ECAP加工したMg-8 mass%Li合金の超塑性変形によるミクロ組織変化

抄録

  Microstructural change of a Mg-8 mass%Li alloy processed by a deformation combining the extrusion and equal-channel angular pressing (ECAP) during superplastic deformation has been investigated by optical microscopic observation and SEM/EBSP analysis. The maximum tensile elongation to failure of up to about 970% was attained at a temperature of 473 K and with a strain rate of 1.0×10^-4 s^-1. The elongated α phase in the cross section of tensile specimens was observed to change an equiaxed shape during superplastic deformation. Twinning was observed in the α phase in the initial stages of superplastic deformation. The frequency of high angle boundaries defined as above 15° misorientation increased with increasing the elongation to failure in order to advance a dynamic recrystallization during superplastic deformation. Cavities were formed in the boundaries between α and β phases from elongations of about 600%. The area fraction of cavities in the cast+extrude+ECAP condition was smaller than in the cast+extrude condition. These microstructural changes play an important role for superplastic deformation of the Mg-8 mass%Li alloy.