Effect of strain path change on the texture evolution after cold rolling and recrystallization of Nickel-40 wt. %Cobalt alloy

Effect of change in strain path during rolling and its role in the development of crystallographic texture after deformation and then after recrystallization has been studied for Ni-40wt%Co which is a medium stacking fault energy (SFE) material. Results indicate that textures developed after unidirectional rolling and multi-step cross-rolling are quite different which also leads to different texture evolution after recrystallization. Microstructure of both UDR and cross rolled sample shows band type of feature within the grain having lesser band density for cross rolled samples. After recrystallization unidirectionally rolled sample shows bimodal whereas cross rolled sample shows equiaxed grain size distribution. But all the samples show large fraction of annealing twins. It has been shown that in all cases formation of annealing twins have large influence on the texture transition after recrystallization.


Introduction
The mechanisms of evolution of deformation texture in face centred cubic (FCC) metals and alloys have been investigated by many researchers [1][2][3][4][5][6][7][8]. One of the most important material property that controls the micro-mechanisms of deformation in FCC materials is the stacking fault energy (SFE). It is well known that rolling texture in a closed packed face centred cubic materials show texture transition from Cu (or pure metal) type to Bs (or alloy type) with a decrease in stacking fault energy. In general, high and medium SFE material shows Cu type texture which consist of prominent Cu {112} <111> component, a continuous spread of orientation from Bs {110} <112> to S {123} <634> and a weak Goss {110} <001> components. Low SFE material shows Bs type texture which consists of strong Bs {110} <112> and Goss {110} <001> components. This transition in deformation texture with a change in SFE is also accompanied by deformation heterogeneities such as deformation twins, microbands and shear bands. In a low SFE material, it was found that twinning occurs in the initial stage of deformation process, at the intermediate level twinning activity ceases and later at high strain formation of shear bands take place. Experiments conducted on single crystals revealed that Cu orientation is very prone to shear banding, whereas Goss orientation is not susceptible to heterogeneities during deformation [9,10].
Apart from the material property, the processing variables are also equally important in the formation and stability of a particular texture [5,11,12]. It was found that change in strain path during rolling has pronounced effect on the evolution of texture [13][14][15]. Rotation of the 1234567890''"" sample after every pass during rolling rotates the substructure along the new rolling direction. It leads to destabilizations of the substructures formed during the previous pass of rolling and thereby alters the evolution of texture.
Formation of recrystallization texture strongly depends on the initial deformation texture which is dependent upon the stacking fault energy of material and the processing parameters of deformation such as, mode of deformation, strain, strain path etc. Previous investigations on the development of recrystallization texture show that Cu type rolling texture typically results in cube recrystallization texture and Bs type rolling texture usually recrystallize in brass orientation [16]. Annealing twin is also an important factor which contributes to the formation of nuclei during early stages of recrystallization and later development of texture which was not present in the initial deformed material [17].
The aim of the present work is to understand the effect of strain path on the development of texture during deformation and subsequent recrystallization in Ni-40wt. %Co alloy. Ni-Co alloys form completely substitutional FCC solid solution up to 65 wt. %Cobalt. Stacking fault energy of the system decreases with cobalt addition from high SFE of (pure Ni, 150 mJ m -2 ) to very low SFE of (Ni-60Co, 30 mJ m -2 ) [18]. Ni-40Co alloy, used in the present work is a medium SFE material with SFE value is about 60 mJ m -2 . The micro-mechanisms of texture evolution in Ni-40Co are elaborated in this investigation.

