The effect of heat treatment on the microstructural changes and hardness of cast Ni₅₀Ti₃₀Zr₂₀ alloy

¹ Department of Materials Science and Metallurgical Engineering, University of Pretoria, South Africa
² Vilnius Gediminas Technical University, Institute of Building Materials Laboratory of Composite Materials, Lithuania

Corresponding author: shangasepaul@gmail.com

Abstract

The nickel-based shape memory alloys (SMAs) provide attractive properties inter alia lightweight and simple compact mechanical designs requiring low actuating force levels. However, there is a limit to their functional properties such as super-elasticity and shape memory effects, as they cannot perform well above 100°C. Thus, there is a need to improve these properties for applications of these alloys beyond their maximum transformation temperatures, by alloying them with elements like Zr. The aim of this study is to examine how heat treatments affect microstructural evolution is crucial for SMAs designed for high-temperature application. As microstructures are closely related to mechanical properties, analysis of the changes in mechanical properties through various heat treatment processes may provide key information on how to design alloys with superior mechanical properties. In order to create a homogeneous microstructure and eliminate chemical segregation, six samples (5 × 10 × 10 mm) were homogenised at 1000°C. Samples were then aged for one hour and three hours at heat treatment temperatures of 550°C, 750°C, and 800°C. The samples were analysed using SEM, XRD and Vickers hardness. The results show a microstructure with a (Ti, Zr)₂Ni phase that precipitated due to heat treatment and resulted in increased hardness.