First of all, I would like to express my sincere gratitude to my supervisor Dr.Pinaki Prasad Bhattacharjee for giving me the opportunity to work under his guidance in this field. Finally, I would like to express my deepest gratitude to the Almighty without his blessing, this project would not have been possible. The UCR processed material showed heterogeneous microstructure compared to the MSCR and TSCR(45º) processed materials.
The deformation texture of L12/FCC in the MSCR-treated material matches that of cryo-rolled FCC materials quite well, while the texture of both phases in the TSCR(45º)-treated material appears relatively weak. After recrystallization at 800 °C, the UCR-treated material showed a rather novel heterogeneous microstructure, while the MSCR and TSCR (45°)) treated materials showed an ultrafine micro-duplex structure. The UCR treated material showed a much higher hardness compared to the MSCR and TSCR treated materials (45º)) attributed to its new heterogeneous microstructure.
Overview
Objective of the present work
Eutectic High Entropy Alloys (EHEAs)
Thermo-mechanical processing of AlCoCrFeNi 2.1 EHEA
Annealing the 90% cold-rolled material at 800°C results in a duplex microstructure consisting of disordered FCC and precipitate-free B2 phases with equiaxed morphologies and with significant resistance to grain growth. Significant improvement of the tensile properties compared to the cast alloy could be achieved by thermomechanical machining characterized by high tensile strength (≥1000 MPa) with tensile strength exceeding 10%. The heterogeneous microstructure of the cryo-rolled and annealed material results in a simultaneous increase in strength (Yield Strength/YS MPa, Ultimate Tensile Strength/UTS: 1562±33 MPa) and ductility (elongation to failure/ef ~14±1%) as compared to the cast as well as cold-rolled and annealed materials.
Effect of strain-path on microstructure, texture and properties
The UCR processed material shows copper-type texture, while the textures of different cross-rolled materials are characterized by different rotated copper components. With annealing, the UCR processed material shows the lowest grain size while the highest grain size is observed in the TSCCR(45°) processed material. This indicates greater availability of nucleation sites in the UCR processed material compared to the TSCCR(45°) processed material.
The differences in annealed grain size can be attributed to substructure destabilization and misorientation build-up, which reduces the number of potential nuclei in the TSCCR(45°) machined material. It was pointed out that the cross-rolled products show a higher hardness value compared to conventional UCR machined material in HCP metals such as Ti [27]. However, in some cases the UCR-treated materials show lower hardness compared to different cross-rolled samples [28].
Novelty of the work
The annealing textures of the differently processed materials are characterized by the presence of α-fiber (ND//<110>) and the absence of preferential nucleation and growth [26]. These conflicting reports indicate that the effect of the stress path on the mechanical behavior of materials needs to be understood in depth.
Preparation of starting material
Processings
Cryo-rolling
Isochronal Annealing
Characterization
Evolution of microstructure and texture during deformation
The remarkable differences in the microstructure of the three processed materials can be understood from the EBSD image quality (IQ) maps (Fig.4.2). The IQ map of UCR-processed materials (Fig. 4.2(a)) shows heterogeneous microstructure with a narrow lamellar region (marked by circle) together with fine fragmented regions. However, the IQ maps for MSCR (Fig. 4.2(b)) and TSCR (45º) (Fig. 4.2(c)) treated materials depict highly fragmented microstructure such that lamellar eutectic regions are not observed in contrast to UCR treated material.
The phase fraction of the constituents L12/FCC and B2 phases of 90% cryo-rolled EHEA processed by three different routes is compared in Fig.4.3. The phase fraction (calculated using Image J software) does not show large variations in all three processed materials including as-cast EHEA. Fig.4.3: Phase fraction in cast and 90% cryo-deformed EHEA processed by different rolling routes.
The evolution of texture in the L12/FCC phase of the 90% cryodeformed material treated by the three different routes is shown by the relevant ODF sections in Fig.4.4. The important deformation and recrystallization texture components of the L12/FCC phase of the HEAs are summarized in Table 1. The φ2 = 45º section of the ODF reveals the presence of the Cu component, whereas the φ2 = 65º section confirms S -component, which is quite weak.
However, the section φ2 = 45º also in this case confirms the complete absence of the Cu component. The texture of phase B2 in 90% cryo-rolled materials processed by three different processing methods is shown in Figure 4.5. The φ2=45° ODF section of the B2 phase in the UCR-treated material (Figure 4.5(a)) shows a slightly displaced {111}<011>component lying at the intersection of the ND and RD fibers.
The intensities of the contour lines show that the texture is weakened after MSCR processing.
Evolution of microstructure, texture and hardness during annealing
Fig.4.7: (a) Change in phase fraction and (b) hardness with annealing temperature in the EHEA processed by the three different routes. The hardness of the EHEA increases significantly after cryo-rolling with three processed routes compared to as-cast material. Remarkably, the hardness value of the UCR processed material is much greater than that of MSCR and TSCR(45º) processed materials after annealing at 800ºC.
