interface [START_REF] Tholence | High-temperature corrosion of cast irons and cast steels in dry air[END_REF][START_REF] Méndez | Effect of silicon and graphite degeneration on high-temperature oxidation of ductile cast irons in open air[END_REF]. The iron oxide nodules are clearly associated with graphite or carbide particles in the case of SGI [START_REF] Ekström | Influence of Cr and Ni on high-temperature corrosion behavior of ferritic ductile cast iron in air and exhaust gases[END_REF] while their formation on Fe-Si alloys is still unclear [START_REF] Liu | Transition between different oxidation modes of binary Fe-Si alloys at 600-800°C in pure O2[END_REF]. High Si contents favour silica ( ) formation at the metal-oxide interface that acts as diffusion barrier [START_REF] Atkinson | The diffusion of Fe3+ in amorphous SiO2 and the protective properties of SiO2 layers[END_REF] and reduces oxidation kinetics even if it is discontinuous [START_REF] Svedung | The influence of silicon on the oxidation properties of iron[END_REF]. Further, silica formation is favoured when temperature increases from 700°C to 800°C and this leads to a decrease in oxidation kinetics of Fe-Si alloys [START_REF] Liu | Transition between different oxidation modes of binary Fe-Si alloys at 600-800°C in pure O2[END_REF]. Accordingly, increasing the oxidation temperature of Hi-Si SGI up to 800°C in air does not have a strong effect on the amount of oxide formed. However, this has a marked effect on the scale morphology which changes from an even layer to a highly uneven one [START_REF] Lekakh | Quantitative µ-CT analysis of scale topology formed during oxidation of high SiMo cast iron[END_REF][START_REF] Lekakh | Prevention of high-temperature surface degradation in SiMo cast irons by Cr and Al alloying[END_REF]. The oxide scale evenness and its formation rate are also affected by water vapour [START_REF] Lekakh | Quantitative µ-CT analysis of scale topology formed during oxidation of high SiMo cast iron[END_REF] and previous work [START_REF] Ebel | High-temperature oxidation of a high silicon SiMo spheroidal cast iron in air with in situ change in H2O content[END_REF] related the effect of water vapour to the promotion of internal oxidation of silicon within the metallic matrix instead of the formation of dense silica at the metal-oxide interface.

In the case of SGI, decarburization refers to the dissolution of graphite particles in the matrix and this occurs when carbon is oxidized into gaseous carbon oxide at the specimen surface. While it remains unclear if carbon oxidation takes place at the metal-oxide interface, in the oxide scale or at the oxide-gas interface, elemental carbon or the gaseous carbon oxide resulting from carbon oxidation has to diffuse through the oxide scale for decarburization to keep proceeding. This phenomenon has been observed above 700°C for low-Si cast-iron [START_REF] Yamada | Some behaviors and characteristics of decarburized layer in spheroidal graphite cast iron[END_REF] and above 800°C for Hi-Si SGI [START_REF] Bastid | Microstructural evolution of spheroidal graphite cast iron at high temperature: Consequences on mechanical behaviour[END_REF], and its rate increases with temperature. The interaction between oxidation and decarburisation are two-fold: on the one hand, decarburisation affects the oxide morphology and the oxidation rate, as reported by Lekakh et al. [START_REF] Lekakh | Quantitative µ-CT analysis of scale topology formed during oxidation of high SiMo cast iron[END_REF][START_REF] Lekakh | Prevention of high-temperature surface degradation in SiMo cast irons by Cr and Al alloying[END_REF]; on the other hand, the formation of oxides could limit the decarburisation rate assuming that Si-rich oxides also act as an effective diffusion barrier for carbon. Accordingly, water vapour was found to fasten decarburization on Hi-Si SGI at 800°C as it promotes internal oxidation of silicon rather than formation of a protective dense silica scale [START_REF] Ebel | High-temperature oxidation of a high silicon SiMo spheroidal cast iron in air with in situ change in H2O content[END_REF].

Thermal cycling below the eutectoid temperature has no effect on oxide scale adhesion and graphite structure beside decarburization for long exposure time at peak temperature (1 hour) [START_REF] Brady | Long-term oxidation of candidate cast iron and stainless steel exhaust system alloys from 650 to 800°C in air with water vapor[END_REF]. However, recent results with a short exposure time at a peak temperature of 800°C (60 seconds) have shown that thermal cycling can trigger coarsening and degeneration of the graphite particle shape [START_REF] Ebel | Coarsening and dendritic instability of spheroidal graphite in high silicon cast iron under thermal cycling in the ferritic domain[END_REF]. Very little work has been devoted to oxidation under thermal cycling with such short exposure time at peak temperature while this should be more representative of the use of Hi-Si cast iron for high temperature applications like passenger car exhaust systems. The present work thus focused on the effect of thermal cycling with short exposure time and large number of cycles on oxidation of Hi-Si SGI with a peak temperature at 800°C. Samples after 1000, 2000 and 3000 cycles were characterized using various techniques, and compared to a reference sample that was oxidized under isothermal conditions at 800°C. In the discussion of the results, a schematic is proposed for explaining the observation of a repetitive columnar zone at the interface between the matrix and the inner oxide scale that appears to be instrumental in the complex mechanism of cast iron oxidation.

Materials & Methods

Material preparation

The material was a Hi-Si near-eutectic SGI with 4.45 wt.% Si that was labelled alloy #4 in the previous work where all casting details are given [START_REF] González-Martínez | Effects of high silicon contents on graphite morphology and room temperature mechanical properties of as-cast ferritic ductile cast irons. Part i -microstructure[END_REF]. Ten plate-like samples, 20 mm in height, 10 mm in width and 2 mm in thickness were cut out from the casting for thermal cycling tests and a 1 mm diameter hole was drilled for hanging. Two more samples were prepared, a thick one (6 mm in thickness) and a thin one (2 mm in thickness) for isothermal oxidation and for high-temperature X-ray diffraction (HT-XRD). Before testing, the samples were polished with SiC papers (1200 grit), then cleaned with acetone and finally dried.

Actual dimensions of each specimen were measured using a Vernier calliper (± 10 μm accuracy), and those specimens for oxidation experiments were also weighted using a Sartorius Genius ME215-P precision balance with a weighing capacity of 210 g (readability of 50 µg and reproducibility of 20 µg according to the documentation).

Oxidation tests

The isothermal heat treatment was carried out in a resistance furnace set at 800°C ± 2°C.

The sample referred to as "isothermal" in the following was held for 50 hours in the furnace, after which it was removed and cooled in the open air.

Thermal cycling was performed using an in-house facility that has been previously described [START_REF] Pegoraro | Spheroidal graphite coalescence during thermal cycling in the ferritic domain of a high-silicon cast iron studied by optical microscopy and X-ray computed tomography[END_REF]. The 10 plate-like samples are suspended to a holder with thin platinum wires, with one of the samples fitted with a thermocouple to continuously record temperature changes over time. The sample holder is attached to a mechanical cylinder that moves it up and down. At the lower position of the cylinder, the specimens are air cooled by a fan. At the upper position, the specimens are heated in a resistance furnace. The whole system is open to the air.

An automatic cycling procedure was defined, consisting of: 1) Moving the specimen holder to the upper position; 2) Maintaining the holder in the upper position until the specimens spent 60 seconds in the temperature range of 800 ± 10°C; 3) Moving the holder to the lower position; 4) Holding the specimens in front of the fan until the specimen temperature is lowered to 70-90°C. The parameters of this thermal cycling procedure are the holding time in the upper position in the furnace, , and the holding time in the lower position in front of the cooling fan, . This process was repeated up to 3000 times, though interrupted every 200 cycles for measuring the mass of the specimens. For this purpose, all the specimens were removed from the holder and were weighted using the same Sartorius precision balance. The mass change of each specimen, , was normalized with its external original surface, S, to obtain specific mass variations ( ).

After completion of 1000 cycles, 3 specimens were removed from the system and stored for analysis. This was repeated at 2000 cycles and after 3000 cycles the last three samples were withdrawn. Thus, at the end of 3000 cycles, three specimens had undergone 1000 cycles, three more 2000 cycles and three others had completed 3000 thermal cycles.

For constant thermal cycling parameters and , preliminary tests demonstrated that the maximum temperature of the specimens increased and the minimum temperature decreased after removing three samples from the holder. Therefore, the parameters and were reduced after 1000 cycles and again after 2000 cycles, so that the control temperature does not exceed 810°C upon heating and does not fall below 70°C upon cooling. The appropriateness of the new parameters was checked with the first cycles following the changes. Typical temperature-time curves during the first, second and third periods of 1000 cycles are illustrated in Fig. 1-a. The time spent in the 800°C +/-10°C range after completion of 3000 cycles actually reaches 56 hours, which is comparable to the duration of isothermal heat treatment (50 hours). However, while these two heattreatments result in roughly the same duration at peak temperature, thermal cycling with such short exposure time at peak temperature adds non-negligible oxidation time at intermediate temperature during heating and cooling steps with the total duration of the exposure at temperature in between 690°C and 790°C reaching 80 hours after 3000 cycles. This is seen with the histogram of cumulated durations spent in the range 690-810°C after 3000 cycles that is presented in Fig. 1-b.

