Ti2AlNb-based alloys are under continuous development as promising candidates for advanced automotive and aerospace applications due to their good creep resistance, low density and optimal balance of strength and elongation at the elevated temperature.
Ti2AlNb-based alloys can be deformed not only into sheet or rod, but also into complex types of parts such as turbine engine blades. However, deformation processing of Ti2AlNb-based alloys is inevitably performed at high temperatures because of their limited plasticity at room temperature.
Therefore, investigation into the flow behavior and reasonable deformation criteria of Ti2AlNb-based alloys is quite crucial for design and control of industrial hot working processes.
In recent years, some scholars have developed constitutive models and processing maps of as-cast Ti2AlNb-based alloys and powder metallurgy Ti2AlNb-based alloys. These powder metallurgy Ti2AlNb-based alloys showed microstructure with coarse grain due to higher sintering temperature and longer sintering time.
Unfortunately, few researchers have endeavored to construct the constitutive model and processing maps of the fine-grained (FG) Ti-22Al-25Nb alloy by mechanical alloying (MA) and subsequent spark plasma sintering (SPS) using elemental powders.
Sim Kyong Ho, a researcher at the Faculty of Materials Science and Technology, has established suitable constitutive models to predict the high–temperature flow behavior of FG Ti–22Al–25Nb alloy fabricated by MA and subsequent SPS. He has also developed processing maps to describe the reasonable deformation criteria for hot working of FG Ti–22Al–25Nb alloy.
First, in order to obtain true stress-strain curves of FG Ti-22Al-25Nb alloy, isothermal uniaxial compression tests were conducted at different deformation conditions of 950 – 1 070℃ and 0.001 – 1 s-1.
Second, constitutive models for FG Ti–22Al–25Nb alloy were developed by using the modified Johnson–Cook model and the strain–compensated Arrhenius type model based on the corrected experimental data.
Finally, DMM–based processing maps were constructed to determine reasonable parameters of hot working processes for FG Ti–22Al–25Nb alloy.
Conclusively, the following conclusions were drawn:
(i) The modified Johnson–Cook model for the FG Ti–22Al–25Nb alloy showed good prediction accuracy at the reference temperature and strain rate. However, the predictability was lowered in other deformation conditions. The AARE and R2 values of the predicted and friction–corrected flow stress were 9.78 % and 0.985 8, respectively.
(ii) The strain–compensated Arrhenius type model for FG Ti–22Al–25Nb alloy exhibited excellent predictability under most deformation conditions. The AARE and R2 values of the predicted and friction–corrected flow stress were 4.19 % and 0.992 7, respectively. Compared with the modified Johnson–Cook model, the strain–compensated Arrhenius type model is more suitable for describing the high–temperature flow behavior of FG Ti–22Al–25Nb alloy.
(iii) Based on the processing maps of FG Ti–22Al–25Nb alloy, flow instability was predicted to occur at temperatures lower than 990℃ and strain rates higher than 0.1s–1. The reasonable parameters of hot working processes for FG Ti–22Al–25Nb alloy were in the temperature range of 1 020 – 1 070℃ and the strain rate range of 0.001 – 0.32s–1.
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