Thermomechanical Fatigue of Mar-M247: Extension of a Unified Constitutive and Life Model to Higher Temperatures

Abstract

The predictive capability of the Sehitoglu–Boismier unified constitutive and life model for Mar-M247 Ni-base superalloy is extended from a maximum temperature of 871 °C to 1038 °C. The unified constitutive model suitable for thermomechanical loading is adapted and calibrated using the response from isothermal cyclic experiments conducted at temperatures from 500 °C to 1038 °C at different strain rates with and without dwells. The flow rule function and parameters as well as the temperature dependence of the evolution equation for kinematic hardening are established. Creep and stress relaxation are critical to capture in this elevated temperature regime. The coarse-grained polycrystalline microstructure exhibits a high variability in the predicted cyclic response due to the variation in the elastic response associated with the orientation of the crystallographic grains. The life model accounts for fatigue, creep, and environmental damage under both isothermal and thermomechanical fatigue (TMF).

Crystal Viscoplasticity of a Ni-Base Superalloy in the Aged State

“…So much so that the presence of an aged microstructure can result in a 62X difference in LCF life from that of the as-heat treated condition undergoing identical TMF conditions [32]. As such, the full utility of this microstructure-sensitive CVP model framework would be as an input for life models as an aid in designing gas turbine engine components [40][41][42][43].…”

High temperature low cycle fatigue characterization of equiaxed MAR-M-247

“…The ageing procedure enabled precipitation of the γ' phase up to approximately 60% by volume, optimal for peak mechanical properties [5]. Other key observable features include grain boundary carbides, which provide additional strengthening by suppressing grain boundary sliding at elevated temperatures [7]. Additionally, coarse primary carbides (white features) which play an important role in fracture as the initiation site of cracks on MC-γ interfaces [14].…”

“…Constitutive modeling the response of MAR-M-247 to thermally-assisted LCF conditions indicative of GT operating regimes has been documented previously. Brindley [7] studied the application of the Sehitoglu-Boismier unified thermo-viscoplasticity constitutive model up to 1038 • C, showing its capability of assessing the stable stress-strain response for fully-reversed cycling. The mechanical strain ranges considered in his study were one for 980 • C (∆ε = 0.8%) and two for 1038 • C (∆ε = 0.7 and 0.8%).…”

MAR-M-247 creep assessment through a modified theta projection model

“…The casting and heat treatment procedures produced a nominally equiaxed microstructure, with a mean grain size of approximately 1 mm. Other key observable features include grain boundary carbides, which provide additional strengthening by suppressing grain boundary sliding at elevated temperatures [13]. Huo et al [15] found that for creep diffusion mechanisms, n = 1, in the low stress regime, with the creep rate being inversely proportional to the grain size.…”