Forming and control of pores by capsule-free hot isostatic pressing in NiTi shape memoryalloys

Abstract

In this study, the pore evolution process of porous NiTi shape memory alloys (SMAs)fabricated by capsule-free hot isostatic pressing (CF-HIP) was investigated by adoptingdifferent fabricating parameters. It is shown that porous NiTi SMAs with homogeneouspore distribution and nearly spherical pore shape can be prepared by CF-HIP undersuitable conditions. In addition, two novel pore structures were produced, one with asandwich-like structure, and another with controlled gradient of porosity along the radialdirection, with pore size increasing from the outside towards the center of the specimen.The former was obtained by balancing the pressure in the HIP chamber and thegas pressure resulting from the gas expansion in the green sample. The latterresulted from the formation of air bubbles in the liquid phase during the sintering.