Formation and structures of bulk nanocrystalline intermetallic alloys studied by transmission electron microscopy

The evolution of nanocrystalline structures in bulk intermetallic alloys made by high-pressure torsion (HPT) was investigated using transmission electron microscopy methods. HPT deformation of long-range ordered Ni3Al induces a heterogeneous formation of a nanocrystalline (NC) structure accompanied with disordering of the nanograins. At a strain of 40,000%, about 90% of the volume is transformed to the NC structure. From the existence of retained coarse-grained (CG) volumes, it is concluded that the two different structures carry different strains during HPT. The weak texture of the NC structure indicates that an inter-granular deformation process is occurring. In NiTi shape memory alloys subjected to HPT, grain refinement and texture formation is followed by amorphization. Nanocrystalline debris that has survived the deformation is embedded in an amorphous matrix. During devitrification of the amorphous phase these nanocrystallites act as nucleation sites. The nuclei trigger the formation of a nanocrystalline structure of a preferred crystallographic orientation that corresponds to that of the original texture after HPT deformation.