Study on the mechanical properties gradient in surface oxygen diffusion hardened layer of Ti6Al4V alloy

This paper studies the mechanical properties gradient in surface oxygen diffusion hardened (SODH) layer of Ti6Al4V alloy by four-point bending test, nano-indentation test and Vickers hardness measurement test. The relationships of hardness-oxygen content and hardness-yield stress were also investigated. Tests results show that the outmost ≈25 μm deep surface layer has the highest oxygen content, hardness and yield strength under compression monotonic loading but the lowest cracking strength before yielding under tension loading. However, from ≈50 μm deep position to substrate, the yield strengths under tensile and compressive loading are almost the same. Test result also shows that, with further decrease of oxygen content deeper down in the SODH layer, the hardness and yield strength decrease correspondingly. It is also found that there are linear relationships between oxygen content and Vickers hardness or nano-hardness. The above relationships can be used to estimate the oxygen concentration distribution in the SODH layer by the simple test method of hardness measurement. Since the yield stress also has a linear relationship with hardness, a valid estimate of the yield stress distribution in SODH layer can be obtained as well. The high hardness and compressive yield strength but low cracking tensile strength of the outmost ≈25 μm depth SODH layer can be employed to explain why surface oxygen diffusion treatment can improve greatly the wear resistance of titanium alloy but reduce its tensile mechanical properties and fatigue resistance.