No longer unachievable

Materials World magazine
,
1 Sep 2017

Mike Ellis looks into the uncharted world of 'unachievable alloys' and a new high entropy alloy under development. 

Most known for its work in the titanium family of alloys, making conventional alloys more economically using energy efficient solid-state processing methodology, Metalysis, based in South Yorkshire, UK, is also manufacturing the next generation of ‘unachievable alloys’. These new alloy recipes, previously considered too niche, difficult to produce or expensive to qualify for most R&D programmes are aimed at weighing commercial applications for alloy compositions of the future.

A range of R&D scale programmes particularly focused on high-entropy alloys (HEAs) and high-performance alloys for aerospace, and broader engineering applications are being explored for prominent industries in which some existing alloys have reached their limits of commercial performance. 

Components made from HEAs can stand up to the rigours of corrosive and abrasive environments in power generating applications where present materials of choice fail, leading to plant downtime inefficiencies. Ni-based alloys used today are ripe for replacement, with HEAs providing options to run engines to higher temperatures, enabling increased gas turbine efficiencies. Liquid storage tanks operating at cryogenic temperatures – essential for superconducting magnets – could benefit from replacing commonly used alloys that are brittle and prone to failure. 

In most cases, the manufacture of HEAs involves high temperatures to put all of the alloying elements into the liquid phase. This can create numerous problems and restrict which HEAs are made – what if an alloy requires the combination of low melting point or low boiling point elements with refractory elements? What if there are significant liquid density differences between the constituents, causing melt segregation? By overcoming these problems, the production of ‘unachievable alloys’ is made possible.

Concept to R&D

Metalysis' technology could transform the way metals and alloys are produced in the future, taking the idea from a concept to R&D and then small scale production, offering modular production in the range of tens to hundreds of tonnes per annum. The introduction of an oxide to the cathode of an electrochemical cell in a bath of molten calcium chloride at 800–1,000˚C causes the oxygen in the metal oxide to be ionised, migrating the carbon anode where it reacts to form CO/CO2. When a pure metallic oxide is used, a pure metal is produced. When a mixture of oxides is placed on the cathode, an alloy is produced.

A major advantage is the ease with which fine ceramic oxides can be blended to produce an alloy that is homogeneous at a microscopic scale. Fine metal powders are very difficult to blend due to their pyrophoricity, but the method makes production simpler and economically viable. The metal remains in the solid state throughout the process, with no melting, as metal powder is produced directly from its oxide. It is more environmentally friendly than traditional melting, with only small levels of CO2 produced because calcium chloride is used as the electrolyte. This results in low levels of toxicity – similar to table salt – and components that can be recycled for repeated use.

By using a solid-state, low-temperature reduction process, it is possible to make new industrial materials. These powders are an ideal feedstock as the ‘ink’ for metal 3D printing and additive manufacturing of numerous components and cold-sprayed surfaces. For example, engineers and metallurgists looking beyond nickel-based superalloys for replacement gas turbine materials could benefit from the next generation of superalloys to allow engines to operate at higher temperatures with increased efficiency. 

While an end user may want a new lightweight, refractory and corrosion-resistant alloy, one may not exist in practice, and prove difficult or impossible to make using conventional metallurgical casting techniques. Metalysis' process enables the different alloying elements, density, refractory characteristics and corrosion resistance of that alloy to be varied and, in theory, optimised for any given application. By producing small amounts of trial alloys in relatively short periods to assess this, these alloys can be analysed for optimum alloy composition, which can be made in large volumes.

The HEA backdrop

HEAs are a relatively recent addition to the world’s alloy portfolio. They have emerged primarily through the work of Yeh and Cantor, published in 2004, and differ compositionally from classical engineering alloys. They are not based on one majority component into which minority additions are made, unlike Cupronickels, nominally Cu80Ni20, bronze, traditionally based on Cu90Sn10, and steel based on the Fe-C alloys. The most well researched HEA is the Cantor alloy. 

Broadly, HEAs often contain alloying elements in near-equiatomic ratios. The concept is that their high configurational entropies of mixing stabilise solid solution phases in preference to the precipitation of potentially embrittling intermetallic phases. This means that HEAs should, and often do, exhibit a unique microstructural stability, as well as a variety of unusual properties that arise specifically from their complex compositions. Professor Dr Dierk Raabe, at the Max Planck Institute, Germany, has implied that if you mix equal amounts five or more of the circa 60 periodic elements used in materials, there are 1040 possible combinations. If you extend this with 5% incremental variation in compositions, this rises to 10120 combinations. The challenge for metallurgists is to use computational techniques, knowledge and experience to select the compositions that constitute the right ‘unachievable alloys’ of the future.

New opportunities for alloy design and use

Academic interest in HEAs has been gaining momentum for some time. In 2004, approximately 10 articles focused on the subject, but come 2015, more than 200 emerged. Major reviews have been published in recent years including Dan Miracle of the US Air Force Research Laboratory in Ohio and Oleg Senkov of UES Inc, USA, a second by Tsai and Yeh respectively from the National Chung Hsing and National Tsing Hua Universities, Taiwan, and a third by Ed Pickering at the University of Manchester and Nick Jones from the University of Cambridge, UK. 

From a development perspective, Miracle said, ‘In recent times we have run out of ideas for new high-temperature alloys,’ suggesting these materials open up new significant opportunities for alloy design and use. With many exotic or ‘unachievable alloys’ now off-patent, some remain difficult and expensive to make by traditional techniques. 

Four aspects of Metalysis’ technology aid the selection process. These are: 

  • The ability to make ‘unachievable alloys’ where constituent elements have large differences in their liquid (melt) densities, thus preventing segregation in the melt as the alloy is produced in the solid-state.
  • The ability to make ‘unachievable alloys’ where constituent elements have large differences in their melting points.
  • The ability to make ‘unachievable alloys’ containing elements where boiling points are below the melting point of other constituents or when vapour pressures are high at the melting point. 
  • The ability to tailor the particle size of the particulate product specific manufacturing technique, producing less waste from non-conforming product and manufacturing.

In a recent R&D programme, Metalysis’ chosen alloy of manufacture was a quinternary Al20Ti20Cr20Nb20Ta20 HEA for which the maximum difference in alloying elements' melting points and densities was 2,357oC (Al and Ta) and 12.6 g/cm3 (Al and Ta) respectively. A resulting powder composition is Al17.5Ti21.5Cr18.5Nb20.4Ta22.1 and after sintering a metallographic analysis shows a homogeneous alloy with little porosity. Back scattered electron images were taken using EDX and showed the resulting alloy appears to be an Al:Ti matrix with Cr-rich and Ti:Nb-rich regions that could be dendritic. Using the average compositions of this HEA, the alloy density can be calculated to equal a theoretical volumetric density of 8.4g/cm3, for production capacity of 40T per year.

This poses a flexible, efficient and innovative approach to alloy design, aiding in economically attractive development and manufacturing of a new family of alloys that can have metallurgical, physical and mechanical properties tailored to all parts of the specific application. The challenge remains to select and progress through approvals the optimum new alloys from a huge number of possible combinations. Achieving this will be vital to getting previously unachievable and high-entropy alloys into materials applications, where they could prove invaluable for industrial materials in future. 

Dr Mike Ellis is Metalysis’ Director of Strategic Projects.

www.metalysis.com