AN INVESTIGATION OF THE ROLE OF A LIQUID-PHASE IN AL-CU-MG METAL-MATRIX COMPOSITES EXHIBITING HIGH-STRAIN RATE SUPERPLASTICITY

Tensile experiments were performed under conditions of true constant strain rate on three Al-Cu-Mg metal matrix composites containing 20vol.% of either beta-Si3N4 whiskers or alpha-Si3N4 particulates with diameters of 0.2 or 1.0 mum, respectively. The tests were conducted at temperatures from 758 to 833 K and at strain rates up to 10(2) s-1. The results show that each composite exhibits a strain rate sensitivity of >0.3 at strain rates in the vicinity of approximately 10(-1)-l s-1 and with associated superplastic elongations of > 200% or even up to > 800% for the composite with particulates having a diameter of 1.0 mum. It is concluded that high ductilities are achieved at effective testing temperatures above the solidus temperature of the matrix alloy but the results are not consistent with a rheological model based on deformation in a semi-solid matrix. It is suggested instead that the presence of a liquid phase at the interfaces between the reinforcement and the matrix serves to relieve the stress concentrations due to sliding and thereby to limit the development of internal cavitation.