Effect of Strain Rate on the Formation of Fine-Grained Microstructure in a Cast Aluminum Alloy 7475 upon High-Temperature Upsetting

摘要

Microstructural changes occurring upon high-temperature deformation (T= 490 deg C, epsilon > = 10~(-2) s~(-1)]) of a cast aluminum alloy 7475 have been analyzed. Two ranges differing in the mechanical and structural behaviors of the material were distinguished in the investigated interval of strain rates. At epsilon > = 10~(-2) s~(-1) the material undergoes rapid softening at the initial deformation stage with a subsequent stress stabilization occurring at the steady-state stage of plastic flow at high degrees of deformation. The structure changes are characterized by the development of deformation bands leading to the formation of a new fine-grained structure. The new grains are formed as a result of continuous structural reactions that are similar in type to the "continuous" dynamic recrys-tallization. At epsilon > = 10~(-2) s~(-1), deformation does not lead to softening of the material, and the steady-state stage of plastic flow is reached at early stages of deformation. The structure changes at such strain rates are caused mainly by the development of dynamically equilibrium subgrain structure. No formation of new grains is observed up to relatively high degrees of deformation (epsilon = 1.4). The critical conditions and the mechanisms of the formation of fine-grained structure are discussed in terms of the evolution of the strongly misoriented dislocation substructures that are formed due to the deformation localization upon the development of grain-boundary sliding in the initially coarse-grained material.