SURFACE ENGINEERING AND CHEMICAL CHARACTERIZATION IN ION-NITRIDED TITANIUM AND TITANIUM-ALLOYS

The chemical and physical characteristics of ion-nitrided surface layers, obtained on α-β titanium alloys, are examined and correlated both with the working conditions adopted in the ion-nitriding process and with the alloy chemical composition. Besides the influence of the working parameters on the morphology and on the microstructures of the ion-nitrided surface layers, mainly the alloy element distributions both in surface coatings and in the substrate are analysed for five α-β titanium alloys of industrial use, and for titanium c,p. as reference, ionnitrided at various treatment temperatures. The nitriding process forms, on titanium alloy parts, high-hardness surface layers consisting of TiN (δ phase) and Ti2N (e{open} phase) nitrides and an interstitial solid solution of nitrogen in the close-packed hexagonal lattice of titanium (α phase). The presence and the extent of these phases as well as the ion-nitrided layer morphology are essentially determined by the alloy chemical composition and the working parameters. In particular a low-temperature treatment produces an extended nitrogen diffusion in the matrix beneath a thin continuous nitrided layer, while a high-temperature treatment produces prevalently a continuous nitrided surface layer. The alloy element distribution appears differentiated in the various phases and may be correlated with the different affinity of these elements with nitrogen. © 1990 Chapman and Hall Ltd.