Periodic Reporting for period 5 - ExtendGlass (Extending the range of the glassy state: Exploring structure and property limits in metallic glasses)
Periodo di rendicontazione: 2023-04-01 al 2023-10-31
The project focused on ‘Extending the range of the glassy state: Exploring structure and property limits in metallic glasses’. Conventional metals and alloys are crystalline, and are chosen for structural applications because of their good mechanical properties. Choice of novel compositions and processing routes can allow alloys to form without crystal structure – they are ‘amorphous’ or ‘glassy’. These metallic glasses moved from laboratory curiosities to engineering materials when iron-based compositions showed good soft-magnetic properties. Later, it became clear that their mechanical properties can also be impressive: e.g. the property known as ‘damage tolerance’ (the product of yield stress and fracture toughness), is better than for any other known material. For too long, it was thought that a metallic glass of a given composition has a given set of properties. Rather, a glass of a given composition can have a range of properties, depending on how it is made or subsequently processed. This project focused on extending the range of the glassy state through innovative processing, in particular by thermomechanical treatments: rejuvenation (to higher energy) offers improved plasticity; relaxation (to lower energy) offers access to ultrastable states. The research aimed to explore the consequences of a wider range of glassy states, particularly for mechanical properties and for phase stability/crystallization. The improved properties offer prospects for materials usage that is more energy-efficient and therefore sustainable. For metallic glasses, the environmental benefits are clearly seen in their use as soft-magnetic materials, for example in power-distribution transformers – but the benefits can be spread even wider.
The project focused on the following topics:
Material Processing
We have developed new compositions of metallic glasses. Our novel aluminium-based glassy alloys are precursors to nanoscale partially and fully crystallized materials that show exceptional ratios of strength to density and excellent thermal stability. Our development of iron-based high-entropy metallic glasses has been fruitful in extending the composition range of glass formation, and in obtaining very stable nanoscale structures that show ultra-high hardness without any obvious embrittlement.
We have demonstrated that constrained uniaxial compression gives extreme rejuvenation of metallic glasses ― this progress exceeds our expectations at the start of the project: see ‘Progress beyond the state of the art’, below.
A key idea underlying the project was that temperature cycling (mostly from room temperature down to liquid-nitrogen temperature, 77 K) would change the structures of metallic glasses. This idea has been amply verified by subsequent work. The effects of cryothermal cycling on metallic glasses have now been explored much further – and it is clear that remarkable improvements in properties (especially in mechanical properties such as toughness) can be achieved.
We have shown that electrical Joule heating can achieve ultrafast heat treatments and thermal cycling of metallic glasses. Heating rates of over 100,000 K/s can be reached, and thousands of thermal cycles can be performed. The base temperature is 77 K (liquid-nitrogen bath), and heating can be up to the melting point of the metallic sample.
We have shown that ultrafast heating is useful to obtain glass/crystal nanocomposites with some remarkable properties, such a high strength maintained to high temperature. Our results open up possibilities for further alloy development.
Materials Characterization
We have shown that high-resolution transmission electron microscopy can be used to detect nanoscale phase separation and voiding, and to obtain quantitative information on the degree of relaxation/rejuvenation of metallic glasses.
Modelling and Atomistic Simulation
We have used atomistic simulation to detect new ‘atomic rattling’ ultrafast relaxation processes, relevant for the onset of plastic flow.
We have shown that classical nucleation theory can account for an unrecognised crystal nucleation regime in a glass-forming system. This is needed to understand a wide range of phenomena and in planning future research, for example to optimize glass-forming ability.
Magnetic materials
In the final phase of the project, we explored the prospects for obtaining ‘tetrataenite’ in a metallic-glass-forming alloy. Tetrataenite is an iron-nickel phase that is potentially a good permanent-magnet material. As it does not contain rare-earths, it has attracted much interest. It is found in some meteorites, but no way has so far been found to manufacture it at scale on earth. A possible synthesis route (simple casting) had been reported in the literature. Our aim was to build on that work by developing better optimized compositions and processing. After much detailed investigation, we concluded that the interpretation of the results in the published work was wrong. Although disappointing, this has helped to clarify research in this area and to suggest directions for future research.
Overview of exploitation and dissemination
There is as yet no commercial exploitation of the results. But links have been made (small-scale ongoing research and consulting) in two areas: (i) soft-magnetic iron-based metal glasses for transformers cores and device applications; (ii) precious-metal-based bulk metallic glasses for jewellery. In area (i), the interest is in making metallic glasses less brittle, especially after the heating treatments necessary to optimize the magnetic properties. In area (ii), the interest is in developing hallmark-standard alloys for jewellery (including watches) that are scratch-resistant and tarnish-resistant.
