Thermomechanical Processing, Microstructure and Texture Studies of TiHy 600 Alloy (a Near Alpha Titanium Alloy)

K, Basanth Kumar and Dey, Suhash Ranjan (2018) Thermomechanical Processing, Microstructure and Texture Studies of TiHy 600 Alloy (a Near Alpha Titanium Alloy). PhD thesis, Indian institute of technology Hyderabad.

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Abstract

The titanium IMI 834 alloy is near α (hcp) titanium alloy, widely utilized for gas turbine engine parts such as compressor disc. It shows good creep and fatigue properties at high temperature (operating temperature: 600 °C). Low dwell fatigue life is identified to cause early failure in titanium based aero engine parts. The main reason for the decreasing dwell fatigue life is associated with the presence of large micro texture regions (macro zones) in near alpha titanium alloys. Hence, the degree of random micro texture with fine grain microstructure should be increased in order to improve the dwell fatigue life of near alpha titanium alloys. Thermomechanical processing is a useful technique to generate different kind of texture and microstructure of titanium alloys. Thermomechanical processing conditions are greatly influence the microstructural evolution and mechanical behavior. The main objective of the current Ph.D. work is to perform the hot deformation at various strain rates (10-3-10s-1) in different phase regions 900 °C (α rich region), 950 °C (α+β region), 975 °C (α+β rich region), 1000° C (rich β region) and 1050 °C (β region) of near alpha titanium alloy (TiHy 600 alloy) up to 50% deformation, mainly to generate its processing maps and to compare it with the generated microstructures and textures. Following targets are identify during the processing of TiHy 600 alloy. • Flow behaviour characteristic studies of hot compressed TiHy 600 alloy using peak stress values and further utilizing the Avrami model to verify the DRX fraction from the experiments. • Developing the processing maps through DMM (Dynamic Material Modeling) approach at various true strain values from 0.3 to 0.6. Correlating the processing map (0.6 true strains) domains with microstructures of related processing parameters (temperature and strain rate). • Evolution of microstructure and texture studies of TiHy 600 alloy at 900 °C (α rich region) with various strain rates (10-3-10 s-1). • Texture analysis of reconstructed parent β (bcc) phase at 1050 °C with various stain rates ((10-3-10 s-1) and its deformation studies through texture. TiHy 600 alloy which is similar to titanium IMI 834 alloy is received in the form of pan cake (12 mm height and 250 mm diameter) from TAG (Titanium Alloy Group) at DMRL Hyderabad (Defense Metallurgical Research Laboratory). It showed bimodal microstructure with primary alpha (αp) (50%) and transformed beta (β) having secondary alpha lamellar (αs) (50%). Hot compression is the best processing method to get the workability conditions of the near alpha titanium alloys with introducing large strains in one step. The thermo-mechanical simulator Gleeble 3800 is utilized for the hot compression of as received TiHy 600 alloy. Flow behaviour characteristics of TiHy 600 alloy is studied by developing the true stress-true strain curves up to 50% deformation at constant strain rates with various phase regions (α region, α+β region, α+β rich region, rich β region and β region). Dynamic recrystallization (DRX) is noticed to be the dominant mechanism at lower strain rate 10-3 s-1 at 900 °C and 975 °C and for 10-1 s-1 strain rate at 1000 °C. Dynamic recovery (DRV) is observed at higher temperature 1050 °C (β region) at strain rates of 10-3-10 s-1. The peak stress increases with decreasing deformation temperatures and increasing strain rates. TiHy 600 alloy material constants (A, α and n of the Arrhenius equation), obtained by using hyperbolic-sine law, are determined from the peak stress values of the flow curves. Activation energy (Q) values of 50% deformed TiHy 600 alloy are calculated at various regions such as α rich region (900 °C), α+β region (950-1000 °C) and β region (1050°C) which are 384 kJ/mol, 307 kJ/mol to 255 kJ/mol and 251 kJ/mol, respectively. The high activation energy of 384 kJ/mol for α region indicates hard hcp phase character which results in undergoing DRX phenomenon. Similarly, low activation energy of 251 kJ/mol for β region indicates soft bcc character which leads to undergoing DRV phenomenon. Predicted peak stress values are consistent with the experimentally obtained peak stress values. According to the Zener-Holloman parameter, the peak flow stress follows the unified constitutive equation observed from the plot of lnZ vs. ln [sinh (ασp)]. The developed Cingara equation is consistent with the experimental stress curves, as verified from the correlation coefficient factor. Dynamic recrystallization (DRX) fraction is calculated from the Avrami equation for 900 °C, 950 °C and 975 °C at 10-3 s-1 strain rate conditions and is compared with the DRX fraction obtained from the EBSD studied microstructures with one example taken is 900 °C, with 10-3 s-1 strain rate condition. Processing maps are developed at 0.3, 0.4, 0.5 and 0.6 true strain with clear deformation of stable and unstable domains at different temperatures and strain rates. Microstructure studies at selective domains in 0.6 true strain processing map are performed on image quality maps with grain boundaries. Hot compression at 900°C (α rich region) in unstable domain ([A] 900 °C-1 s-1) mostly resulted into new fine dynamic recrystallization equiaxed α grains along grain boundaries of large deformed α grains and large amount of low angle grain boundaries (LAGB) misorientation. Several fine DRX grains (more than higher strain rate deformed microstructure) among large deformed equiaxed αp grains and less amount of LAGB misorientation in the large equiaxed αp grains are observed in stable domain ([B] 900 °C-10-3 s-1) of α region deformed microstructure. Stable domains ([C] 950 °C-10-2 s-1 to [D] 975 °C-10-3 s-1) deformation in the α+β region had resulted in deformation of both α phase and β phase. The 950 °C deformed at 10-2 s-1 strain rate microstructure contained large LAGBs inside large equiaxed αp grains and is drastically reduced in 975 °C and 10-3 s-1 strain rate deformed microstructure. Deformation in stable domain ([E] 1000 °C-10-1 s-1) resulted in very small fraction of equiaxed αp grains with no LAGB and mostly remaining αs laths formed from the deformed β. Another stable domain ([F] 1050 °C-10-1 s-1) clearly showed only fine αs laths formed from β. Texture studies of hot compressed TiHy 600 alloy at 900 °C (α rich region hcp phase) with various strain rates (10-3-10 s-1) are performed. Deformation grains and recrystallization grains are segregated based on grain size diameter chart to study the texture analysis of both the grains separately. Both the grains found having similar oriented texture components but only differed in their texture intensities. At lower strain rate (10-3 s-1) the major texture component found is fiber texture <=> . Higher strain rate (10 s-1) showed similar texture component like lower strain rate (10-3 s-1) with an additional texture component which is present due to generation of twins having misorientation angle of 65° around axes. Crystallographic texture studies of the reconstructed parent (bcc β) orientation maps of (1050 °C) TiHy 600 alloy after hot compression at various strain rates (10-3-10 s-1) is studied. Reconstructed β (bcc) phase microstructure contained coarser β grains. Deformed texture studies of parent (bcc β) phase are studied with various strain rates (10-3-10 s-1). It showed one major texture component at lower strain rate (10-3 s-1), <100> ║compression direction (CD). Further increasing of the strain rate (10 s-1) to higher value showed {100} ׀׀ CD along with <111> ׀׀ CD. At lower strain rate (10-3 s-1) <100> orientated grains are activated due to having the possibility of more than five crystallographic slip systems. Increasing the strain rate activated less preferable orientation of <111> grains having the possibility of only three crystallographic slip systems along with the <100> oriented grains.

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