Processing and Characterization of NiTi Porous SMA by Elevated Pressure Sintering

Abstract

Currently, three methods are commonly used for producing porous NiTi shape memory alloys (SMAs) from elemental powders. These include conventional sintering, Self-propagating High temperature Synthesis (SHS), and sintering at elevated pressure via a Hot Isostatic Press (HIP). Conventional sintering requires long heating times and samples are limited in shape and pore size. SHS is initiated by a thermal explosion ignited at one end of the specimen, which then propagates through the specimen in a self-sustaining manner. One of the difficulties with SHS is the inability to control intermetallic phases. This work will focus on the fabrication and characterization of porous NiTi SMA material produced from elemental powders via HIPping. Porous NiTi SMA was produced from elemental Ni and Ti powders at elevated temperature and pressure using a HIP. Small and large pore specimens containing average pore sizes ranging from 20 μm up to 1 mm have been produced by slightly varying the HIPping sintering temperatures and times. Quasi-static and dynamic loading experiments are conducted on various samples produced using the presented methodology and their shape recovery and energy absorption characteristics are measured during the forward and reverse phase transformation and detwinning. Their phase transformation characteristics were found using calorimetric measurements and their composition has been studied using optical and electron microscopy and microprobe X-ray analysis.