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

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 degrees 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 degrees 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.