Effects of addition of magnesium on interface structure and high-strain-rate superplasticity in Si3N4-reinforced Al-alloy composites

The mechanical properties and interface structure of fine-grained Al-Cu all oy and Al-Cu-Mg alloy composites, reinforced with Si3N4 whiskers (Si(3)N(4) w) and particles (Si(3)N(4)p), are investigated to reveal the role of magne sium addition in high-strain-rate superplasticity. The AI-Cu alloy composit es, which exhibit lower elongations (<100%), have clear interfaces between the Al-matrix and Si3N4 crystals without any interfacial reaction. On the o ther hand, the interfaces in the Al-Cu-Mg alloy composites, which exhibit h igh elongations (>280%), show strong reaction between the Al-matrix and Si3 N4 crystals. The result suggests that the addition of magnesium causes the reaction between the Al-matrix and Si3N4 crystals, and consequently that pa rtially melting of the reaction phases at the tensile-testing temperature r esults in the relaxation of stress concentration and suppresses the develop ment of microcracks and cavities at the interfaces during superplastic defo rmation. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science! Ltd : All rights reserved.