Mechanical behavior of the in situ composite alloys in the Al-Ni-Ti system near the L1(2) phase field

The Vickers microhardness (VHN) test at room temperature and compressive tests at temperatures up to 1000 degrees C were carried out on the three-phase composite alloy, consisting of the Ll(2), face-centered cubic (fee) Al2TiNi, and Al2Ti intermetallic phases, in the Al-Ti-Ni system. The microhardness tests indicated that the fee Al2TiNi phase was very hard and brittle. Comparatively, the Ll(2), phase was softer and more crack resistant. A considerable hardening was noticed due to the precipitation of Al2Ti within Ll(2). In addition, the VHN of the Ll(2) 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 fee Al2TiNi phase. These cracks propagated catastrophically at a stress barely approaching yield stress, resulting in nil ductility. This behavior was observed up to 800 degrees C. Between 900 degrees C and 950 degrees C, brittle-to-ductile transition in compressive behavior was observed for the three-phase alloy. Compressive ductility of the order of 80 pet was observed at 1000 degrees C. The mechanism of dynamic recrystallization was found to be operative at 1000 degrees C. Metallographic investigation revealed new recrystallized grains in the primary Ll(2) 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.