Direct gaseous nitridation of the Ti–6Al–4V alloy by nitrogen

Abstract

The gaseous nitridation of the Ti–6Al–4V alloy in a furnace with nitrogen improves strongly the superficial microhardness of the alloy from 405 to 2133 HV_0.3. For temperatures between 1175 and 1300°C, it leads to a very complex structure composed (in cross-section, from the surface to the heart) of several layers: (i) stoichiometric TiN, (ii) sub-stoichiometric δ-TiN_0.7 (without vanadium and aluminium), (iii) a thin film of alloy enriched in vanadium and aluminium, (iv) an α-(Ti, N) phase, containing aluminium (from TiAl_0.12N_0.25 to TiAl_0.12N_0.12) and (v) the Ti–6Al–4V alloy with needles of the α-(Ti, N) phase together with aluminium and traces of nitrogen (less than TiAl_0.12N_0.05). The reaction, which is not influenced by the pressure of nitrogen, involves a gaseous diffusion through the superficial layers and is governed by the volumic diffusion of nitrogen in the α-phase, giving parabolic kinetics with the rate law:

where Δm/S is the gain of mass per surface unit (g cm⁻²), R the gas constant and T the temperature (K).