Study On Shot Peening And XRD Characterization Of TC4 Titanium Alloy And Titanium Matrix Composite

Titanium alloys and titanium matrix composites(TMCs) have wide application prospects in the fields of aerospace, automobile and other industries because of their good properties such as high specific strength and good ductility. As an important technology of surface treatment methods, shot peening(SP) can improve surface properties of materials. In order to explore other potential properties of titanium alloys and TMCs, the shot peened TC4(Ti-6Al-4V) and(TiB+TiC)/TC4 were investigated in this work.The results of residual stresses showed that the compressive residual stress(CRS) were introduced in the surface layer after SP. Under 0.3mmA + 0.15 mmA SP intensity, the max CRS were-813,-857 and-859 MPa for TC4, 5%(TiB+TiC)/TC4 and 8%(TiB+TiC)/TC4 respectively. The depths of CRS layer were about 300, 250 and 250 μm respectively. Under different SP intensities, the depth of surface CRS layer was improved with increasing SP intensity. During the process of stress peening, the CRS was improved significantly, and the increment of CRS was proportional to the prestress.The thermal relaxation of residual stress were investigated, and the CRS was relaxed in the whole deformation layer. At beginning, the relaxation was obvious, then the residual stress was stable. The process of relaxation can be described using Zener-Wert-Avrami function. The composite’s activation parameter m, the activation enthalpy ?H and the activation energy of relaxation QRS, were 0.4483,2.92 e V and 282 kJ/mol respectively. The value of QRS was higher than the activation energy of self diffusion of α-Ti and β-Ti. The residual stress relaxation of TC4 under the cyclic loading conditions showed that the relaxation was more obvious under larger loading, and the stable CRS was smaller.The program of ANSYS with three-dimensional dynamic model was utilized to analysing the distribution of residual stress after SP, because the residual stresses in and around the reinforcements could not be measured directly. The results showed that the tensile residual stresses appeared in the reinforcements, however, the CRS appeared in the matrix, the max CRS and tensile residual stress were-1511 MPa and +1155 MPa respectively, which revealed the higher yield strength of reinforcements. This type of stress distribution reflected the effect of reinforcements, keeping the adverse tensile stress in the reinforcements, retarding the damage of matrix.In order to investigate the microstructure of peened layer, using Voigt line profile analysis method, the domain sizes, microstrain and dislocation densities were obtained. After SP, the domain size was smallest on the surface, about 50 nm. The domain size of titanium alloy was smaller than that of composite, and the microstrain of titanium alloy was bigger than that of composite. The average value of dislocation density was improved after SP, which was caused by the effect of SP and the reinforcements. Utilizing the Rietveld method to analyze the microstructure after shot peening, the variations were similar to the results by Voigt method.The microstructure variation of peened layer under high temperature showed that during isothermal annealing, the recrystallization activation energies of composite and titanium alloy were 341 kJ/mol and 294 kJ/mol respectively. The relaxation activation energies were obtained according to the analysis of microstrain relaxation, and those of composite and titanium alloy were 288 kJ/mol and 273 kJ/mol respectively. According to above analysis, the results showed that the thermostability of composite was higher than that of titanium alloy because of the existence of reinforcements.According to investigation on the hardness of peened layer, the hardness on surface was max, then decreased with the increase of depth. On the surface, the hardness of 8%(TiB+TiC)/TC4 was biggest, which was 630 Hv. It was improved 52% compared to the unpeened sample. After isothermal annealing, at 450 oC, the hardness were smaller than those at 350 oC. At the same temperature, the hardness of titanium alloy was smaller than that of composite. Because the existence of reinforcements improved the hardness.Utilizing in-situ XRD method to investigate the yield strength of peened surface of TC4, the results showed that the yield strength of surface of TC4 was 1080 MPa, which was improved about 27% comparing to that before SP. The fatigue properties showed that the fatigue strength of TC4 was improved from 620 MPa to 550 MPa after SP while the fatigue life was 107 cycles, which was enhanced about 13%. The cross section of fatigue fracture of peened TC4 alloy revealed that the crack originated in subsurface, which verified that SP could inhibit the initiation of crack on surface.