Intermetallic compound and martensitic transformation in Ag/Cu co-modified Ti6Al4V alloy

Titanium and its alloys are promising materials for engine production, special pump manufacturing, and bone implants. However, the abrasion resistance and fracture toughness of Ti alloys are insufficient to withstand extreme high loading conditions and harsh environments. Surface modification is an effective way to improve the performance of Ti alloys. In this study, Ag and Cu were added to Ti6Al4V alloy through friction stir processing to modify its surface. The different ratios of the Ag/Cu powder mixture were designed to optimize the surface modification of Ti6Al4V, and the relationship between intermetallic compounds and martensitic transformation was analyzed. The martensitic transformation was induced by AgCuTi phases. The submicron-size martensite was distributed outside the AgCuTi particle, while the nano-laminar martensitic band was generated inside the AgCuTi particle. Under the co-modification of Ti alloy using Ag and Cu, the surface of Ti6Al4V alloy exhibited higher hardness (4.3 GPa) values and lower modulus (100.6 GPa), which gave the credit to the formation of AgCuTi and martensitic transformation. The alloy specimen was subjected to short heat treatment at 600 °C for 30 min to further modify the gradient structure. After the short heat treatment of the Ag/Cu co-modified specimen, a lower modulus (88.8 GPa) and a high level of hardness (4.1 GPa) were achieved. In addition, superelastic strain recovery was also enhanced, showing the further induced martensitic transformation. This work provides new insight into the surface modification of titanium alloys using the FSP method to achieve the ideal hardness and elastic modulus balance.