Morphology of nitrated layers in titanium alloys of different structural classes

Abstract

This paper generalizes the results of investigations of the effect of nitrogen on titanium alloys at temperatures of isothermal exposure of 800 – 1100°C and gas pressures of 10⁵ – 10⁻¹ Pa. As a result of nitration, a gas-saturated area appears on the surfaces of the alloys (in addition to the nitride area). Its qualitative and quantitative characteristics depend on the phase composition of the alloys. The gas-saturated area consists of two layers. For all the investigated alloys, the first layer (adjacent to the nitride area) can be described as the α-titanium structure stabilized with nitrogen. If the saturation temperature is lower than the temperature of the polymorphic transformation, then the structure of the second layer is identical to the alloy matrix. As a result of nitration in the (α+β)-β-region, this structure undergoes the β → α transformation and, for α- and pseudo-α-alloys, turns into an α-grain of smaller size but with higher etchability compared to the α-structure of the first layer. For (α+β)-alloys, this is mainly the α-phase (α-plates) in the β-transformed structure. For β-alloys, the morphology of the gas-saturated area does not depend on the saturation temperature and is identical to other structural types nitrated in the α-region.