Plastic instability in Ti–6Cr–5Mo–5V–4Al metastable β-Ti alloy containing the β-spinodal decomposition structures

Metastable β-titanium (Ti) alloys have been extensively used in aerospace industry owing to the outstanding mechanical properties. Temperature arising sometimes becomes unavoidable in service and thus it is necessary to evaluate plastic behavior of the alloys at elevated temperatures prior to applications. Here, plastic stability of metastable β-titanium alloys containing the β-spinodal decomposition structures at different temperatures is investigated for the first time using Ti–6Cr–5Mo–5V–4Al alloy as prototypical material. It is found that a serrated plastic flow with little frequency accompanied by overall strain softening appears in the tensile stress-strain curves at 298 K, while the serrated plastic flow occurs much more frequently and overall strain hardening capability is recovered appropriately when the tensile temperature is increased to 573 K. Microstructural characterizations reveal that ordinary dislocation slip dominates the alloy plasticity at all the testing temperatures. Dislocations strongly interact with the β-striations and dislocation debris are left around the striation interfaces. The resulting local stress accumulation destroys the β-striations in the form of shear bands so that sudden release of an avalanche of dislocations in the soft channels leads to stress drop. Subsequent dynamic reconstruction of the β-striations with the assistance of thermal activation especially at elevated temperature hardens the prior soft channels, which results in the following stress rise. The striking difference on deformation characteristics at 298 K and 573 K is that more homogeneous plasticity conferred by dense deformation bands at the latter, which is the direct reason on high serrated frequency and overall strain hardening. These findings offer valuable insights into plastic behavior of metastable β-Ti alloys, which will have important implications for the applications.