Effect of temperature on deformation modes of Ti834 during dwell fatigue testingWednesday (06.11.2019) 14:35 - 14:55 Part of:
Near-α titanium alloys that are widely used for compressor disc applications are sensitive to reductions in fatigue life when the load is held at relatively high stresses for a period of time. This reduction in fatigue life is often exacerbated when cycling at temperatures below 200°C, which is referred to as Cold Dwell Fatigue. This type of failure in titanium alloys has been a matter of investigation for more than four decades in the aerospace industry. However, the excellent corrosion resistance, low density and mechanical properties of titanium still makes the replacement of this metal a difficult task for the fabrication of components for the high-pressure part of a gas jet engine.
Even though titanium alloys such as Ti834 has being designed to deal with temperature up to 600°C, its dwell fatigue failure has been linked with the presence of clusters of similarly orientated regions in the microstructure of the material. The failure source is typically observed as subsurface intragranular cracks through primary alpha grains leading to quasi-cleavage facets, which are nearly normal to the loading direction and therefore unfavorably orientated for slip to occur. Its formation mechanism is often described by the Stroh pile up model, where a combination neighboring of a hard-soft orientated grain needs to coexist. The soft orientation grain deforms leading to a dislocation pile up at the interface boundary promoting slip transfer into the hard grain.
In this work, the deformation modes of Ti834 subjected to dwell fatigue at different temperatures are assessed with in-situ EBSD dwell fatigue testing. The main objective being to understand how deformation is accommodated in unfavorably orientated grains as a function of temperature. High-resolution EBSD data sets were assessed up to 550 dwell fatigue cycles. Most slip bands were formed within the first 50 cycles in both soft orientated alpha colonies and primary alpha grains with the vast majority of them being prismatic slip. After approximately 500 cycles, there was evidence of slip transfer to hard-orientated grains using basal slip, which was most observable at above 150°C. These observations are then compared with full scale low cycle dwell fatigue testing.