Abstract
The high temperature deformation of a fine-grained aluminum alloy processed by rapid solidification powder metallurgy was investigated. Region I of low rate sensitivity with a high activation energy and a negative grain size exponent (in contrast with the positive exponent of the superplastic region) was observed. These observations support the existence of a threshold stress for superplastic flow which varies inversely with grain size and depends strongly on temperature. The restricted mobility of the grain boundaries pinned by the dispersed particles during the superplastic flow is suggested to be the origin of the threshold stress.
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