Influence of Thermal Treatment on Crystallization and Course of Martensitic Transformation in Ni25Ti50Cu25 Alloys Produced via High-Energy Ball Milling

Abstract

The alloy with the chemical composition of Ni25Ti50Cu25 and a weight of 20 g was produced via high-energy ball milling that lasted 100 h, 120 h, and 140 h. After grinding, the powders consisted of an amorphous-nanocrystalline mixture formed from solid solutions based on alloying elements and a phase being a precursor of the intermetallic ß phase. This phase undergoes the reversible martensitic transformation in NiTiCu alloys. Crystallization occurred in several stages, ranging from 361 to 556 °C. Using short-term annealing followed by the X-ray quantitative analysis, the crystallized phases sequence and their weight percentages were determined after each stage. Annealing of the crystallized alloy lasted from 5 min to 20 h in the temperature ranges from 700 to 1000 °C, which revealed the quantitative evolution of the equilibrium phases and the martensitic transformation evolution. Moreover, following thermal treatment steps, the transformation changed from the B2 ↔ B19′ to the B2 ↔ B19, as expected for the Ni25Ti50Cu25 alloy.