Influence of Milling Time on Structural and Microstructural Parameters of Ni50Ti50 Prepared by Mechanical Alloying Using Rietveld Analysis

Nanostructured Ni50Ti50 powders were prepared by mechanical alloying from elemental Ni and Ti micrometer-sized powders, using a planetary ball mill type Fritsch Pulverisette 7. In this study, the effect of milling time on the evolution of structural and microstructural parameters is investigated. Through Rietveld refinements of X-ray diffraction patterns, phase composition and structural/microstructural parameters such as lattice parameters, average crystallite size < L >, microstrain <epsilon(2)>(1/2), and stacking faults probability (SFP) in the frame of MAUD software have been obtained. For prolonged milling time, a mixture of amorphous phase, NiTi-martensite (B19'), and NiTi-austenite (B2) phases, in addition to FCC-Ni(Ti) and HCP-Ti(Ni) solid solutions, is formed. The crystallite size decreases to the nanometer scale while the internal strain increases. It is observed that, for longermilling time, plastic deformations introduce a large amount of stacking faults in HCP-Ti(Ni) rather than in FCC-Ni(Ti), which are mainly responsible for the observed large amount of the amorphous phase.