Metallic glass roadmap

Soft materials are usually defined as materials made of mesoscopic entities, often self-organised, sensitive to thermal fluctuations and to weak perturbations. Archetypal examples are colloids, polymers, amphiphiles, liquid crystals, foams. The importance of soft materials in everyday commodity products, as well as in technological applications, is enormous, and controlling or improving their properties is the focus of many efforts. From a fundamental perspective, the possibility of manipulating soft material properties, by tuning interactions between constituents and by applying external perturbations, gives rise to an almost unlimited variety in physical properties. Together with the relative ease to observe and characterise them, this renders soft matter systems powerful model systems to investigate statistical physics phenomena, many of them relevant as well to hard condensed matter systems. Understanding the emerging properties from mesoscale constituents still poses enormous challenges, which have stimulated a wealth of new experimental approaches, including the synthesis of new systems with, e.g. tailored self-assembling properties, or novel experimental techniques in imaging, scattering or rheology. Theoretical and numerical methods, and coarse-grained models, have become central to predict physical properties of soft materials, while computational approaches that also use machine learning tools are playing a progressively major role in many investigations. This Roadmap intends to give a broad overview of recent and possible future activities in the field of soft materials, with experts covering various developments and challenges in material synthesis and characterisation, instrumental, simulation and theoretical methods as well as general concepts.

Absence of ultrasonic‐vibration‐induced plasticity in metallic glacial glasses

An amorphous phase in Nd–Fe–Al system was formed in an extremely wide composition range of 0 to 90 at% Fe and 0 to 93 at% Al by melt spinning. Based on the information on the amorphous formation, ferromagnetic Nd90−xFexAl10 bulk amorphous alloys with high coercive force at room temperature were obtained by a copper mold casting method. The maximum diameter of the cylindrical amorphous samples with a length of 50 mm is about 7 mm for the 20%Fe alloy and about 4 mm for the 30%Fe alloy. Neither glass transition nor supercooled liquid region is observed in the temperature range before crystallization, being different from previous bulk glassy alloys exhibiting a wide supercooled liquid region before crystallization. The onset temperature of crystallization (Tx) and melting temperature (Tm) are measured to be 778 and 863 K, respectively, for the Nd70Fe20Al10 alloy. The resulting reduced ratio of Tx⁄Tm is as high as 0.90 and the temperature interval between Tx and Tm is as small as 85 K. The extremely high Tx⁄Tm and small ΔTm(=Tm−Tx) values are the reason for the achievement of the large glass-forming ability. The bulk amorphous Nd70Fe20Al10 alloy has a ferromagnetism with the Curie temperature (Tc) of about 600 K which is much higher than the highest Tc (about 480 K) for the Nd–Fe binary amorphous alloy ribbons. The remanence (Br) and intrinsic coercive force (iHc) for the bulk Nd60Fe30Al10 alloy are 0.122 T and 277 kA/m, respectively, in the as-cast state and 0.128 T and 277 kA/m, respectively, in the annealed state for 600 s at 600 K. The Br and iHc decrease to 0.045 T and 265 kA/m, respectively, for the crystallized Nd60Fe30Al10 sample consisting of Nd+Al2Nd+δ phases and the maximum hard magnetic properties are achieved in the amorphous state. The hard magnetic properties for the bulk amorphous alloys are presumably due to the homogeneous development of ferromagnetic clusters with large random magnetic anisotropy. The finding of the bulk amorphous alloys exhibiting hard magnetic properties at room temperature is promising for the future development as a new type of metallic amorphous permanent magnet.

On the fatigue response of bridging ductile fibers in an MoSi 2 intermetallic composite : Deve, H.E., Spearing, S.M. and Maloney, M.J. Mater. Sci. Eng. A (28 Feb. 1993) A160 (2), 209–214

