Deformation mechanism of bimodal microstructure in Ti-6Al-4V alloy: The effects of intercritical annealing temperature and constituent hardness

Abstract

The so-called bimodal microstructure of Ti-6Al-4V alloy, composed of primary α grains (αp) and transformed β areas (βtrans), can be regarded as a “dual-phase” structure to some extent, the mechanical properties of which are closely related to the sizes, volume fractions, distributions as well as nano-hardness of the two constituents. In this study, the volume fractions of primary α grains (vol.%(αp)) were systematically modified in three series of bimodal microstructures with fixed primary α grain sizes (0.8 μm, 2.4 μm and 5.0 μm), by changing the intercritical annealing temperature (Tint). By evaluating the tensile properties at room temperature, it was found that with increasing Tint (decreasing vol.%(αp)), the yield strength of bimodal microstructures monotonically increased, while the uniform elongation firstly increased with Tint until 910 °C and then drastically decreased afterwards, thereby dividing the Tint into two regions, namely region I (830−910 °C) and region II (910−970 °C). The detailed deformation behaviors within the two regions were studied and compared, from the perspectives of strain distribution analysis, slip system analysis as well as dislocation analysis. For bimodal microstructures in region I, due to the much lower nano-hardness of βtrans than αp, there was a clear strain partitioning between the two constituents as well as a strain gradient from the αp/βtrans interface to the grain interior of αp. This activated a large number of geometrically necessary dislocations (GNDs) near the interface, mostly with <c+a> components, which contributed greatly to the extraordinary work-hardening abilities of bimodal microstructures in region I. With increasing Tint, the αp/βtrans interface length density gradually increased and so was the density of GNDs with <c+a> components, which explained the continuous increase of uniform elongation with Tint in this region. For bimodal microstructures in region II, where the nano-hardness of βtrans and αp were comparable, neither a clear strain-partitioning tendency nor a strain gradient across the αp/βtrans interface was observed. Consequently, only statistically stored dislocations (SSDs) with <a> component were activated inside αp. The absence of <c+a> dislocations together with a decreased volume fraction of αp resulted into a dramatic loss of uniform elongation for bimodal microstructures in region II.