Microstructures and hardness of ultrafine-grained Ni3Al

The microstructural evolution of the ultrafine-grained intermetallic compound Ni₃Al is studied as a function of annealing at different temperatures. The ultrafine microstructure is produced by a high plastic torsional straining. Transmission electron microscopy, X-ray diffraction and differential scanning calorimetry are used to characterize the microstructural evolution and microhardness is used to determine mechanical behaviour. The as-deformed microstructure exhibits an almost fully disordered crystalline structure with coherent domain size of about 18 nm, a strong torsional texture and high internal elastic strains. On annealing the as-deformed samples at different temperatures, the recrystallization of the material into a granular type structure containing non-equilibrium grain boundaries is first observed. This is followed by the transformation from non-equilibrium grain boundaries with simultaneous grain growth. This transformation is correlated with an increase of hardness. A new concept of non-equilibrium grain boundaries transparency is presented to interpret this singular behaviour. The results are compared to those obtained on an ultrafine-grained Al-1.5% Mg alloy produced by the same technique and which exhibits the same mechanical behaviour.