Effect of Milling Time on Crystallization Sequence and Microstructure of NiTi Alloys Produced Via High-Energy Ball Milling

A 20 gram batch weight of NiTi alloy, with a nominal equiatomic composition, was produced by mechanical alloying with milling times of 100, 120, and 140 hours. The differential scanning calorimetry was used to analyze the progress of the crystallization process. The X-ray diffraction examined the crystal structure of the alloy at individual crystallization stages. The observation of the powders microstructure and the chemical composition measurement were carried out using a scanning electron microscope equipped with an energy-dispersive detector. After the milling process, the alloy revealed an amorphous-nanocrystalline state. The course of the crystallization process was multi-stage and proceeded at a lower temperature than the pure amorphous state. The applied production parameters and the stage heat treatment allowed to obtain the alloy showing the reversible martensitic transformation with an enthalpy of almost 5 J/g.