Abstract
Using nanoindentation we have investigated the local strain rate sensitivity in dual-phase Ti alloys, Ti–6Al–2Sn–4Zr-xMo (x = 2 and 6), as strain rate sensitivity could be a potential factor causing cold dwell fatigue. Electron backscatter diffraction (EBSD) was used to select hard and soft grain orientations within each of the alloys. Nanoindentation based tests using the continuous stiffness measurement (CSM) method were performed with variable strain rates, on the order of 10−1 to 10−3s−1. Local strain rate sensitivity is determined using a power law linking equivalent flow stress and equivalent plastic strain rate. Analysis of residual impressions using both a scanning electron microscope (SEM) and a focused ion beam (FIB) reveals local deformation around the indents and shows that nanoindentation tested structures containing both α and β phases within individual colonies. This indicates that the indentation results are derived from averaged α/β properties. The results show that a trend of local rate sensitivity in Ti6242 and Ti6246 is strikingly different; as similar rate sensitivities are found in Ti6246 regardless of grain orientation, whilst a grain orientation dependence is observed in Ti6242. These findings are important for understanding dwell fatigue deformation modes, and the methodology demonstrated can be used for screening new alloy designs and microstructures.
Highlights
Titanium alloys are used extensively in aerospace applications where fatigue is one of the most significant issues to contend with and manage
We focussed on grain hard and soft grain orientations from both Ti6242 and Ti6246, as the local load shedding process near the interface of a rogue grain combination is thought to play a principal role in facet formation [8]
These maps were searched for colonies which are hard and soft, based upon evaluation of the f angle that describes the angle between the c axis and the sample surface normal in Bunge convention
Summary
Titanium alloys are used extensively in aerospace applications where fatigue is one of the most significant issues to contend with and manage. Cold dwell susceptible alloys are those which suffer from a significant reduction in fatigue life due to the inclusion of a short load-hold (~120s) at low to moderate temperatures (up to ~200 C). This can reduce the number of cycles to failure by a decade or more and this is called the “dwell debit”. Dwell fatigue failure is mitigated by use of dwell insensitive alloys and careful maintenance schedules, but management of this phenomena costs the aerospace industry significantly (~£100 m/year). It is important to understand microstructural contributions towards the dwell process to enable more cost effective component management strategies and engineering new materials that are dwell insensitive
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