Effect of microstructure refinement on high strain rate superplasticity in a PM 2024Al-Fe-Ni alloy

Improvement of high strain rate superplasticity (HSRS) of a high strength PM 2024Al-3Fe-5Ni alloy has been examined in relation to the microstructure refining through a modification of heating condition to testing temperatures. The alloy was fabricated by an air atomization technique, followed by extrusion at 623 K and warm rolling at 523 K. After the warm rolling, the alloy exhibited ultra fine-grained structure stabilized with a fine dispersion of intermetallic particles. The high strain rate superplastic properties for the present alloy were remarkably improved by the microstructural refinement, even at the optimum superplastic temperature of 773 K, which is close to the solidus temperature of about 780 K for the alloy. The finer the sizes of matrix grains and intermetallic particles, the higher values of total elongation and strain rate sensitivity were obtained at a high strain rate range of 10^-1-10 s^-1 and at a wide temperature range of 713-773 K. The stress concentration during sliding at the particle/matrix interface was considered to be diffusionally relaxed, because the particles were sufficiently small. A constitutive equation has been obtained through an analysis of high strain rate superplasticity data on the alloy. The high strain rate superplastic mechanism for the alloy is probably lattice diffusion controlled grain boundary sliding.