Grain Boundary Phase Transitions in the Al-Mg System and Their Influence on High-Strain Rate Superplasticity

摘要

Grain boundary (GB) wetting phase transitions were studied, for the first time, in Al-Mg polycrystals (Mg contents of 5, 10, 15 and 20 wt.%; temperature range 490-630 ℃). It was observed that above 598 ℃ all GBs in the solid Al-rich phase were wetted by a liquid phase. Below 540 ℃ no GBs wetted by a liquid phase were observed. The maximal and minimal temperatures of the GB wetting phase transitions, T_(wmax) and T_(wmin), were determined. Between 540 and 598 ℃ the percentage of the wetted GBs gradually increases from 0 to 100 %. Grain boundary presetting or premelting is given as an explanation for the hitherto not understood high-strain rate superplasticity observed for ternary Al-Mg-X alloys in a narrow temperature range just below the bulk solidus line.