On the strain hardening abilities of α+β titanium alloys: The roles of strain partitioning and interface length density

Abstract

In this study, the strain hardening ability of an α+β titanium alloy (Ti–6Al–4V) was systematically investigated by comparing the tensile properties of different microstructures at room temperature, including the lamellar microstructure, bi-lamellar microstructure, equiaxed microstructure and bimodal microstructure. The lamellar and equiaxed microstructures, mostly comprised of α phase (either α lamellae or equiaxed α grains) and a few retained β phase, were characterized by limited strain hardening abilities, regardless of the colony size and grain size. On the other hand, the bi-lamellar and bimodal microstructures obtained by intercritical annealing of the lamellar and equiaxed microstructures in α+β two-phase region followed by water quenching, generally possessed higher strain hardening abilities than the lamellar and equiaxed microstructured counterparts. In addition, the evolution tendencies of uniform elongation (as determined by the strain hardening rate and true stress) with intercritical annealing temperature were different between the bi-lamellar and bimodal microstructures. In the bi-lamellar microstructure, the uniform elongation continuously decreased with the increase of the intercritical annealing temperature while in the bimodal microstructure, a peak uniform elongation was obtained at an intermediate annealing temperature (910 °C). Both microstructures can be regarded as ‘dual-phased’ microstructures, as composed of primary α lamellae/grains and transformed β regions (an aggregate of secondary α lamellae and retained β). Due to the different nano-hardness between primary α lamellae/grains and transformed β regions, a plastic strain partitioning between the two components was found in both microstructures. The strain gradient at the interface between the two components introduced geometrically necessary dislocations, which contributed to the strain hardening ability. It is therefore believed that the interface length density was a critical microstructural parameter in determining the strain hardening abilities of bi-lamellar and bimodal microstructures, which explained the different intercritical annealing temperature dependences of uniform elongation in both microstructures.