An active machine learning approach for optimal design of magnesium alloys using Bayesian optimisation.

In the pursuit of magnesium (Mg) alloys with targeted mechanical properties, a multi-objective Bayesian optimisation workflow is presented to enable optimal Mg-alloy design. A probabilistic Gaussian process regressor model was trained through an active learning loop, while balancing the exploration and exploitation trade-off via an acquisition function of the upper confidence bound. New candidate alloys suggested by the optimiser within each iteration were appended to the training data, and the performance of this sequential strategy was validated via a regret analysis. Using the proposed approach, the dependency of the prediction error on the training data was overcome by considering both the predictions and their associated uncertainties. The method developed here, has been packaged into a web tool with a graphical user-interactive interface (GUI) that allows the proposed optimal Mg-alloy design strategy to be deployed.

On the early stages of localised atmospheric corrosion of magnesium-aluminium alloys.

The surface film on pure magnesium and two aluminium-containing magnesium alloys was characterised after 96 h at 95% RH and 22 °C. The concentration of CO2 was carefully controlled to be either 0 or 400 ppm. The exposed samples were investigated using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and electron microscopy. The results showed that when the alloys were exposed to the CO2-containing environment, aluminium cations (Al3+) was incorporated into a layered surface film comprising a partially "hydrated" MgO layer followed by Mg(OH)2, and magnesium hydroxy carbonates. The results indicated that aluminium-containing magnesium alloys exhibited considerably less localised corrosion in humid air than pure magnesium. Localised corrosion in the materials under investigation was attributed to film thinning by a dissolution/precipitation mechanism.

Oxidation and electrical properties of chromium-iron alloys in a corrosive molten electrolyte environment.

Chromium-iron (CrFe) binary alloys have recently been proposed to serve as the "inert" anode for molten oxide electrolysis (MOE). Herein, the effects of anodic polarization on physical and functional properties of CrFe anodes in the corrosive environment of MOE are studied via empirical observations and theoretical calculations. The findings indicate that the alloys form an inner chromia-alumina solid-solution covered by an MgCr2O4 spinel layer. A survey into the electrical properties of the detected oxides suggests that the layered oxide scale function as an efficient conductor of electricity at elevated temperature. The formation mechanism of the oxides is also investigated.

Experiment-based modelling of grain boundary ß-phase (Mg2Al3) evolution during sensitisation of aluminium alloy AA5083.

An empirical model for the evolution of ß-phase (Mg2Al3) along grain boundaries in aluminium alloy AA5083 (Al-Mg-Mn) during isothermal exposures is proposed herein. Developing a quantitative understanding of grain boundary precipitation is important to interpreting intergranular corrosion and stress corrosion cracking in this alloy system. To date, complete ab initio models for grain boundary precipitation based upon fundamental principles of thermodynamics and kinetics are not available, despite the critical role that such precipitates play in dictating intergranular corrosion phenomena. Empirical models can therefore serve an important role in advancing the understanding of grain boundary precipitation kinetics, which is an approach applicable beyond the present context. High resolution scanning electron microscopy was to quantify the size and distribution of ß-phase precipitates on Ga-embrittled intergranular fracture surfaces of AA5083. The results are compared with the degree of sensitisation (DoS) as judged by nitric acid mass loss testing (ASTM-G67-04), and discussed with models for sensitisation in 5xxx series Al-alloys. The work herein allows sensitisation to be quantified from an unambiguous microstructural perspective.

A closer look at the in vitro electrochemical characterisation of titanium alloys for biomedical applications using in-situ methods.

