On the Microstructural Evolution of Metastable Beta Alloys During Gaseous Nitriding

Abstract This research investigates the nitriding kinetics and microstructural evolution during gaseous nitriding treatments of two metastable beta titanium alloys, Ti-15Mo (wt.%) and Ti 15-3 (Ti 15V-3Cr-3Al-3Sn wt.%). These are compared with commercially pure Grade 4 Titanium and two-phase α+β Grade 5 Titanium (Ti-6Al-4V wt.%) alloys. Nitriding treatments of 16 h durations, and at temperatures in the span 800-1050 °C, were carried out in a thermal analyzer for in-situ analysis. Following microstructural characterization combined with XRD, formation of a compound layer comprising of TiN and Ti2N ranging from a few microns to tens of micron in thickness was observed in all the alloys. Consistent with previous studies, 10-60 μm thick diffusion zones entirely consisting of HCP titanium were found below the compound layer. In addition, nitrogen ingress to the bulk alloy led to the formation of coarse alpha laths along grain boundaries along with a concurrent coarsening of these grains, more pronounced in the metastable beta alloys. Such grain coarsening and its deleterious effect on bulk mechanical properties can potentially be avoided by optimization of the nitriding process at sub-transus temperatures.