Microstructure development during high strain torsion of NiAl

The microstructure development was investigated in torsion deformed NiAl. High strain torsion of solid bars was done with a Paterson rock deformation machine at temperatures between 700 K and 1300 K under a confining pressure of 400 MPa. The maximum shear strains and shear strain rates applied were 19 and 2.2 × 10-4 s-1, respectively. The shear stress-shear strain curves are characterized by a peak at low shear strains, which is followed by softening and a steady state at high shear strains. Increasing shear strain leads to grain refinement, with the average grain size decreasing with temperature. Moreover, a steady state grain aspect ratio and inclination of the grain long axis with respect to the shear plane is observed. With increasing shear strain, the fraction of low angle grain boundaries goes over a maximum and approaches a steady state of about 20-40%. The development of the microstructure is characterized by two different temperature regimes. Up to 1000 K., con tinuous dynamic recrystallization characterized by limited grain growth takes place, leading to a transformation of low into high angle grain boundaries. At temperatures above 1000 K., discontinuous dynamic recrystallization occurs by massive grain growth. The results are qualitatively discussed on the basis of models dealing with dynamic recrystallization