Browsing by Author "Stiehler, Martin E."

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    Caught in the act: The structural pathway of liquid metals to vitrification monitored in situ by synchrotron X-ray diffraction.
    (Cranfield University, 2021-09) Stiehler, Martin E.; Georgarakis, Konstantinos; Jolly, Mark R.
    When a metallic melt is undercooled fast enough below its liquidus temperature, crystallisation can be avoided and a metallic glass, i.e. a metallic solid with amorphous structure, be formed. This kind of solidification is called vitrification. The prerequisites for this phenomenon are still not clear. An extensive review of the available relevant literature was carried out. To reveal the structural changes taking place at the atomic scale during undercooling and vitrification, data obtained by ultrafast synchrotron X-ray diffraction during aerodynamic-levitation experiments of different metallic-glass forming liquids was analysed. The complete pathway from temperatures well above the liquidus temperature during undercooling and vitrification down to temperatures well below the glass-transition temperature Tg was studied. During undercooling, a non-linear evolution of structural metrics in real as well as in reciprocal space takes place. Especially the height of the first maximum in the structure factor can be described by a structural analogue to the Curie-Weiss law. This behaviour was also found in published data re-analysed here. Indications of universal behaviour among the investigated alloys below a certain temperature as well as for a liquid-liquid crossover in Ti₄₀Cu₃₄Pd₁₄Zr₁₀Sn₂ were found. Small differences in the temperature dependence of the structural behaviour among the different alloys are possibly related to their different glass-forming abilities. To facilitate the analysis of the real-space structure the novel concept of the anti-shell was introduced. Temperature affects different length scales differently. Below Tg the structural behaviour is dominated by the Debye-Waller factor as well as by normal thermal-expansion behaviour. Above Tg an apparent negative thermal expansion of the first nearest-neighbour distance can be attributed to the influence of the structure-forming processes. In addition to short- and medium-range order, a third structural range for distances beyond the third nearest-neighbour is proposed. A disordering of the atomic structure of metallic glasses by the introduction of further alloying elements, facilitated by emergent effects among the components, could be demonstrated. The importance of the influence of global electronic interactions on structure formation was shown as well as their limitation to distances beyond the third nearest-neighbour.
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    The effect of Ni or Co additions on the structure of Zr60Cu30Al10 bulk metallic glass revealed by high-energy synchrotron radiation
    (Elsevier, 2022-04-13) Stiehler, Martin E.; Panagiotopoulos, Nikolaos T.; Keeble, Dean S.; Ivanov, Yurii P.; Menelaou, Melita; Jolly, Mark R.; Greer, A. Lindsay; Georgarakis, Konstantinos
    The effect of substituting Cu by elemental additions of Ni or Co on the atomic structure of the Zr60Cu30Al10 ternary bulk metallic glass (BMG) is studied using high-energy synchrotron radiation X-ray diffraction. Analyses of the structural features in reciprocal and real space using the structure factors S(Q) and pair-distribution functions (PDF) point to an increase in the structural disorder for the Ni- or Co-bearing quaternary alloys. This is consistent with the “confusion principle” since upon alloying the initially nearly identical atomic sizes of Cu, Ni and Co diversify due to local electronic interactions. In real space, the disordering is manifested by a reduced deviation from the average particle density visible in the nearest-neighbour (NN) atomic shell structure over the complete short- and medium-range order region. Despite their similar atomic size, enthalpies of mixing with the main alloy elements and apparent disordering of the structure, the additions of Ni or Co have different effects on thermal stability of the ternary “mother” alloy.
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    In-situ monitoring the structural pathway of a Ti-based alloy from metallic liquid to metallic glass
    (Elsevier, 2025-04-25) Georgarakis, Konstantinos; Stiehler, Martin E.; Hennet, Louis; Guo, Yaofeng; Antonowicz, Jerzy; Louzguine-Luzgin, Dmitri V.; Jolly, Mark R.; Andrieux, Jérôme; Vaughan, Gavin B. M.; Greer, A. Lindsay
    A metallic glass is formed when a molten metallic alloy is cooled rapidly enough that crystallisation is avoided. However, the way the atomic structure of the liquid converts to that of the glass is generally unknown. The main challenge is the sufficiently fast experimental acquisition of structural data in the undercooled liquid regime necessitated by the high cooling rates needed to avoid crystallisation. In the present study, using aerodynamic levitation, the Ni-free Ti-based alloy Ti40Zr10Cu34Pd14Sn2 was vitrified in-situ in a high-energy synchrotron X-ray beam while diffraction data were acquired during cooling from above the liquidus temperature Tliq to well below the glass-transition temperature Tg. The structure in the undercooled liquid regime shows an accelerated evolution. Both the local order in the short (SRO) and medium range (MRO) increases rapidly as the undercooled liquid approaches Tg, below which the amorphous structure “freezes”. Nevertheless, distinct differences between the evolution of SRO and MRO were observed. The structural rearrangements in the undercooled liquid are found to be correlated with a rapid increase in viscosity of the metallic liquid upon cooling. The new findings shed light on the evolution of the atomic structure of metallic liquids during vitrification and the structural origins of the sluggish kinetics that suppress nucleation and growth of crystalline phases.
