On the design and feasibility of refractory metal-base Superalloys

Over the last 60 years, the evolution of nickel-base superalloys has enabled successive generations of gas turbine engines to operate at progressively higher temperatures. However, despite continued research activity, capability enhancement has become incremental and it seems unlikely that nickel-base superalloys will be able to support the requirements of future engine designs. Therefore, to enable a step change in operating temperatures, it is necessary to identify and develop new alloy systems, which, in addition to higher temperature capability, also have the correct balance of mechanical and environmental properties. Here, we outline an alloy design philosophy and report on the initial characterisation of one of the potential alloy systems. High temperature properties are dominated by the melting temperature and crystal structure of the principal element. Thus, only 11 elements offer capability above that of nickel-base alloys. However, if terrestrial abundance and cost are also considered, then only the bcc refractory metals remain as viable options. Intrinsic environmental resistance above 1000˚C can be afforded only by the formation of protective silica or alumina scales, requiring the incorporation of at least one of these elements in reasonable concentrations. In addition the required balance of mechanical properties is only likely to be achieved by the production of a microstructure containing a fine dispersion of small intermetallic precipitates, which have a coherent superlattice structure of solid solution matrix. The simplest materials identified by this approach are ternary refractory metal-base alloys, e.g. Ta‑Al‑Co. However, the phase equilibria of these systems, particularly in the refractory rich corners, are poorly defined. To address this issue and explore the potential of these materials, a series of alloys in the Ta-rich corner of the Ta‑Al‑Co system have been created and characterised following 500 hour heat treatments at temperatures between 1000 and 1300 ̊C. As part of this work the first conclusive evidence of a large-unit-celled Ta2AlCo phase was obtained, which may give potential for refractory metal-base superalloys

A Comprehensive Case Study of Macrosegregation in a Steel Ingot

This is the author accepted manuscript. The final version is available from Springer via http://dx.doi.org/10.1007/s11663-015-0386-yA 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.This work was undertaken as part of a Project sponsored by Rolls-Royce Power Nuclear plc in collaboration with Sheffield Forgemasters International

An assessment of the high-entropy alloy system VCrMnFeAlx

A major consideration when choosing materials for nuclear reactors is the future radioactive waste produced by the irradiation of their component parts. Only a handful of structural metals can be considered ‘low activation’ in a fusion environment. In recent work, we showed that the low-activation multicomponent equiatomic alloy VCrMnFe comprises a single BCC (A2) phase at 1200°C. Here, we examine its stability on ageing at lower temperatures, and the effect of Al additions (to create VCrMnFeAlx alloys) to destabilise the sigma phase and form strengthening superlattice structures. It is found that substantial volume fractions of sigma phase form after ageing VrCrMnFe at 600°C and 800°C for 1000 h. The addition of Al was found to destabilise the sigma phase, as predicted using thermodynamic modelling, with it being eliminated at all temperatures with additions of 6.6 at% Al. Increasing Al additions also led to the formation of superlattice structures: B2 and L21 (Heusler). Higher Al content had a slight increasing effect on the alloys’ hardness, but also embrittled the alloys (at room temperature). Significant hardening was produced by nano-segregation induced in the higher Al x = 0.25, 0.5 and 1.0 alloys after aging at 600 °C. This alloy system presents an attractive opportunity to fine-tune the composition to obtain a balance of ductility and high-temperature strength and stability. Of particular interest was the formation a two-phase basket weave cube-on-cube orientated, coherent, microstructure in VCrMnFeAl1.0 after aging at 800 °C

Reduced Dwell-Fatigue Resistance in a Ni-Base Superalloy After Short-Term Thermal Exposure

The effect of short-term thermal exposure on microstructure and dwell-fatigue resistance of Ni-base superalloy 718Plus was investigated. Contrary to previous studies performed after long-term exposure, an increase in the dwell-fatigue crack growth rate was observed, which was connected to a small increase in the size of the hardening precipitates. The proposed controlling mechanism was the stress relaxation rate at the crack tip, and based on this a schematic model for the development of the properties during exposure is presented

