Russell Goodall

Prediction and validation of quaternary high entropy alloys using statistical approaches

Abstract Prediction of the crystalline structure formation of high entropy alloys is addressed in a novel way by applying principal components analysis to their thermodynamic and electronic parameters. In the simplest form, it shows an excellent discrimination between both face and body centred cubic structures when taking into account the valence electron concentration and enthalpy of mixing. Our analysis indicates that there is a stronger correlation between the formation of multiprincipal components alloys and these parameters than with entropy. The successful prediction of a multiphase structure in TiMnFeNi and the discovery of two novel four component HEAs, MnFeCoNi and TiVMnNb, lends credence to this approach.

Microstructural tailoring of open-pore microcellular aluminium by replication processing

The replication process is presented and discussed with emphasis on methods for microstructural tailoring of open-pore microcellular aluminium-based foams, highlighting methods it offers for control of principal foam mesostructural and microstructural parameters: pore volume fraction, pore shape, pore size(s), as well as the composition and microstructure of the metal making the foam.

The Effect of Electronic Structure on the Phases Present in High Entropy Alloys

Multicomponent systems, termed High Entropy Alloys (HEAs), with predominantly single solid solution phases are a current area of focus in alloy development. Although different empirical rules have been introduced to understand phase formation and determine what the dominant phases may be in these systems, experimental investigation has revealed that in many cases their structure is not a single solid solution phase, and that the rules may not accurately distinguish the stability of the phase boundaries. Here, a combined modelling and experimental approach that looks into the electronic structure is proposed to improve accuracy of the predictions of the majority phase. To do this, the Rigid Band model is generalised for magnetic systems in prediction of the majority phase most likely to be found. Good agreement is found when the predictions are confronted with data from experiments, including a new magnetic HEA system (CoFeNiV). This also includes predicting the structural transition with varying levels of constituent elements, as a function of the valence electron concentration, n, obtained from the integrated spin-polarised density of states. This method is suitable as a new predictive technique to identify compositions for further screening, in particular for magnetic HEAs.

Formation of microporous NiTi by transient liquid phase sintering of elemental powders

Porous metallic structures are attractive for biomedical implant applications as their open porosity simultaneously improves the degree of fixation and decreases the mismatch in stiffness between bone and implant, improving bonding and reducing stress-shielding effects respectively. NiTi alloys exhibit both the shape memory effect and pseudoelasticity, and are of particular interest, though they pose substantial problems in their processing. This is because the shape memory and pseudoelastic behaviours are exceptionally sensitive to the presence of oxygen, and other minor changes in alloy chemistry. Thus in processing careful control of composition and contamination is vital. In this communication, we investigate these issues in a novel technique for producing porous NiTi parts via transient liquid phase sintering following metal injection moulding (MIM) of elemental Ni and Ti powders, and report a new mechanism for pore formation in the powder processing of metallic materials from elemental powders.

Microstructure, Strength and Creep of Aluminium-nickel Open Cell Foam

Al–6.4 wt% Ni open-cell foams produced by replication are tested in compression and in tension at room temperature and creep tested in tension at 450°C. The mechanical behaviour of this foam at room temperature is consistent with that observed for other open-cell metallic foams. A strong dependence on relative density as well as dependence on stress and temperature of the steady-state creep rate are captured by the model described by Mueller et al. [Scripta Mater. 57 (2007) p.33], itself a simplification of variational estimates by Ponte Castañeda and Suquet [Adv. Appl. Mech. 34 (1998) p.171]. The foam exhibits two regimes of creep, separated by a critical stress; these features can be interpreted on the basis of the microstructure of the Al–6.4 wt% Ni alloy making the foam. This microcellular eutectic Al–Ni exhibits attractive creep failure resistance compared to other open-cell aluminium alloy foams.

Porous metals: foams and sponges

Abstract: This chapter describes the processing and properties of metals containing significant fractions of porosity, processed using powders. The basic concepts used in porous materials research are introduced and the different types of processing techniques that have been explored are surveyed. The reported property data for different foams are collated and used to illustrate the range of properties that have been achieved and methods to predict the properties of porous metals from elementary knowledge about their structure are discussed. Finally, the outlook for porous metals research and some likely future directions of fruitful enquiry are suggested.

Cast aluminium single crystals cross the threshold from bulk to size-dependent stochastic plasticity

Metals are known to exhibit mechanical behaviour at the nanoscale different to bulk samples. This transition typically initiates at the micrometre scale, yet existing techniques to produce micrometre-sized samples often introduce artefacts that can influence deformation mechanisms. Here, we demonstrate the casting of micrometre-scale aluminium single-crystal wires by infiltration of a salt mould. Samples have millimetre lengths, smooth surfaces, a range of crystallographic orientations, and a diameter D as small as 6 μm. The wires deform in bursts, at a stress that increases with decreasing D. Bursts greater than 200 nm account for roughly 50% of wire deformation and have exponentially distributed intensities. Dislocation dynamics simulations show that single-arm sources that produce large displacement bursts halted by stochastic cross-slip and lock formation explain microcast wire behaviour. This microcasting technique may be extended to several other metals or alloys and offers the possibility of exploring mechanical behaviour spanning the micrometre scale.

Testing the Fracture Behaviour of Chocolate.

In teaching the materials science aspects of physics, mechanical behaviour is important due to its relevance to many practical applications. This article presents a method for experimentally examining the toughness of chocolate, including a design for a simple test rig, and a number of experiments that can be performed in the classroom. Typical data for some of these experiments are given, along with reflection on the activity.

Uniaxial deformation of microcellular metals: model systems and simplified analysis

Microcellular aluminium can be produced by a process known as replication; this involves the infiltration of a packed bed of NaCl particles which are subsequently leached after metal solidification. The resulting material features a uniform distribution of equisized pores, the shape and volume fraction of which can be tailored, as can the composition and microstructure of the metal making the resulting metal “sponges”. These display a regular uniaxial stress-strain behaviour, at both room and elevated temperature, which is interpreted using standard composite models for Young’s modulus coupled with variational predictions for non-linear deformation of two-phase composites by Ponte-Castaneda and Suquet, adapted and simplified for the specific case at hand.

Structure and Properties of Some CoCrFeNi-Based High Entropy Alloys

The understanding of the structure and the stability of high entropy alloys is still incomplete and the mechanism behind the composition-property relationship is unclear. One reason is that few systematic and accurate determinations of the composition-dependent structure on the atomic level and of the physical properties have been made. In this paper we report on the structure and physical properties of CoCrFeNi and CoCrFeNi-X, (X=Pd, Sn, Ru) alloys of equimolar composition using different experimental techniques (microscopy, neutron and X-ray diffraction, atom probe tomography, Mossbauer spectroscopy, calorimetry). The results show that i) the alloys are not completely homogeneous as is generally suggested in existing literature; ii) they do not form a perfect solid solution; iii) their structure is not single phase, even not either fcc or bcc.