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    Medium entropy alloys for biomedical applications
    (Cranfield University, 2024-06-07) Zhang, Jiacheng; Stiehler, Martin E.; Syed, Adnan; Jorge Jr, Alberto Moreira; Jolly, Mark R.; Georgarakis, Konstantinos
    High entropy alloys (HEAs) is a rapidly emerging class of metallic materials consisting of four or more elements in equimolar or quasi-equimolar compositions. These alloys often have simple crystal structures and tailorable properties attracting significant interest for different applications. Common metallic materials for orthopaedic and dental implants include stainless steel, Co-Cr and Ti- alloys. Although these materials are widely in use, issues relevant to biocompatibility and suspected toxicity and the elastic modulus mismatch compared to that of hard tissue have been raised in recent years. High entropy alloys specifically designed for bio-medical applications can offer solutions to overcome these limitations. Bio-HEAs have emerged in the last couple of years and currently receive increasing scientific attention. In this work, we discuss on the design of new entropic alloys using only non-toxic elements such as Ti, Zr, Nb, Ta and Mo. We use a systematic approach to investigate the effect of additional elements on the microstructure and properties of the alloys starting from the binary Ti-Nb and extending to the ternary Ti-Zr-Nb, the quaternary Ti-Zr-Nb-Ta and the Ti-Zr-Nb-Ta-Mo alloy. The alloy design is building on previous work on beta Ti- alloys which has shown promising trends for reducing the elastic modulus of implant materials. The alloys were produced by arc-melting and suction casting under Ar inert atmosphere. X-ray diffraction, and scanning electron microscopy were employed to reveal their crystal structure and microstructure. respectively. The developed alloys exhibit BCC crystal structure and a dendritic microstructure in their as-cast condition. The addition of Zr and Mo was found to increase the hardness of the alloys.
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    Metallic glasses–novel materials for applications in nuclear fusion
    (Institute of Electrical and Electronics Engineers (IEEE), 2024-09) Stiehler, Martin E.; Georgarakis, Konstantinos
    Novel alloys like metallic glasses (MGs) hold the potential to revolutionize many scientific and industrial sectors including nuclear fusion (NF). These metallic materials with amorphous atomic structure possess outstanding mechanical properties in combination with high corrosion resistance and high radiation tolerance. They can be made and shaped in unique ways impossible for conventional crystalline materials. MGs also offer high compositional flexibility and can be tailored to various specific applications. Here, we give a brief overview on how the fusion sector can benefit from MGs and, as examples, highlight some specific use cases. We also outline a pathway for the future development of MGs specifically designed for the extreme environments occurring in NF.
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    On the impact of global interactions on the structure of metallic glasses
    (Elsevier, 2018-12-12) Stiehler, Martin E.; Jolly, Mark R.; Georgarakis, Konstantinos
    The influence of global interactions between the static atomic structure and the valence electrons on structure formation in binary Al-(Ni,Cu,Zr), Zr-(Ni,Cu) and ternary Al-(Ni,Cu)-Zr metallic glasses is investigated over wide concentration ranges and discussed in terms of a Hume-Rothery-like theory by analysing and comparing data available in the literature. The results suggest that global interactions lead to an improvement of thermal stability and glass-forming ability. A complete understanding of structure formation in the considered alloys is assumed to be possible only by taking into account both local and global effects.
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    On the undulatory behaviour of metallic glass foils: a novel spring-type behaviour
    (Cranfield University, 2022-11-08) Shahin Elzoubi, O.; Panagiotopoulos, Nikolaos T.; Stiehler, Martin E.; Salonitis, Konstantinos; Georgarakis, Konstantinos
    The undulatory behaviour is a unique type of mechanical response that was recently observed for metallic glass foils in geometric confinement. It is manifested when normal load is applied on the top of an arc-shaped thin foil of metallic glass; the foil then deforms elastically and its shape changes by progressively increasing the number of formed sinusoidal arcs. This behaviour results from a combination of successive elastic bending and buckling events and can be utilized for developing novel types of non-linear springs. In this work, the undulatory behaviour of a Ni-Fe-Si-B-Mo metallic glass foil has been systematically studied and compared with that of the previously reported Fe-Cr-Si-B foil. The results indicate that the alloy composition and the foil thickness can significantly affect the load required for the formation of the harmonic undulations. The initial geometry of the formed sinusoidal arc including its amplitude and boundary length, can also be used to tune the load and displacement response of the foils. Upon unloading, the foil returns to its initial shape, as long as the loading remains in the elastic deformation range of the metallic glass. The findings suggest that the undulatory behaviour of thin metallic glass foils can be potentially exploited for a wide range of engineering applications including micro-springs, sensors, actuators, and shock absorbers.