Aşırı deforme titanyumun termo-mekanik davranışı

Thermo-mechanical processing of metallic materials has attracted noticeable interest due to the fact that these processing methods can be used to improve the mechanical properties. In this investigation, ultrafine grained commercial purity titanium was fabricated utilizing equal channel angular extrusion as a severe plastic deformation technique. Compression tests were performed on severely deformed titanium at various temperatures of 600–900°C and at strain rates of 0.001–0.1/s. It was observed that severe plastic deformation via equal channel angular extrusion can considerably enhance the flow strength of grade 2 titanium deformed at 600 and 700°C. Post-compression microstructures showed that a fine-grained structure can be retained at a deformation temperature of 600°C. The strain rate sensitivity during hot compression of severely deformed Ti was shown to be strongly temperature dependent, where strain rate sensitivity increased with the increase in deformation temperature. High temperature workability of severely deformed grade 2 titanium was analyzed based on the flow localization parameter. According to the flow localization parameter values, deformation at and below 700°C is prone to flow localization. The flow response of the ultrafine grained microstructure is modeled using the Arrhenius type, dislocation density based and modified Johnson-Cook models. The validities of the models were demonstrated with the reasonable agreement in comparison to the experimental stress-strain responses. In order to investigate the influence of purity level on hot characteristics and dynamic softening mechanisms of severely deformed titanium, compression tests were also conducted on severely deformed grade 4 titanium at similar temperatures and strain rates. It was seen that the effects of deformation rate and temperature are significant on obtained flow stress curves of both grades. Higher compressive strength exhibited by grade 2 titanium at relatively lower deformation temperatures was attributed to the grain boundary characteristics in relation with its lower processing temperature. However, severely deformed grade 4 titanium demonstrated higher compressive strength at relatively higher deformation temperatures (above 800°C) due to suppressed grain growth via oxygen segregation limiting grain boundary motion. Constitutive equations were established to model the flow behavior, and the validity of the predictions was demonstrated with decent agreement accompanied by average error levels less than 5% for all the deformation conditions. Severely deformed grade 4 titanium was less stable at the temperature range of 600-800°C. Therefore, warm deformation behavior and microstructure evolution of severely deformed grade 4 titanium were studied at temperatures of 300-600°C and at strain rates of 0.001-0.1/s in order to reveal the pertinent softening mechanisms such as dynamic recovery and dynamic recrystallization. Results suggest that severe plastic deformation is capable of increasing the strength of grade 4 up to 500°C. Above this temperature the severely deformed microstructure was seen to demonstrate complete recovery. The strain rate sensitivity within the warm tension of severely deformed titanium was shown to be strongly temperature dependent, where strain rate sensitivity increased with the increase in deformation temperature. With the rise of temperature, void coalescence and growth of dimples takes place which was attributed to the higher rate of diffusion and growth of recrystallized grains at higher deformation temperatures. Studying monotonic behavior of severely deformed titanium is not the only aim of this research work. Another important aspect is the fatigue behavior. Therefore, cyclic deformation response of coarse-grained and ultrafine-grained grade 4 titanium was probed by low cycle fatigue experiments at elevated temperatures up to 600°C and at strain amplitudes of 0.2-0.6%. It was found that cyclic stability strongly depends on grain size and volume fraction of high angle grain boundaries. Severely deformed titanium showed an improvement in fatigue performance at or below 400°C. Electron backscatter diffraction assisted microstructural findings were seen to stand in reasonable agreement with cyclic mechanical results, where micrographs revealed the occurrence of recrystallization and grain growth at 600°C. Cyclic characteristics of two processing routes, defined as route-8E and 8Bc, were also compared. Accordingly, cyclic deformation response of severely deformed titanium was not sensitive to changing routes in the examined range of temperatures and strain amplitudes. Last but not least, the impact of severe plastic deformation on the tensile and damping properties of titanium grade 4 was also explored to complete this study more effectively. Grain refinement via severe plastic deformation enhanced the strength at room temperature while this effect diminished at a high temperature of 600°C. Ultrafine-grained titanium showed an improvement in damping capacity over its coarse-grained counterpart. High damping capacity of the former was rationalized with the contributions of increased dislocation density and high angle grain boundary fraction. As a summary of this research, thermo-mechanical behavior of severely deformed titanium investigated in detail by demonstrating the thermal stability under monotonic and cyclic loading. It was found that severely deformed grade 2 and grade 4 titanium are monotonically stable up to the homologous temperature of 0.45 and 0.40, respectively. Severely deformed grade 4 titanium also showed cyclic stability up to the homologous temperature of 0.35. These findings form the base for the utilization of ultrafine-grained titanium in high temperature applications.