G031012 Effect of annealing in nitrogen gas on fatigue strength in p-type titanium alloy Ti-15Mo-5Zr-3Al

Abstract

The effect of annealing treatment in nitrogen gas on fatigue strength was studied using the bulk P-type titanium alloy, Ti-15Mo-5Zr-3Al. Annealing treatment in nitrogen gas was performed at 1200℃ for 0.5 and 1 hour. The hardness of annealed specimen surface was increased due to the formation of TiN, a phase precipitation resulting in needle-like Widmannstaetten structure and the solid solution of nitrogen. Four-point bending fatigue tests were performed, and the fatigue strength of annealed specimen decreased compared with that of untreated one. Fractographic analysis revealed that the fracture surface was covered by flat facets due to the crack growth along grain boundary with TiN precipitates and through coarse grains. Re-solution treatment at 850℃ for 1 hour dissolved TiN along the grain boundaries, while the fatigue strength of re-solution treated specimen was still lower than that of the untreated one because grain coarsening occurred by re-solution treatment.