Superplastic Constitutive Equation For Metal With Fine Grain And Its Application

Based on the metallographical research results for the mechanismof superplastic deformation a constitutive equation of the structural superplasticity of polycrystalline metal is developed from the viewpoint of micro-mechanics here. Some very sensitive microstructural parameters to superplastic deformation, Such as grain size and cavity volume fraction, are entered into the constitutive equation,therefore the constitutive equation can be used to study the large superplastic deformation process including grain growth, dynamic recrystallization and micro-void damage development. There are two points to be emphasized as follows. The evolution equation of the grain size is derived from the micro-mechanism analysis for the physical process of both grain growth and dynamic recrystallization rather than from some assumptions. Another point is that the void ellipticity as a shape parameter is contained in this constitutive equation to consider both the void extension and closing. In addition we also give a constitutve equation for superplastic deformation of particle reinforced composites and apply it to discuss the influence of the particle volume fraction on superplastic behaviours of the materials.As a capability examine and application of presented constitutive equation, by using it we give the analysis and calculation for some important experimental phenomina due to dynamic changes of micro-structures and typical processings of superplastic forming, for example, influence of grain size and strain rate on superplasticity. grain growth, dynamic recrystallization and stress hardening and softing due to the coarsing and refinement of the grain.The comparison of theoretical analysis and experimental results shows that the constitutive equation presented here basically reflects the main physical characteristics of superplastic deformation of polycrystalline metal.Finally, we discuss the experimental results about both micro-structure changes and macroscope mechanical behaviour of high strength Al-alloy under large superplastic torsion and give a theoretical analysis for the observed phenomina of stress softing,instability of crack extension and axial stress effect by using above constitutive equation. The calculated results agree with experiments observation.It shows the validity of the constitutrve equation on the one hand, and the coupling relation between micro-structure and macro-mechanical behaviour of materials during large superplastic deformition on the other hand.