Novel reduced-activation TiVCrFe based high entropy alloys

The conditions inside next-generation nuclear reactors are likely to be extreme. High-performance materials will be required, and there is still great scope for developing new and improved alloys. High-entropy alloys (HEAs) are potential candidates, and it is likely that such alloys will need to be based around low-activation elements that tend to form body-centred cubic structures. Whilst there have been a number of investigations assessing the capability of CALPHAD databases to predict the phases formed in face-centred cubic HEAs, their applicability to less studied systems is not well known. Here, two low-activation HEAs are produced; TiVCrMnFe and Si0.1TiVCr0.5Fe, and their microstructures assessed and compared to CALPHAD predictions. The microstructures of both alloys comprised a C14 Laves phase and a B2 phase following casting, and a C14 Laves phase and a BCC solid solution (A2) phase following holding at 1200 °C, with high proportions of both constituent phases found in each case. It was found experimentally that the Laves phases in both alloys were quaternary intermetallics of Fe, V, Cr and Ti, with Fe, V and Cr likely disordered across the Fe site. These observations were compared with predictions made using four CALPHAD databases. The accuracy of the predictions varied markedly between the databases. It was found that the Laves phase was underpredicted, often severely, by the databases that did not account for the quinary nature of phase. Removing Ti from TiVCrMnFe resulted in an equiatomic VCrMnFe alloy that formed a single BCC phase at 1200 °C

The microstructural evolution of CM247LC manufactured through laser powder bed fusion

Numerous challenges persist with the additive manufacturing of high γ′ containing Ni-based superalloys such as CM247LC. Currently, significant cracking occurs during deposition of CM247LC components using laser powder bed fusion and during post-processing. Whilst post-deposition procedures seek to eliminate or minimise cracks, current procedures do not produce a microstructure suitable for service. This study systematically investigates the microstructural evolution of CM247LC manufactured using laser powder bed fusion following multiple post processing treatments. Phase and textural changes after each processing step were consistent with previous studies, although an additional Hf-rich and Cr-depleted segregation zone was identified along intercellular boundaries in the as-deposited condition, believed to be associated with the cracking propensity. Compositional modification of CM247LC including removal of Hf, reduction of C and addition of Nb eliminated the segregation zone but these changes were associated with an increased susceptibility to solidification and liquation cracking

Simulation of Channel Segregation During Directional Solidification of In—75 wt pct Ga. Qualitative Comparison with In Situ Observations

International audienceFreckles are common defects in industrial casting. They result from thermosolutal convection due to buoyancy forces generated from density variations in the liquid. The present paper proposes a numerical analysis for the formation of channel segregation using the three-dimensional (3D) cellular automaton (CA)—finite element (FE) model. The model integrates kinetics laws for the nucleation and growth of a microstructure with the solution of the conservation equations for the casting, while introducing an intermediate modeling scale for a direct representation of the envelope of the dendritic grains. Directional solidification of a cuboid cell is studied. Its geometry, the alloy chosen as well as the process parameters are inspired from experimental observations recently reported in the literature. Snapshots of the convective pattern, the solute distribution, and the morphology of the growth front are qualitatively compared. Similitudes are found when considering the coupled 3D CAFE simulations. Limitations of the model to reach direct simulation of the experiments are discussed

Effect of age, sex and gender on pain sensitivity: A narrative review

© 2017 Eltumi And Tashani. Introduction: An increasing body of literature on sex and gender differences in pain sensitivity has been accumulated in recent years. There is also evidence from epidemiological research that painful conditions are more prevalent in older people. The aim of this narrative review is to critically appraise the relevant literature investigating the presence of age and sex differences in clinical and experimental pain conditions. Methods: A scoping search of the literature identifying relevant peer reviewed articles was conducted on May 2016. Information and evidence from the key articles were narratively described and data was quantitatively synthesised to identify gaps of knowledge in the research literature concerning age and sex differences in pain responses. Results: This critical appraisal of the literature suggests that the results of the experimental and clinical studies regarding age and sex differences in pain contain some contradictions as far as age differences in pain are concerned. While data from the clinical studies are more consistent and seem to point towards the fact that chronic pain prevalence increases in the elderly findings from the experimental studies on the other hand were inconsistent, with pain threshold increasing with age in some studies and decreasing with age in others. Conclusion: There is a need for further research using the latest advanced quantitative sensory testing protocols to measure the function of small nerve fibres that are involved in nociception and pain sensitivity across the human life span. Implications: Findings from these studies should feed into and inform evidence emerging from other types of studies (e.g. brain imaging technique and psychometrics) suggesting that pain in the older humans may have unique characteristics that affect how old patients respond to intervention