Characterization Methods

Four specimens, i.e. the isothermally heat-treated specimen and one specimen for each cycling period (1000, 2000 and 3000 cycles), were prepared for metallographic observations. A Secotom 50 Struers micro-chainsaw was used to make a vertical cut along each sample. One half was kept for surface observation and the other one was hot mounted and polished using SiC paper and diamond cloths. Polished cross-sections were cleaned in an ultrasonic bath in ethanol before observation. Surface and cross section observations were performed with an optical Zeiss AX10 microscope. Additional observations were performed using a FEI Quanta 450 scanning electron microscope (SEM) equipped with a Bruker Quantax electron dispersive spectrometer (EDS). Imaging was performed with accelerating voltage between 5 and 12 kV. EDS composition measurements were then made quantitative using the PhiRhoZ method after calibration.

Raman spectroscopy was used to perform point analyses and mappings on cross sections.

The setup used is a confocal RAMAN Labram HR 800 Horiba Yvon Jobin microscope coupled to a 532 nm solid laser with a power of 12 mW. Attenuation filters for lowering the laser power by a factor of ten were used to avoid any degradation of the samples. In this facility, the sample is positioned under an Olympus BX 41 microscope with an objective x 100 which gives a lateral resolution of 0.72 µm and an axial resolution of 2.61 µm. A silicon standard enabled the equipment to be frequency calibrated using the first order line of silicon at 520.7 cm -1 with an accuracy of 1cm -1 . The spectrum of each point was acquired with a spectral resolution of 1 cm -1 and collected with a quantum well detector cooled to -60°C (CCD Synapse).

The maps were recorded using an XYZ motorized table having a position accuracy of 0.1 µm in each direction. Mapping was done with a step of 1 µm in both X and Y directions, and the spot size of the laser was estimated to about 1 µm. To adjust the focus of the laser, the system is equipped with an autofocus whose amplitude range is optimized according to the roughness of the studied area. The spectrum for each point of the maps was obtained by accumulating three successive spectra during 30 second each. Data processing was afterwards performed using the Labspec 6 software.

In-situ high temperature XRD experiments

In addition to the isothermal oxidation and thermal cycling experiments, in-situ X-ray diffraction measurements were carried out at high temperature to investigate the early steps of oxidation and possible phase transformations.

Two experiments were carried out: experiment n°1 with measurements at temperatures between 100 and 750°C, and experiment n°2 with four successive thermal cycles between 100°C and 800°C. For each experiment, XRD acquisition was carried out during the hold steps at constant temperature as depicted on Fig. 2. Heating and cooling rates were 1 °C/s, and the holding time at each temperature was limited to the acquisition time. Both experiments were performed in argon at atmospheric pressure, i.e., corresponding to an oxygen partial pressure P(O2) of about 10 -5 atm.

HT-XRD measurements were performed using a high-temperature X-ray diffraction setup consisting in a Bruker D8 Discover diffractometer equipped with a Cu (1.5418 A) X-ray tube operated at 40 kV and 40 mA, a nickel filter, a solid-state 1-D Lynx-eye detector and a high temperature furnace. These measurements enabled identification of the phases at the specimen surface as well as their lattice parameters, crystallite size and micro-strain evolution during high temperature oxidation.

Before running the experiments on Hi-Si SGI, the pseudo-Voigt peak broadening related to the instrument configuration was evaluated using a standard powder sample from the National Institute of Standards and Technology (NIST). This method enabled the determination of the Caglioti parameters (U, V, W for Gaussian components and X, Y for the Lorentz components) in order to perform the deconvolution of the signal due to the instrumental configuration and to obtain the quantification of the physical parameters such as average crystallite size and micro-strain.

After this calibration, measurements were performed on Hi-Si SGI specimens with a mounting, from = 40° to 102°, with a step of 0.02° for an acquisition time of 1 seconds per point. The total acquisition time was 2400 seconds (40 minutes). With these parameters, the X-ray penetration depth in ferrite is evaluated to increase from 1 µm at 40° to 4.5 µm at 102° during the scan.

Phase identification, phase lattice parameter, crystallite size (Lorentz contribution) and micro-strain (Gaussian contribution) determinations were performed on each diffraction pattern using the Topas software [START_REF] Coelho | TOPAS and TOPAS-academic: An optimization program integrating computer algebra and crystallographic objects written in C++[END_REF] following the Rietveld Method [START_REF] Rietveld | A profile refinement method for nuclear and magnetic structures[END_REF]. For convenience, background was subtracted from the diffraction patterns before plotting.

Results

The in-situ HT-XRD experiments are presented first because these results are key to understanding many other observations. This is followed by metallographic analysis of the oxide scales and then oxidation and decarburization kinetics are presented.

In situ high temperature X-ray diffraction

The diffraction patterns obtained during in-situ HT-XRD experiment n°1 up to 750°C (see thermal cycle on Fig. 2-a) presented only ferrite peaks. On the contrary, the diffraction patterns recorded during in-situ HT-XRD experiment n°2 (see the thermal cycles in Fig. 2-b) that are displayed on Fig. 3-a clearly show that austenite peaks were detected along with ferrite peaks when the specimen was heated up to 800°C. These austenite peaks disappear and only ferrite is detected after cooling again the specimen down to 100°C. As expected, some oxidation took place and hematite was also clearly detected on all diffraction patterns after the second heating to 800°C (step 5 and later steps). The formation of austenite was unexpected as the three-phase ferrite-austenite-graphite field for the nominal composition of the studied alloy is located at temperature higher than 860°C [START_REF] Ebel | Coarsening and dendritic instability of spheroidal graphite in high silicon cast iron under thermal cycling in the ferritic domain[END_REF]. This important observation will be discussed later. During both experiments n°1 and n°2, the first heating up to 750°C or 800°C led to a drop in micro-strain in ferrite which is manifested by a decrease of the width of the ferrite peak observed between steps 2 and 3 on fig. 3-a. It is correlated to a drop in micro-strain (Fig. 3b ) and to a large increase of its average crystallite size as calculated by the Rietveld method (not shown here). The measurement of micro-strain also reveals that the formation of austenite at 800°C is not accompanied by micro-strain during the first heating to 800°C (step 3). However, micro-strain in austenite is identified during the second thermal cycle (step 5) and the micro-strain level then increases with further thermal cycling. After the second cooling down to 100°C (step 6), some micro-strain is also identified in the ferrite phase that may be inherited from austenite. In the same way as for austenite, the level of micro-strain detected in ferrite increases with the number of cycles (steps 8 and 10). Based on these measurements, it can be assumed that the austenite formed during isothermal oxidation is micro-strain free and that the micro-strain identified in austenite is only linked to thermal cycling.

The shift in the ferrite peaks positions with temperature enabled to determine the evolution of its lattice parameter with temperature. Its thermal expansion coefficient was thus obtained from both experiments n°1 and n°2, and the average value was evaluated at 16.10 -6 °C-1 .

Oxide scale constitution

Observation of specimen cross-sections on Fig. 4 shows that isothermal oxidation for 50 hours resulted in the formation of oxide nodules whereas cyclic oxidation resulted in the formation of an oxide scale covering the whole specimen surface with an even thickness.

However, both oxide scales have the same duplex structure that was characterized by EDS microanalysis profiles which showed that the external part is iron-rich while the internal part is iron-and silicon-rich. Voiding and some cracking are observed in the outer Fe-oxide layer. The graphite nodules observed in the internal oxide layer indicates that the Fe-Si rich oxide layer was inward growing while the outer Fe-oxide was outward growing. The dotted line in each micrograph of Fig. 4 locates the interface between these inner and outer oxide scales. The above observations are consistent with previous works on Si-rich cast irons from Ekström et al. [START_REF] Ekström | Influence of Cr and Ni on high-temperature corrosion behavior of ferritic ductile cast iron in air and exhaust gases[END_REF] or Tholence et al. [START_REF] Tholence | High-temperature corrosion of cast irons and cast steels in dry air[END_REF] and others, but a new feature was evidenced that is the columnar structure at the interface between the matrix and the inner oxide scale.

This columnar structure is located in Fig. 4 for the micrographs of the cycled samples, while it is less apparent but still present on the isothermal sample. Such a feature could be seen on a micrograph in a study by Lekakh et al. [START_REF] Lekakh | Improving high-temperature performance of high Si-alloyed ductile iron by altering additions[END_REF] (their Fig. 10) but was not commented by these authors. It will be further detailed in the next sub-section. Raman spectroscopy was used to identify oxides in the scale and in the columnar structure on specimen cross-sections. The Raman spectra obtained after 3000 cycles are presented on Fig. 5. The bands were identified using literature information for fayalite [START_REF] Chopelas | Single crystal Raman spectra of forsterite, fayalite, and monticellite[END_REF] and iron oxides [START_REF] De Faria | Raman microspectroscopy of some iron oxides and oxyhydroxides[END_REF]. These spectra reveal that the outward growing iron-rich oxide scale consists of two layers: the thin outermost part of the scale is hematite (point E) while the thicker inner part is magnetite (point D). In the inward growing Fe-Si oxide scale, both magnetite and wüstite are detected alongside fayalite (points B and C). At the metal-oxide interface, fayalite is clearly identified (point A) along with other unidentified bands.

