New estimate of phase sequence in diffusive layer formed on plasma nitrided Ti-6Al-4V alloy

Ti-6Al-4V alloy was glow discharge nitrided at 680 °C/4 h and 750 °C/3 h at 2.5 mbar nitrogen pressure. Surface diffusive layers formed after such processes were characterized with TECNAI transmission microscope equipped with EDAX microanalysis system. Thin foils were cut out with Quanta 200 FIB. The X-ray phase analysis was performed using Bruker D2 Phaser. The performed experiments proved, that the diffusive layers obtained after these treatments are formed of four distinct sub-layers, i.e. nano-crystalline δ-TiN, fine-crystalline δ′-Ti2N, medium-coarse columnar grains filled with plates of α″-Ti martensite, to be followed by medium-coarse roughly equiaxed Ti3Al intermetallic bordering with coarse α-Ti substrate grains. The δ-TiN and δ′-Ti2N nitride sub-layers are much thinner than the subsequent metallic ones and therefore the nitrides are minor phases, while martensite and the Ti3Al are the major phases of the diffusive layer. The plasma nitriding process - promoting nitrogen diffusion into the metallic substrate - “pushes out” aluminium from the surface area. The depletion in aluminium of the upper part of the diffusive layer stimulates β-Ti → α″-Ti martensite transformation in the metallic middle sub-layer, while enrichment in this element of the sub-layer nearest the substrate causes α-Ti → Ti3Al diffusion driven transformation. Therefore, the aluminium is pushed out of most of the diffusive layer including α″-Ti martensite.