Mechanical behavior of thein situ composite alloys in the al-ni-ti system near the l12 phase field

The Vickers microhardness (VHN) test at room temperature and compressive tests at temperatures up to 1000 °C were carried out on the three-phase composite alloy, consisting of the Ll₂, facecentered cubic (fcc) Al₂TiNi, and Al₂Ti intermetallic phases, in the Al-Ti-Ni system. The microhardness tests indicated that the fcc Al₂TiNi phase was very hard and brittle. Comparatively, the L1₂ phase was softer and more crack resistant. A considerable hardening was noticed due to the precipitation of Al₂Ti within L1₂. In addition, the VHN of the L1₂ phase was found to increase with the combined content of nickel and titanium without the presence of any observable precipitates. Under compressive loading at room temperature, microcracks nucleated in the fcc Al₂TiNi phase. These cracks propagated catastrophically at a stress barely approaching yield stress, resulting in nil ductility. This behavior was observed up to 800 °C. Between 900 °C and 950 °C, brittle-to-ductile transition in compressive behavior was observed for the three-phase alloy. Compressive ductility of the order of 80 pct was observed at 1000 °C. The mechanism of dynamic recrystallization was found to be operative at 1000 °C. Metallographic investigation revealed new recrystallized grains in the primary L1₂ matrix. However, the oscillatory nature of the true stress-true strain curve could not be explained with the help of the existing model of dynamic recrystallization. S. BISWAS, formerly Graduate Student, Department of Mechanical Engineering, University of Waterloo