Ej Pickering

High-entropy alloys: a critical assessment of their founding principles and future prospects

High-entropy alloys (HEAs) are a relatively new class of materials that have gained considerable attention from the metallurgical research community over recent years. They are characterised by their unconventional compositions, in that they are not based around a single major component, but rather comprise multiple principal alloying elements. Four core effects have been proposed in HEAs: (1) the entropic stabilisation of solid solutions, (2) the severe distortion of their lattices, (3) sluggish diffusion kinetics and (4) that properties are derived from a cocktail effect. By assessing these claims on the basis of existing experimental evidence in the literature, as well as classical metallurgical understanding, it is concluded that the significance of these effects may not be as great as initially believed. The effect of entropic stabilisation does not appear to be overarching, insufficient evidence exists to establish the strain in the lattices of HEAs, and rapid precipitation observed in some HEAs suggests their diffusion kinetics are not necessarily anomalously slow in comparison to conventional alloys. The meaning and influence of the cocktail effect is also a matter for debate. Nevertheless, it is clear that HEAs represent a stimulating opportunity for the metallurgical research community. The complex nature of their compositions means that the discovery of alloys with unusual and attractive properties is inevitable. It is suggested that future activity regarding these alloys seeks to establish the nature of their physical metallurgy, and develop them for practical applications. Their use as structural materials is one of the most promising and exciting opportunities. To realise this ambition, methods to rapidly predict phase equilibria and select suitable HEA compositions are needed, and this constitutes a significant challenge. However, while this obstacle might be considerable, the rewards associated with its conquest are even more substantial. Similarly, the challenges associated with comprehending the behaviour of alloys with complex compositions are great, but the potential to enhance our fundamental metallurgical understanding is more remarkable. Consequently, HEAs represent one of the most stimulating and promising research fields in materials science at present.

On the precipitation of delta phase in ALLVAC ® 718Plus

ALLVAC 718Plus is a new commercial superalloy derived from Inconel 718, but possessing a higher temperature capability whilst employing the same philosophy regarding the microstructure. Many articles have been published describing various heat treatments exploiting the precipitation of intermetallic phases at grain boundaries to optimize the mechanical properties over a range of testing conditions. The requirement to further improve the mechanical properties of this alloy drives our interest in the precipitation mechanism of the delta and eta phases found in this alloy. We report the presence of finely layered structures composed of two phases, delta and eta, with distinct structures and chemistries. Possible pathways to explain this precipitation in 718Plus are considered as follows: (i) the sequential formation of the delta from eta phase and (ii) the simultaneous precipitation of both eta and delta facilitated via solute rejection. Both can result in the formation of those small delta layers observed in HRSTEM. We discuss which is most likely by comparing the relative alignment of the phases by image processing and the analysis of the HRSTEM images, and propose formation mechanisms consistent with the distinctive dislocation structures observed at the interface.

A Comprehensive Case Study of Macrosegregation in a Steel Ingot

Abstract A case study is presented that examines the macrosegregation and grain structure present in a 12-tonne steel ingot, which was cast for experimental purposes. Details of the casting procedure were well documented and the resulting ingot was characterized using a number of techniques that measured chemical segregation, shrinkage, and porosity. The formation of the porosity and segregation patterns is discussed in reference to the particular grain structure observed in the ingot. It is hoped that this case study can be used as a tool for the validation of future macromodels.

Macrosegregation and Microstructural Evolution in a Pressure-Vessel Steel

This work assesses the consequences of macrosegregation on microstructural evolution during solid-state transformations in a continuously cooled pressure-vessel steel (SA508 Grade 3). Stark spatial variations in microstructure are observed following a simulated quench from the austenitization temperature, which are found to deliver significant variations in hardness. Partial-transformation experiments are used to show the development of microstructure in segregated material. Evidence is presented which indicates the bulk microstructure is not one of upper bainite, as it has been described in the past, but one comprised of Widmanstätten ferrite and pockets of lower bainite. Segregation is observed on three different length scales, and the origins of each type are proposed. Suggestions are put forward for how the segregation might be minimized, and its detrimental effects suppressed by heat treatments.