The columnar structure

It should be noted that the above-mentioned columnar area is clearly apparent in both the SEM and mapping images of Fig. 6. It is seen that protrusions of oxide grow inward at the metal oxide interface, and they were observed after both isothermal heat treatment and thermal cycling. After isothermal oxidation, these oxide protrusions are however only clearly apparent at the metal-oxide interface of some of the iron-rich oxide nodules as illustrated with Fig. 7. Because these are the areas where the formation of austenite evidenced by in-situ HT-XRD is expected, EDS microanalysis was carried out to check for local changes in substrate composition. Using EDS line profiles (not shown), a gradient in chemical composition of the matrix was measured over approximately 5 microns below the metal-oxide interface, i.e., twice as much as the length of the protrusions. While the Si content of the matrix in the bulk is 4.2 ± 0.1 wt.% Si (7.9 ± 0.2 at.% Si), the Si content at the metal-oxide interface in between the oxide protrusions is 2.5 ± 0.3 wt.% Si (4.8 ± 0.7 at.% Si). It will be seen in the discussion section that such silicon depletion does favour the formation of austenite at 800°C. Fig. 7. Back scattered electron image of an oxide nodule after 50 hours at 800°C.

After cyclic oxidation, the oxide protrusions formed a regular cell-like columnar structure that was observed along most of the metal-oxide interface. Fig. 8 offers a closer look at the metal-oxide interface after thermal cycling. As the protrusions show different grey levels, there may be different oxides in their structure and silicon-rich precipitates are observed in the matrix in between them. The spacing between these protrusions is typically 2-3 microns and does not seem to be affected by the number of thermal cycles, while the thickness of the columnar structure increases slightly with the number of thermal cycles as can be seen in Fig. 4. It is also noted in Fig. 8-a that ghosts of this columnar zone form layers within the inner oxide scale. In an attempt to identify the phases in the columnar structure, a Raman mapping at high magnification was recorded that is displayed on Fig. 9. Iron oxide is detected in the metallic matrix at the tip of some but not all of the oxide protrusions. In between the protrusion tips and the inner oxide scale, fayalite was also detected, either alone or with iron oxide. Due to poor signal in this area, this iron oxide could not be properly identified with Raman spectroscopy. Indeed, the peak related to the red colour on the mapping is in the range 640 -670 cm -1 and can be attributed to either wüstite or magnetite [START_REF] Hazan | Characterization of AISI 4340 corrosion products using Raman spectroscopy[END_REF]. Accordingly, this iron oxide is thus denoted in Fig. 9. Note that, however, only wüstite is theoretically expected at the metal-oxide interface at 800°C.

In a previous work on Hi-Si SGI, magnetite and fayalite were identified using both Raman spectroscopy and XRD after grinding the surface to remove the outer scale [START_REF] Ebel | High-temperature oxidation of a high silicon SiMo spheroidal cast iron in air with in situ change in H2O content[END_REF]. Using the same grinding technique, wüstite was clearly identified from XRD patterns along with fayalite by Méndez et al. [START_REF] Méndez | Effect of silicon and graphite degeneration on high-temperature oxidation of ductile cast irons in open air[END_REF]. It is worth noting that wüstite is observed deeper in the scale than magnetite, which could explain why it was not reported in some other previous works on Hi-Si SGI if the grinding was not prolonged enough. EDS mappings of Fig. 6 showed that the oxide protrusions in the columnar structure are mainly Si-rich with some parts containing little iron. This result is consistent with the identification of fayalite and iron oxide on the Raman mapping. The EDS mappings also show that the tips of the oxide protrusions where no oxide could be identified on Raman spectra are mainly silicon rich and therefore could be amorphous silica. As a matter of fact, transmission electron microscopy (TEM) is needed to clearly identify silica [START_REF] Lekakh | Prevention of high-temperature surface degradation in SiMo cast irons by Cr and Al alloying[END_REF] which was not attempted in the present study.

Finally, and as already noted for the isothermal sample, EDS point analysis in the matrix located between the oxide protrusions of the columnar structure showed silicon depletion compared to the matrix located further in the substrate.

Evaluation of oxidation and decarburization kinetics

As described in the Materials and Methods section, the samples were removed from the test rig every 200 cycles to be weighted. No debris from oxide spallation could be observed during thermal cycling and the oxide scale remained adherent during all handling operations. It is therefore assumed that the oxide scale remained adherent during the whole experiment and that oxide spallation was negligible.

Weight monitoring performed during the thermal cycling experiment is presented on Fig. 10-a. As expected, it shows clearly that the specific mass change, denoted apparent specific mass change mapp/S, increased with the number of cycles due to the formation of oxide scale. In this graph, the bars represent the scattering of the measurements which were performed on 9 samples for up to 1000 cycles, 6 samples between 1200 and 2000 cycles and finally 3 samples for the remaining cycles. The scattering increased because of this decreasing number of data and certainly also because of differences in the overall oxidation kinetics between samples. The apparent specific mass gain for the isothermal sample is also shown in Figure 10-a for comparison with the results at 3000 cycles.

The oxide scale thickness was measured in several locations after 1000, 2000 and 3000 thermal cycles, and after isothermal oxidation. As the oxide scales showed an even thickness after thermal cycling, the thickness measurements reported on Fig. 10-b are related to the overall oxide scale thickness. On the contrary, for the isothermal heat-treated specimen, oxide thickness measurements were only performed on oxide nodules. During cross-section preparation of this latter specimen, the oxide scale heavily spalled so that the number of measurements was limited. Therefore, the thickness of the oxide nodules shown in Fig. 10-b for the isothermal sample may be more indicative than statistically relevant. It is noted however that this value of 130 µm is of the same order than the 150 µm obtained by Lekakh et al. on a Hi-Si SGI with similar silicon content (4.2 wt.%) oxidized in the same conditions (temperature, atmosphere) but for a longer time of 100 h [START_REF] Lekakh | Quantitative µ-CT analysis of scale topology formed during oxidation of high SiMo cast iron[END_REF]. We will come back to this difference later.

Comparison of the specific mass variation and of the thickness of the oxide scale resulting from thermal cycling and isothermal oxidation on Fig. 10 show large differences. The oxide scale measured after 3000 thermal cycles is approximatively twice thicker than the oxide nodules resulting from isothermal oxidation. Such a difference can be clearly seen with the micrographs in Fig. 4. As a result, Fig. 10-a shows that the change in apparent specific mass after 3000 short thermal cycles is approximately twice as high as after isothermal oxidation. The apparent specific mass variation obtained through weight measurements is not a good metric for oxidation of SGI at 800°C because it is the sum of the specific mass gain due to oxide formation and the specific mass loss due to decarburization :

(1)

Oxide scale thickness is a more suitable metric if the thickness of the oxide scale formed is even over the whole specimen surface as is the case for the experiments with thermal cycling. Assuming oxidation kinetics follows a parabolic rate with a constant that depends on the temperature T and is constant over time t for a given temperature, the change in oxide scale thickness can be expressed as [START_REF] Young | High Temperature Oxidation and Corrosion of Metals[END_REF]:

(2)

In case of thermal cycling, the thickness of the oxide scale grown during one thermal cycle of duration tcycle is thus obtained by time integrating Eq. ( 2). This leads to:

(3)

It will be further assumed that the parabolic oxidation rate follows an Arrhenius law which is written:

(4)
For calculations, each thermal cycle was discretized into n time steps. Using i as the increment for time within a thermal cycle and j as the increment for the number of thermal cycles, the oxide scale thickness can be defined as:

(5)

For any thermal cycle j, the initial oxide scale at t=0 (i=0) is set to the oxide scale computed at the end (i=n) of the previous cycle (j-1), i.e.:

For the first cycle, the initial oxide scale thickness was arbitrarily set to 100 nm. Using the experimental thermal cycles for describing the change , the oxide thickness could then be calculated as a function of the number of thermal cycles for any activation energy and any pre-exponential term of the Arrhenius law describing the oxidation kinetics.

A minimization procedure was applied to determine the parameters and for which the calculated oxide thickness has the best fit with the oxide thickness measured after 1000, 2000 and 3000 thermal cycles. The result of the minimization procedure gave A0=5.96•10 -6 m 2 •s -1 and Ea=1.58•10 5 J•mol -1 . With these values, it is seen in Fig. 11 that the calculated thickness values are in good agreement with experimental results. Interestingly, the calculation for isothermal conditions also led to a good agreement with the experimental value as shown in Fig. 11. This suggests that the increased oxidation during thermal cycling as compared to isothermal condition is mostly due to the heating and cooling stages and not to a change in the oxidation mechanism. The Arrhenius law that has been identified for the parabolic oxidation rate can be discussed by comparison with literature data for silicon [START_REF] Deal | General relationship for the thermal oxidation of silicon[END_REF] and for iron [START_REF] Chen | Review of the high-temperature oxidation of iron and carbon steels in air or oxygen[END_REF] isothermal oxidation that are indicated along the corresponding lines in Fig. 12. This figure shows that the present estimate for Hi-Si SGI is quite close to that for pure iron oxidation, with a slower value that can probably be associated to some positive effect of silicon. However, the present analysis does not show an effect of the formation of silica in slowing down the oxidation kinetics between 700 and 800°C, in line with the presence of water in air and possibly also because of the detrimental effect of the columnar zone. Using again the fact that the oxide layer has even thickness during cyclic oxidation, the specific mass gain due to oxidation can be derived considering an equivalent oxide layer with a density , a molar mass and a number of moles of oxygen, , per mole of oxide. With being the molar mass of oxygen, the specific mass gain due to oxidation is given as:

