Roles of Uniaxial, Shear and Cyclic Loading on the Microstructure and Texture Evolution during Plastic Deformation of Ti6Al4V Alloy

Abstract

The objective of the present research was to investigate the effects of uniaxial, shear, and cyclic loading on the microstructure and texture evolution of Ti6Al4V (Ti64) alloy during plastic deformation. Ti64 alloy samples were subjected to both uniaxial tension and compression in the temperature range of 298 873 K for different degrees of deformation. During uniaxial tension, refinement of both and grains was observed during deformation at all temperatures. This has been attributed to the continuous dynamic recovery and recrystallization (CDRR) mechanism. A Burgers orientation relationship (BOR) between and grains was also observed in the alloy. The co-linearity of BOR between and grains was found to refine the grains, and that of non-BOR refined the grains. The crystallographic texture of the samples was observed to be along the off-basal orientation after deformation. The Schmid factor value for pyramidal <c+a> slip was found to be the highest among all the slip systems during the uniaxial tension of the alloy. Stage III hardening was observed in the samples during deformation at all the studied temperatures, and an athermal hardening rate was found up to 4 % deformation of the samples. During uniaxial compression, a significant amount of deformation twins were observed in the samples beyond 20 % reductions at temperatures of 298 K and 673 K only. Such twins were further found to be of non-Schmid type and were confirmed through VPSC (Visco-plastic self-consistent) simulation. It was further found that the deformation bands formed during deformation facilitated the twin’s formation inside the grains. The texture strength of (0001) pole increased up to a 30 % reduction at all temperatures of deformation. However, the dominant texture strength remained along (1 2 10) pole, which was also the starting texture of the sample before being subjected to uniaxial compression. An initial decrease and subsequent increase in hardening rate was observed in the samples deformed at room temperature. The maximum decrease in the rate of hardening was observed at a temperature of 673 K up to 18 % strain, followed by a relative increase thereafter. Further, the observed decrease in the rate of hardening was found to be relatively lower at a temperature of 873 K compared to that at 673 K. Similarly, free-end torsion tests were conducted to understand the microstructure and texture evolution in the alloy during shear deformation. Torsion tests were conducted at temperatures of 298 K, 673 K, and 873 K for different strains ranging from 0.22 to 0.99. A nearequiaxed homogeneous microstructure was observed after deformation at all temperatures. Dynamically recrystallized (DRX) grains were observed in the samples at higher deformation temperatures, and these were found to be significant at 873 K for strains > 0.66. Dynamic transformation from → was also observed at a deformation temperature of 873 K. The initial fiber texture got rotated towards the ideal B fiber orientation and reached at the ideal C2 fiber orientation after deformation for a maximum strain of 0.99. The deformation texture was further simulated through the Visco plastic self-consistent (VPSC) method, and it was found that the basal slip was the dominant deformation mode, followed by prismatic and both pyramidal <c+a> I and II slip systems. Ti64 alloy was also subjected to low cycle fatigue (LCF) to examine the role of cyclic deformation on the microstructure and texture evolution in the alloy. The LCF tests were carried out at room temperature for different strain amplitudes ranging from 0.5 % to 2 %. Cyclic softening behavior was observed at all strain amplitudes, and it was found to increase with increasing the strain amplitude. It was observed that softening behavior at low strain amplitudes (a ≤ 1 %) was due to the combined effect of both back stress and friction stress. However, at higher strain amplitudes (a > 1 %), it was attributed to the significant decrement in friction stress, which was further related to the sub-grain formation. Only (1012) type tensile twins were formed during the deformation, and this was found to be significant at a higher strain amplitude of 2 %. The [1010] grains were further found to have substantial cyclic deformation than the other grains, such as [2110] and [0001]. The dominant texture was found along [1010] during the LCF tests. However, the sample at a higher strain amplitude of 2 % also had prominent [0001] texture along with [1010] texture. Fractographic examination revealed a dimple fracture surface at lower strain amplitude (0.5 %), whereas a quasi-cleavage type fracture was observed at higher strain amplitudes (1 % & 2 %).