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FIG.1 T.E.M. showing the Y,O, dispersion obtained with the REP process Advanced High Temperature PM Titanium Alloys A collaborative EURAM research pro- ject aimed at developing new high performance titanium alloys is cur- rently underway at RAE. IMI and Rolls Royce (UK), and GEZUS. ONERA and SNEGMA (France) with the objective of raising the properties of titanium alloys in aero engines operating at temperatures up to and above 600C. Two researchers at ONERA- T. Khan and S. Naka - stated recently in an appraisal of the research programme that the upper temperature limit of 600C for conven- tional titanium alloys had almost been reached with the IMI834 alloy. and that if the properties of such tita- nium alloys are to be extended new processing routes must be developed. Two routes currently under study in the USA and in Europe involves the usc of PM technology to develop rapidly solidified titanium alumini- des. and dispersion strengthened Ti alloys. The main thrust of the EURAM pro- ject will focus on the latter materials because of improvements in strength which the collaborators believe can be obtained at elevated temperature due to the athermal character of'Oro- wan-type' strengthening mechanism for which stable dispersed oxide par- ticles constitute efficient barriers to dislocation motion. Yttria has been found to have extended solubility in titanium alloys when processed by rapid solidification. and research at ONERA has already established that a fine rare earth oxide dispersion can be achieved even at the moderate solidification rate of about 10 4 Ks', This produced a fine and homogen- eous dispersion of YZ03 particles as can be seen in Fig. 1. The major imp- lication of this development is that the tried and tested rotating electrode process (REP)can be used to produce dispersion strengthened Ti alloy powders more economically than can be achieved using other methods such as melt extraction. The stability of the small oxide dis- persoids «50 nrn] was also found to be remarkably stable and coherent with the matrix during hot consoli- dation or subsequent heat treatment in the a-phase. It was also found that the size stability of these particles was excellent during heat treatment in the a-phase field even after severe prior cold rolling, but that the dispersion coarsens very rapidly in the losing coherency and attaining a typical size of 0.25 - 0.5I1m. Fig. 2(a) and (b) show the variation of yield stress with temperature for the binary Ti-Y alloys and ternary Ti-AI- Y alloys following consolidation in the a-phase field. However. whilst a substantial improvement can be obse- rved in the dispersion strengthened alloys in the temperature range 20-550C. the -improvement was less pronounced at higher temperatures. which the researchers potentially attribute to grain boundary sliding. PROGRAMME OBJECfIVES Having studied dispersion strengthe- ned binary and ternary titanium alloys used in industrial applications, the researchers are aiming to identify the needs and requirements for high strength. high temperature titanium alloys for the two European aero engine manufacturers in the pro- gramme - Rolls Royce and SNECMA. SNECMA for example is said to have shown interest in the type high strength alloys for usc in the interrne- O"o2(MPa) O"o2(MPa) Ti-13AI-1.84Y(160<¢<2OO l-lm 1000 HIPed at 800C 400 + Ti-1.84Y(12S<¢<16O l-lm) x Ti-6AI-0.74Y • Extruded at 800C (160<¢<2OO l-lm) / 800 Extruded at 830C Ti-1.84Y(¢<40l-lm) 600 i:;, x HIPed at 800C 200 • 400 100 200 HIPed at 800C 0 • -roC -roC 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 a b FIG.2Temperature dependence of yield stress for (0) binary aJ/oys (products consolidated at BOOC) (b) ternary alloys. MPR February 1990 diate temperature range (450-550C). whilst Rolls Royce has indicated greater interest in the near a-type alloys which could push the tempe- rature capability well beyond 600e. According to Khan and Naka the app- ropriate alloy composition for the near a titanium base matrix has been deter- mined recently by extensive work done at IMI.and the based matrix is derived from the 11'.11 550 alloy. In both cases YZ03 will be directly incorporated in the matrix instead of yttrium in order to maintain the initial strength level of the base alloy. Ingot! electrode production for use in REP has been achieved both at GEZUS (a+p base alloy). and IMI (near a-base alloy). and ONERAhas produced the first batches of powder. The powder is being consolidated by extrusion. Some of the as-produced a-base tita- nium alloy powder containing yttria will be mixed with boron particles prior to consolidation. This is expec- ted to hinder (or minimise) grain boundary sliding at elevated tempe- ratures. Work will also be done on establishing the parameters which influence and control the coarsening of the second phase particles. and tensile and creep tests arc to be carried out at appropriate temperatures. 91
