Oxygen/nitrogen-assisted embrittlement of titanium alloys exposed at elevated temperature

D Texier,V Velay,Q Sirvin,B Dod,R Roumiguier,B Mazères,M Salem,D Monceau,E Andrieu

doi:10.1051/matecconf/202032106004

Abstract

Due to high solubility of oxygen and nitrogen in titanium alloys, the influence of the diffusion zone on the macroscopic tensile properties of pre-oxidized annealed Ti-6Al-4V tensile specimens was examined at room temperature. Thin microtensile specimens were prepared with different thicknesses ranging from 100 µm to 500 µm and then exposed at 750°C for durations between 5 and 200h. A dedicated gripping technique was developed in the present study to investigate the brittleness of such pre-oxidized and ultrathin specimens at room temperature. Tensile testing was paired with digital image correlation techniques to assess both macroscopic deformation and full-field strain maps. High temperature pre-oxidation treatments significantly decreased the ductility of the specimen and the tensile strength of the materials (yield strength and ultimate tensile strength). Fractographic examinations revealed typical brittle fracture features in the oxygen/nitrogen-affected diffusion zone in the periphery of the cross-section while the fracture remained ductile in the core of the specimen for most of the specimens. Some specimens fully failed in a brittle manner for “(pre-ox. duration)1/2/thickness” configurations with ratio equal or higher than 0.45 h1/2.µm-1.

Highlights

Titanium alloys are increasingly used in the manufacture of intermediate/high temperature structural components
It is worth reminding that the oxide layer of the pre-oxidized specimens was removed prior to tensile testing and the cross-section took into account the metal recession due to oxidation
The strain to failure (STF) is large enough and the tensile strengths (yield strength (YS) and ultimate tensile strength (UTS)) are close enough to be compared with the ones of the pre-oxidized specimens

Summary

Introduction

Titanium alloys are increasingly used in the manufacture of intermediate/high temperature structural components. The high solubility of these interstitial elements leads to their ingress in the metal, by diffusion from the metal/oxide interface during high temperature dwells and to the formation of an oxygen/nitrogen-affected diffusion zone (ONADZ) for depths typically one order of magnitude thicker than the oxide layer [3,4]. This gradient of chemical composition could alter the microstructure of Ti alloys but in a greater manner impairs the mechanical properties of the base material by: 1. Decreasing the ductility [6,14,15]

Results

Discussion

Conclusion