Characterising the microstructure and mechanical properties of aerospace β-21S titanium alloy during thermal ageing

Abstract

A relatively new titanium alloy, nominally b-21S or Timetal 21S (Ti-15 Mo-3 Nb-3 Al- 0.2 Si), is a seen as a viable material for the design of advanced titanium composites due to its improved oxidation, strength and creep resistance. Dr. Michael Bermingham is presently working with a confidential manufacturer to assess the suitability of the alloy, for aerospace engine exhaust applications, at typical operating temperatures between 250-600dC. However, the metallurgical stability or ageing response of this alloy over such a temperature range has not previously been studied in depth. This body of work therefore aims to characterise changes in the microstructure and mechanical properties of the b-21S alloy during low temperature isothermal ageing at 300 and 450dC, representing typical operating temperatures. Optical microscopy, scanning electron microscopy and microhardness techniques were employed for discrete ageing times up to 9days. Analysis of microhardness testing data suggested that the ageing behaviour of the alloy was significantly different across the two experimental temperatures. In the case of the 450dC aged samples, a severe rate of hardening was observed after an incubation period of 60mins with a peak hardness of 491Hv (sy a1473MPa) obtained at 72hours. This represents a 71% increase when compared to the as received, solution treated sample. In comparison, an incubation period of up to 8hrs was noted at 300dC, with a peak hardness of 405Hv (sy a1215MPa) obtained at 9days. There was no evidence of over-ageing. The hardness of the alloy is dependent on the decomposition of the b phase into fine intragranular a phase particles which prevent the movement of dislocations or crystallographic defects causing plastic deformation. Ageing of the alloy at 450dC results in fine dispersions of a nucleated within the centre of the grains which rapidly grow towards the grain boundaries over time. There is a fine layer of grain boundary a and adjacent precipitate free zone. The precipitates exhibit a lenticular morphology after 8hrs which coarsens over time as larger particles grow preferentially to smaller ones. At 300dC, growth of the equilibrium a phase is relatively slow due to insufficient thermal activation energy, meaning small atomic mobility and diffusion rate. It is hypothesised that an intermediate phase, o or br, may have first formed. These nano-sized transitional phase precipitates serve as precursors for the nucleation of a platelets where there is insufficient thermal energy for direct decomposition of the b phase to equilibrium a. Rapid increases in the hardness of the alloy, due to the nucleation of fine precipitates, are intrinsically proportional to reductions in the ductility. Embrittlement and cracking may therefore be increasing likely at temperatures around 450dC, as well as at 300dC where the alloy is exposed to longer ageing periods.