Surface characteristics and high cycle fatigue behaviors of Ti47Al2Cr2Nb alloy subjected to different surface processes and thermal exposure

The Ti47Al2Cr2Nb alloy fatigue specimens were prepared by investment casting, and three kinds of surface processes were applied to fatigue specimens. These three processes were sand-blasting (SB), sand-blasting and shot-peening (SBSP) and sand-blasting and mechanical grinding (SBMG). The surface characteristics before and after thermal exposure at 700 °C for 24 h were investigated. The high cycle fatigue behaviors of specimens after thermal exposure were evaluated. The results showed that the surface roughness (Ra) values after SB, SBSP and SBMG processes were 1.71, 0.99 and 0.06 μm, respectively. After thermal exposure, they increased slightly for all three processes. The SB process caused work hardening in the near-surface region and the work hardening reached saturation after the SB process. Due to the mechanical grinding process removing an uncertain thick hardening layer, the maximum hardness after SBMG process was noticeably lower than those after SB and SBSP processes. After thermal exposure, the maximum hardness after all three processes significantly recovered. Deformed layers were formed beneath SB and SBSP surfaces. After thermal exposure, a depth-dependent gradient microstructure was formed in the surface layer. The SBMG specimens had the highest fatigue limit of 350 MPa. This was attributed to the SBMG specimens having very smooth surfaces and keeping some work hardening in the surface layers. The fatigue fractures were typical brittle cleavage fractures. The fracture modes included interlamellar and trans-lamellar fractures. The dislocation slip and mechanical twins were the main deformation mechanisms.