The molar volume of the oxide scale can be derived from the thickness of base metal consumed through the oxidation process considering it relates to the thickness of the inward growing scale. With VM, and being the molar volume, the molar mass and density of the matrix, respectively, the equivalent molar volume of the oxide can be determined from the following balance:

For calculations, the molar volume of the Fe-Si matrix was evaluated using the density value of 7.570 g•cm -3 that was previously estimated [START_REF] Ebel | Coarsening and dendritic instability of spheroidal graphite in high silicon cast iron under thermal cycling in the ferritic domain[END_REF]. The molar mass was set to 53.5 g•mol -1 considering a binary Fe-Si alloy with 4.45 wt.% Si. The molar volume of the oxide offering the best fit with the data was obtained using the molar mass and density of wüstite, respectively 71.8 g•mol -1 and 5.7g•cm -3 . Inserting this data in Eq. ( 7) and using the average experimental thickness measurements plotted on Fig. 11, the specific mass variation due to oxidation was computed for 1000, 2000 and 3000 cycles and is plotted with upwards triangles in Fig. 13. It is seen that these estimates are higher than the measured apparent specific mass values, which suggests that the mass loss due to decarburization, mC, cannot be neglected. Based on Eq. ( 1), this mass loss is such that and its estimates are plotted with downwards triangles in Fig. 13. The mass loss due to decarburization evaluated in this way appears more or less constant at -5 mg•cm -2 , a value that was reached during the first 1000 cycles. that some graphite particles are missing. A closer look at the metal-oxide interface with the greater depth of field provided by SEM observations on Fig. 8 confirms that cavities can be observed at the previous location of graphite particles. Thus, decarburization took place during heat treatment and carbon from the dissolved graphite particles diffused towards the metal-oxide interface and escaped as gaseous species. This process has recently been studied in detail by Lekakh et al. [START_REF] Lekakh | Quantitative µ-CT analysis of scale topology formed during oxidation of high SiMo cast iron[END_REF][START_REF] Lekakh | Prevention of high-temperature surface degradation in SiMo cast irons by Cr and Al alloying[END_REF].

Measuring the distance between the metal-oxide interface and the farthest cavities would quantify the extent of decarburization. However, both cavities resulting from decarburization and apparently intact graphite particles can be observed at similar depth below the oxide-metal interface. This means that graphite particles can dissolve in the matrix at different rates, making it difficult to assess decarburisation using metallographic observations. While the real decarburization depth could hardly be quantified, the fact that the central part of the studied samples was not decarburized [START_REF] Ebel | Coarsening and dendritic instability of spheroidal graphite in high silicon cast iron under thermal cycling in the ferritic domain[END_REF] suggests should be of the same order as the oxide scale, i.e., 150 µm, for both isothermal and cyclic oxidation. Such a value is much lower than the one observed after 100 h isothermal oxidation at 800°C by Lekakh et al. [START_REF] Lekakh | Prevention of high-temperature surface degradation in SiMo cast irons by Cr and Al alloying[END_REF] in which work the decarburized depth was several times thicker than the actual oxide scale. In fact, the present experiments were performed in still air, whereas Lekakh et al. imposed an air flow of 0.5 L.min -1 to avoid gas starvation.

Such conditions may have favored oxidation of carbon and its loss as gas species.

Accordingly, less oxygen would have been available for further oxidation of the cast iron matrix which is in accordance with the lower oxidation rates in Lekakh's experiments than in the present ones as noticed above in relation with Fig. 10-b.

Discussion

The oxide scales observed in this work after isothermal holding at 800°C and thermal cycling show the same duplex structure, and their growth kinetics could be described with the same parabolic law. In agreement with literature, the continuous and even scale observed after thermal cycling results from lateral growth of oxide nodules until they coalesce. From the results described above, it can be concluded that the highlighted columnar structure is important in the oxidation mechanism of Hi-Si SGI. This discussion is thus intended to elaborate on the origin of this structure and the probable coupled effect of iron oxide growth, silica formation, carbon and silicon redistribution, as well as austenite formation.

Atkinson and Gardner [START_REF] Atkinson | A theoretical analysis of the oxidation of Fe-Si alloys[END_REF] have described the formation of silica during oxidation of Fe-Si alloys as resulting from silicon being rejected in front of the iron oxide nodules growing inward as silicon cannot enter into iron oxides. The silicon content in the matrix in contact with the Fe-rich oxide scale thus increases until it becomes high enough for silica to precipitate, which then forms a protective layer. In the case of cast irons, the silica layer is certainly effective in slightly slowing down oxidation kinetics but it gets easily disrupted and overtaken by the fast growing iron oxide nodules. When this happens, silica should react with wüstite to form fayalite because it is impossible for silica, wüstite and the matrix to be in equilibrium at a single oxygen pressure for a given temperature.

A key result of the in-situ HT-XRD experiments performed in this work is the detection of the phase transformation of the ferritic matrix into austenite close to the specimen surface at 800°C. Differential thermal analysis performed on the same alloy demonstrated that the phase transformation of ferrite does not take place below 860°C for the bulk material [START_REF] Ebel | Coarsening and dendritic instability of spheroidal graphite in high silicon cast iron under thermal cycling in the ferritic domain[END_REF].

Austenite formation during oxidation is thus guessed to be associated with the silicon depletion of the matrix due to the formation of silica that has already been described in the literature for Fe-Si model alloys [START_REF] Logani | The development of the wüstite-fayalite scale on an iron-1.5 wt. silicon alloy at 1000°C[END_REF]. On the calculated isotherm section of the relevant phase diagram in Fig. 14, it is seen that austenite could appear at 800°C provided the local silicon content decreases below about 3 wt.%. The extremities of the arrow in Fig. 14 correspond to the composition of ferrite far from the oxide scale, namely 4.45 wt.% Si, and to the average EDS measurement at the metal oxide interface at 2.5 wt.% Si that was reported above. This shows effectively that austenite could have formed during both isothermal and cyclic oxidation when the temperature reaches 800°C.

However, silicon depletion is not the only condition for austenite to form at 800°C. As shown with Fig. 14, austenite requires carbon for growing. As the solubility of carbon in ferrite is negligible, the carbon required for austenite to grow can only come from graphite.

In other words, growth of austenite in the silicon depleted ferritic matrix is also controlled by carbon diffusion from graphite to the surface through the ferritic matrix. The carbon that is thus temporarily fixed by the austenite does not escape as gaseous species, which may explain the lower decarburisation observed in the present work compared to the results reported by Lekakh et al. In turn, if the conditions are such that most of the carbon escapes as gas, austenite cannot form as easily and this could explain why the columnar structure was not observed in previous works. It seems therefore that the experimental conditions selected in the present work were particularly appropriate to highlight the mechanism by which the columnar structure appears, and that this mechanism must be related to the formation of austenite. showing that only ferrite is stable for Si contents higher than 3 wt.% and that austenite appears for lower Si contents. The calculations were made with the Thermocalc software and the TCFE8 database [START_REF] Andersson | Thermo-calc & DICTRA, computational tools for materials science[END_REF]. The red arrow illustrates the change of composition from the bulk ferrite to the area in between oxide protrusions.

From the above discussion, a schematic for the formation and development of the complex oxide scales that have been observed may be proposed. During early oxidation, iron oxides form on top of a thin silica layer while a silicon gradient develops in the substrate as illustrated with the graded contrast in the scheme of Fig. 15-a. When the sample is heated to 800°C, small austenite grains eventually nucleate and develop at the interface between the matrix and the silica layer. As austenite grows, it contracts with respect to ferrite and this gives rise to stresses exerted on the silica layer which are illustrated with the solid arrows in Fig. 15-b. Such stresses lead eventually to ruptures in the silica layer shown with thick vertical dark lines, and to stress release in both phases. Through these cracks, the iron oxide may rapidly overtake the original silica layer, pushing away and alongside particles of silica.

During this stage, the silica layer transforms in a mix of fayalite and iron oxides (see Fig. Once the silica layer at the interface between the substrate and the internal iron oxide scale has completely transformed into FexOy and fayalite, fayalite stops growing. At this point, the process of silica layer formation at the metal/oxide interface can start again until the formation of austenite leads to its cracking here and there. The fact that this is a repetitive process is demonstrated by Figure 16, where it can be seen that the columnar zone is not continuous along the boundary between the matrix and the inner oxide layer, as well as by the clearly separated columnar-type layers in the inner oxide layer. Furthermore, the observation that the thickness of the columnar structure increases with cycling is certainly in line with the fact that these protrusions are so little developed in the isothermally oxidised sample. However, further work seems necessary to clarify whether the effect of cycling is due to stress accumulation, increased silicon consumption or austenite formation, or else a coupling of these phenomena with temporary carbon fixing.

Conclusion

To test the high silicon cast iron used for automotive exhaust manifolds, rapid thermal cycling oxidation up to 800°C was performed and compared to a reference sample oxidised at constant temperature. Optical observations, micro-analyses and Raman spectroscopy showed that the outer and inner oxide layers have the same constituents in cyclic and isothermal oxidation. The experimental cyclic oxidation kinetics could be described by considering an equivalent oxide layer following a parabolic growth law which was found to apply also to the isothermal oxidation test. It could thus be concluded that the oxidation mechanism is the same under both conditions.

