High temperature fatigue crack growth in powder processed nickel based superalloy U720Li

Abstract

Fatigue crack propagation at elevated temperatures (650 and 725°C) has been studied in a powder processed high strength nickel based superalloy, U720Li, in air and vacuum environments, using trapezoidal loading waveforms. Constant load (increasing D K) tests were used to generate da/dN ± D K curves, while interrupted constant D K tests were used to study crack propagation paths and mechanisms. At 650°C in air there is little effect of any dwell at maximum load while at 725°C such a dwell causes a signi® cant increase in the crack growth rate coupled with a transition from mixed mode to fully intergranular crack growth. In vacuum the growth rates were signi® cantly lower, and the dwell caused little or no change in crack growth rate at 650°C, but an increase at 725°C. The crack path in vacuum changed from fully transgranular at 650°C to incorporate increasingly mixed mode growth at 725°C with dwell. The intergranular failure in vacuum was through creep cavitation of grain boundaries, while in air, static failure of oxygen embrittled boundaries dominated.MST/4835

Thermal activation of fatigue crack growth: Analysing the mechanisms of fatigue crack propagation in superalloys

“…Extensive microstructural data is available for all alloys considered here ( [16,17,18,19,26,27,29,30,31,32,33,34] and references therein) and for most alloys fractography data has been published. FCG fractography analysis for N18, RR1000, Udimet 720 low interstitial (U720Li) and U720Li Large Grain (U720Li LG) variant [16,17,26,27] has been interpreted as showing that intergranular failure was promoted in: (1) air cf.…”

“…FCG fractography analysis for N18, RR1000, Udimet 720 low interstitial (U720Li) and U720Li Large Grain (U720Li LG) variant [16,17,26,27] has been interpreted as showing that intergranular failure was promoted in: (1) air cf. vacuum, then considering both environments (but to a lesser degree in vacuum): (2) at longer dwells (3) higher temperatures and (4) higher ∆K-levels.…”

“…To investigate the model, FCG data on N18 [26,27], RR1000 [28], Udimet 720 low interstitial (U720Li) [16,17] and U720Li Large Grain (U720Li-LG) variant [27], Waspaloy [29], CH99 [30], Rene95 and Haynes230 [18] are analysed. U720Li is a variant of the U720 alloy designed to minimise the formation of topologically closed packed phases which are detrimental to the properties, and U720Li-LG is an experimental (large grain size) variant produced by solution heat treating U720Li at a somewhat higher temperature.…”

“…The mechanisms responsible for this temperature and time dependency have been discussed in a range of works (see e.g. [8,9,10,11,12,13,14,15,16,17,18,19] and references therein). Generally 3 main groups of mechanisms are cited for this temperature dependence: i) environmental effects including dynamic embrittlement involving atomic oxygen [8,9] and/or oxidation of grain boundaries, γ' (Ni 3 (Al,Ti)) [19] and grain boundary carbides [13,14] (the latter two processes are stress assisted), ii) creep (especially at grain boundaries) and iii) changes in the monotonic mechanical properties of the material, most notably the yield strength.…”

Microstructure variation effects on room temperature fatigue threshold and crack propagation in Udimet 720Li Ni‐base superalloy

“…In particular, this paper concentrates on the effects of microstructure on fatigue threshold and long crack propagation at room temperature. Other complementary papers 4–6 detail the effects of microstructure on long fatigue crack growth at elevated temperatures in these alloy variants as well as other turbine disc alloys, and two further papers focus on fatigue crack initiation and early crack growth behaviour in these alloy variants at room temperature 7 and elevated temperature 8 . Data from these other publications are referenced for comparison purposes in this paper, where appropriate.…”