Comparison of time-dependent crack growth mechanisms in a range of turbine disc alloys

Everitt, S., Pang, H.T., Starink, M.J. and Reed, P.A.S. (2010) Comparison of time-dependent crack growth mechanisms in a range of turbine disc alloys. International Conference on Fatigue Damage of Structural Materials VIII, Hyannis, United States. 18 - 23 Sep 2010.

Abstract

Turbine discs experience a range of temperature and load conditions in service, but the increasing operating temperatures expected for new, more efficient engine designs, mean that increasingly their time-dependent fatigue response is of more and more importance. A variety of alloying and heat treatment approaches have now been developed to improve the high temperature fatigue performance of turbine disc alloys (whilst maintaining an appropriate balance of their other required properties). In this study we have reviewed the microstructures achieved in a range of sub-solvus and super-solvusheat treated PM alloys: U720Li, RR1000, N18 and LSHR alloys. The microstructures are examined in terms of grain and gamma prime size (?') variations, whilst fatigue crack growth (FCG) rates for all materials have been obtained at 650ºC and 725ºC in air and in vacuum. These show that RR1000 provides the best performance at the most severe conditions, i.e. the highest temperatures and longest dwells. In general failure modes become increasingly intergranular with increasing temperature and ?K. Some of the variations in FCG rate between the alloys are due to reduction in grain boundary oxidation processes with increased grain size, but more subtle interplays between grain boundary character, alloy composition and slip character are also clearly important.The comparison between air and vacuum conditions is especially helpful in separating out creep and environmental contributions, and the vacuum conditions have some service relevance for sub-surface defect growth. An apparent activation energy analysis approach is also presented to further assess the micromechanistic variations between these alloys. This allows a more explicit analysis of the mechanistic dependence of F.C.G rate changes between these two temperatures, where 650oC (a current possible service temperature) is compared to 725oC (a much higher, aspirational. target service temperature).

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Contributors

Author: P.A.S. Reed

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