Reversible Negative Thermal Expansion Response and Phase Transformation Behavior of a Ti-Rich Ti₅₄Ni₄₆ Alloy Prepared by Rapid Solidification

Abstract

In this study, a Ti-rich Ti–Ni alloy (Ti₅₄Ni₄₆) was prepared by rapid solidification technique through vacuum suction casting into a water-cooled copper mold. The microstructure, thermal expansion, and phase transformation behavior of the alloy were studied systematically. The results show that the rapidly solidified Ti₅₄Ni₄₆ alloy exhibits negative thermal expansion (NTE) response in both vertical and horizontal measuring directions upon heating and cooling. The discrepancy in the NTE response between the two mutually perpendicular directions of the alloy is small, indicating an implicit anisotropic NTE behavior. A one-to-one correspondence exists between the characteristic temperatures of phase transformation and NTE, as well as between their changes during thermal cycling. It is conclusive that the NTE strains generated upon heating and cooling originate from the volume changes accompanying the forward and reverse martensitic transformations in Ti₅₄Ni₄₆ alloy. Characteristic temperatures of both phase transformation and NTE of the alloy rapidly shift to lower temperatures due to the multiplication of dislocations during the initial approximately 20 thermal cycles, and then tend to be relatively unchanged in subsequent thermal cycling as the transformation-induced defects reach saturation. The absolute values of the coefficient of thermal expansion of the NTE stage upon heating and cooling decrease rapidly during the initial approximately 20 thermal cycles, and thereafter become relatively stable with the increase of thermal cycle number, which is mainly attributed to the decrease of the effective fraction of the B19′ martensite participating in the forward and reverse martensitic transformations.