Improving High-Temperature Strength of Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si Titanium Alloy by Cryogenic Pre-treatment and Laser Peening

The plastic deformation behavior and microscopic strengthening mechanism of Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si (TC6) titanium alloy treated by the combination of cryogenic pre-treatment (CT) and laser peening (LP) during a high-temperature tensile testing were investigated. The residual stress and full-width-half-maximum (FWHM) values were determined by X-ray diffraction. In addition, micro-structural evolution in the subsurface was characterized by optical microscopy (OM) and transmission electron microscopy (TEM). It was found that the yield strength of CT-LPed samples was 14.09, 29.46, and 23.37 pct higher than that of the LPed samples at 400 °C, 500 °C, and 600 °C, respectively, due to less thermal relaxation of compressive residual stress (CRS) and more refined grains. Furthermore, widely distributed dislocation walls and tangled dislocations in CT-LPed samples verified the strengthening effects induced by the combination of CT and LP. A novel “paw-shaped” structure which was adhered stably on the grain boundaries was a direct evidence of the strengthening effects. The unmovable dislocation jogs generated by multi-direction dislocation movement and the cross-slip structure were believed to be two important factors to improve the high-temperature strength of TC6 titanium alloy.