In Situ SEM Investigation of Deformation Processes and Fracture Behavior in Bimodal Ti-6Al-3Nb-2Zr-1Mo Alloy

The deformation mechanism, crack initiation, propagation and fracture behavior of a new near-alpha titanium alloy Ti80 (Ti-6Al-3Nb-2Zr-1Mo) with a bimodal microstructure consisting of primary alpha (alpha(p)) grains and transformed beta (beta(trans)) matrix is systematically investigated by in situ tensile testing in SEM. The results show that alpha(p) preferentially undergoes plastic deformation (single slip) in the early stage of deformation. As the deformation continues, the crack nucleates in the hard-to-deform region near the U-shaped notch, and then advances through alpha(p) to form small cracks, which tend to initiate at the alpha(p)/beta(trans) interface and slip bands region. However, slip transfer occurs between some alpha(p) and adjacent beta(trans), and multi-slip or cross-slip is generated in alpha(p), which plays a role in coordinating the inhomogeneity of local plastic deformation and delaying crack initiation. In the crack propagation stage, the main crack and microcracks are connected to promote crack growth. The crack tends to propagate along the alpha(p)/beta(trans) interface or slip bands inside alpha(p). The alpha(p) boundaries play the role of deflecting the crack propagation direction. However, a relatively flat crack path is formed from a macroscopic point of view due to the small particle size of alpha(p).