Stresses In Superalloys Research Articles

Numerically efficient microstructure-based calculation of internal stresses in superalloys

According to the classical Eshelby inclusion problem, we introduce a new linear relation to calculate internal stresses in γ/γ′ microstructures of superalloys via an effective stiffness method. To accomplish this, we identify regions with almost uniform deformation behavior within the microstructure. Assigning different eigenstrains to these regions results in a characteristic internal stress state. The linear relation between eigenstrains and internal stresses, as proposed by Eshelby for simpler geometries, is shown to be a valid approximation to the solution for complex microstructures. The fast Fourier transformation method is chosen as a very efficient numerical solver to determine the effective stiffness matrix. Numerical validation shows that this generalized method with the effective stiffness matrix is efficient to obtain appropriate internal stresses and that it can be used to consider the influence of internal stresses on plasticity and creep kinetics in superalloys.

Formation of nanograins in Ni-Co based superalloys compressed quasistatically at high temperature

In this study, the performance of hot compressive deformation was investigated in two newly developed Ni-Co based superalloys having different stacking fault energy (SFE). It is interesting to discover that nanograins (NGs) can be produced in superalloy deformed at high temperature and relatively low strain rate. With the decrease of SFE and strain rate, the formation of NGs will be promoted obviously, leading to the negative strain rate sensitivity of compressive stress in superalloys. The NGs, generated through the fragmentation of subgrains by microtwins, may become a candidate choice for dynamically strengthening superalloys at high temperature.

On the prediction of the yield stress of unimodal and multimodal γ′ Nickel-base superalloys

A new model for the yield stress in superalloys accounting for unimodal and multimodal γ′ size distributions is presented. A critique of the classic models on γ′ shearing is presented and important features not previously considered are incorporated in our model. This is extended to account for multimodal particle size distribution effects by weighting the individual particle contribution to the total strength. This analysis is focused on powder metallurgy alloys. The yield stress and particle strengthening are predicted for eight superalloys containing wide variations in initial microstructure, composition and at temperatures up to 700°C. We demonstrate through a theoretical approach that the strength of alloys with multimodal γ′ is lower than those with unimodal γ′ radius in the vicinity of 10–30nm. For the first time, a parameter-free physics-based model is able to predict the yield stress in superalloys with complex microstructures, including unimodal and multimodal γ′ size. This has been possible by removing limitations inherent to the classical models. Such approach also enables critical evaluation of the relevant factors contributing to the yield strength of polycrystalline superalloys.

Investigation of Stresses in Superalloys by Analysis of γ/γ'-Microstructure

The response of the structural parameters to applied stress was investigated in a conic single-crystal specimen of nickel-base superalloy CMSX-4 crept at 1100°C for 300 hours. The investigation was performed by Fourier analysis of scanning electron microscope images and by X-ray diffraction. Measurement of the period of the y/y'-microstructure (total thickness of the γ- and γ'-rafts) and the γ/γ'-misfit along the specimen axis showed clear correlations of these parameters with the change of applied stress along the specimen axis. At stress levels > 40 MPa the measured change of misfit was described by a model proposed by the authors earlier. It is intended to continue the investigation to develop a method for the estimation of stresses acting in turbine blades during operation at high temperatures.