Microstructural and constitutive behavior of superplastic titanium alloy (titanium)-(6 aluminum)-(4 vanadium)

Microduplex titanium alloy Ti-6Al-4V displays superplastic deformation under specific conditions of temperature ({dollar}approx{dollar}900{dollar}spcirc{dollar}C), microstructure ({dollar}approx{dollar}5{dollar}mu{dollar}m) and strain rate({dollar}approx{dollar}10{dollar}sp{lcub}-3{rcub}{dollar}/s). Microstructural effects on superplastic behavior center on grain growth via static and deformation enhanced mechanisms. In order to model a part forming process, a constitutive relation is required which includes microstructural effects and is valid over the appropriate range of temperatures and strain rates. After the superplastic deformation is modeled, various numerical techniques can be used to optimize process parameters (e.g., reduce forming time under uniform deformation constraint) by applying instability analysis.; This study models superplastic deformation and forming of Ti-6-4. The investigation was conducted in two stages. First, the microstructural evolution and flow stress behavior are quantified under constant (average) strain rate conditions. This data is used with superplastic and dislocation creep activation energies to generate a constitutive relation relating the strain rate to the applied stress. Experimental data is obtained over a temperature range of 875 to 925{dollar}spcirc{dollar}C, at strain rates from 10{dollar}sp{lcub}-4{rcub}{dollar} to 10{dollar}sp{lcub}-2{rcub}{dollar}/s to determine the constitutive relation parameters and check the validity of the relation. The constitutive relation can then be used to predict corresponding behavior under other (variable strain rate) deformation conditions.; In the second discussion, concepts for optimizing deformation paths were explored through numerical modeling and examined experimentally. It is shown that there is little opportunity to delay the onset of non-uniform strain in the tensile tests due to non-uniform stress states, but that subsequent necking can be limited by control of the post-uniform deformation process. A concept is proposed and verified experimentally for designing a variable strain rate path suitable for minimizing non-uniform deformation while deforming as rapidly as possible.