SURFACE PHASE DEFINITION BY MICRODIFFRACTOMETRY IN ION-NITRIDED ALPHA-BETA-TI ALLOYS

X-ray microdiffraction (XR-mu-D) was used to characterize the surface layers of a-beta-Ti alloys (Ti-6Al-4V, Ti-4Al-4Mo-2Sn, Ti-5Al-2Fe and Ti-4Al-2Mn) ion nitrided at 1273 K for 12 h. On the alloys Ti-4Al-4Mo-2Sn, Ti-5Al-2Fe and Ti-4Al-2Mn the ion-nitrided layers consist of the phases-delta (TiN(x)), epsilon (Tin2N) and a (nitrogen solid solution in a-Ti); the phases-delta and alpha have already been identified by conventional surface diffractometry. The alloy Ti-6Al-4V presents only the phases-delta and alpha. It is of particular interest to note that the epsilon-phase was detected by means of both X-ray diffraction and XR-mu-D in the surface layers of the same alloys after ion nitriding and annealing at 1073 K. The presence of the epsilon-phase in alloys that were ion nitrided but not annealed seems to be correlated with the alloy chemical composition and with the differentiated alloy element distribution between the hardened layer and the metal matrix, which involves diffusion of the beta-stabilizing elements (iron, molybdenum and vanadium) in the matrix with their maximum amounts in the interface zone.