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The assessment of local lattice strains in alloys using total scattering
The highly-strained lattice hypothesis in high-entropy alloys (HEAs) has led to an interest in local distortions created in substitutional solid-solution alloys. In this work, the use of total scattering for the assessment and analysis of local lattice strains in alloys is considered. Using two theoretical models, the variation in the width of peaks in the pair distribution function (PDF) with changes in composition, ordering and atomic radius is presented. Key practical considerations for the successful analysis of local lattice strains using this technique are discussed, with particular reference to sample preparation, instrumental and data processing effects. Further, the mitigation of errors in local-strain measurements caused by differences in the scattering length of constituent atoms is presented. This is concluded with a proposed methodology for the analysis of local strains using this techniqu
The influence of Al: Nb ratio on the microstructure and mechanical response of quaternary Ni-Cr-Al-Nb alloys
The influence of Al:Nb ratio on the microstructure and properties of Ni–Cr–Al–Nb alloys has been investigated following long-term exposure at elevated temperatures. The γ′ volume fraction, size and lattice misfit were seen to increase with a larger Al:Nb ratio, although these changes resulted in reduced hardness. The change in the critical resolved shear stress (CRSS) associated with strong dislocation coupling was determined to be the dominant strengthening mechanism and increased with decreasing Al:Nb ratio. A distribution of tertiary γ′ was observed to be necessary in maximising the mechanical properties of these alloys.This work was supported by the EPSRC/Rolls-Royce Strategic Partnership (EP/H022309/1 and EP/H500375/1).This is the final published version, which can also be found on the Elsevier website at: http://www.sciencedirect.com/science/article/pii/S0921509314007369
A new approach to the analysis of short-range order in alloys using total scattering
In spite of its influence on a number of physical properties, short-range order in crystalline alloys has received little recent attention, largely due to the complexity of the experimental methods involved. In this work, a novel approach that could be used for the analysis of ordering transitions and short-range order in crystalline alloys using total scattering and reverse Monte Carlo (RMC) refinements is presented. Calculated pair distribution functions representative of different types of short-range order are used to illustrate the level of information contained within these experimentally accessible functions and the insight into ordering which may be obtained using this new method. Key considerations in the acquisition of data of sufficient quality for successful analysis are also discussed. It is shown that the atomistic models obtained from RMC refinements may be analysed to identify directly the Clapp configurations that are present. It is further shown how these configurations can be enhanced compared with a random structure, and how their degradation pathways and the distribution of Warren-Cowley parameters, can then be used to obtain a detailed, quantitative structural description of the short-range order occurring in crystalline alloys.Science and Technology Facilities CouncilThis is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.actamat.2016.05.03
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Detailed Analysis of the Solution Heat Treatment of a Third-Generation Single-Crystal Nickel-Based Superalloy CMSX-10K<sup>®</sup>
Abstract
A detailed analysis of the response of as-cast third-generation single-crystal nickel-based superalloy CMSX-10K® to solution heat treatment (SHT) has been carried out, alongside an SHT optimization exercise. The analysis was conducted through microstructural characterization, differential scanning calorimetry, and compositional homogeneity measurements, quantifying (i) the dissolution and microstructural evolution of the inter-dendritic constituents, (ii) the shift in thermo-physical characteristics of the material, and (iii) the change in compositional homogeneity across the microstructure, in order to gain further understanding of these phenomena during the progression of the SHT. During the early stages of SHT, the coarse cellular γ′/narrow γ channel inter-dendritic constituents which were the last areas to solidify during casting, progressively dissolve; homogenization between these inter-dendritic areas and adjacent dendritic areas leads to a rapid increase in the incipient melting temperature T
IM. The fine γ/γ′ morphology which were the first inter-dendritic constituents to solidify after primary γ dendrite solidification were found to progressively coarsen; however, subsequent dissolution of these coarsened γ/γ′ inter-dendritic areas did not result in significant increases in the T
IM until the near-complete dissolution of these inter-dendritic areas. After the final SHT step, residual compositional micro-segregation could still be detected across the microstructure despite the near-complete dissolution of these remnant inter-dendritic areas; even so the T
IM of the material approached the solidus temperature of the alloy.The authors would like to acknowledge funding through the EPSRC/Rolls-Royce Strategic Partnership (EP/H500375/1 and EP/M005607/1). The authors also wish to express appreciation to Dr. Chris Hayward at the School of Geosciences, University of Edinburgh for carrying out the EPMA composition measurements and to Mr. Kevin Roberts of Dept. of Materials Science and Metallurgy for assistance in carrying out the solution heat treatment runs. Requests for access to the underlying research data should be directed to the corresponding author and will be considered against commercial interests and data protection.This is the final version of the article. It was first available from Springer via http://dx.doi.org/10.1007/s11661-015-3252-
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