Evolution of Phases and their Influence on Shape Memory Effect by Varying Sintering Parameters of NiTi Alloys

NITINOL is the most popular and economic shape memory alloy (SMA) used in various industries. For temperature associated applications, the shape memory effect (SME) is the major phenomenon for the shape and strain recovery of materials after deformation. The machinability and cold working ability of NiTi alloys are poor compared to conventional alloys. So to reduce the post-processing of the finished product with good homogenization of elements, the powder technology route is suitable for making these SMAs. Due to the simplicity and cost-effectiveness, the uniaxial press and sinter process was used for pellet making. In this paper, before sintering 450, 475, 500, 525, 550, 575 and 600 MPa compaction pressures were analysed by their green densities and 600 MPa compacted pellet yielded the better density. Sintering was done at 950, 1000, 1050, 1100 and 1150 degrees C with the variation of sintering time 0.5, 1, 1.5 and 2 h. The XRD and SEM studies showed that samples sintered at 950 degrees C have oxide phases with elemental Ni and Ni3Ti phases. For samples sintered at 1000, 1050 and 1100 degrees C, NiTi and Ti2Ni formed as major phases with minor phases of beta Ti and Ni4Ti3 precipitates formed inside the NiTi matrix. It was difficult to detect the minor phases for the sample sintered at 1150 degrees C. Needle shaped martensitic NiTi forms inside the austenitic NiTi matrix and was detected at a very high magnification of 5000x in SEM. From the Differential Scanning Calorimetry study, it was found that an increase in sintering time and sintering temperature results in a faster shape memory response.

Automated summary

NITINOL is a popular and cost-effective shape memory alloy used in various industries. Utilizing powder technology route for manufacturing these SMAs can reduce post-processing, and the uniaxial press and sinter process can yield better density.