温間変形した7075アルミニウム合金における微細粒組織の形成と超塑性変形

抄録

Hot deformation of prior warm-worked 7075 aluminum alloy was studied by means of 2-step tensile testing at 798 K and at various strain rates, and metallographic observations. The Al alloy samples before hot deformation contained high density of dislocations and fine particles precipitated on them, which existed quite stably at 798 K. Strain rate dependence of flow stresses as well as flow curves changes clearly in the three regions of strain rate, \dotε; i. e. region I (lower \dotε), II (medium \dotε) and III (higher \dotε). Typical superplasticity taking place in region II leads to the total elongation to fracture of over 750% and the stress exponent of 1.7. Rapid work softening after a peak stress can result from the fine grains evolved in high density dislocation matrices and subsequent gradual work hardening from grain coarsening taking place at high strains. The average size of new grains can be expressed only by flow stresses developed during hot deformation. It is concluded that hot deformation behaviors of 7075 aluminum are affected sensitively by the change of grain structures and then hot deformation in region II can be controlled mainly by grain boundary sliding.