The effects of changes, in the dislocation sub-structure, upon the strength and toughness of stoichiometric polycrystalline material were studied. The dislocation sub-structure was introduced by a thermomechanical processing which involved the hydrostatic extrusion, at 573K, of polycrystalline material that had previously been cast and conventionally extruded at 1173K. Texture analysis had revealed that the beneficial stress-state, which was created by hydrostatic extrusion near to the ductile/brittle transition, produced a change in texture. Complementary transmission electron microscopy and optical metallography revealed that recrystallization did not occur during or after such low-temperature deformation processing. The dislocation sub-structure which was retained after hydrostatic extrusion led to an increase in both the compressive strength and fracture toughness. Subsequent hot micro-hardness experiments, and high-temperature annealing studies, revealed that removal of the dislocation sub-structure, from samples which had been hydrostatically extruded at 573K, occurred during exposure to 973K. This was accompanied by a reduction in both the strength and toughness.

Effects of Dislocation Sub-Structure on Strength and Toughness in Polycrystalline NiAl Processed via Low-Temperature Hydrostatic Extrusion J.J.Lewandowski, B.Berger, J.D.Rigney, S.N.Patankar: Philosophical Magazine A, 1998, 78[3], 643-56