Systematic Evaluation of Microstructural Effects on the Mechanical Properties of ATI 718Plus( ^® ) Alloy

Abstract

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Environmental and Dwell Effects on the Damage Tolerance Properties of ATI 718Plus® Alloy

“…In addition, comparative mechanical testing was also performed on Waspaloy prepared in the standard heat treatment condition, W1. [4,7]. The typical variation in grain structure for both billet materials demonstrated uniform equiaxed grain morphology with grain size ranging between 21 to 64 µm.…”

“…Dwell fatigue cracking in HT2 occurred For Ni-based superalloys such as 718Plus and Waspaloy, fatigue crack propagation may be either time or cycle dependent or some combination thereof, largely governed by such variables as temperature and environment [12,13]. The main mechanism for crack propagation in a cycle-dependent case is the cyclic plastic deformation immediately ahead of the crack tip, usually resulting in a transgranular type of fracture [4]. In the case of timedependent fatigue crack propagation, environmental effects (i.e.…”

“…However, it was shown in previous work that the presence of varying amounts of grain boundary δ phase (0.06A f to 0.12A f ) in four microstructural variants of 718Plus had minimal effect on the dwell-fatigue crack propagation resistance. In fact they showed identical dwell-FCGR behaviour [5,7]. Recent studies performed by Gabb et al, have examined the effect of modifying the microstructure of 718Plus via grain boundary engineering (GBE) practices while maintaining grain boundary size features [15].…”

“…More recent work completed by the authors has revealed that the axial-fatigue crack nucleation life for a range of microstructural conditions of 718Plus have all demonstrated superior crack nucleation resistance compared to Waspaloy when tested under dwell and non-dwell conditions to temperatures as high as 704ºC [4]. It was also shown that the presence of low and high fractions of grain boundary δ phase had a negligible effect on the crack nucleation resistance of 718Plus.…”

“…During thermal exposure, two significant microstructural changes were observed: 1) the formation and growth of the δ phase and 2) coarsening of the ultrafine γ′ hardening phase [6]. It was shown that the δ phase had minimal affect on the dwell-fatigue resistance of 718Plus; therefore, the improved dwell-fatigue crack propagation resistance was most likely related to the observed coarsening of the γ′ hardening phase, which was observed to double in size after thermal exposure [4]. This structure results in crack-tip blunting due to the significantly reduced plastic deformation resistance of the over-aged precipitate condition.…”