Influence of γ′ Precipitate Size and Distribution on LCF Behavior of a PM Disk Superalloy

Abstract

The influence of γ' precipitate distribution on tensile and low cycle fatigue (LCF) behaviors of a powder metallurgy (PM) disk superalloy was investigated at 450°C. Four γ' particle distributions were obtained through various cooling paths and/or aging treatments in coarse grain size superalloy N18. The mechanical tests show that the main influence of the intragranular microstructure concerns the 0.2% yield stress (0.2%YS) and the ultimate tensile stress. Wide variations of the 0.2%YS affect the mean stress under non symmetrical loading but have only little effect on fatigue life, the lower the 0.2%YS, the longer the fatigue life. The fatigue life of N18 at 450°C is independent of the intragranular microstructure as long as the mean stress effect is correctly taken into account. As expected with the coarse grain size N18, no crack initiation at pores or inclusions was observed.A precipitation model was coupled with a critical resolved shear stress calculation providing 0.2%YS value for a given heat treatment sequence. Finally, this computation procedure was implemented in a numerical modeling of the LCF life of a disk taking into account the heat treatment applied to its wrought preform.

γ′ precipitation kinetics in the powder metallurgy superalloy N19 and influence of the precipitation latent heat

“…Our precipitation model is based on N. Milhet-Gayraud PhD work [1] with recent modifications carried out by G. Boittin [2]. This model is now implemented in Z-set software [5].…”

“…In this context, Onera is developing microstructural modelling in strong relationship with mechanical modelling. Successful results on the N18 γ /γ superalloy have already been published [2].…”

“…The size distribution of its γ precipitates was first modelled in the late 1980's using a binary alloy model [1]. The precipitation model has been recently revised, upgraded in a pseudo-binary alloy model and used in a multi-scale mechanical model for the calculation of the fatigue life in low cycle fatigue conditions [2]. We here present how the new calibration of the pseudo-binary model for the N19 superalloy has been accelerated through the use of the Thermo-Calc R software, its TCNI5 thermodynamic database and its MOBNI2 mobility database.…”

Local strain redistribution in a coarse-grained nickel-based superalloy subjected to shot-peening, fatigue or thermal exposure investigated using synchrotron X-ray Laue microdiffraction

“…In the absence of such data for the N18 alloy, the LCF tests were interrupted after 300 cycles (i.e., 1 h 40 min). This value corresponds to approximately 25% of the fatigue lifetime of an unpeened specimen [4]. After the thermal and mechanical treatments, the test sections were cut to obtain cylinders with 10 mm height (see Fig.…”

“…Owing to the complex thermal and mechanical repeated cyclic loading conditions during the engine service, the initial stress field may relax, be redistributed or evolve, particularly at high temperature (see [2] and references therein). Nonlinear elasto-viscoplastic models coupled to creep and a fatigue damage model [3] enable the identification of fatigue critical zones and crack initiation time, depending on the thermal and mechanical history of the disk [4]. The improvement of such lifetime analysis is nowadays realized through the development of methods that take into account residual stresses and can predict their static and cyclic relaxation [5].…”