Influence of phase morphology, static recrystallization, and crystallographic texture on room temperature tensile properties of Ti-6Al-4V alloy: Comparison between post-tested equiaxed, bimodal, and lamellar microstructures

The influence of phase fractions, morphology, recrystallization, and crystallographic texture on the room temperature uniaxial tensile properties were investigated in a Ti-6Al-4V (Ti-64) alloy. The hot rolled and annealed Ti-64 alloy was heat treated to generate three different microstructures (equiaxed, bimodal, and lamellar) with varying volume fractions and dimensions of constituent phases (equiaxed primary & alpha;, transformed & alpha; laths, and & beta; laths). The tested tensile specimens were characterized using techniques such as electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and X-Ray diffraction-based bulk texture analysis to determine the deformation mechanism of the alloy. Among three different microstructures, the equiaxed microstructure (& alpha;/& beta;-900/950-FC) exhibited a superior strength-toughness combination, while the lamellar microstructure (& beta;-1050-AC) displayed poor tensile properties. The bimodal microstructure (& alpha;/& beta;-900/950-AC) showed intermediate tensile properties. Recrystallized & alpha; grains and their connectivity promoted an extended steady-state flow during deformation and ductile fracture. The bimodal microstructure work hardened faster as compared to the other two microstructures. TEM investigation on the post-tested microstructures showed that events such as dislocation multiplication and dislocation-dislocation interactions, formation of slip bands across the & alpha; grains, multiple slip and dislocation pile-ups contribute to hardening mechanisms during deformation. Whereas, slip transfer across & alpha;/& alpha; boundary and breaking of & beta; laths are the events of softening mechanisms. The slip transfer across two adjacent & alpha; grains happens when the misalignment between grains is less than 10 degrees. Equiaxed microstructures exhibited maximum slip transfer due to interconnected primary & alpha; grains. Bimodal microstructures (& alpha;/& beta;-900/950-AC) exhibited slip transfer when a higher fraction of interconnected primary & alpha; is present in the microstructure. In contrast, the lamellar microstructure showed no evidence of slip transfer. The prism slip is the dominant active slip system in the equiaxed and bimodal microstructures. However, the lamellar microstructure with no primary & alpha; showed only the prismatic and pyramidal slip activities.