Mechanisms of hardness increase for composite surface layers during laser gas nitriding of the Ti6Al4V alloy

Titanium matrix composite (TMC) TiN_X/Ti surface layers characterized by high microhardnesses in a range from 1000 HV0.2 up to 2330 HV0.2 were produced by the Laser Gas Nitriding (LGN) of Ti6Al4V substrate in a pure nitrogen environment by a diode laser beam with unique characteristics. Despite the high hardness, crack-free surface layers were produced in a wide range of processing parameters. It was found that the maximum values of hardness and its depth profile on the cross-section of the surface layer should be considered in relation not only to the population and volume fraction of the dendritic precipitations of titanium nitrides TiN, but also in relation to the atomic ratio N/Ti in the TiN_X compounds. Conditions favorable for stoichiometric δ-TiN nitrides precipitation exist in the vicinity of the liquid/gaseous nitrogen boundary, because of the highest temperatures in this region of melt pool and the highest concentration of atomic nitrogen. Therefore, the population of stoichiometric titanium nitrides is the highest directly under the top surface.