Rheological Law and Mechanism for Superplastic Deformation of Ti–6Al–4V

Abstract

The behaviors of and mechanisms acting in Ti–6Al–4V alloy during low-temperature superplastic deformation were systematically studied by using a Gleeble-3800 thermocompression simulation machine. Focusing on the mechanical behaviors and microstructure evolution laws during low-temperature superplastic compression tests, we clarified the changing laws of the strain rate sensitivity index, activation energy of deformation, and grain index at varying strain rates and temperatures. Hot working images based on the dynamic material model and the deformation mechanism maps involving dislocation quantity were plotted on the basis of PRASAD instability criteria. The low-temperature superplastic compression-forming technique zone and the rheological instability zone of Ti–6Al–4V were analyzed by using hot processing theories. The dislocation evolution laws and deformation mechanisms of the grain size with Burgers vector compensation and the rheological stress with modulus compensation during the low-temperature superplastic compression of Ti–6Al–4V were predicted by using deformation mechanism maps.

Influence of strain rate and temperature on the deformation mechanisms of a fine-grained Ti-6Al-4V alloy

“…These two parameters indicate that the superplastic effect is probably weak. Moreover the activation energy value calculated (Q = 436kJmol-1 ) is clearly higher than activation energy involving self-diffusion mechanisms [17,35] and tends to values linked to d yn amic recrystallization which is generally associated with the dislocations activity /storage. The softening shown before failure is similar to the behaviour reported in the literature [14,25] that is related to dislocations activity.…”

“…Liu et al described a mechanism of GBS controlled by lattice diffusion at 890 °c with a high strain rate. By applying high temperatures, the dislocations mobility is favoured leading ta the formation of large grain boundaries and thus ta a grain boundary slip [17].…”

Microstructure Evolution and a Unified Constitutive Model of Ti-55511 Alloy Compressed at Stepped Strain Rates

“…During the isothermal compression, the alloy undergoes complex deformation mechanisms such as work hardening (WH), dynamic recovery (DRV) and dynamic recrystallization (DRX), etc. [9][10][11][12], which often influence flow stress and microstructures. A unified constitutive model is one ideal method to describe the relationship between flow behavior, deformation parameters and microstructure evolution.…”

“…The WH and DRV processes were modeled in early research [29,30], and the dislocation density was introduced as the main ISV in the Kocks-Meching (K-M) model. With the development of physical-based ISV models, some other complicated deformation mechanisms were also considered, such as DRX [14,[31][32][33], spheroidization [15,34,35] and precipitation [11], which can greatly influence the flow stress. Recently, many investigations have been made to introduce these physical mechanisms into constitutive models.…”

“…Liang et al [10] identified the relationship between the critical dislocation density for DRX and the saturated dislocation density of the Ti-55511 alloy. Tang et al [11] constructed a physical-based ISV model, in which the precipitation, static recovery/recrystallization and precipitation were coupled, to predict flow stress and microstructure during the multi-stage hot compression of the Al-Zn-Mg-Cu alloy. Tang et al [14] and Lin et al [25] established a unified ISV model considering the DRX fraction and average grain size to predict the flow stress and microstructure change of Ni-based superalloys.…”