The role of liquid phase on superplasticity in metals and ceramics

The presence of a small amount of liquid phase in a superplastic metal or ceramic can change its deformation properties. This has been illustrated in ceramics and metallic alloys (including conventional alloys, mechanically alloyed materials, and metal-matrix composites). In the present paper, the effects of liquid phases on the microstructure, flow properties, and fracture behavior in superplastic ceramics and metal alloys are discussed and reviewed. Our recent results from the study of the effect of strain rate on the elevated-temperature tensile properties of an Al-0.75wt%Pb Alloy will also be presented. In this case, samples were tested from 100°C to 350°C, i.e. at temperatures both below and above the melting point of Pb, and from a conventional superplastic strain rate of 10^-4 s^-1 to a strain rate of 1 s^-1. It was found that at a low strain rate (10^-4 s^-1), a ductile to brittle transition was observed at temperatures between 320 and 330°C, within which temperature range liquid Pb begins to form at grain boundaries. This promotes grain-boundary cavitation and, thus, premature failure. At a high strain rate (1 s^-1), on the other hand, even at temperatures above the melting point of Pb, grain-boundary cavitation is greatly suppressed and ductility significantly increases, m this case, the fracture of the sample is determined by neck instability. These results are directly compared with those obtained from pure aluminum.