So far the project researchers (other than the PI himself) appear as co-authors on a total of 36 papers in print in international scientific journals. The researchers have also brought prominence to the project through face-to-face presentations at international conferences (an average of 3 per year, except in the depth of the COVID lockdown (when there were roughly 2 on-line meetings per year).
Material Processing
We have developed new compositions of metallic glasses. Our novel aluminium-based glassy alloys are precursors to nanoscale partially and fully crystallized materials that show exceptional ratios of strength to density and excellent thermal stability. Our development of iron-based high-entropy metallic glasses has been fruitful in extending the composition range of glass formation, and in obtaining very stable nanoscale structures that show ultra-high hardness without any obvious embrittlement.
We have demonstrated that constrained uniaxial compression gives extreme rejuvenation of metallic glasses ― this progress exceeds our expectations at the start of the project: see ‘Progress beyond the state of the art’, below.
A key idea underlying the project was that temperature cycling (mostly from room temperature down to liquid-nitrogen temperature, 77 K) would change the structures of metallic glasses. This idea has been amply verified by subsequent work. The effects of cryothermal cycling on metallic glasses have now been explored much further – and it is clear that remarkable improvements in properties (especially in mechanical properties such as toughness) can be achieved.
We have shown that electrical Joule heating can achieve ultrafast heat treatments and thermal cycling of metallic glasses. Heating rates of over 100,000 K/s can be reached, and thousands of thermal cycles can be performed. The base temperature is 77 K (liquid-nitrogen bath), and heating can be up to the melting point of the metallic sample.
We have shown that ultrafast heating is useful to obtain glass/crystal nanocomposites with some remarkable properties, such a high strength maintained to high temperature. Our results open up possibilities for further alloy development.
Materials Characterization
We have shown that high-resolution transmission electron microscopy can be used to detect nanoscale phase separation and voiding, and to obtain quantitative information on the degree of relaxation/rejuvenation of metallic glasses.
Modelling and Atomistic Simulation
We have used atomistic simulation to detect new ‘atomic rattling’ ultrafast relaxation processes, relevant for the onset of plastic flow.
We have shown that classical nucleation theory can account for an unrecognised crystal nucleation regime in a glass-forming system. This is needed to understand a wide range of phenomena and in planning future research, for example to optimize glass-forming ability.
Magnetic materials
In the final phase of the project, we explored the prospects for obtaining ‘tetrataenite’ in a metallic-glass-forming alloy. Tetrataenite is an iron-nickel phase that is potentially a good permanent-magnet material. As it does not contain rare-earths, it has attracted much interest. It is found in some meteorites, but no way has so far been found to manufacture it at scale on earth. A possible synthesis route (simple casting) had been reported in the literature. Our aim was to build on that work by developing better optimized compositions and processing. After much detailed investigation, we concluded that the interpretation of the results in the published work was wrong. Although disappointing, this has helped to clarify research in this area and to suggest directions for future research.
Overview of exploitation and dissemination
There is as yet no commercial exploitation of the results. But links have been made (small-scale ongoing research and consulting) in two areas: (i) soft-magnetic iron-based metal glasses for transformers cores and device applications; (ii) precious-metal-based bulk metallic glasses for jewellery. In area (i), the interest is in making metallic glasses less brittle, especially after the heating treatments necessary to optimize the magnetic properties. In area (ii), the interest is in developing hallmark-standard alloys for jewellery (including watches) that are scratch-resistant and tarnish-resistant.
So far the project researchers (other than the PI himself) appear as co-authors on a total of 36 papers in print in international scientific journals. The researchers have also brought prominence to the project through face-to-face presentations at international conferences (an average of 3 per year, except in the depth of the COVID lockdown (when there were roughly 2 on-line meetings per year).
There is a pre-eminent achievement in the project – demonstration that metallic glasses can be rejuvenated to the extent that they show strain-hardening and thereby suppress shear-banding. At the start of the project, it was not known whether or not this would be possible at all. Metallic glasses have many attractive mechanical properties, but when deformed they show strain-softening leading to sharp, highly undesirable, localization of flow. In contrast, conventional crystalline engineering alloys show strain-hardening that prevents this problematic localization and this underlies their success as engineering materials. The project aimed to find a way to process metallic glasses so that they also can show the desirable strain-hardening. We have shown that this is possible in a bulk glass that is stable at room temperature. Furthermore, the processing (uniaxial compression under constraint) is simple. Metallic glasses can become brittle after various treatments, and this greatly limits their usefulness. We also found that this embrittlement can be reversed: likely to be very useful in industrial processing of these materials, particularly for soft-magnetic applications relevant for the energy transition.