Microstructures and mechanical properties of NiAl–Ni₂AlHf alloys

The microstructure and mechanical properties of several Ni–Al–Hf alloys in the composition range between NiAl (β) and Ni₂AlHf (Heusler phase) have been studied. The volume fraction of Heusler phase, V_f, in these alloys varies from about 15 to 96%. The lattice misfit between the β and Heusler phases in two-phase alloys is larger than 5%, indicating no coherency between them. The yield strength increases with increasing V_f at all temperatures to 1000°C. Compressive ductilities of 4 and 7% were obtained for the alloy with V_f of 15% at room temperature and 500°C, respectively, but they decreased to 0% with increasing V_f to 96%. The corresponding fracture mode is basically transgranular cleavage. However, all the alloys can be deformed extensively without fracture at 1000°C. The hardness of the Heusler alloy is very high (8.3 GPa) at room temperature, and it decreases gently with temperature to 600°C, followed by a rapid decrease to 1000°C. The brittleness and high hardness of the Ni₂AlHf Heusler phase at low temperatures are interpreted in terms of internal lattice distortion resulting from its crystal structure. The thermally activated process of deformation takes place above 600°C, which is responsible for the rapid drop of the hardness of the alloys.