Microstructure examination of the Zr65Al10Ni10Cu15 bulk metallic glass after superplastic gas-pressure forming

Abstract

Microstructure and crystallinity during superplastic gas-pressure forming of the Zr65Al10Ni10Cu15 bulk metallic glass sheet fabricated by squeeze copper-mold casting method was examined. A small degree of crystallization (5.7-8.7 vol.%) was found to occur during the forming, though the sum of heating and holding time for the sample and forming time was controlled to be much less than the onset time of crystallization predicted by the T.T.T diagram of the metallic glass sheet. The degree of crystallization depended on position of the deformed body, which was highest at the apex where the largest straining took place, indicating acceleration of crystallization kinetics assisted by plastic flow. The strain-rate-change test at 696K performed in tension repeated for four times consecutively on a given sample suggested that superplastic-flow behavior of the metallic glass sheet would not have been significantly affected by crystallization during hot gas-blow forming.