Mechanical attrition of intermetallic TiNi powder was carried out in two different ball mills and studied as a function of milling temperature. The microstructural changes with milling time were monitored by X-ray diffraction, TEM and DSC. The more energetic Spex shaker mill provided a higher degree of lattice strain and rapidly refined the grain size into the nanometre regime. Amorphization was observed in the Spex mill with a linear increase in the milling time for amorphization with increasing milling temperature. No amorphization was observed in the less energetic vibratory mill, and the grain size saturated to a constant value of 15nm after more than 60h of milling. A critical grain size for amorphization, of 4 to 5nm, was estimated from the temperature-dependent studies in the Spex mill. The grain-boundary energy (706mJ/m2), estimated from the vibratory mill experiments, and the above critical grain sizes (5nm) for amorphization were used to calculate the enthalpy supplied by the nanocrystalline grain boundaries. The calculated value of 4.1kJ/mol was comparable to the measured enthalpy of crystallization of 3.2kJ/mol. It was concluded that the nanocrystalline grain-boundary energy was responsible for driving the crystalline-to-amorphous phase transformation induced by mechanical attrition in TiNi.

The Influence of Mill Energy and Temperature on the Structure of the TiNi Intermetallic after Mechanical Attrition. K.Yamada, C.C.Koch: Journal of Materials Research, 1993, 8[6], 1317-26