Experimental Procedure
An ingot of nickel-cobalt alloy containing 40 wt. %Co was melted in vacuum arc melting furnace. During melting magnetic stirring was used to ensure composition uniformity. Quantitative composition analysis was carried out by energy dispersive spectroscopy (EDS). To break the cast dendritic structure and to weaken the initial solidification texture, the as-cast ingot was cross-rolled to 50% thickness reduction. After that, the rolled material was further annealed at 700 °C for 6 hours, to get completely homogenised strain free equiaxed microstructure having random texture. It was taken as starting material for our study. The initial material was then subjected to 70% unidirectional rolling (UDR) and multistep cross rolling (CR), equivalent to a true strain of ~1.2 at room temperature. A schematic representation of rolling schedules followed in the present study is shown in Fig.1. Both the Unidirectional and the cross rolled material was then subjected to isothermal annealing at 550 °C for 30 minutes for recrystallization. Bulk texture measurements for all the rolled and recrystallized samples were carried out by the Schulz reflection method, using an X-ray texture goniometer with Co Kα radiation (D8 Discover, Bruker AXS, Germany). Four incomplete pole figures (α = 0-75°) were measured from the 111, 200, 220 and 311 peaks on the rolling plane of the sample. Because of coarse grain for initial sample texture measurements were done at multiple locations to improve the statistics of the measurements. The three-dimensional orientation distribution function (ODFs) was calculated using Labotex® software [19].
The microstructure of all rolled and recrystallized samples was taken in a field emission scanning electron microscope, equipped with EBSD system. All microstructure were taken on the transverse plane of rolled samples. EBSD data has been rotated to ND-plane to see the important texture component. Samples for EBSD scans were mechanically polished using silicon carbide emery sheets and further electropolished. The starting material shows random texture because of prior cross rolling and annealing as mentioned earlier. After 70% UDR, it shows the development of strong Bs and Goss component with weak Cu and S component. It also shows some amount of Goss twin orientation which developed as a result of unidirectional rolling. On the other hand, the 70% Cross rolled sample shows weaker texture as compared to UDR which is also reported by many authors [20]. It shows remarkable Goss and {110} <111> (A) component. The development of 'A' component after cross rolling has also been reported for copper [20]. The intensity of Cu and S component was found very weak as compared to UDR sample. Cross rolled samples also show retention of Bs, G/B and rotated Goss component which was present in the initial material. The volume fraction of important deformation and recrystallization texture component was calculated after  After recrystallization of 70% UDR sample shows strong Brass, cube, Goss and Rt-cube component in almost equal proportion. It can also be seen that the intensity of Cu, Bs, S and Goss component went down as compared to deformed samples whereas the intensity of cube and Rt-cube component got increased. The 70% Cross rolled and recrystallized sample shows completely different texture as compared to UDR recrystallized samples. The development of strong rotated Goss component is clearly noticed, which was not present in the UDR recrystallized sample. It also shows little increases in Cu and S component as compared to cross rolled sample which was decreasing when UDR sample was recrystallized. One similarity were found in texture transition during recrystallization of both UDR and cross rolled sample is that Goss component always decreases after recrystallization.

Results and Discussion
To get the better understanding of texture transition upon rolling and after recrystallization, important fibres were plotted from the ODFs. The fibre plots are shown in Fig. 4. The α and τ-fibres show orientations <110> || ND and <110> || TD respectively. The α and τ-fibres are quite inhomogeneous. The βfibre also does not show much variation in differently processed samples except for 70% cross rolled sample which exhibits a maximum between S/Bs and Bs orientation. The 70% UDR sample shows very high intensity near Bs and Goss orientation in αfibre plot but after recrystallization, their intensity decreases. However, it maintains a broad peak between G/Bs and Bs orientation. The retention of some Brass orientation after recrystallization can be explained by nucleation with orientation close to those of deformed matrix. It can take place by twinning into crystallographic identical variants influenced by annealing twin since lots of annealing twins can be observed in the microstructure after recrystallization (Fig. 6c) [21]. The 70% UDR recrystallized sample also shows a remarkable peak between CuT and Goss orientation with some intensity at Rt-Cube orientation in τfibre which cannot be ignored. The presence of CuT orientation could be attributed to the annealing twin of Cu orientation which was present in the deformed matrix. The development of rotated cube (Rt-C) component may take place by transformation of CuT orientation since it possesses 3 twin relation with it. On the other hand, 70% cross rolled sample shows a strong peak near Goss and {110} <111> (A) orientation in αfibre and very high intensity between S/Bs and Brass orientation in βfibre. The reason behind the occurrence of high intensity between S/Bs and Brass orientation could be destabilization of substructure because of rotation of sample after subsequent passes of rolling. Since Ni40Co is a medium SFE material, initial unidirectional passes leads to develop some Brass orientation but because of rotation of sample previous orientation destabilize and try to rotate towards other very preferential rolling texture component S orientation. As a result finally, high intensity develops at a location between S/Bs and Brass orientations which can be seen in βfibre (Fig 4c). After recrystallization of 70% cross rolled sample it shows high intensity near brass orientation and a maximum at rotated Goss (Rt-G) orientation in αfibre. It can also be seen that there is a sharp decrease in Goss orientation after recrystallization. However, near Bs orientated grains recrystallized in the same manner as explained earlier for the unidirectional rolled samples through the formation of annealing twin, Volume Fraction