Microstructures and Martensitic Transformation Behavior of a (Ti plus Hf)-Rich Ti-49Ni-12Hf Alloy

Abstract

Microstructures, deformation behavior, hot workability, and martensitic transformation behavior of Ti-49Ni-12Hf alloy were investigated using electron probe micro-analysis, differential scanning calorimetry (DSC), tensile tests, and scanning electron microscopy. The Ti-49Ni-12Hf alloy homogenized at various temperatures consisted of a Ti-Ni-Hf matrix and (Ti,Hf)(2) Ni particles. The volume fraction of the (Ti,Hf)(2) Ni particles decreased from 5.2% to 3.1% on increasing the homogenization temperature from 1173 K to 1273 K. The DSC peak temperatures corresponding to martensitic transformation decreased with the increase in homogenization temperature, which was ascribed to the dissolution of (Ti,Hf)(2) Ni into the matrix. The fracture stress increased from 604 MPa to 1245 MPa and the fracture strain increased from 4.6% to 12.1% on increasing the homogenization temperature from 1173 K to 1273 K, which was attributed to the decreased volume fraction of (Ti,Hf)(2) Ni particles. The (Ti,Hf)(2) Ni particles initiated cracks during tensile loading, which propagated through the boundary between the (Ti,Hf)(2) Ni particles and the matrix. Ti-49Ni-12Hf alloy ingots homogenized at temperatures higher than 1173 K were hot rolled without significant cracking, whereas those homogenized at 1173 K were fractured.