However, the rapid cyclic oxidation favours the formation of a columnar zone at the substrate/oxide interface which has never been reported but could also be observed to a small extent on the isothermal sample. In addition, in-situ high-temperature X-ray diffraction showed the formation of austenite at 800°C associated with silicon depletion at the substrate surface due to the precipitation of silica, which then transforms into fayalite.

This results in the repetitive formation of successive layers, the remnants of which appear in the inner oxide scale. A probable mechanism for this repetitive process has been proposed, which is based on the formation of silica, its mechanical breakdown due to the precipitation of austenite, followed by an overgrowth of rapidly growing iron oxides. This process is also thought to occur during isothermal oxidation, but is highlighted by cycling.

Funding

None

Fig. 1 .

 1 Fig. 1. a: Examples of thermal cycle measured between 0 and 1000 cycles, 1000 and 2000 cycles and between 2000 and 3000 cycles. b: Histogram of the cumulated time spent in the range 690 -810°C after 3000 cycles considering the evolution of the thermal cycles.

Fig. 2 :

 2 Fig. 2: Temperature profiles of the in-situ HT-XRD experiment n°1 (a) and experiment n°2 (b) carried out in this work on Hi-Si SGI. Red lines indicate the XRD acquisition steps.

Fig. 3 .

 3 Fig. 3. Part of the in-situ X-ray diffraction patterns of experiment n°2 showing the most significant ferrite and austenite peaks (a) and the micro-strains identified with the Rietveld method in these phases during thermal cycling (b).

Fig. 4 .

 4 Fig. 4. Optical micrographs of the oxide scale after isothermal holding (a) and 1000 (b), 2000 (c) and 3000 (d) cycles. Dotted lines represent the initial specimen surface and separate the outward growing and inward growing oxide scales.

Fig. 5 .

 5 Fig. 5. Light micrograph (a) with the locations of the Raman spectra (b) obtained on the oxide scale cross-section after 3000 cycles. The Raman spectra are labelled according to the locations indicated in a.

Fig. 6 .

 6 Fig. 6. Characterization of the metal-oxide interface after 3000 cycles: SEM backscattered electron image (a), Si EDS map (b), Fe EDS map (c) and O EDS map (d). 1, 2, 3 denote the different constituents of the inward growing oxide.

Fig. 8 .

 8 Fig. 8. Back scattered electron image of the oxide scale after 3000 cycles: inward growing oxide scale (a) and columnar structure at metal-oxide interface (b). The micrograph in b is a close up of the squared area in a.

Fig. 9 .

 9 Fig. 9. Raman map obtained at the metal-oxide interface after 3000 cycles: iron oxide is red (see text), fayalite is blue, pink areas contain both iron oxide and fayalite.

Fig. 10 .

 10 Fig. 10. Evolution of the apparent specific mass (a) and total oxide scale thickness (b) during cyclic oxidation. The specific mass variation and oxide thickness resulting from the 50 h isothermal heat-treatment are reported at 3000 cycles for comparison purpose. Points are related to mean values of measurements while the height of the error bars is twice the standard deviation of the related measurements.

Fig. 11 .

 11 Fig. 11. Comparison of the experimental oxide scale thickness measured during thermal cycling (TC_exp) and isothermal heat-treatment (Iso_exp) with the results from the oxidation model for thermal cycling (TC_num) and isothermal heat-treatment (Iso_num).

Fig. 12 .

 12 Fig. 12. Comparison of the parabolic rate of formation of silica on Si, iron oxides on Fe and the oxide scale formed on Hi-Si SGI in this work.

Fig. 13 . 9 )

 139 Fig. 13. Comparison of the experimental apparent specific mass with the calculated specific mass resulting from oxidation and the mass loss from decarburization.

Fig. 14 .

 14 Fig. 14. Isothermal 800°C section of the Fe-C-Si-Mn phase diagram with 0.25 wt.% Mn

  15c), and carbon is rejected on the side of the protrusions where it further stabilizes austenite.

Fig. 15 .

 15 Fig. 15. Schematic diagram showing the formation of protrusions of iron oxides along the interface between the cast iron substrate and the inner oxide layer during oxidation at 800°C.

Fig. 16 .

 16 Fig. 16. SEM micrograph of a sample having been cycled 3000 times showing the disruption of the columnar zone and its ghosts within the inner oxide scale.

For complementing Raman results, EDS microanalysis maps were recorded on the same sample on the inward growing oxide scale. Fig.6shows an example with a SEM image and the corresponding Si, Fe and O mappings. The mappings indicate that the inward growing oxide scale presents three constituents: Si and O-rich areas that could be silica (1); Fe, Si and O-rich areas that could be fayalite (2); O-poor, Si-poor and Fe-rich areas that could be wüstite or magnetite[START_REF] Lacaze | The austenite to pearlite/ferrite transformation[END_REF].

Acknowledgments

Azterlan (Bizkaia) kindly provided the material used in this study. Special thanks are due to Yannick Thebault for SEM observations and EDS measurements. The quality of the experimental work performed by Mariana Alves Pegoraro during her stay at CIRIMAT is greatly acknowledged as well as the support of Ronan Mainguy.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations


Tilahun Yeshambel
email: tilahun.yeshambel@uog.edu

Josiane Mothe
email: josiane.mothe@irit.fr

Yaregal Assabie
email: yaregal.assabie@aau.edu.

Morphologically Annotated Amharic Text Corpora

Keywords: Information systems ~ Information Retrieval, Information systems ~ Document representation, Information systems ~ Information retrieval, Corpus, Morphological annotation, Underresourced language, Amharic

In information retrieval (IR), documents that match the query are retrieved. Search engines usually conflate word variants into a common stem when indexing documents because queries and documents do not need to use exactly the same word variant for the documents to be relevant. Stemmers are known to be effective in many languages for IR. However, there are still languages where stemmers or morphological analyzers are missing; this is the case for Amharic which is the working language of Ethiopia. Morphological analysis is the key to derive stems, roots (primary lexical units) and grammatical markers of words such as person, tense and negation markers. This paper presents morphologically annotated Amharic lexicons as well as stem-based and root-based morphologically annotated corpora which could be used by the research community as benchmark collections either to evaluate morphological analyzers or information retrieval for Amharic. Such resources are believed to foster research in Amharic IR.

Introduction

A core component of information retrieval system is querydocument matching which the system uses to retrieve document for a given query. Because queries and documents do not need to use exactly the same word variant for the documents to be relevant, search engines usually apply word stemming. Stemming is the process of conflating the variants of a word (most often inflected forms) to their stem or base responsible for the word meaning. Search engines can then treat words with the same stem as synonyms in the query-document matching process. For example, the English word variants developer, develops, developed, undeveloped, etc. may all be considered as synonyms to the stem or root "develop". Other natural language processing applications such as text classification and categorization require word stemming and rooting as pretreatments [START_REF] Gaustad | Accurate stemming of Dutch for text classfication[END_REF]. Automatic morphological analyzers for word stemming and rooting often rely on rule-based algorithms, such as Porter's stemmer, which is the reference for English [START_REF] Porter | An algorithm for suffix stripping[END_REF]. Other rule-based stemmers have been developed for different languages such as Portuguese [START_REF] Orengo | A stemming algorithm for the Portuguese language[END_REF] or Arabic [START_REF] Aljlayl | On Arabic search: improving the retrieval effectiveness via a light stemming approach[END_REF]. Rule-based algorithms are, however, difficult to implement in the case of morphologically complex languages [START_REF] Hovy | Towards a science of corpus annotation: A new methodological challenge for corpus linguistics[END_REF]. Corpus-based stemming and rooting are alternatives to rule-based stemming; they rely on word similarity, n-gram frequencies and dictionaries or lexicons [START_REF] Majumder | YASS: Yet another suffix stripper[END_REF] [START_REF] Singh | A novel unsupervised corpus-based stemming technique using lexicon and corpus statistics[END_REF] but need resources and references to optimize their hyperparameters.

The development and evaluation of such algorithms need textual corpora and references [START_REF] Jiaul | A fast corpus-based stemmer[END_REF] [START_REF] Melucci | A novel method for stemmer generation based on hidden Markov models[END_REF] [10] [START_REF] Daud | Urdu language processing: a survey[END_REF]. In the case of well studied languages such as English, evaluation forums like TREC 1 [START_REF] Harman | Overview of the second text retrieval conference (TREC-2)[END_REF], CLEF 2 [START_REF] Ferro | CLEF 15th birthday: past, present, and future[END_REF], and NTCIR 3 [START_REF] Kando | The NTCIR workshop : the 1 st evaluation workshop on Japanese text retrieval and cross-lingual information retrieval[END_REF] are used to develop and evaluate these algorithms on different tasks. For digitally underresourced languages, tools and reference corpora are usually not available, which is the case for Amharic. In such cases, annotated lexicons are possible alternatives to serve as references for automatic morphological analyzers [START_REF] Chaudhury | Annotated Bangla news corpus and lexicon development with POS tagging and stemming[END_REF]. Although Amharic is the working language of Ethiopia, it is still an under-resourced language characterized by lack of standard digital resources required for natural language processing and IR.

In this paper, we present a collection which consists in two lexicons of 170,000 morphologically annotated Amharic terms where both stems and roots are annotated, as well as corpora of texts where documents have been re-written using these lexicons. These texts are part of the 2AIRTC, the Amharic Adhoc Information Retrieval Test Collection where documents, queries and query relevance are provided [START_REF] Yeshambel | 2AIRTC: The Amharic adhoc information retrieval test collection[END_REF].

