Role of stress-induced martensite on damage behavior in a metastable titanium alloy

In general, stress-induced martensite (SIM) is frequently employed to enhance the strain hardening behavior of Ti alloys through a so-called transformation-induced plasticity (TRIP) effect. However, the SIM does not necessarily lead to the improved mechanical performance of Ti alloys with the underlying mechanism unclear yet. The present work investigates the damage behavior of a typical model metastable Ti-1023 alloy with dominant SIM using uniaxial tensile test and nano-indentation measurement. The full beta microstructure with varied grain size (70 μm-350 μm) is obtained in the present Ti-1023 alloy through careful heat treatments. The integrated mechanical and microstructural characterizations indicate that the SIM displays a dual impact on the damage behavior of the present Ti-1023 alloy. In particular, the SIM could either facilitate the nucleation of cracks or inhibit the propagation of cracks depending on the martensitic lath spacing. The above dual-impact of SIM on the damage of Ti-1023 alloy can be rationalized based on the competing role of martensitic lath spacing on the dislocation pile-up and damage nucleation at lath boundaries. The dislocation-based plasticity could additionally assist the crack blunting. The effect of the beta stability on the damage behavior is found to be indirectly determined by the martensitic lath spacing. Since the martensitic lath spacing is mainly governed by the beta grain size, the present work suggests that the grain boundary engineering should be harnessed to fully explore the potential of SIM in developing strong and ductile/tough Ti alloys for broad industrial applications.