Quantifying the effect of oxygen on micro-mechanical properties of a near-alpha titanium alloy

H M Gardner,P A J Bagot,K Fox,C M Magazzeni,P Gopon,D E J Armstrong,J Wade,D Rugg,M P Moody,A Radecka

doi:10.1557/s43578-020-00006-3

H M Gardner, P A J Bagot + Show 8 more

Open Access

https://doi.org/10.1557/s43578-020-00006-3

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Abstract

Titanium alloys are widely used in the aerospace industry, yet oxygen ingress can severely degrade the mechanical properties of titanium alloy components. Atom probe tomography (APT), electron probe microanalysis (EPMA) and nanoindentation were used to characterise the oxygen-rich layer on an in-service jet engine compressor disc, manufactured from the titanium alloy TIMETAL 834. Oxygen ingress was quantified and related to changes in mechanical properties through nanoindentation studies. The relationship between oxygen concentration, microstructure, crystal orientation and hardness has been explored through correlative hardness mapping, EPMA and electron backscatter diffraction (EBSD). It has been found that the hardening effects of microstructure and crystallography are only significant at very low-oxygen concentrations, whereas interstitial solid solution hardening dominates by order of magnitude for higher oxygen concentrations. The role of microstructure on oxygen ingress has been studied and oxygen ingress along a potential α/β interface was directly observed on the nanoscale using APT.

Highlights

Titanium alloys have been incorporated into jet engine design since the early 1950s due to their corrosion resistance and high specific fatigue strength, which helps minimise total engine weight
Since the conditions experienced by both these samples were equivalent to the intermediate in-service temperatures described by Satko et al [5], no β denudation was observed and the oxygen ingress depth is considered to be the depth to which the mechanical properties are detrimentally affected
Correlative hardness mapping, electron backscatter diffraction (EBSD) mapping and electron probe microanalysis (EPMA) mapping have been used to find the relative contributions of oxygen concentration, crystal orientation and microstructure to hardness throughout the oxygen-rich layer

Summary

Introduction

Titanium alloys have been incorporated into jet engine design since the early 1950s due to their corrosion resistance and high specific fatigue strength, which helps minimise total engine weight. This improves fuel efficiency, providing cost savings to the operator, as well as reducing CO2 emissions. Greater use of titanium alloys in compressor discs is currently temperature-limited since above ~ 480 °C, oxygen can start to diffuse through the surface oxide layer on components [1] This leads to the detrimental formation of a brittle, high-oxygen content surface layer. At temperatures approaching 600 °C, the amount of oxygen embrittlement and the thickness of the oxygen-rich layer formed can be sufficient to promote crack formation under cyclic loading conditions over thousands of hours, limiting the use of the alloy at high temperatures [2,3,4]

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