Detection of macrosegregation in a large metallic specimen using XRF

Abstract The characterisation of macroscopic chemical segregation in engineering components over length scales of many metres can be an arduous task. This report investigates the implementation of a technique that is capable of mapping long range variations in the chemical composition of metal components, without the need for extensive sample preparation. The capability of the method is optimised for large production components in a setting where process parameters such as measurement time and minimal surface preparation are of importance. Hence, a readily available hand held X-ray fluorescence instrument and analysis software are used to map macrosegregation in a low alloy steel slab.

The Consequences of Macroscopic Segregation on the Transformation Behavior of a Pressure-Vessel Steel

It is important that the material used to produce high-integrity pressure vessels has homogeneous properties which are reproducible and within specification. Most heavy pressure vessels comprise large forgings derived from ingots, and are consequently affected by the chemical segregation that occurs during ingot casting. Of particular concern are the compositional variations that arise from macrosegregation, such as the channels of enriched material commonly referred to as A-segregates. By causing corresponding variations in microstructure, the segregation may be detrimental to mechanical properties. It also cannot be removed by any practically feasible heat treatments because of the large scale on which it forms. Here we describe an investigation on the consequences of macrosegregation on the development of microstructure in a pressure-vessel steel, SA508 Grade 3. It is demonstrated that the kinetics of transformation are sensitive to the segregation, resulting in a dramatic spatial variations in microstructure. It is likely therefore that some of the scatter in mechanical properties as observed for such pressure vessels can be attributed to macroscopic casting-induced chemical segregation.

Application of criterion for A-segregation in steel ingots

Abstract A recently developed criterion for A-segregation in steel ingots and castings, based on a dimensionless Rayleigh number, is applied to case studies of two steel ingots. In both cases, experimentally obtained locations of A-segregation are correlated against Rayleigh numbers predicted through casting simulations. It is found that the value of 17±8 previously reported as the critical value for the Rayleigh criterion agrees well with the results of these case studies, but that 6 is an appropriate critical value if a conservative lower bound is sought. Limitations and sources of sensitivity in the application of the criterion are highlighted.

The Consequences of Macroscopic Segregation on the Transformation Behaviour of a Pressure-Vessel Steel

It is important that the material used to produce high-integrity pressure vessels has homogeneous properties which are reproducible and within specification. Most heavy pressure vessels comprise large forgings derived from ingots, and are consequently affected by the chemical segregation that occurs during ingot casting. Of particular concern are the compositional variations that arise from macrosegregation, such as the channels of enriched material commonly referred to as A-segregates. By causing corresponding variations in microstructure, the segregation may be detrimental to mechanical properties. Given the scale of the pressure vessel casting, the segregation cannot be removed by practically feasible heat treatments.Here we describe an investigation on the consequences of macrosegregation on the development of microstructure in a pressure-vessel steel, SA508 Grade 3. It is demonstrated that the kinetics of transformation are sensitive to the segregation, resulting in a dramatic spatial variations in microstructure. It is likely therefore that some of the scatter in mechanical properties as observed for such pressure vessels can be attributed to macroscopic casting-induced chemical segregation.Copyright

Research data supporting: "The Effect of Zirconium on the Omega Phase in Ti 24Nb-[0-8]Zr (at.%) Alloys"

The files here correspond to the raw TEM, APT and XRD data that are presented in the figures of the manuscript.

Accompanying data for "On the effect of Nb on the microstructure and properties of next generation polycrystalline powder metallurgy Ni-based superalloys"

The data has been labelled with the same designations as used in the corresponding paper. Raw data is provided from differential scanning calorimetry and scanning electron microscopy following the various heat treatments and oxidation analyses. In addition, data is provided from the tensile, creep and dwell tests performed and the outcomes of the mechanical property modelling are also given.