On the Behavior of Ti Alloys during Deforming in Near Superplastic Regimes

Abstract

Traditionally, conditions under which superplastic deformation of poly crystalline material takes place include certain ranges of temperatures, strain rates and some limitations on the grain size. However neither exact criteria, nor mathematically formulated and determined domain of superplasticity are currently developed. In this article attempts are made to formulate the notion of the superplastic domain and its boundaries. To illustrate the behavior of the material near the boundary of the domain the results of the experiments on Ti-6A1-4V alloy with p-annealed microstructure are presented

Experimental study on microstructural variation in near alpha titanium alloy during non-optimal regime of superplastic deformation

“…This observation is also in agreement with the experimental data a longitudinal direction; b transverse direction 4 Microstructure at central portions of tensile specimen after lower constant strain rate (1610 24 s 21 ) test a longitudinal direction; b transverse direction 5 Microstructure at central portions of tensile specimen after strain rate jump test a longitudinal direction; b transverse direction 6 Microstructure at central portions of tensile specimen after higher constant strain rate (5610 24 s 21 ) test reported by other authors on Ti-6Al-4V titanium alloy. 9,12 There are many ways to calculate strain rate sensitivity parameter m, but calculation of this parameter using strain rate jump test provides the value of m at a constant value of strain, microstructure and temperature. 20 The values of m are plotted against true strain for the strain rate jump test of near alpha titanium alloy in Fig.…”

“…8 As a result, the microstructure of the materials does not change actively during optimal superplastic flow. 9 Based on various deformation mechanisms operating during superplastic forming, the constitutive equation as described by many researchers is as follows s~f e, de dt ,T,d where s is the flow stress, e is the strain, de/dt is the strain rate, T is the temperature and d is the average grain size of the material. 10 In real technological application, maintenance of these narrow ranges of condition required for optimal superplastic flow from beginning to end is a very difficult and challenging task.…”

“…10 In real technological application, maintenance of these narrow ranges of condition required for optimal superplastic flow from beginning to end is a very difficult and challenging task. 9 In superplastic forming industry, deformation process is carried out not at optimal but closes to the boundary of optimal superplastic condition that is called near superplastic regimes for which microstructure changes significantly during the deformation process. 11 For computer simulation of these industrial processes, it is very much essential to develop a constitutive equation that can also take into account the major changes in microstructure that takes place during non-optimal process of superplastic deformation.…”

“…The accommodation mechanisms observed in optimal superplastic deformation are mainly grain boundary migration, recrystallisation, grain rotation, diffusional mass transport and slip in grains 8. As a result, the microstructure of the materials does not change actively during optimal superplastic flow 9. Based on various deformation mechanisms operating during superplastic forming, the constitutive equation as described by many researchers is as follows where σ is the flow stress, ϵ is the strain, d ϵ /d t is the strain rate, T is the temperature and d is the average grain size of the material 10.…”

“…Based on various deformation mechanisms operating during superplastic forming, the constitutive equation as described by many researchers is as follows where σ is the flow stress, ϵ is the strain, d ϵ /d t is the strain rate, T is the temperature and d is the average grain size of the material 10. In real technological application, maintenance of these narrow ranges of condition required for optimal superplastic flow from beginning to end is a very difficult and challenging task 9. In superplastic forming industry, deformation process is carried out not at optimal but closes to the boundary of optimal superplastic condition that is called near superplastic regimes for which microstructure changes significantly during the deformation process 11.…”