Amorphisation and intermetallic nanophase formation in ball-milled Al-Ti-Si studied through positron lifetime spectroscopy

Abstract

Alloys of Al-Ti-Si of different compositions were prepared by ball-milling and were characterised by X-ray diffraction. A characteristic transformation to an amorphous phase during continued ball-milling is highlighted in the context of the production of excess free volume at the grain boundaries and interfaces, as the grains undergo nanometrical size reduction. Nanocrystalline intermetallic alloys appear to develop when the alloys are milled for 20-30 h. Positron lifetime measurements are carried out on samples of various compositions and after different hours of milling. The observed annihilation characteristics are found to differ from the normally expected trend during the reduction of the grain sizes. It is observed that the alloy with larger Al content is less susceptible to amorphisation whereas the said transformation is achieved with less hours of milling with increasing Ti content. The amorphisation process in fact begins when the average grain size falls to typical nanometric values where the fractional excess free volume associated with the atoms on the grain boundaries increases asymptotically. The nanoparticles of the intermetallic alloys appear to agglomerate due to the energy dissipated by the thermalising positrons.