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

¹ Institut Clément Ader (ICA) - UMR CNRS 5312 ; Université de Toulouse ; CNRS, IMT Mines Albi, INSA, ISAE-SUPAERO, UPS ; Albi, France
² CIRIMAT, Université de Toulouse, CNRS, INP-ENSIACET, 4 allée Emile Monso, BP 44362, 31030, Toulouse Cedex 4, France
³ MIDIVAL ; Sainte-Foy d’Aigrefeuille, France
⁴ AIRBUS ; Toulouse, France

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 h^1/2.µm-1.