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European Microscopy Congress 2020 Abstract Database

On the structural study of rejuvenation/relaxation in bulk metallic glasses

On the structural study of rejuvenation/relaxation in bulk metallic glasses

Session

PSA.2 - Metals & Alloys

Authors

Dr Iurii Ivanov (1), Prof A L Greer (1)

Affiliations

1. University of Cambridge, Department of Materials Science & Metallurgy

Keywords

metallic glasses, electron diffraction, high resolution TEM, radial distribution function

Abstract text

Bulk Metallic glasses (MGs) are amorphous solids formed by cooling a metallic melt fast enough to avoid crystallization. Due to their non-crystalline structure MGs exhibit interesting physical, magnetic and mechanical properties which can be superior to those of crystalline alloys. For instance, since MGs lack the microstructural features associated with conventional polycrystalline alloys, such as grain and twin boundaries or multiphase dispersions, they have the highest known yield stresses of all metallic materials due to the absence of crystallographic slip systems. It is increasingly appreciated that their disordered nature offers, even for a single composition, access to a wide range of structures and properties. This range can be characterized by energy ‒ relaxation (ageing) taking the glass to lower-energy configurations, and rejuvenation to higher-energy configurations. There is a growing interest in rejuvenation as a route to improving the plasticity of BMGs, as their lack of ductility is the main impediment to their wider exploitation in engineering applications. Rejuvenation of BMGs can be induced in several ways, for example by fast re-quenching, by heavy plastic deformation, or by thermal cycling [1].

Despite the fact that MGs lack long-range atomic order, they are not completely disordered. But the characterization of the structural changes in rejuvenation of a BMG remains challenging. In situ quenching experiments by synchrotron X-ray studies indicated a significant increase in the intensity of the first peak in radial distribution function due to the liquid/solid transition [2]. Potentially, this can be used as a possible structural parameter for different state of MGs. Significant progress has been made in the characterisation of the MGs by transmission electron microscopy (TEM) methods. In particular, fluctuation electron microscopy has been proposed as a tool to characterise the structural inhomogeneities of MGs on the nanoscale [3].

Here we present a method by which BMG rejuvenation/relaxation can be characterized by a combination of transmission electron microscopy (TEM) methods: electron diffraction (ED) and aberration-corrected high-resolution TEM (HRTEM). 

The samples were prepared to a final thickness of 30−50 nm by focused ion-beam milling using a Helios Nanolab FIB/SEM. Final polishing was conducted at a reduced voltage of 2 kV to minimize the level of implanted gallium in the final specimen and the damage caused by gallium implantation. TEM studies were carried out with a Tecnai Osiris (FEI) with field-emission gun operated at 200 kV, equipped with Super-X windowless EDX detector. Selected-area electron diffraction (SAED) patterns and associated images were acquired with a Gatan US1000 CCD camera. The size of the SAED aperture was set to be ~200 nm; the camera length was set to cover the high-q range up to 12.2 Å^‒1; the acquisition time was 1 s. High-resolution TEM (HRTEM) studies were carried out with a Titan G2 60–300 microscope (FEI), equipped with a high-brightness field-emission gun (X-FEG) and Cs image-corrector. The radial distribution functions (RDFs) of the as-cast and rejuvenated samples were directly computed from the SAED patterns. For each specimen, the analysis was based on more than ten independently acquired SAED patterns. The details of the sample preparation can be found in [4,5,6]. To study quantitatively the atomic-scale structural ordering, the HRTEM images were analysed using custom DigitalMicrograph scripts based on evaluation of the local autocorrelation function [7].

The structural indication of the rejuvenation/relaxation is related to the change in the average volume per atom attributed to the decrease/increase of BMGs density under rejuvenation/relaxation. The combination of the Differential Scanning Calorimetry (DSC) and TEM techniques was used to establish structure – macroscopic property relationship for BMGs undergoing relaxation (low temperature annealing) and rejuvenation by cryo-thermal cycling [4] and as a result of constrained loading in compression [8]. Furthermore, the last technique could rejuvenate BMG samples sufficiently to enable strain-hardening [5]. The structural changes correlate well with the changes in the macroscopic heat of relaxation and mechanical properties. In particular it supports the link between strain-hardening and structural relaxation. 

In complement to ED data, HRTEM allows extraction of such information locally (at nanoscale) with atomic resolution. It was shown that the variance of the image autocorrelation calculated locally is very sensitive to the average interatomic distances of the specimen as well as to the level of heterogeneity.

References

[1] Y Sun et al., Nature Reviews Materials 1 (2016), p.16039.

[2] KF Kelton et al., PRL 90 (2003), p. 195504.

[3] PM Voyles, DA Muller, Ultramicroscopy 93 (2002), p. 147–159.

[4] SV Ketov, et al., NPG Asia Materials 10 (2018), p. 137–145.

[5] J Pan, YuP Ivanov et al., Nature 578 (2020), p. 559-562. 

[6] SV Ketov, YuP Ivanov et al., Materials Today Advances 1 (2019), p. 100004.

[7] B Sarac, YuP Ivanov et al., Nature Communications 9 (2018), p. 1333.

[8] J. Pan et al., Nature Communications 9 (2018), p. 560.