Kevin J. Laws

Ca-Mg-Zn bulk metallic glasses as bioresorbable metals.

A series of six unique Ca-based bulk metallic glasses were synthesized and characterized. The glasses were designed to consist solely of the biocompatible elements Ca, Mg and Zn, with the view to their potential use as bioresorbable metals for orthopaedic applications. The alloys had a critical casting thickness of up to 4.5 mm. Mechanical and thermophysical testing revealed a Youngs modulus (stiffness) of ∼40 GPa. Glass transition temperatures ranged from 119 to 129°C, above which the alloys can be formed like a thermoplastic polymer. In vitro biocorrosion testing using a combination of polarization and mass loss techniques revealed that the corrosion rate of these alloys is relatively rapid, although, in some cases, it may be tailored through alloy composition.

Quantitative in vitro assessment of Mg65Zn30Ca5 degradation and its effect on cell viability

A bulk metallic glass (BMG) of composition Mg(65) Zn(30) Ca(5) was cast directly from the melt and explored as a potential bioresorbable metallic material. The in vitro degradation behavior of the amorphous alloy and its associated effects on cellular activities were assessed against pure crystalline magnesium. Biocorrosion tests using potentiodynamic polarization showed that the amorphous alloy corroded at a much slower rate than the crystalline Mg. Analysis of the exchanged media using inductively coupled plasma optical emission spectrometry revealed that the dissolution rate of Mg ions in the BMG was 446 μg/cm(2)/day, approximately half the rate of crystalline Mg (859 μg/cm(2)/day). A cytotoxicity study, using L929 murine fibroblasts, revealed that both the BMG and pure Mg are capable of supporting cellular activities. However, direct contact with the samples created regions of minimal cell growth around both amorphous and crystalline samples, and no cell attachment was observed.

Accelerated discovery of metallic glasses through iteration of machine learning and high-throughput experiments

Coupling artificial intelligence with high-throughput experimentation accelerates discovery of amorphous alloys. With more than a hundred elements in the periodic table, a large number of potential new materials exist to address the technological and societal challenges we face today; however, without some guidance, searching through this vast combinatorial space is frustratingly slow and expensive, especially for materials strongly influenced by processing. We train a machine learning (ML) model on previously reported observations, parameters from physiochemical theories, and make it synthesis method–dependent to guide high-throughput (HiTp) experiments to find a new system of metallic glasses in the Co-V-Zr ternary. Experimental observations are in good agreement with the predictions of the model, but there are quantitative discrepancies in the precise compositions predicted. We use these discrepancies to retrain the ML model. The refined model has significantly improved accuracy not only for the Co-V-Zr system but also across all other available validation data. We then use the refined model to guide the discovery of metallic glasses in two additional previously unreported ternaries. Although our approach of iterative use of ML and HiTp experiments has guided us to rapid discovery of three new glass-forming systems, it has also provided us with a quantitatively accurate, synthesis method–sensitive predictor for metallic glasses that improves performance with use and thus promises to greatly accelerate discovery of many new metallic glasses. We believe that this discovery paradigm is applicable to a wider range of materials and should prove equally powerful for other materials and properties that are synthesis path–dependent and that current physiochemical theories find challenging to predict.

The Prediction of Glass-Forming Compositions in Metallic Systems - The Development of New Bulk Metallic Glasses

A novel methodology of predicting specific compositions for glass forming alloys based on elemental cluster selection, liquidus lines, atomic packing efficiency and ab initio calculations is presented and discussed. The proposed composition selection model has lead to the discovery of a number of novel, soon to be reported Mg, Cu, Zn and Ag-based bulk metallic glasses. The proposed model may also be used to explain high glass forming ability and physical properties of known BMG compositions and to pin-point new or superior BMG compositions in existing glass forming systems. Further, the aforementioned model shows strong correlations between proposed elemental clusters, glass forming ability and BMG ductility. This model has also shown applicable adaptation to known ceramic oxide glass forming systems.

