In the present work, combined mechanical and microstructural experiments were carried out to investigate the thermomechanical behavior of a slightly Ni(Pd)-rich Ni24.3Ti49.7Pd26 alloy (near stochiometric Ni–Ti basis with Pd replacing Ni). Initial microstructure and phase information were determined and related to the macroscopic stress–strain–temperature response. The role of deformation during initial cycling on post cycling behaviors (i.e., development of TWSME) was also examined in context of the microscopic mechanisms observed through CTX0294885 diffraction and TEM analysis.
2. Material and methods
An ingot of nominal composition Ni24.3Ti49.7Pd26 (at.%) and weighing ∼500 g was prepared by vacuum induction melting of high purity elements (99.98 Ni, 99.995 Pd, 99.95 Ti) in a graphite crucible under a protective argon atmosphere. The melt was cast into a cylindrical copper mold (25.4 mm diameter × 102 mm long) with a built-in conical hot-top to accommodate shrinkage during solidification. The resulting ingot was vacuum homogenized for 72 h at 1050 °C and furnace cooled. The ingot was subsequently sealed in a mild steel can (after removing the hot top) and extruded at 900 °C through an area reduction ratio of 7:1. Test specimens were machined from the extruded rod (Ext. 159) with the tensile axis parallel to the extrusion direction and subjected to an aging treatment of 24 h at 400 °C followed by furnace cooling. This will be referred to as the “aged” condition in subsequent sections.