Understanding the factors influencing yield strength on Mg alloys

Taking the Hall–Petch relationship as a starting point, the factors contributing towards Mg alloy strengthening are analysed, and their relative importance quantified. Solid-solution strengthening is modelled employing a power-law approach. The effects of various processing schedules are reviewed, showing that these play a relatively minor role. Grain refinement effects are described employing thermodynamic and kinetic formulations via the interdependence theory approach. The effects of rare earths are examined, showing that their major contribution is towards grain size control, an effect often in conflict with solid-solution strengthening. A computational approach is proposed, successfully modelling 104 grades reported in the literature. The approach may aid in tailoring and designing Mg alloys for yield strength.The authors wish to acknowledge nancial support from the Accelerated Metallurgy Project, which is co-funded by the European Commission in the 7th Framework Programme (Contract NMP4-LA-2011-263206), by the European Space Agency and by the individual partner organisations.This is the original submitted version of the manuscript. It does not include changes arising from peer-review or editing. The final published version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S1359645414003279#

Scandium-based hexagonally-closed packed multi-component alloys

Since their early development, High-Entropy Alloys have fueled the investigation of exotic metal combinations. Here, we present a strategy for the rational design of a library for multi-component alloys based on six hcp-structured metals. Seven five- and six-component equimolar alloys based on Co, Gd, Y, Sc, Ti and Zr were prepared via induction melting and characterized by PXRD, SEM–EDX and Vickers hardness. They all present ternary hexagonal phases (ScTiZr or GdScY) co-existing with one or more cubic phases and intermetallic compounds. Both ScTiZr and GdScY appear promising as the starting point for new single-phase High-Entropy Alloys families

Anisotropy in mechanical properties and fracture behavior of an oxide dispersion Fe20Cr5Al alloy

Anisotropy of fracture toughness and fracture behavior of Fe20Cr5Al oxide dispersion-strengthened alloy has been investigated by means of compression tests, hardness tests, and wedge splitting test. The results show a small effect of the compression direction on yield strength (YS) and strain hardening. The YS is minimum for longitudinal direction and maximum for the tangential direction. The transverse plastic strain ratio is similar for tangential and longitudinal directions but very different from that in normal direction. Hardness depends on the indentation plane; it is lower for any plane parallel to the L-T plane and of similar magnitude for the other orthogonal planes, i.e., the L-S and T-S planes. Macroscopically, two failure modes have been observed after wedge-splitting tests, those of LS and TS specimens in which fracture deviates along one or two branches normal to the notch plane, and those of LT, TL, SL, and ST specimens in which fracture propagates along the notch plane. Besides LT and TL specimens present delaminations parallel to L-T plane. Both, the fracture surface of branching cracks and that of the delaminations, show an intergranular brittle fracture appearance. It is proposed that the main cause of the delamination and crack branching is the alignment in the mesoscopic scale of the ultrafine grains structure which is enhanced by the 〈110〉- texture of the material and by the presence in the grain boundaries of both yttria dispersoids and impurity contaminations. An elastoplastic finite element analysis was performed to study what stress state is the cause of the branches and delaminations. It is concluded that the normal to the crack branches and/or the shear stress components could determine the crack bifurcation mechanism, whereas the delamination it seems that it is controlled by the magnitude of the stress component normal to the delamination plane. © The Minerals, Metals & Materials Society and ASM International 2014.Peer Reviewe

A criterion for the formation of high entropy alloys based on lattice distortion

Lattice distortion in high entropy alloys (HEAs) is one of their main crystallographic features. Its description is possible by means of unit cell parameter and bulk modulus variations of their constituent elements. The balance of forces acting on the lattice atoms under such distortion is related to the formation of a solid solution of a given crystal structure. This leads to the definition of a novel criterion for selecting HEA compositions. The main existing families of HEAs have been classified under this approach, in addition to an extensive list of multicomponent alloys including intermetallics and bulk metallic glasses. Criteria reported in the literature have been revised with the multicomponent alloy database used in this work which, together with the proposed approach, can be used to improve our understanding of HEAs formation

Solid solution formation rules and crystal structure indicators on high entropy alloys

High Entropy Alloys (HEAs) are multicomponent systems incorporating several elements in a nearly equiatomic configuration. The content of each solute can typically vary between 5 and 35 at%. The high entropy associated to mixing several elements can inhibit the formation of intermetallic phases in favour of FCC or BCC solid solutions. Existing rules for predicting HEAs formation are at an incipient form, generally not offering information on the crystal structure. In this work, the interatomic spacing mismatch and bulk modulus mismatch across the lattice are considered for predicting the occurrence of HEAs. The work follows similar approaches to predict the formation of intermetallic phases and bulk metallic glasses, allowing the prescription of FCC or BCC HEAs occurrence. A statistical analysis on the reliability of the complete set of rules for predicting HEAs has been achieved by analysing approximately 400 different compositions

Damage evolution around primary carbides under rolling contact fatigue in VIM-VAR M50

The presence of stress raisers in bearing steels is known to be highly detrimental to rolling contact fatigue performance. In VIM-VAR (vacuum induction melted-vacuum arc remelted) M50, primary carbides inherited from the solidification stage can develop damage in the form of butterflies, possibly leading to failure by spalling. A ball-on-rod rolling contact fatigue test rig was used to induce damage under different loading conditions. The butterfly distribution as well as the spatial variation of number, size, and orientation relative to the racetrack surface was investigated. The effect of the load on the butterfly formation is also detailed. Finally, the nucleation and growth of butterflies was observed throughout a series of tests suspended after different number of stress cycles. The trends unveiled in this paper could highly benefit the development and validation of models aiming at predicting the effect of stress raisers on the performance of materials subjected to rolling contact fatigue

Discovery of new materials and heat treatments:Accelerated Metallurgy and the case of ferrous and magnesium alloys

Traditionally, the discovery of new materials has been the result of a trial and error process. This has resulted in an extremely time-consuming and expensive process. Models for guiding the discovery of new materials have been developed within the European Accelerated Metallurgy project. The application of statistical techniques to large materials datasets has lead to the discovery of unexpected regularities among their properties. This work focuses on mechanical properties. In particular, the interplay between yield strength, ultimate tensile strength and elongation. A methodology based on principal component analysis, and Kocks-Mecking modelling has led to a tool for finding optimal compositional and heat treatment scenarios. The model is first presented for wide ranges of alloys, and the application to the discovery of new magnesium and ferrous alloys is outlined

Understanding the factors controlling rolling contact fatigue damage in VIM-VAR M50 steel

Sub-surface initiated spalling remains a key factor in determining the ultimate life of properly maintained bearings. In its early stages, spalling is manifested by the development of cracks and accompanying microstructure alterations, so-called butterflies, around the microstructure inhomogeneities. Base upon a unique three-dimensional microscopic characterisation of a large population of butterflies in VIM-VAR M50 samples that underwent rolling contact fatigue under different experimental conditions, the key factors determining butterfly nucleation and growth has been identified. The work identifies the conditions for crack nucleation and growth, and quantitatively relates them to microstucture. The model encompasses the sub-surface stress field and the microstructural parameters of the material leading to crack growth. Outputs of numerical evaluation of the model show good agreement with experimental data concerning number density, depth distribution and size distribution of butterflies across the wide range of fatigue test conditions