Microstructure and stored energy evolutions during rolling of Cu60Zr20Ti20 bulk metallic glass

摘要

The microstructure and stored energy of Cu60Zr20Ti20 bulk metallic glass rolled at cryogenic temperature in a wide strain rate range 1.0 x 10(-4) - 5.0 x 10(-1) s(-1) have been investigated. As the specimen is rolled to be thinner, the stored energy first increases linearly, and then saturates above a critical thickness reduction at lower strain rates, or decreases at high strain rates. At the initial stage of rolling, no phase transformation except shear bands appears in the glass. Phase transformation occurs only when the specimen is severely deformed at strain rates higher than 1.0 x 10(-4) s(-1). As strain rate increases, the critical strain for the stored energy to saturate increases, but the critical strain for phase separation to occur decreases, and meanwhile the type of the phase transformation changes from phase separation to nanocrystallization. The stored energy does not change with the Occurrence of phase separation, but decreases due to nanocrystallization. It is proposed that coalescence of more free volume in shear bands into nanovoids should be principally responsible for the Saturation of the stored energy, which balances the results from the increase in shear band number at higher strains.