Microstructure, texture and recrystallization of heavily rolled CoCrFeNi medium entropy alloy

The microstructure and texture have been characterized in a CoCrFeNi medium entropy alloy after cold rolling to 85% and subsequent annealing at 550°C for 130 min. It is observed that the as-rolled microstructure is heterogeneous and comprises extended regions of either 〈110〉//ND or 〈111〉//ND orientations and volumes of mixed orientations. The microstructure also contains a large number of shear bands acting as preferential nucleation sites for recrystallization during annealing. Recrystallization generates annealing twins, which results in new components along the 〈110〉//ND fiber and other new orientations, thus making the texture of recrystallized regions fairly weak. Orientations demonstrating increased intensities in the recrystallization texture are identified as {113}〈332〉 and {236}〈385〉 components.


Introduction
High entropy alloys (HEAs) and medium entropy alloys (MEAs) with high concentrations of several elements in their chemical compositions have attracted significant attention in the past two decades because these materials represent a paradigm shift in alloy design.Despite unconventional chemical compositions, the microstructure and mechanical properties of these new alloys can be controlled using approaches developed for traditional alloys dominated by one core element.Partial recrystallization after heavy deformation is one of the methods used to produce good combinations of strength and ductility in traditional metallic materials [1,2], and this approach has also been applied to control the mechanical behavior in several HEAs and MEAs [3][4][5].Obviously, to design conditions with improved combinations of mechanical properties, the deformed microstructure and the recrystallization process in HEAs and MEAs have to be analyzed.The aim of the present work is to characterize the microstructure and texture in a face centered cubic (FCC) CoCrFeNi medium entropy alloy both in a heavily rolled condition and after partial recrystallization.

Material and Experimental methods
An ingot of the equiatomic CoCrFeNi alloy was produced from metal billets melted in a vacuum induction furnace.The ingot was then re-melted in a vacuum arc melter, followed by homogenization at 1200°C, multipass cold rolling to 85% thickness reduction and annealing at 550°C for 130 min.The rolled and annealed conditions were investigated in the plane containing the rolling direction (RD) and the normal direction (ND).Transmission electron microscopy (TEM) images were taken using a JEOL JEM 2100 microscope.A Zeiss Sigma 300 scanning electron microscope was applied for electron channeling contrast (ECC) imaging and electron backscatter diffraction (EBSD).
Step sizes of 20-40 nm were used for orientation mapping of the as-rolled microstructure, while the annealed microstructure was mapped with step sizes of 40-80 nm.In addition, large areas in the cold-rolled sample were covered by EBSD with a greater step size (3 µm) to enable statistical texture analysis.Recrystallized grains in the annealed sample were identified by at first selecting regions with grain orientation spread ≤0.6° surrounded by boundaries with misorientations >5° (where twins were considered to be individual crystallites).Within these regions recrystallized grains were defined as those with a minimum size of 0.3 µm (calculated disregarding twin boundaries).Low angle boundaries (LABs) and high angle boundaries (HABs) were defined as those with 2-15° and >15° misorientations, respectively.Area fractions of individual texture components were calculated applying a 15° deviation from the exact {hkl}〈uvw〉 orientations.Fiber textures were defined within 15° of the corresponding fiber axes.

As-rolled material
One of the prominent features in the deformed microstructure are shear bands (SBs) oriented at 20-45° to the RD (figure 1a-c).The microstructure contains a high dislocation density and deformation twins (figure 1d) though the frequency of twins varies significantly from region to region.Deformation twins are especially frequent in 〈111〉//ND-oriented regions, where twin bundles are aligned close to the RD.While the average spacing between twin boundaries measured in TEM images is small (~30 nm), twin lamellae can extend over several micrometers.In contrast to the comparatively long twin lamellae, subgrains within the SBs are both narrow and short (see figure 1b).The fine boundary spacing makes successful indexing of EBSD patterns from the SBs rather difficult as is evident from clusters of dark gray pixels in figure 1c representing non-indexed patterns.Despite the reduced indexing rate in the SB regions, there is a strong indication that orientations ranging from the G {110}〈001〉 component to the Bs {110}〈112〉 component are present here more frequently than other standard texture components.Orientations different from the standard rolling texture components are also frequently observed within the SBs.These orientations can mostly be classified as random.Furthermore, subgrains of the D {113}〈332〉 component and the BR {236}〈385〉 component are occasionally found within the SBs.Orientations along the 〈110〉//ND fiber comprising the G and Bs components are present not only in the SBs, but also in broad horizontal bands.In addition to the extended regions of either 〈110〉//ND or 〈111〉//ND orientations, the deformed microstructure contains regions of mixed orientations.
The orientation distribution function (ODF) calculated based on the EBSD data from large sample areas demonstrates a brass-type texture with high intensities of the G and Bs components and a pronounced 〈111〉//ND fiber represented by the E {111}〈110〉 and F {111}〈112〉 components (figure 2a).In this deformation texture, orientations along the 〈110〉//ND fiber occupy 45% of the area.The summed area fraction of orientations along the 〈111〉//ND is lower, 31% (see figure 3).Correspondingly, the area fraction of orientations which do not belong to any of these fibers is 24%.

