Experimental and theoretical investigations on morphological changes of γ′ precipitates in Ni -Al single crystals during uniaxial stress-annealing

Abstract

Influences of the external tension and compression on the morphological changes of γ′ precipitates during coarsening in an Ni-15 at. % Al single crystal were experimentally investigated, and theoretical evalutions on the energetically favourable shape and its orientation were also derived based upon the anisotropic elasticity theory. The shape transformation with growth in size is experimentally found to follow the sequence: cuboid→rod→ plate, under all annealing conditions, i.e. compression-annealing, tension-annealing and external stress-free annealing. However, the external stresses give preferential orientation to such microstructures. The tension of the [001] direction aligns the rods and plates along the tension axis, while the compression aligns them on a plane perpendicular to the compression axis. Theoretical evaluations based upon the anisotropic elasticity for the energetically favourable shapes and orientations of γ′ are consistent with experimental results except in the appearance of rods after compression-annealing. This discrepancy is understood by a conception that the shape transformation is influenced not only by energetics in the elastic strain and interfacial energies, but also by elastic interaction energy.