EFFECTS OF THE LIQUID-PHASE ON THE HIGH-TEMPERATURE TENSILE DUCTILITY- FROM EMBRITTLEMENT TO SUPERPLASTICITY

Inclusions of liquid phase are generally considered to cause liquid-me tal embrittlement. On the other hand, there have been some reports imp lying the enhancement of tensile elongation by liquid inclusions. In t his paper, the possibility of elongation enhancement by liquid and its underlying mechanism were investigated in a model binary system of Al -Bi and a composite of(Al-Mg)-Si3N4. Clear evidence of elongation enha ncement was observed in the Al-Bi alloy. Liquid inclusions of Bi were found to assist stress accommodation processes and to delay cavitation failure. In the (Al-Mg)-Si3N4 composite, a close correlation was foun d between the incipient melting temperature and an optimum temperature for high-strain-rate superplasticity. The liquid phase was observed a long grain boundaries and interfaces of the composite, suggesting a si milar stress accommodation process to that of the Al-Bi alloy. The int erface microstructure was also studied by high-resolution transmission electron microscopy to understand the detailed mechanism of high-stra in-rate superplasticity.