MICROSTRUCTURES AND HARDNESS OF ULTRAFINE-GRAINED NI3AL

The microstructural evolution of the ultrafine-grained intermetallic c ompound Ni3Al is studied as a function of annealing at different tempe ratures. The ultrafine microstructure is produced by a high plastic to rsional straining. Transmission electron microscopy, X-ray diffraction and differential scanning calorimetry are used to characterize the mi crostructural evolution and microhardness is used to determine mechani cal behaviour. The as-deformed microstructure exhibits an almost fully disordered crystalline structure with coherent domain size of about 1 8 nm, a strong torsional texture and high internal elastic strains. On annealing the as-deformed samples at different temperatures, the recr ystallization of the material into a granular type structure containin g non-equilibrium grain boundaries is first observed. This is followed by the transformation from non-equilibrium into equilibrium grain bou ndaries with simultaneous grain growth. This transformation is correla ted with an increase of hardness. A new concept of non-equilibrium gra in boundaries transparency is presented to interpret this singular beh aviour. The results are compared to those obtained on an ultrafine-gra ined Al-1.5% Mg alloy produced by the same technique and which exhibit s the same mechanical behaviour.