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On the influence of crystallography on creep of circular notched single crystal superalloy specimens
(2020)
The present work contributes to a better understanding of the effect of stress multiaxiality on the creep behavior of single crystal Ni-base superalloys. For this purpose we studied the creep deformation and rupture behavior of double notched miniature creep tensile specimens loaded in three crystallographic directions [100], [110] and [111] (creep conditions: 950 °C and 400 MPa net section stress). Crystal plasticity finite element method (CPFEM) was used to analyze the creep stress and strain distributions during creep. Double notched specimens have the advantage that when one notch fails, the other is still intact and allows to study a material state which is close to rupture. No notch root cracking was observed, while microstructural damage (pores and micro cracks) were frequently observed in the center of the notch root region. This is in agreement with the FEM results (high axial stress and high hydrostatic stress in the center of the notched specimen). Twinning was observed in the notch regions of [110] and [111] specimens, and <112> {111} twins were detected and analyzed using orientation imaging scanning electron microscopy. The present work shows that high lattice rotations can be detected in SXs after creep fracture, but they are associated with the high strains accumulated in the final rupture event.
The thermal shock behavior of three commercial-advanced ceramics (SSiC, MgO–PSZ, Al2O3) is characterized in air and vacuum applying a laser thermal shock. The available testing system permits the reproducible setting of defined temperature profiles in thin disks and allows a heating-up thermal shock in various media. Due to the accurate determination of the time- and space-resolved temperature distribution, the local stress state can be calculated as a function of time. It is shown that the thermal shock strength is highest for SSiC and lowest in Al2O3 with MgO–PSZ in between. The approach presented in this work allows quantifying the stress state at failure in terms of tangential tensile stress. The investigated environment does not affect the thermal shock resistance under the studied experimental conditions.
Quite a number of numerical models for hydrogen-assisted cracking in different kind of steels are existing reaching from simple analytical models to more complex two- and three-dimensional finite element simulations. These numerical models have been used to simulate the processes of hydrogen-assisted cracking in homogeneous microstructure. This paper contributes to numerical simulation of hydrogen-assisted cracking in heterogeneous microstructure, e.g., in a duplex stainless steel microstructure consisting of two phase fractions. If hydrogen is absorbed during welding or during service, i.e., due to cathodic protection, hydrogen is leading to material embrittlement and leads to hydrogen-assisted cracking. In order to improve understanding of the mechanisms of hydrogen-assisted cracking in duplex stainless steels, a numerical model has been created that operates at the mesoscale and enables simulation of stress–strain distribution as well as cracking in the various phases of a metallic material. Stress–strain distribution and hydrogen-assisted cracking in the duplex stainless steel 1.4462, consisting of approximately equal portions of ferrite and austenite, was simulated using the finite element program ANSYS. It was shown by numerical simulation that higher local stresses and strains are present at ferrite and austenite than the global stresses and strains in the duplex stainless steel, while the highest plastic deformations occur at austenite and the highest stresses can be found in small ferrite bars surrounded by ductile austenitic islands. By analyzing the stress–strain distribution in the duplex microstructure, crack critical areas in the ferrite can be identified. Hydrogen-assisted cracking was modeled assuming high hydrogen concentrations and regarding the local mechanical load in each phase of the duplex stainless steel. The mesoscale model qualitatively reflects the crack initiation and propagation process in the ferritic and austenitic phase of the duplex stainless steel.