Nitridation of Ti6Al4V alloy under ultrasonic impact treatment in liquid nitrogen

X-ray photoelectron spectroscopy and scanning electron microscopy with energy dispersive X-ray microanalysis were employed to study the effect of mechanical treatment on the surface chemical state, composition and morphology of commercial Ti6Al4V alloy. It has been demonstrated that ultrasonic impact treatment of Ti6Al4V in a liquid nitrogen (LN2) environment results in substantial grain refinement and formation of nitrides and oxynitrides of all the alloy components. The mechanochemical synthesis of the nitrides and oxynitrides from LN2 was found to efficiently occur not only within the impacted area but outside it as well. The layer with a high content of incorporated nitrogen and average atomic ratio N/Ti = 0.94 was estimated to be at least ∼2 μm thick. As a result of the combined effect of cryogenic deformation, nanoscale grain refinement and nitriding, a 3-fold increase in the microhardness of Ti6Al4V alloy is obtained. The impact treatment in LN2 was observed to be accompanied by material transfer from the hardened steel pin to the alloy followed by incorporation of the pin components into Ti6Al4V and formation of the FeN, FeC and TiC bonds.