Titanium (Ti) and its alloys are widely used in several biomedical applications, particularly as permanent orthopaedic implants. Electrochemical testing provides a means to perform accelerated corrosion testing, however whilst results from polarisation testing for Ti and its alloys to date have been generally useful, they are also rather limited on the basis of several reasons. One reason is that the polarisation curves for Ti and its alloys in simulated body fluids all appear rather similar, and they do not present a classical 'breakdown' or pitting potential, making discrimination between alloys difficult. Of practical relevance however, are two key issues; (1) how do Ti alloys respond to a breakdown event? (i.e. do they readily 'repassivate'?), and, (2) what is that actual rate of Ti ion loss from exposure to physiological conditions? The answers to these questions are probed herein. Several Ti alloys of either unique composition or different fabrication method were studied, including commercially pure Ti (cp-Ti), Ti-6Al-4V, Ti-29Nb-13Ta-4.5Zr (TNTZ), selective laser melted Ti-6Al-4V, direct laser deposited cp-Ti, Ti-35Nb-15Zr, and Ti-25Nb-8Zr. Results reveal that both fabrication method and alloying influence 'repassivation' behaviour. Furthermore, atomic emission spectroelectrochemistry as applied to cp-Ti indicated actual dissolution currents of ∼2-3µA/cm-2 (i.e. ∼9µm/yr) in the range of the corrosion potential, also revealing such dissolution is persistent, even with cathodic polarisation, and definitively revealing that the presence of hydrogen peroxide and albumin activate anodic dissolution of Ti. STATEMENT OF SIGNIFICANCE: We believe the paper makes a significant and important contribution to the field of permanent implant biomaterials. Whilst we concede that the paper does not include any in vivo work, the timeliness of the work, and the completely new nature of the findings, we believe carries the impact required for Acta Biomaterialia. Key highlights include:All of the above combine to produce a manuscript that we believe has wide appeal, and can be used as both a port of reference to those working with Ti biomaterials, and also those wishing to apply useful characterisation techniques to their own work (with two very novel methods demonstrated herein, along with the unique information they provide).

Controlling the corrosion and cathodic activation of magnesium via microalloying additions of Ge.

The evolution of corrosion morphology and kinetics for magnesium (Mg) have been demonstrated to be influenced by cathodic activation, which implies that the rate of the cathodic partial reaction is enhanced as a result of anodic dissolution. This phenomenon was recently demonstrated to be moderated by the use of arsenic (As) alloying as a poison for the cathodic reaction, leading to significantly improved corrosion resistance. The pursuit of alternatives to toxic As is important as a means to imparting a technologically safe and effective corrosion control method for Mg (and its alloys). In this work, Mg was microalloyed with germanium (Ge), with the aim of improving corrosion resistance by retarding cathodic activation. Based on a combined analysis herein, we report that Ge is potent in supressing the cathodic hydrogen evolution reaction (reduction of water) upon Mg, improving corrosion resistance. With the addition of Ge, cathodic activation of Mg subject to cyclic polarisation was also hindered, with beneficial implications for future Mg electrodes.

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.

Controlling initial biodegradation of magnesium by a biocompatible strontium phosphate conversion coating.

A simple strontium phosphate (SrP) conversion coating process was developed to protect magnesium (Mg) from the initial degradation post-implantation. The coating morphology, deposition rate and resultant phases are all dependent on the processing temperature, which determines the protective ability for Mg in minimum essential medium (MEM). Coatings produced at 80 °C are primarily made up of strontium apatite (SrAp) with a granular surface, a high degree of crystallinity and the highest protective ability, which arises from retarding anodic dissolution of Mg in MEM. Following 14 days' immersion in MEM, the SrAp coating maintained its integrity with only a small fraction of the surface corroded. The post-degradation effect of uncoated Mg and Mg coated at 40 and 80 °C on the proliferation and differentiation of human mesenchymal stem cells was also studied, revealing that the SrP coatings are biocompatible and permit proliferation to a level similar to that of pure Mg. The present study suggests that the SrP conversion coating is a promising option for controlling the early rapid degradation rate, and hence hydrogen gas evolution, of Mg implants without adverse effects on surrounding cells and tissues.

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 Young's 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.

Assessing the corrosion of biodegradable magnesium implants: a critical review of current methodologies and their limitations.

Magnesium (Mg) and its alloys have been intensively studied as biodegradable implant materials, where their mechanical properties make them attractive candidates for orthopaedic applications. There are several commonly used in vitro tests, from simple mass loss experiments to more complex electrochemical methods, which provide information on the biocorrosion rates and mechanisms. The various methods each have their own unique benefits and limitations. Inappropriate test setup or interpretation of in vitro results creates the potential for flawed justification of subsequent in vivo experiments. It is therefore crucial to fully understand the correct usages of each experiment and the factors that need to be considered before drawing conclusions. This paper aims to elucidate the main benefits and limitations for each of the major in vitro methodologies that are used in examining the biodegradation behaviour of Mg and its alloys.