Cao Chunxiao

Hot deformation mechanism and process optimization for ti-alloy ti-6.5al-3.5mo-1.5zr-0.3si during α+β forging based on murty criterion

Abstract To investigate the hot deformation and optimize the process configuration for Ti-alloy Ti-6.5Al-3.5Mo-1.5Zr-0.3Si in α+β forging process, the processing maps (P-maps) are developed for the alloy with α+β microstructure based on Murty instability criterion. The P-maps are constructed by the experimental results obtained through the isothermal and constant strain rate compression test at the temperatures of 780–990 °C and the strain rates of 0.001-70 s−1. The relationship between the deformation mechanism as well as deformation defect and deformation thermomechanical parameter is studied by the generated P-maps. The results show that the superplasticity deformation would occur at the temperature range of 780–990 °C and the strain rate range of 0.001-0.01 s−1. In addition, flow instability including cracks and cavities in β phase, flow localization and adiabatic shear bands would occur at the above mentioned temperature range under the strain rates greater than 0.01 s−1. According to the P-maps, microstructure and flow stress status, the preferred deformation thermomechanical parameters are identified, e.g. the temperature range of 850–940 °C and the strain rate range of 0.001-0.01 s−1. The optimum deformation thermomechanical parameter is around the temperature of 900 °C and the strain rate of 0.001 s−1.

Thermal Stability of TG6 Titanium Alloy and Its Partial Resumption at High Temperature

Abstract The tensile properties of TG6 titanium alloy forging disc as solution and aging treatment (STA) state and STA plus 600 °C, 100 h exposed state were tested at various temperatures. The results show that the tensile strength decreases while the ductility increases with the test temperature increasing. Compared with as STA state, the room temperature tensile plasticity of the specimen subjected to a long-term thermal exposure decreases remarkably. However, once the temperature rises over 150 °C, the tensile ductility increases rapidly, namely, the thermal stability gets a recovery to a great extent. The loss of thermal stability of TG6 titanium alloy may be mainly attributed to the coherent precipitation of (α2 phase and surface oxidation. When the temperature is above 150 °C, the resumption of the thermal stability would come from the change of dislocation slip mode. Upon temperature rising, the slip mode of the dislocation changes from concentrated slip mode by cutting the coherent α2 particles to cross slip mode, which will promote a more homogeneous plastic deformation and result in the improvement of tensile ductility, showing the characteristic of the macro-transferring from a planar slip to a wave slip on the fracture surface.

FLOW SOFTENING MECHANSIM OF A Ti ALLOY WITH LAMELLAR STRUCTURE DURING SUBTRANSUS DEFORMATION

The flow stress has a considerable flow softening after a peak strain hardening at very low strains for Ti alloys with lamellar structure during subtransus deformation.In order to study the mechanism of such flow softening behavior,the deformation behavior of TC11 Ti alloy with a lamellar structure was studied using isothermal hot compression tests under a temperature range of 890—995℃and a strain rate range of 0.01-10 s~(-1).Theoretical calculation shows the Hall-Petch strengthening effects induced byα/βinterface as well as the twin boundary inαlamellar are far more significant than that of the colony boundary.The flow softening can be related to reduction of Hall-Petch strengthening effects due to transfer from the hard slip mode to the soft one.