Mechanism‐based life model for out‐of‐phase thermomechanical fatigue in single crystal Ni‐base superalloys

Abstract

A B S T R A C T This paper describes an enhanced physics-inspired model to predict the life of the secondgeneration single crystal superalloy PWA 1484 experiencing out-of-phase (OP) thermomechanical fatigue (TMF). Degradation due to either pure fatigue or a coupling between fatigue and environmental attack are the primary concerns under this loading. The life model incorporates the effects of material anisotropy by utilizing the inelastic shear strain on the slip system having the highest Schmid factor while accounting for the effects of temperature-dependent slip spacing and stress-assisted γ depletion. Both conventional TMF and special bithermal fatigue (BiF) experiments were conducted to isolate and therefore better understand the interactions between these degradation mechanisms.The influences of crystallographic orientation, applied mechanical strain range, cycle maximum temperature and high temperature hold times were assessed. The resulting physics-inspired life estimation model for OP TMF and OP BiF predicts the number of cycles to crack initiation as a function of crystal orientation, applied strain amplitude and stresses, temperature, cycle time (including dwells), and surface roughness within a factor of 2. N i = Number of cycles to crack initiation A, m, n, g, S, B, b, k, K, u, p = Empirical constants ε in = Inelastic strain range t = Time Q = Activation energy R = Universal gas constant T = Absolute temperature T eff = Absolute effective cycle temperature T(t) = cycle absolute temperature as a function of time X crit = Critical crack length N s = Number of slip bands C = Slip band spacing X = γ depletion depth D 0 = Diffusion coefficient v = Frequency t h = Hold time Correspondence: Richard W. Neu.

Failure of metals II: Fatigue

“…Furthermore, the loads and temperatures are not necessarily in phase with each other which is a very important feature of TMF. In recent years, research aimed ultimately at developing a life prediction methodology has been carried out in our laboratories and the evolution of this work has been reported in numerous publications [204][205][206][207][208][209][210][211][212][213]. The most important findings are summarized in this section.…”

Microstructural aspects of fatigue in Ni-base superalloys

“…Based on all of the preceding, and noting that out-of-phase TMF is generally most damaging in Ni-base superalloys, the model refined and extended a previous environmental-based model for LCF in polycrystalline René 80 [44]. The basis of the model is shown in figure 36 [78,80]. The fundamental notion is that oxides formed at the high-temperature/low-strain part of the cycle are cracked by impingement of slip bands formed at the low-temperature/high-strain part.…”

“…In recent years, research aimed ultimately at developing a life prediction methodology has been carried out in our laboratories and the evolution of this work has been reported in numerous publications [72][73][74][75][76][77][78][79][80]. The most important findings are reprised in this section but it is appropriate to note that the work was motivated, in part, by simple experiments designed to simulate TMF in René 80 [78,80]. [81].…”

“…where A and u are constants determined from precipitate coarsening, t is the total precipitate coarsening time, r o is the initial equivalent precipitate size, γ sl in is the inelastic strain on the slip system of interest, T eff is the effective temperature (defined below), Q sl is the slip activation energy, Q ox is the oxidation activation energy, σ max is the maximum tensile stress in the stabilized cycle (occurring at the cycle minimum temperature), τ min is the shear stress for slip band The data designated as 'correlated data' were used to develop the prediction (solid line) [78,80].…”

Modelling of TMF Crack Initiation in Smooth Single-Crystal Superalloy Specimens

“…Out-of-phase TMF loading conditions were investigated by Amaro et al [3] for the single-crystal nickel-base superalloy PWA1484. In this paper a mechanism-based crack initiation model was developed and validated.…”