The rest of this paper is organized as follows. Section 2 gives background information about Amharic language and its morphology. Section 3 discusses related work. In Section 4, we present the morphological annotation process, the structure of the morphological tags and the details of our resources. Section 5 discusses about the implications of using the resources in Amharic IR. Finally, we draw conclusions and highlight future directions in Section 6.

Amharic language

Amharic is the working language of the government of Ethiopia, currently having a population of over 110 million, and serves as the lingua franca of the country. It is widely used as a medium of communication in governmental, religious, educational, social and business institutions of the country. Amharic is a Semitic language and uses Ethiopic alphabet for writing. The writing system has 34 base characters which change their shapes into seven different forms due to vowels. Owing to its Semitic characteristics, Amharic is known to have a rich and complex morphological structure. According to Assabie [START_REF] Assabie | Development of Amharic morphological analyzer[END_REF], thousands of surface words can be generated from a base form. Amharic words are inflected for person (first, second, third), gender (feminine, masculine), number (singular, plural), case (subjective, objective, possessive), definiteness (definite, indefinite), tense (past, present, future), aspect (perfective, imperfective), politeness (impolite, polite), etc. [18][31]. This is achieved by adding prefixes, infixes and suffixes. The grammatical relations like subject, object, and syntactic information might be indicated morphologically at word level. For instance, the word ይሰጣታል /jɨsəţatalɨ 'he will give her'/ is composed of the subject marker for imperfect tense ይ…ኣል /jɨ…ʔalɨ/, imperfect verbal stem ሰጥ /səţɨ/ and the object marker ኣት /ʔatɨ/. Verbs exist in perfective, imperfective, jussive, gerund, and infinitive forms. Each form has its own stem template. Stems of verbs can be classified as basic and derived stems. The basic stems of verbs are modified either internally (by inserting infix) or externally (by attaching prefix) to form derived stems. The derived stems include passive, causative, infinitive, and reduplicative. Passive stems are formed by attaching the prefix 'ተ /tə/' on basic stems, causative stems are derived variably by attaching the prefix 'አ /ʔə/', 'አስ /ʔəsɨ/', or 'አት /ʔətɨ/', and infinitive stems are formed by adding the prefix 'መ /mə/' on basic stems. Moreover, derived stems can be formed by reduplicating a character of basic stem. Both types of stems may be preceded by many prefixes and followed by many suffixes. The orthography of Amharic language can be the combination of one or more functional words and inflectional morphemes. Amharic morphemes play significant roles both in morphology and syntax. Most Amharic words are composed of a basic form and many attached affixes. Prefixes can be the prepositions (ከ /kə/, በ /bə/, etc.) or genitive (የ /jə/), negations (አል /ʔəlɨ/), and conjunctions (እንደ /ʔɨnɨdə/, etc.) while suffixes include plural marker (ኦች /ʔotʃɨ/ or ዎች /wotʃɨ/), possessive (ኤ /ʔe/, ኡ /ʔu/, አችን /ʔətʃɨnɨ/, ሽ /ʃɨ/, etc.), or a definite marker (ኡ /ʔu/, ው /wɨ/, ዋ /wa/), and connectors (ና /na/). Gender, number, case, and definite markers can be suffixed to the stem of nouns. The sets {ኢት /ʔitɨ/, ዋ /wa/, ኡ /ʔu/, ው /wɨ/}, {ኦች /ʔotʃɨ/, ዎች /wotʃɨ/, አን /ʔənɨ/, እየ /ʔɨjə/, አት /ʔətɨ/}, {ን /nɨ/, ዬ /je/, ኤ /ʔe/, ዎ /wo/, ህ /hɨ/, ሽ /ʃɨ/, ኡ /ʔu/, ዋ /wa/, አችሁ /ʔətʃɨhu/, አችን /ʔətʃɨnɨ/, አቸው /ʔətʃəwɨ/}, and {ኡ /ʔu/, ዋ /wa/, ው /wɨ/, ኢቱ /ʔitu/, ይቱ /jɨtu/} are gender, number, case, and definite markers, respectively. The most common suffixes to derive adjectives are አማ /ʔəma/ and አዊ /ʔəwi/ from nouns, and ኢ /ʔi/ from verbs. The language has a number of lexical variations and clitics [START_REF] Demeke | Manual annotation of Amharic news items with part-of-speech tags and its challenges[END_REF]. Sometimes, there is no clear demarcation between clitics and content word in the orthography. The clitics such as prepositions and conjunctions, which have syntactic roles, indicate grammatical relations with the content words. An Amharic content word can represent a phrase, a clause or a sentence [START_REF] Epherem | Morphosyntactically annotated Amharic treebank[END_REF] [START_REF] Mekonnen | Contemporary Amharic corpus: Automatically morpho-syntactically tagged Amharic corpus[END_REF]. From a computational point of view, segmenting a word into its morphemes is very crucial in many Amharic IR and NLP applications. For instance, Amharic IR systems require words in documents and queries to be segmented correctly into their stems, roots and affixes. However, separating morphemes from surface words is a challenging task. This problem has a negative impact on the performance of different applications as it results in vocabulary mismatch problem for words generated from the same root form. Therefore, Amharic raw text needs to be morphologically analyzed to get the desired results.

Related work

Information retrieval and natural language processing have benefited a lot from the advancement of automatic tools relying on corpora. This has triggered a renewed interest to create annotated corpora that can be used for training and measuring the validity, accuracy and effectiveness of IR and NLP components and systems as described below. Grubenmann et al. [START_REF] Grubenmann | SB-Ch: A Swiss German corpus with sentiment annotations[END_REF] built annotated Swiss German text corpus for sentiment analysis manually. The corpus consists of more than 200,000 phrases from Facebook comments and online chats. Similarly, Maher et al. [START_REF] Maher Itani | Corpora for sentiment analysis of Arabic text in social media[END_REF] annotated an Arabic corpus for sentiment analysis manually. The corpus was annotated with five labels (positive, negative, dual, neutral, and spam). Dukes and Habash [START_REF] Dukes | Morphological annotation of quranic Arabic[END_REF] built the first publicly accessible annotated Quranic Arabic corpus. The annotation involves morphological segmentation, part of speech (POS) tagging, and syntactic analysis using dependency grammar. It was carried out by automatic morphological tagging using diacritic edit-distance followed by manual verification in online Marcus et al. [START_REF] Marcus | Building a large annotated corpus of English: The Penn Treebank[END_REF] constructed a large Treebank corpus consisting of 4.5 million English words and made it accessible to the research community. The annotation was done automatically and then corrected by human annotators. Zinkevičius et al. [START_REF] Rimkutė | Morphological annotation of the Lithuanian corpus[END_REF] built a morphologically annotated Lithuanian corpus automatically with an analyzer. The annotated corpus contains a set of morphological features such as lemmas, prefixes, and suffixes for 1 million words. With the growing need of automatic text processing, there is also an increased interest in building Amharic corpora. A POS tagged corpus that contains 1,065 text documents having 202,671 words was built manually by the Ethiopian Languages Center at Addis Ababa University [START_REF] Demeke | Manual annotation of Amharic news items with part-of-speech tags and its challenges[END_REF]. This corpus exists in both Ethiopic and Romanized version known as SERA forms. It has been used to develop a chunker [START_REF] Ibrahim | Amharic sentence parsing using phrase chunking[END_REF] and a machine learning POS tagger [START_REF] Yifiru | Part-of-speech tagging for under-resourced and morphologically rich languages: the case of Amharic[END_REF]. Seyoum et al. [START_REF] Epherem | Morphosyntactically annotated Amharic treebank[END_REF] created the morpho-syntactically annotated Amharic Treebank using a semi-automatic approach to develop a text parser. The corpus contains 5,000 annotated sentences, out of which 1,000 sentences were manually annotated with POS tags, morphological information, and syntactic relations of words. These sentences have been used for training a machine learning system to annotate the other sentences automatically. Fifty six POS tags were compiled based on the morpho-syntactic properties of words. The universal dependence approach is applied and clitics are separated from content words manually. Gezmu et al. [START_REF] Mekonnen | Contemporary Amharic corpus: Automatically morpho-syntactically tagged Amharic corpus[END_REF] built a POS tagged corpus consisting of 25,199 documents using syntactic information of words. Each word in the corpus was tagged automatically using HornMorpho analyzer [START_REF] Gasser | HornMorpho: a system for morphological processing of Amharic, Oromo, and Tigrinya[END_REF] and manual intervention was made to correct erroneous results. The morphological analyzer generates the derived stems of nonverbal words rather than basic stems. For verbs, it generates only roots rather than stems producing incorrect representations. This leads to the generation of similar representations for many semantically non-related words. On the other hand, since the collection is simply set of annotated documents without topic set and relevance judgment, the whole corpus is not suitable for IR experiments. Although morphological properties have been used to create POS tagged corpora, there is no morphologically annotated large Amharic corpus to date. In this work, we construct two lexicons with 170,000 words annotated with their composition based on stems and roots, as well as two corpora annotated with these morphological features.

Construction of morphologically annotated Amharic lexicons and corpora

The main problems facing the development of Amharic IR and NLP applications are the morphological complexity of the language and lack of resources. Thus, this work aims at addressing these important issues. In this section, we present how we created both the stem-based and root-based morphologically annotated lexicons and corpora.

