A theoretical treatment on the oxide-controlled dwell fatigue crack growth of aγ'strengthened nickelbased superalloys is presented.In particular,this study investigates the influence of an externally applied load...A theoretical treatment on the oxide-controlled dwell fatigue crack growth of aγ'strengthened nickelbased superalloys is presented.In particular,this study investigates the influence of an externally applied load and variations in theγ'dispersion on the grain boundary oxide growth kinetics.A dislocation-based viscoplastic constitutive description for high temperature deformation is used to simulate the stress state evolution in the vicinity of a crack at elevated temperature.The viscoplastic model explicitly accounts for multimodalγ'particle size distributions.A multicomponent mass transport formulation is used to simulate the formation/evolution of an oxide wedge ahead of the crack tip,where stress-assisted vacancy diffusion is assumed to operate.The resulting set of constitutive and mass transport equations have been implemented within a finite element scheme.Comparison of predicted compositional fields across the matrix/oxide interface are compared with experiments and shown to be in good agreement.Simulations indicate that the presence of a fineγ'size distribution has a strong influence on the predicted ow stress of the material and consequently on the relaxation in the vicinity of the crack-tip/oxide wedge.It is shown that a unimodal dispersion leads to reduced oxide growth rates(parabolic behavior)when compared to a bimodal one.Stability conditions for oxide formation are investigated and is associated with the prediction of compressive stresses within the oxide layer just ahead of the crack tip,which become progressively negative as the oxide wedge develops.However,mechanical equilibrium requirements induce tensile stresses at the tip of the oxide wedge,where failure of the oxide is predicted.The time taken to reach this critical stress for oxide failure has been calculated,from which dwell crack growth rates are computationally derived.The predicted rates are shown to be in good agreement with available experimental data.展开更多
This study investigates how hetero-zone boundaries between soft and hard regions affect strain incom-patibility and hetero-deformation-induced(HDI)strengthening in heterostructured metallic materials.We focused on Cu-...This study investigates how hetero-zone boundaries between soft and hard regions affect strain incom-patibility and hetero-deformation-induced(HDI)strengthening in heterostructured metallic materials.We focused on Cu-based alloy(C194)/stainless steel(SS304)clad materials,featuring distinct soft and hard domains separated by a single boundary,to quantify the HDI strengthening.Despite the typical strain incompatibility caused by mechanical incompatibility at the boundary,the precise microstructural char-acteristics remain unclear.To address this,we varied the initial grain sizes of C194 while keeping the grain size of SS304 constant to explore a relation between strain incompatibility and HDI strengthening.Our results show that HDI strengthening is not directly proportional to the mechanical incompatibility at the C194/SS304 boundary.Additionally,this hetero-zone boundary induces two stages of HDI strength-ening during uniform elongation,driven by newly generatedα’-martensite due to the TRIP effect.These analyses,supported by both experimental and computational methods,correlate with the evolution of geometrically necessary dislocations at the hetero-zone boundaries.Our findings offer valuable guidance for designing hetero-zone boundaries by considering microstructural features such as grain sizes of each interior component and shapes/morphologies/volume fractions of hetero-zones.展开更多
文摘A theoretical treatment on the oxide-controlled dwell fatigue crack growth of aγ'strengthened nickelbased superalloys is presented.In particular,this study investigates the influence of an externally applied load and variations in theγ'dispersion on the grain boundary oxide growth kinetics.A dislocation-based viscoplastic constitutive description for high temperature deformation is used to simulate the stress state evolution in the vicinity of a crack at elevated temperature.The viscoplastic model explicitly accounts for multimodalγ'particle size distributions.A multicomponent mass transport formulation is used to simulate the formation/evolution of an oxide wedge ahead of the crack tip,where stress-assisted vacancy diffusion is assumed to operate.The resulting set of constitutive and mass transport equations have been implemented within a finite element scheme.Comparison of predicted compositional fields across the matrix/oxide interface are compared with experiments and shown to be in good agreement.Simulations indicate that the presence of a fineγ'size distribution has a strong influence on the predicted ow stress of the material and consequently on the relaxation in the vicinity of the crack-tip/oxide wedge.It is shown that a unimodal dispersion leads to reduced oxide growth rates(parabolic behavior)when compared to a bimodal one.Stability conditions for oxide formation are investigated and is associated with the prediction of compressive stresses within the oxide layer just ahead of the crack tip,which become progressively negative as the oxide wedge develops.However,mechanical equilibrium requirements induce tensile stresses at the tip of the oxide wedge,where failure of the oxide is predicted.The time taken to reach this critical stress for oxide failure has been calculated,from which dwell crack growth rates are computationally derived.The predicted rates are shown to be in good agreement with available experimental data.
基金financially supported by the National Research Foundation of Korea(NRF)grant funded by the Korean government(MSIP)(NRF-2021R1A2C3006662)and(NRF-2022R1A5A1030054)Gang Hee Gu was supported by the Basic Science Research Pro-gram‘Fostering the Next Generation of Researchers(Ph.D.Candi-date)’through the NRF funded by the Ministry of Education(RS-2023-00275651).
文摘This study investigates how hetero-zone boundaries between soft and hard regions affect strain incom-patibility and hetero-deformation-induced(HDI)strengthening in heterostructured metallic materials.We focused on Cu-based alloy(C194)/stainless steel(SS304)clad materials,featuring distinct soft and hard domains separated by a single boundary,to quantify the HDI strengthening.Despite the typical strain incompatibility caused by mechanical incompatibility at the boundary,the precise microstructural char-acteristics remain unclear.To address this,we varied the initial grain sizes of C194 while keeping the grain size of SS304 constant to explore a relation between strain incompatibility and HDI strengthening.Our results show that HDI strengthening is not directly proportional to the mechanical incompatibility at the C194/SS304 boundary.Additionally,this hetero-zone boundary induces two stages of HDI strength-ening during uniform elongation,driven by newly generatedα’-martensite due to the TRIP effect.These analyses,supported by both experimental and computational methods,correlate with the evolution of geometrically necessary dislocations at the hetero-zone boundaries.Our findings offer valuable guidance for designing hetero-zone boundaries by considering microstructural features such as grain sizes of each interior component and shapes/morphologies/volume fractions of hetero-zones.