The texture evolution in the L12/FCC phase of the annealed EHEA is summarized in Figure 4.8. The φ2 = 0º section of the ODF of the UCR processed material annealed at 800ºC (Fig.4.8(a)) shows the intensity near the G component. The G component is enhanced with increasing annealing temperature (Fig. 4.8(b)), but upon further annealing at 1200°C the G/B component emerges as the strongest recrystallization texture component (Fig. 4.8(c)).
Figure 4.8: Sections of the ODF φ2=0° of the L12/FCC phase in annealed EHEA processed by different routes (see Table 1 for legends). Figure 4.9: φ2=45° section of ODFs of phase B2 in annealed EHEA processed by the second route (see Table 2 for legends). The φ2 =45º ODF section of the UCR-treated material annealed at 800 °C (Figure 4.9(a)) shows the presence of twisted cube components.
However, after annealing at 1000ºC (Fig.4.9(b)), the ODF of the B2 phase shows the presence of common RD and ND components. The ODF section in the 1200ºC annealed material shows very similar texture to that of the 1000ºC material confirming no significant change in texture at higher temperature. In the case of MSCR processed material, the {001}<110> component remains present in the deformed state in the MSCR processed material even after annealing at 800°C (Fig.4.9(d)) along with a strong ND fiber .
The texture of the B2 phase in the TSCR material (45°) is quite weak in the annealed state at 800 °C (Fig. 4.9(g)), but in this case after annealing the formation of a strong ND fiber detected. at 1000 °C (fig.
Evolution of deformation microstructure and texture
Thus, the orientations of the a-fibers will converge to the Bs orientation (stable orientation during unidirectional rolling) and then further rotate toward the {011<111> orientation when the RD is rotated 90°. The deformation texture of the L12/FCC phase in the UCR-processed material shows a clear presence of the Bs component. In contrast, the MSCR-processed material shows the development of α-fibers with a strong intensity peak exactly at the BS/BSND or the (011)[755].
Evidently, the deformation texture of the L12/FCC phase agrees very well with the theoretical calculations of Hong et [34] and also with the cross-rolling texture of different FCC materials [23]. 34] for predicting the origin of {011}<755> component in MSCR processed material, simple rotation of the Bs component by 45° does not lead to the observed components in the TSCR(45°) material. This argument was further supported by the study on diagonally rolled Cu (45° roll to the previous RD similar to the TSCCR(45°) route, but the stress in step 1 and step 2 is different than in the present study) developed by other texture as 90° cross-rolled materials [35].
The B2 phase in the UCR machined material shows a strong but slightly displaced {111}<011> component, which has been reported for some cold-rolled B2 phases. The MSCR machined material shows a distinct {001}<110> component, which is typically observed in cross-rolled BCC, such as ferrite in duplex steel [36] and can be followed from the stability analysis of deformation texture components in BCC materials. Thus, the textures of both FCC and B2 phases are significantly different in the TSCR(45°) treated EHEA.
It appears that a 45° rotation around the RD may affect the deformation and slip activities more fundamentally, leading to the observed differences in texture.
Evolution of annealed microstructure and texture
The annealing texture of the B2 phase in the UCR-treated material shows common RD and ND fiber components. Wani, I.S., et al., Microstructure and texture evolution during thermomechanical processing of a high entropy Al0.5CoCrFeMnNi two-phase alloy. Bhattacharjee, P., et al., Microstructure and texture evolution during annealing of a high-entropy equiatomic CoCrFeMnNi alloy.
Sathiaraj, G.D., et al., Effect of heavy cryo-rolling on the evolution of microstructure and texture during annealing of equiatomic CoCrFeMnNi high entropy alloy. Sathiaraj, G.D., et al., The effect of heating rate on microstructure and texture formation during annealing of heavy cold rolled equiatomic CoCrFeMnNi high entropy alloy. Wani, I.S., et al., Tailoring of nanostructures and mechanical properties of AlCoCrFeNi2.1 eutectic high-entropy alloy using thermomechanical processing.
Wani, I.S., et al., Cold rolling and recrystallization textures of the high entropy nanolamellar eutectic alloy AlCoCrFeNi2.1. Bhattacharjee, T., et al., Simultaneous Improvement of Strength and Ductility of AlCoCrFeNi2.1 High Entropy Nanolamellar Eutectic Alloy by Cryo-Rolling and Annealing. Suwas, S., et al., Effect of rolling methods on texture development in pure copper and some copper-based alloys.
Reddy, S., et al., Effect of stress path on microstructure and texture formation in cold rolled and annealed FCC equiatomic CoCrFeMnNi high entropy alloy. Liu, Y.H., et al., Strain Path Dependence of Microstructure and Annealing Behavior in High Purity Tantalum. Stepanov, N., et al., Effect of cryo-deformation on structure and properties of CoCrFeNiMn high-entropy alloy.
Toth, L.S., et al., Development of ferrite rolling structures in low-carbon and extra-low-carbon steels.