Partially recrystallized material
During annealing at 550°C SBs act as preferential nucleation sites for recrystallization (see figure 4).Other nucleation sites are regions of mixed texture components containing a high frequency of HABs.Compared to non-recrystallized regions, recrystallized grains are characterized by higher quality of EBSD patterns (lighter regions in figure 4b).The area fraction of the recrystallized microstructure (fRX) after annealing for 130 min is 38%.Recrystallized regions contain a high frequency of annealing twins (see figure 4c,d).The average recrystallized grain size dRX measured including twin boundaries is 0.5 µm.If the grain size is measured disregarding twin boundaries, dRX is 1 µm.The ODF calculated only for recrystallized grains is shown in figure 2b, where it is seen that the recrystallization texture is much weaker than the rolling texture.In this recrystallization texture, increased intensities are observed for the D component and near the BR component (see their positions in figure 2c).The D and BR components together occupy 24% of the recrystallized area.The fraction of orientations along the 〈110〉//ND fiber is similar, 26%, whereas the fraction of the 〈111〉//ND fiber is only 8%.The comparatively high fraction of the 〈110〉//ND fiber in the recrystallization texture is partly due to fact that some recrystallized grains have orientations of the deformation texture and partly due to new components developing along this fiber, such as the {110}〈111〉, {110}〈221〉 and {110}〈110〉 components.The area fraction of "other" orientations in the recrystallization texture is 42% (figure 3).

Discussion
The microstructure and texture of the heavily rolled CoCrFeNi alloy appears very similar to the microstructure and texture of conventional FCC materials with low stacking fault energies, such as austenitic stainless steels and α-brass [6][7][8].At low rolling strains these materials are deformed by slip, developing a copper-type texture with Bs, S {123}〈634〉 and C {112}〈111〉 components.However, as the strain increases, mechanical twinning begins to operate as an additional deformation mechanism, which strongly reduces the intensity of the S and C components [7].Heavy rolling orients deformation twins almost parallel to the RD, so that {111} twinning planes become closely aligned with the rolling plane, thus producing the 〈111〉//ND fiber.Since homogeneous deformation in the strongly oriented structure with nanotwin lamellae becomes rather difficult, plastic instabilities in the form of profuse SBs develop during heavy rolling.For α-brass it has been demonstrated that shear banding decreases the intensity of the 〈111〉//ND fiber and that the Bs component strengthens during rolling to high strains [7,8].This description of texture evolution in rolled brass is consistent with the texture observed in the heavily rolled CoCrFeNi alloy (see figure 2a).
The greatly refined microstructure within SBs implies that the stored energy is high in these regions, which makes them preferential sites for nucleation of recrystallized grains during annealing.It is suggested that some grains of BR, D or random orientations could nucleate directly from subgrains having these orientations within the SBs.Another mechanism capable of producing these orientations is annealing twinning.It was previously shown that the BR component could originate due to annealing twinning taking place in recrystallized grains nucleated in SBs [9,10].Similarly, our study reveals that the D component can also be generated by annealing twinning in grains nucleated in SBs.In particular, crystallites of the D component can often be produced by twinning in grains having orientations within the spread of the G component.Analysis of the EBSD data from the recrystallized regions in the annealed CoCrFeNi sample indicates that ~90% of all boundaries formed between D-oriented crystallites and G-oriented crystallites are twin boundaries.Furthermore, the new components developed during recrystallization along the 〈110〉//ND fiber (figure 2b) correspond to orientations produced by first-order twinning within crystallites having orientations ranging from G to Bs [11,12].Therefore, it is considered that annealing twinning is an important source of the components observed in the recrystallization texture of the CoCrFeNi alloy.Growing grains of the new orientations can twin further, resulting in multiple twinning, which tends to weaken the recrystallization texture.

Summary
A heterogeneous microstructure with extended regions of either 〈110〉//ND or 〈111〉//ND orientations and volumes with mixed orientations is observed in a heavily rolled CoCrFeNi medium entropy alloy.The deformed microstructure also contains a large number of shear bands, which act as preferential nucleation sites for recrystallization.After annealing at 550°C for 130 min recrystallized grains occupy 38% of the area.The average size of these grains calculated disregarding twin boundaries is 1 µm.Annealing twinning generates crystallites with different orientations, thus making the recrystallization texture fairly weak.Orientations demonstrating increased intensities in the ODF calculated for recrystallized regions are identified as {113}〈332〉 and {236}〈385〉 components.

Figure 1 .
Figure 1.Twin bundles and shear bands in the as-rolled CoCrFeNi sample: (a,b) ECC images showing the microstructure at different magnifications; (c) orientation map where white, black and red lines represent LABs, HABs and twin boundaries, respectively; (d) TEM image from a region containing twin bundles.The RD is parallel to the scale bar in (a-c) and is indicated by the arrowed line in (d).

Figure 2 .
Figure 2. ODFs representing deformation and recrystallization textures in the CoCrFeNi alloy: (a) texture in the as-rolled sample; (b,c) orientations of recrystallized grains in the sample annealed at 550°C for 130 min.Contour lines: 1, 2, 3, 5, 8 × random.In (c) different colors correspond to different texture components defined within 15° of their exact orientations.Orientations other than those in the legend are shown in gray.

Figure 4 .
Figure 4. Microstructure in the sample annealed at 550°C for 130 min (fRX = 38%): (a) ECC image; (b) map representing the quality of EBSD patterns; (c) orientation map where white, black and red lines represent LABs, HABs and twin boundaries, respectively; (d) map highlighting annealing twins.In this map, twin boundaries within recrystallized grains are shown by red lines, while all boundary types in non-recrystallized regions are shown by gray lines.

Figure 3 .
Figure 3. Fractions of different texture components calculated for the as-rolled sample and for recrystallized grains in the sample annealed at 550°C for 130 min.