Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - ExtendGlass (Extending the range of the glassy state: Exploring structure and property limits in metallic glasses)

Teaser

The project focuses 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 many structural applications because of their good mechanical properties...

Summary

The project focuses 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 many 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 rapidly moved from laboratory curiosities to engineering materials when it was found that iron-based compositions have very attractive soft-magnetic properties. Over later years, it has become clear that some of the mechanical properties are also very impressive, indeed in one case (the property known as ‘damage tolerance’, which is the product of yield stress and fracture toughness) better than for any other known material. The idea underlying this research is that, arguably for too long, it has been considered that a metallic glass of a given composition has a given set of properties. Rather, it should be recognised that a glass of one composition can have an exceptionally wide range of properties, depending on how it is made or subsequently processed. This project focuses on extending the range of the glassy state through innovative processing, in particular by thermomechanical treatments: rejuvenation (to higher energy) offers improved plasticity (perhaps even desirable strain-hardening); relaxation (to lower energy) offers access to ultrastable states. The research aims to extend the range of glassy states and to explore the consequences of unusual states, particularly for mechanical properties and for phase stability/crystallization. The improved properties offer prospects for materials usage that is much more energy-efficient and therefore sustainable. For metallic glasses, the environmental benefits are already clearly seen in their use as soft-magnetic materials, for example in power-distribution transformers – but the benefits can be spread even wider.

Work performed

The project focuses on (i) material processing, (ii) materials characterization, and (iii) modelling and simulation of the atomic-level processes involved in the development of superior properties. Taking these areas in turn:

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 ― the progress made on this much 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. There is much more work to do in this area.

Materials Characterization
We have shown that high-resolution transmission electron microscopy 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 a hitherto unrecognised crystal nucleation regime in a glass-forming system. Such analyses are useful in understanding a wide range of phenomena and in planning future research, for example to optimize glass-forming ability.

Final results

There is one pre-eminent achievement in the project so far – the proof-of-concept demonstration that metallic glasses can be rejuvenated to the extent that they acquire the ability to show strain-hardening and thereby suppress shear-banding. At the start of the project it was really not known whether or not this would be possible in any way. To explain the point, we note that metallic glasses have many attractive mechanical properties, but upon plastic deformation they are understood to 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, indeed, is critical to the success of alloys as engineering materials. A key aim of the project has been to find a way to process metallic glasses so that they also can show the desirable strain-hardening. Rather earlier than we had dared to hope, 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 rather simple. Expected results until the end of the project include the extension of this beneficial processing to (i) larger samples, (ii) samples that have been made brittle by earlier treatments such as annealing, and (iii) a wider range of composition-families of metallic glasses.

Website & more info

More info: https://www.mkg.msm.cam.ac.uk/news/extendglassrecruitment.