Potentiodynamic polarisation study of bulk metallic glasses based on the Mg-Zn-Ca ternary system

Abstract Bulk metallic glasses (BMGs) based on the Mg–Zn–Ca ternary system are considered as potential bioresorbable metals for orthopaedic applications as they are comprised solely of biocompatible elements. In the present study, a series of six Ca rich and ten Mg rich BMGs were synthesised and their corrosion behaviours characterised using potentiodynamic polarisation. A complex interaction between the elements in this compositional window was observed, whereby a significant increase in dissolution rate was accompanied by increasing Ca content or decreasing Mg content. Although the addition of Zn had a positive contribution in reducing the rate of corrosion, its effect was not dominant. In addition, a form of passivation was observed in the Mg rich BMGs, although it appears that such passive corrosion behaviour is not directly related to a high Zn content.

Examining the elemental contribution towards the biodegradation of Mg–Zn–Ca ternary metallic glasses

The Mg-Zn-Ca metallic glass system has been the focus of recent studies as a prospective material for biodegradable implants. To date, the influence each alloying element has on the degradation behaviour of this class of alloy is still not well understood. This study employs electrochemical polarisation and in situ impedance spectroscopy coupled with H2 gas collection in simulated body fluid at 37 °C to elucidate the mechanisms by which a series of custom produced Mg-Zn-Ca metallic glasses degrade compared with high purity Mg. The results show that Mg-Zn-Ca metallic glasses provide significantly more noble corrosion potentials and suppressed hydrogen gas evolution relative to high purity Mg. Furthermore, the role each element has in degradation was investigated systematically by varying the concentration of each alloying element. Testing revealed that the complex nature of dissolution in metallic glasses requires testing beyond solely polarisation and hydrogen gas collection to elucidate degradation behaviour in vitro. Practical limits to which the composition may be adjusted in this ternary alloy system, so as to maintain minimal degradation, have been achieved.

Recent progress in high Bs and low Hc Fe-based nanocrystalline alloys

Abstract Fe-based nanocrystalline alloys provide a viable strategy to enhance the energy conversion efficiency for next-generation electrics by utilizing nanotechnology. This paper reviews current research activities that focus on Fe-based amorphous/nanocrystalline alloys with high Bs and low Hc values as well as their applications in industry.

Thermoplastic formability of CaMgZn bulk metallic glasses for biomedical applications

Bulk metallic glasses (BMGs) based on the Mg-Zn-Ca ternary system have shown promise as a potential bioresorbable material. Amongst these, Ca-rich BMGs have demonstrated superior thermoplastic formability. However, corrosion assessments showed that these BMGs can disintegrate within hours. Ca55Mg17.5Zn27.5 was synthesised to improve its corrosion resistance over other Ca-rich BMGs whilst maintaining its thermoplastic formability. In the present study, the thermoplastic formability of Ca55Mg17.5Zn27.5 is compared with Ca-rich and Mg-rich BMGs. A dilatometer was used to measure the associated contraction (softening) in the material as it is heated above its glass transition temperature. Although the enrichment of Zn in Ca55Mg17.5Zn27.5 reduced the temperature range of its supercooled liquid region, this alloy exhibits significantly lower viscosity within this region compared to the other BMGs, deeming it more favourable for complex thermoplastic forming processing.

3D-EBSD Studies of Deformation, Recrystallization and Phase Transformations

A focused ion beam (FIB) coupled with high resolution electron backscatter diffraction (EBSD) has emerged as a useful tool for generating crystallographic information in reasonably large volumes of microstructure. In principle, data generation is reasonably straightforward whereby the FIB is used as a high precision serial sectioning device for generating consecutive milled surfaces suitable for mapping by EBSD. The successive EBSD maps generated by serial sectioning are combined using various post-processing methods to generate crystallographic volumes of the microstructure. This paper provides an overview of the use of 3D-EBSD in the study of various phenomena associated with thermomechanical processing of both crystalline and semi-crystalline alloys and includes investigations on the crystallographic nature of microbands, void formation at particles, phase redistribution during plastic forming, and nucleation of recrystallization within various regions of the deformation microstructure.

Atomistic origin of stress overshoots and serrations in a CuZr metallic glass

In this work we use molecular dynamics simulations to study the stress overshoots of metallic glass Cu