High strain rate superplasticity in metal matrix composites: The role of load transfer

It is well established that metal matrix composites may exhibit high strain rate superplasticity (HSR SP) but the measured activation energies are higher than anticipated for lattice self-diffusion or grain boundary diffusion in the matrix alloys. It is proposed that the occurrence of load transfer introduces an additional dependence on temperature which must be included in any attempt to estimate the true activation energy for flow, Q*. Experimental data are analyzed for four metal matrix composites and it is demonstrated that, by incorporating load transfer for each material, Q* may be reduced to a value close to that anticipated for grain boundary diffusion up to testing temperatures within a fen degrees of the incipient melting temperature. The results of this analysis are consistent with data for mechanically alloyed materials exhibiting HSR SP where no load transfer is expected and where experiments have shown that the measured activation energies are also close to those anticipated for grain boundary diffusion. (C) 1998 Acta Metallurgica Inc.