Methodology

Data sources

The documents are part of the 2AIRTC ad hoc IR collection [START_REF] Yeshambel | 2AIRTC: The Amharic adhoc information retrieval test collection[END_REF].

Recently released and made open access in the IR domain, where query and document relevance are provided along with the document collection from the 2AIRTC collection, we re-used the documents only. The 2AIRTC documents were collected from news agencies (Walta4 , Fana5 , AMM 6 ), Amharic Wikipedia7 , blogs 8 , web, and individuals. Documents are full text and of various genres and topics such as sport, health, religion, history, social issues, culture and economy and thus encompass a large variety of words.

Annotation and evaluation method

We morphologically annotated parts of the 2AIRTC documents semi-automatically. We focus on stem and root extractions since these are two core text operations in many text-based applications such as text mining, translation, IR, and question answering. An Amharic native annotator morphologically segmented unique words from the corpus into their affixes and basic stems or roots. The accuracy of the annotations was then evaluated and corrected by a linguist in the case of inappropriate annotations. This process generates the lexicons. Then, each occurrence of surface words in the corpus was replaced by its corresponding morphologically segmented word to produce the corpora. This process is detailed hereafter.

Morphological annotation process

Morphological annotation is made by extracting unique words from preprocessed Amharic documents. Documents were preprocessed by tokenizing and removing non-Amharic words such as tags for further processing. After preprocessing is made, a total of 170,000 unique words were identified from all documents. Here, the aim was to minimize errors and the time required for annotation as each word would be annotated only once. The annotation process of the stem-based and root-based corpora is shown in Figure 1. The stem-based and root-based morphological forms of each word were created manually by an annotator. Many of the Amharic surface words are constructed from more than one morphological segment called morphemes. Morphemes are divided into prefix, suffix, infix, stem and root. Therefore, the morphological annotation was performed by segmenting each into its morphemes. The general structure of a morphologically annotated word W is:

[ p_ ]* w [ _s ]* where p is a prefix morpheme, "_" is a morphological segment marker, w is the root or stem of W, s is a suffix morpheme, "[…]" denotes optionality, and "*" denotes the possibility of multiple occurrences. The number of prefixes and suffixes varies from one to five [START_REF] Mulugeta | Learning morphological rules for Amharic verbs using inductive logic programming[END_REF]. For example, the word ካልተስማማናቸውም /kalɨtəsɨmamanatʃəwɨmɨ 'if we are not comfortable for them'/ is annotated as follows.

In this example, the Amharic word is split into 7 morphological morphemes (3 prefixes, a stem and 3 suffixes) which are preposition (ከ /kə 'from'/), negation (አል /ʔəlɨ 'not'/), passive form (ተ /tə/), the verbal stem (ስማም /sɨmamɨ 'comfort'/), subject pronoun (አን /ʔənɨ 'we'/), object pronoun (አቸው /ʔətʃəwɨ 'they'/), and the negation marker (ም /mɨ 'not'/). Similarly, definite markers and conjunctions can be suffixed to a word while genitive can be attached as prefix. Therefore, a word can be segmented into a base (stem or root) and affixes like number, gender, tense, aspect, etc. The stem and the root of each word are stored in two lexicons. Each of the stem-based and the root-based morphological lexicons thus contain 170,000 unique entries. The two lexicons are organized into two columns: the surface word and the morphologically annotated form. Therefore, the stem-based morphological lexicon contains unique words and their stembased annotated forms (morphologically segmented basic stems and affixes) whereas the root-based morphological lexicon contains unique words and their root-based annotated forms (morphologically segmented roots and affixes), the details of which are discussed below. As a final resource, the stem-based and the root-based corpora are created based on the stem-based and root-based lexicons, respectively.

Stem-Based Annotation

The stem-based morphological annotation segments surface words into basic stems and affixes. The focus of this work was to segment basic stems from the rest of morphemes. The stem extraction is performed by removing characters, changing the shapes of characters, or simply segmenting stem from the rest of the word. Generally, Amharic words can be classified as derived or non-derived words. The stem-based annotation process is presented below.

Amharic verbs are marked for person, gender, number, tense, subject, object, and negation by attaching a series of affixes. Therefore, their annotation was performed by segmenting a word into its affixes and stem. The following example shows the morphological annotations of the verbs ስላላመጣን /sɨlalamət'anɨ 'since we did not bring'/ and አልተሰበረም /ʔəlɨtəsəbərəmɨ 'it was not broken'/.

ስላላመጣን አልተሰበረም ስለ_አል_አ_መጥ_ኣን አል_ተ_ሰበር_ኧ_ም sɨlə_ʔalɨ_ʔa_mət'ɨ_ʔanɨ ʔəlɨ_tə_səbərɨ_ʔə_mɨ [pre]-[neg]-[cau]-[stem]-[1,p] 9 [neg]-[pas]-[stem]-[3,s,m]-[neg]
In the case of stems derived from basic stems, annotation was made based on basic stems as they are better for conflating more variants than derived stems. Therefore, in this work verbs are annotated morphologically into their basic stems and affixes. Non-derived nouns, adjectives, adverbs and functional words are annotated into their morphemes by segmenting them into their prefixes, stem, and suffixes. For example, the noun ከየቤቱና /kəjəbetuna 'from each house and'/ and the adjective የደጎቹን /jədəgotʃunɨ 'of the (many) generous-[acc]'/ are annotated as shown below.

ከየቤቱና የደጎቹን ከ_እየ_ቤት_ኡ_እና የ_ደግ_ኦች_ኡ_ን kə_ʔɨjə_betɨ_ʔu_ʔɨna jə_dəgɨ_ʔotʃɨ_ʔu_nɨ [pre]-[dis]-[stem]-[def]-[conj] [gen]-[stem]-[p]-[def]-[acc]
Similarly, the adverbial word የነጋችን /jənəgatʃɨnɨ 'of tomorrow-1,p,pos]'/ and the functional word እንደዚያም /ʔɨnɨdəzijamɨ 'like that-[foc]'/ are annotated as follows.

የነጋችን እንደዚያም የ_ነገ_አችን እንደ_እዚያ_ም jə_nəgə_ʔətʃɨnɨ ʔɨnɨdə_ʔɨzija_mɨ [gen]-[stem]-[1,p,pos] [tran]-[stem]-[foc]
Many Amharic words are derived from other word classes by attaching affixes. For example, nouns can be derived by attaching affixes to the base of adjectives, nouns, verbs, stems and roots [START_REF] Alemayehu | Stemming of Amharic words for information retrieval[END_REF]. Accordingly, the verbal noun ለስብራታችን /ləsɨbɨratatʃɨnɨ 'for our state of being broken'/ and the abstract noun በሰውነቶቻችን /bəsəwɨnətotʃatʃɨnɨ 'with our bodies'/ are annotated as follows. Amharic adjectives can also be derived from the stems of nouns, verbs, adverbs, and verbal roots by attaching suffixes. Similarly, derived adverbs are formed from various word classes. Derived adjectives and adverbs are annotated into the stem of their corresponding nouns, verbs, or adjectives. For example, the adjective ጥቁሮቹ /t'ɨk'urotʃu 'the blacks'/ and ሰማያዊ /səmajawi 'blue'/, and the adverbs በፍጥነት /bəfɨt'ɨnətɨ 'quickly'/ are annotated as follows.

ለስብራታችን በሰውነቶቻችን ለ_ስብር_አት_አችን በ_ሰው_ነት_ኦች_አችን lə_sɨbɨrɨ_ʔatɨ_ʔatʃɨnɨ bə_səwɨ_nətɨ_ʔotʃɨ_ʔatʃɨnɨ [pre]-[stem]-[nom]-[1,p,pos] [pre]-[stem]-[nom]-[p]-[
ጥቁሮቹ ሰማያዊ በፍጥነት ጥቁር_ኦች_ኡ ሰማይ_አዊ በ_ፍጥን_ኧት t'ɨk'urɨ_ʔotʃɨ_ʔu səmajɨ_ʔawi bə_fɨt'ɨnɨ_ʔətɨ [stem]-[p]-[def] [stem]-[adj] [pre]-[stem]-[nom]
Amharic has many types of functional words. This includes prepositions, conjunctions, demonstratives, etc. Unlike English, many of them undergo morphological processes. They might merge with each other and other word classes, or affixes might be added as indicated in the following examples. For example, the words ስለዚህም /sɨləzihɨmɨ 'therefore-[foc]'/ and ከሌሎች /kəlelotʃɨ 'from others'/ are annotated as follows.

