Charpy impact behaviors of metastable β-Ti alloys: Transformation induce plasticity versus twining induce plasticity

Abstract

TRansformation Induced Plasticity (TRIP) and TWinning Induced Plasticity (TWIP) have been successfully proposed to overcome the trade-off between strength and ductility of metastable β Ti alloys under static tensile deformation. However, whether they can be directly extended to impact deformation is still controversial due to the high strain rate and the existence of notch. In this work, Ti-4.7Al-6.26Mo-2.85Cr (TRIP Ti alloy) and Ti-8.5Cr-1.5Sn alloy (TWIP Ti alloy) were selected to compare the Charpy impact deformation behaviors and the corresponding deformation mechanisms. It was found that, under Charpy impact deformation, the primary deformation mechanism of TRIP Ti alloy was dislocation slip due to the massive stress-induced martensite (SIM) were inhibited at high strain rate; whereas, in the case of TWIP Ti alloy, the activity of stress-induced deformation twins (SIDTs) was promoted at high strain rate, which resulted in the intersection of different DTs and the formation of secondary DTs. The formation of the hierarchical DTs structures in TWIP Ti alloy not only effectively released the stress concentration to delay the crack initiation, but also remarkably increased the crack propagation path to enhance crack propagation resistance. Specifically, compared with TRIP effect, it seems as though TWIP effect was more efficient at accommodating the imposed high strain rate deformation. These findings enrich the understanding of TRIP and TWIP under impact deformation, and provide references for the design of metastable β titanium alloy with high impact toughness.