Thermoelectric detection of hard alpha inclusion in Ti–6Al–4V by magnetic sensing

Abstract

Hard-alpha inclusions are low-density, hard, brittle regions of spuriously high nitrogen and oxygen content that occur in titanium alloys. Usually, these inclusions are cracked or include voids, so they are easy to detect by non-destructive means. The difficulty for NDT is detecting the uncracked hard-alpha inclusions. This problem is very critical to research since titanium alloys are used in aircraft engines and the material is made more brittle and subject to failure with the added nitrogen and oxygen content. This paper presents experimental data for the magnetic field produced by thermoelectric currents around uncracked hard-alpha inclusions in a Ti–6Al–4V specimen under external thermal excitation for different nitrogen content ranging from 1.6% to 5.9%. According to our preliminary results, the magnetic flux density measurements were found to be rather sensitive to changes in nitrogen content with the exception of the intrinsic material background magnetic signal that affected deeply the detectability of inclusions and imperfections in non-contacting thermoelectric measurements. Hopefully, these results can help identify a non-destructive test method that can detect material inclusions with a level of nitrogen that could be detrimental to Ti–6Al–4V alloy components.