ስለዚህም ከሌሎች ስለ_እዚህ_ም ከ_ሌላ_ኦች sɨlə_ʔɨzihɨ_mɨ kə_lela_ʔotʃɨ [pre]-[stem]-[foc] [pre]-[stem]-[p]
The stems of some words are generated after applying regressive and progressive assimilation. The suffix -ኢ /ʔi/ sometimes palatalizes the last character whereas the suffix -ኢያ /ʔija/ usually palatalizes the final coronal consonants of a with the loss of -ኢ /-ʔi/. For instance, the word ጎጂ /godʒi 'harm'/ is annotated as ጎድ_ኢ /godɨ_ʔi/ whereas the word መጨረሻ /mətʃ'ərəʃa 'end'/ is annotated as መ_ጨረስ_ኢያ /mə_tʃ'ərəsɨ_ʔija/. Amharic has many homonym words for which the morphological annotation is still similar. For example, the stembased morphological annotation of the word ቤቱ /betu 'the house' or 'his house'/ is annotated as ቤት_ኡ where ቤት /betɨ 'house'/ is the stem, and the suffix ኡ /ʔu/ can refer to 'the' or 'his' depending on the word context. The word በጉ /bəgu 'the sheep' or 'a male sheep' or 'foolish'/ can be annotated as በግ_ኡ for the three cases. In such cases, only a single annotation is made for homonym words. However, the morphological annotations of some homonym words can be different due to the use of the word for multiple purposes. For example, the word ቀኑ /k'ənu/ may mean 'the day' or 'they become jealous' whereas the word በቀለ /bək'ələ/ may function as proper name or verb, requiring multiple annotations. As a result, we applied multiple annotations (or context-annotation) to avoid ambiguity for such types of words (see Table 1). Amharic root is the base of stem formation whereas stem is the base of surface word formation. Verbal root is represented as a sequence of radicals (consonants). Morphology provides the templates for the combination of the root consonants with the theme vowels to derive basic stems, which may be actual or potential verbs [START_REF] Leslau | Reference Grammar of Amharic[END_REF]. For example, the stem ሰባር /səbarɨ/ is derived from the root ስ-ብ-ር /s-b-r 'break'/ using a ስ-ኧ-ብ-ኣ-ር /sə-b-a-r-ɨ/ pattern. The radicals are ስ /s/, ብ /b/, and ር /r/ whereas ኧ /ə/ and ኣ /a/ are vowels. The vowels change the shape of the preceding radical in a given root during stem or word formation. Some words can be formed directly from roots. For example, the noun ልብስ /lɨbɨsɨ 'cloth'/ and the adjective ደካማ /dəkama 'weak'/ are derived from the roots ል-ብ-ስ /l-b-s/ and ድ-ክ-ም /d-k-m/, respectively. Affixation applies to the outputs of the patterns to derive verbs with functions such as passive, causative, infinitive, etc. We performed morphological annotation to produce our root-based lexicon that contains roots of words and their affixes. Therefore, surface words are segmented into prefixes, root, and suffixes. For instance, the verb ስለገደለቻቸው /sɨləgədələtʃatʃəwɨ 'since she killed them'/ is morphologically segmented with root form as ስለ_ግ-ድ-ል_ኧች_አቸው /sɨlə_g-d-l_ʔətʃ_ʔətʃəwɨ/. In this case, ስለ /sɨlə 'since'/ is prefix and ግ-ድ-ል /g-d-l 'kill'/ is the root whereas ኧች /ʔətʃɨ/ and አቸው /ʔtʃəwɨ/ are suffixes. Amharic has many words of various word classes that are derived from verbal roots. Thus, the root representation of such words is the verbal root. For example, ዝ-ን-ብ /z-n-b/ and ድ-ክ-ም /d-k-m/ are the verbal roots for the derived noun ዝናብ /zɨnabɨ 'rain'/ and the derived adjective ደካማ /dəkama 'weak'/, respectively. Accordingly, for instance, the noun ለስብራታችን /ləsɨbɨratatʃɨnɨ 'for our state of being broken'/ and the adverb በፍጥነት /bəfɨt'ɨnətɨ 'quickly'/, which are derived from verbal roots, are annotated as follows.

ለስብራታችን በፍጥነት ለ_ስ-ብ-ር_አት_አችን በ_ፍ-ጥ-ን_ኧት lə_s-b-r_ʔat_ʔatʃɨnɨ bə_f-t'-n_ʔətɨ [pre]-[root]-[nom]-[1,p] [pre]-[root]-[nom]
Some words are derived from verbal roots containing weak consonants like ው /w/. In such cases, the weak consonants are included in the root forms as shown in the following annotations of ይሞታል /jɨmotalɨ 'he will die'/ and ለኑሮ /lənuro 'for life'/.

ይሞታል ለኑሮ ይ_ም-ው-ት_አል ለ_ን-ው-ር_ኦ jɨ_m-w-t_ʔalɨ lə_n-w-r_ʔo [3,s,m]-[root]-[future] [pre]-[root]-[nom]
Amharic verbal roots are usually extracted by removing vowels from basic stems directly. However, the roots of some words are formed by palatalizing one or more characters. For example, the root of ቀጭን /k'ətʃ'ɨnɨ 'thin'/ is ቅ-ጥ-ን /q-t'-n/, and the root of ረዥም /rəʒɨmɨ 'long'/ is ር-ዝ-ም /r-z-m/.

On the other hand, the root forms of words that are not derived from verbal roots have similar representations as their basic stems. For example, the non-derived noun በቤቶች /bəbetotʃɨ 'with houses'/, the adjective የደጎቹ /jədəgotʃu 'of the (many) generous'/, and the word ከሌሎች /kəlelotʃɨ 'from others'/ are annotated as follows.

በቤቶች የደጎቹ ከሌሎች በ_ቤት_ኦች የ_ደግ_ኦች_ኡ ከ_ሌላ_ኦች bə_betɨ_ʔotʃɨ jə_dəgɨ_ʔotʃɨ_ʔu kə_lela_ʔotʃɨ [pre]-[root]-[p] [gen]-[root]-[p]-[def] [pre]-[root]-[p]
Like the case of stem-based annotations, the root-based annotations of many ambiguous words can be considered the same way. For example, the annotation of the word ልጁ /lɨdʒu 'the child' or 'his child'/ is ልጅ_ኡ /lɨdʒɨ_ʔu/. However, some ambiguous words may have multiple annotations as presented in Table 2. ) without leaving free space between them. A Python script was used to produce the final morphologically annotated corpora automatically using documents and morphologically analyzed lexicons.

Discussion

The lexicons we built cover a variety of domains and styles. Accordingly, they can serve as important resources in the development of Amharic IR and NLP applications. Many IR and NLP applications need stem or root extraction prior to other processes. We conducted a preliminary analysis on the usefulness of stem-based and root-based retrieval using the corpora we built, the 2AIRTC [START_REF] Yeshambel | 2AIRTC: The Amharic adhoc information retrieval test collection[END_REF] and the Amharic stopword list [START_REF] Yeshambel | Construction of morpheme-based Amharic stopword list for information retrieval system[END_REF] which are all available at https://www.irit.fr/AmharicResources/. We found that rootbased approach is better for retrieving more number of relevant documents. The retrieval effectiveness of root-based and stembased approaches are 0.70 and 0.57, respectively [START_REF] Yeshambel | Amharic document representation for adhoc retrieval[END_REF]. This is due to the fact that root could represent word variants using a single common form. For example, the stopwords ነበር /nəbərɨ 'has existed'/ and ነባር /nəbarɨ 'something known to be in existence for a long time'/ are variants having a common root ን-ብ-ር /n-b-r/. However, they do not have a common stem leading to degradation of retrieval performance in comparison to the use of roots for document representation.

Conclusion

Amharic is a morphologically complex and under-resourced language. In this paper, a considerable effort has been made to create stem-based and root-based morphologically annotated lexicons and corpora. The morphological annotation segments words to their constituent morphemes. The lexicons and annotated corpora could be used by different researchers to extract knowledge in different applications, to model language phenomena, and to train and test algorithms. Future work is directed at adding more vocabulary to the lexicons and extending the corpus by annotating with the contextual meaning of words in a sentence.

Figure 1 .

 1 Figure 1. Morphological annotation process

Table 1 .

 1 Examples of stem-based multiple annotation.

	Word	Stem-based annotation
	የአበበ /jəʔəbəbə/	የ_አበበ /jə_ʔəbəbə 'of Abebe'/ የ_አበብ_ኧ /jə_ʔəbəbɨ_ʔə 'of flowered-[3,s,m]'/
	ማሩን /marun/	ማሩ_ን /maru_nɨ 'Maru-[acc]'/ ማር_ኡ_ን /marɨ_ʔu_nɨ 'the honey-[acc]'/
		ማር_ኡ_ን /marɨ_ʔu_nɨ 'they gave us mercy'/
	4.4 Root-Based Annotation

Table 2 .

 2 Examples of root-based multiple annotations Each annotated word form contains its base (stem or root) and a full set of morphological features indicating inflection and derivations of words. The grammatical features are separated from each other and from the stem or root by underscore ('_'); radicals of a root are separated from each other by hyphen ('-'); and multiple annotations of a word are enclosed in square brackets ('[ ]'

	Words	Root-based annotation
	የአበበ /jəʔəbəbə/	የ_አበበ /jə_ʔəbəbə 'of Abebe'/ የ_እ-ብ-ብ_ኧ /jə_ʔ-b-b_ə 'of flowered-[3,s,m]'/
	ማሩን /marun/	ማሩ_ን /maru_nɨ/ 'Maru-[acc]'/ ማር_ኡ_ን /marɨ_ʔu_nɨ 'the honey-[acc]'/
		ም-ሕ-ር_ኡ_ን /m-h-r_ʔu_nɨ 'they gave us mercy'/
	4.5 The annotated corpora
	Two annotated corpora are created from the initial corpus using
	the two lexicons (stem-based and root-based), where each word
	in the initial corpus is replaced by its annotation from the
	appropriate lexicon. These corpora are monolingual, coded in
	Unicode-8 and International Phonetic Association (IPA) text file
	forms. Each corpus consists of 6,069 full text documents
	consisting of 72,814 sentences constructed 1,592,351
	morphologically annotated words.

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