Structural Evolution and Martensitic Transformation in Fe-Modified NiTi Powders

Abstract

In this study, the effects of 5 wt% Fe addition to pre-alloyed NiTi powder and high-energy ball milling time on the structural characteristics and phase transformation behavior of the NiTiFe system were investigated. Unlike studies based on elemental Ni–Ti mixtures, the use of pre-alloyed NiTi powder in this work allows evaluation of matrix modification rather than new phase formation. Milling was carried out at 300 rpm for different durations, and the powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM)/energy-dispersive X-ray spectroscopy (EDS), and differential scanning calorimetry (DSC) analyses. XRD results showed that increasing milling time led to a significant reduction in crystallite size and the evolution of a nanocrystalline structure. Peak broadening and a slight angular shift indicated increased lattice strain and possible lattice modification during milling. SEM observations revealed that fracture and rewelding mechanisms were active during milling, while EDS mapping demonstrated a more homogeneous distribution of Fe with increasing milling time. DSC results showed that the transformation temperatures of the Fe-containing, milled samples were lower than those of the unmilled, Fe-free NiTi reference. Since these samples differed in both composition and processing history, this shift was attributed to the combined effects of Fe addition and mechanical milling. Within the Fe-containing series, increasing milling time further influenced the transformation enthalpy and peak characteristics. Overall, the findings indicate that the phase transformation behavior of the NiTiFe system can be controlled not only by chemical composition but also by process-induced structural changes associated with mechanical milling.