A miniaturized thin-plate low cycle fatigue test method at elevated temperature

This study aims to develop a high-temperature low cycle fatigue test method using a nonstandard miniature thin-plate specimen in order to characterize the cyclic viscoplasticity behavior of a component material. For demonstration, fully reversed strain-range controlled low cycle fatigue and creep-fatigue tests at 600 C have been performed for a martensitic steel using standard-sized full-scale specimens and miniaturized thin-plate specimens, respectively. Because the displacement is not directly measured from the uniform gauge section of the miniaturized specimen, a geometry-dependent scaling factor is obtained and used to convert the uniaxial strain. The results obtained are shown that the miniaturized test method developed in this work has exhibited a clear possibility to produce comparable low cycle fatigue data with those that are normally obtained by conventional standard specimen tests. K E Y W O R D S low cycle fatigue, miniature thin-plate, scaling factor, unified viscoplasticity mode

Nanoindentation study and mechanical property analysis of nickel-based single crystal superalloys

Based on uniaxial tensile and compressive creeps, the mechanical properties of the third generation nickel-based single crystal superalloy were studied via the combination of nanoindentation, crystal plasticity theory, and finite element analysis. The Young’s moduli and hardness of γ , γ ′ and the topologically close-packed (TCP) phases are related to chemical composition and applied stress. The maximum values of the Von Mises equivalent stress and damage are at the tip of the TCP phase. These values are inversely proportional to the distance to the TCP phase. Furthermore, the rafting of the γ ′ phase can be predicted according to the distribution of the TCP phase and stress

An overview on material parameter inverse and its application to miniaturized testing at elevated temperature

This paper aims to provide a comprehensive guide to the application of the inverse technique to the miniaturized tests, on material parameter identification, without investigating the aspects related to the correct choice of a specific material model. Firstly, a brief introduction to the fundamental principles and procedures associated with the inverse method is given. In general, the strategy described relies on the coupling of the finite element (FE) modelling with an optimization scheme. The FE method allows for the evaluation of the response of the material under various types of tests, test conditions, and complex constitutive equations, and avoids the use of approximation techniques for the interpretation of the experimental results. Then, on this based, an example of FE-based inverse processes using small punch creep tests is given to illustrate the process and capability of identifying the creep damage properties. Finally, several sensitive issues, related to the application of the inverse approach, are addressed

Influence of Cooling Scenarios on the Evolution of Microstructures in Nickel-Based Single Crystal Superalloys

The reprecipitation and evolution of γ’ precipitates during various cooling approaches from supersolvus temperature are studied experimentally and via phase field simulation in nickel-based single crystal superalloys. The focus of this paper is to explore the influence of cooling methods on the evolution of the morphology and the distribution of γ’ precipitates. It is demonstrated that small and uniform spherical shape γ’ particles formed with air cooling method. When the average cooling rate decreases, the particle number decreases while the average matrix and precipitate channel widths increase. The shape of γ’ precipitates which changed from spherical to cubic and irregular characteristics due to the elastic interaction and elements diffusion are observed with the decrease of the average cooling rate. The phase field simulation results are in good agreement with the experimental results in this paper. The research is a benefit for the study of the rejuvenation heat treatment in re-service nickel-based superalloys

Creep Damage Repair of a Nickel-Based Single Crystal Superalloy Based on Heat Treatment

Taking nickel-based single crystal superalloy DD6 as the research object, different degrees of creep damage were prefabricated by creep interruption tests, and then the creep damage was repaired by the restoration heat treatment system of solid solution heat treatment and two-stage aging heat treatment. The results show that with the creep time increasing, the alloy underwent microstructure evolution including γ′ phase coarsening, N-type rafting and de-rafting. After the restoration heat treatment, the coarse rafted γ′ phase of creep damaged specimens dissolved, precipitated, grew up, and became cubic again. Except for the specimens with creep interruption of 100 h, the γ′ phase can basically achieve the same arrangement as the γ′ phase of the original sample. The comparison of the secondary creep test shows that the steady-state creep stage of the test piece after the restoration heat treatment is relatively increased, and the total creep life can reach the same level as the primary creep life. The high temperature creep properties of the tested alloy are basically recovered, and the restoration heat treatment effect is good

First-principles thermodynamics and experimental study of interface oxidation in Ni/Ni3Al structures

Anti-oxidation is one of the significant properties of nickel-based superalloys useful for their potential applications in industry. However, previous research mainly focused on single-phase compounds of NiAl or Ni3Al. In the present study, first-principles density functional theory coupled with thermodynamics analysis are employed to investigate the atomistic oxidation behaviours of the Ni/Ni3Al composites systematically. An oxidation experiment with a DD6 alloy is conducted as well to further confirm the theoretical prediction. Initial surface formation energy analysis shows that the systems composed of Ni(111) and Ni3Al(100)/(111) surfaces are more stable and therefore are selected for further investigation. Thermodynamics calculations indicate that the Ni3Al phase is oxidized first, accompanied by Al-segregation on the top surfaces. This is followed by subsequent oxidation of the Ni phase. Surface oxidation diagrams with respect to the surface formation energies show that oxygen adsorption could enhance Al-segregation to the surface and Ni3Al(111) surfaces tend to be oxidized completely with slightly lower oxygen coverage. Oxidation at the interface is also investigated and the results show that oxygen atoms bind with the upper layers of the Ni3Al phase from the point of view of binding energy. The experimental results provide a reasonable explanation for the selective oxidation of Al atoms at the atomic-scale so as to form a dense anti-oxidation membrane. The present work could serve as a beneficial reference for subsequent investigations of oxidation or adsorption processes of two-phase composites.</p