Metallic glass is a promising metallic material with many unique properties, and also considered as a model system to study the mysteries of amorphous materials. Recently, many experimental and simulation results supported the existence of “flow unit” in metallic glass. In this paper, we review the background, the theoretical and experimental evidences of flow unit model. Flow units are considered as those loosely packed regions embedded inside the elastic matrix and behave like viscous liquid. Compared with the matrix, flow unit regions have low modulus and strength, low viscosity, high atomic mobility and stand in the saddle points on energy landscape. Therefore, flow units can be treated as dynamical defects in metallic glass. The feature, activation and evolution process of flow unit region in metallic glass as well as their correlation with property in metallic glass are also reviewed. Through dynamical mechaincal methods like dynamical mechanical spectra and stress relaxation, flow unit region and its properties can be distinguished and studied. A three-parameter physical model is proposed to describe the mechnical behaivors of flow units. The activations and evolutions of flow unit under different temperature and strain conditions are studied. A three-stage evolution process is found and the relation with mechanical performance and relaxation behavior is established. The characteristics of flow units are also related to various properties of metallic glass, like plasticity, strength, fracture and boson peaks. By using the thermal, mechanical and high pressure aging procedues, the properties of metallic glass can be manipulated as desired through adjusting the density of flow units. We show that the flow unit model not only helps to understand the mechanism behind many long-standing issues like deformation, glass transition dynamic relaxations, and the connection between strucutre and properties and performance of metallic glasses, but also is crucial for tuning and designing the properties of metallic glasses.

By chronopotentiometry and cyclic voltammetry methods it was investigated the corrosion resistance of the tape amorphous alloys Fe 78,5 Ni 1,0 Mo 0,5 B 6,0 Si 14,0 and Fe 73,1 Cu 1,0 Nb 3,0 Si 15,5 B 7,4 in an aqueous solution of0,5 M sulfuric acid. The samples alloyed with Cu and Nb showed more corrosion resistance in the aggressive medium of sulfuric acid. Modification of surface of the investigated amorphous samples by an alternating magnetic field leads to an increasing in their corrosion resistance in acidic medium. Surface oxidations in the presence of cuprous and niobium in Fe–Si–B alloy are two-stage processes. In addition of oxidizing Cu in Cu 2 O and sulfates, Nb adsorbs O 2 to form solid solutions of Nb 6 O, Nb 2 O. Oxides with higher oxidation levels (Nb 2 O 5 ) are covalent compounds with low electron conductivity and they can create a passivating superficial layer. In case of amorphous metallic alloys Fe 78,5 Ni 1,0 Mo 0,5 B 6,0 Si 14,0 and Fe 73,1 Cu 1,0 Nb 3,0 Si 15,5 B 7,4 , alternating magnetic field contributes to a significant exit on surface such elements as ferrum and boron (they form the Fe 3 B compound). In the surface layers of the Fe 73,1 Cu 1,0 Nb 3,0 Si 15,5 B 7,4 alloy modified by magnetization, an increase in at. % of Cu, and Nb is observed. Such elements form dense oxide surface protective layers. Weak alternating magnetic field significantly affects not only diffusion, but also the formation of phases in metallic alloys. Thus, alternating magnetic field has a strong influence on the diffusion processes in solid metallic materials, which causes them to homogenize the phases, change the component composition of the domains. These characteristics are directly related to the chemical activity of the surface of metallic materials; therefore, as main instrumental method for testing the effects of alternating magnetic field can be used electrochemical techniques: chronopotentiometry and voltammetry. Keywords : amorphous metallic alloys, corrosion resistance, sulfuric acid, alternating magnetic field.

Metallic glasses possess attractive properties, such as high strength, good corrosion resistance, and superior soft magnetic performance. They also serve as precursors for synthesizing nanocrystalline materials. In addition, a new class of composites having crystalline phases embedded in amorphous matrix is evolving based on selective crystallization of metallic glasses. Therefore, crystallization of metallic glasses and its effects on properties has been a subject of interest. Previous investigations from our research group related to laser assisted crystallization of Fe-Si-B metallic glass (an excellent soft magnetic material by itself) showed a further improvement in soft magnetic performance. However, a fundamental understanding of crystallization and mechanical performance of laser treated metallic glass was essential from application point of view. In light of this, the current work employed an integrated experimental and computational approach to understand crystallization and its effects on tensile behavior of laser treated Fe-Si-B metallic glass. The time temperature cycles during laser treatments were predicted using a finite element thermal model. Structural changes in laser treated Fe-Si-B metallic glass including crystallization and phase evolution were investigated with the aid of X-ray diffraction, differential scanning calorimetry, resistivity measurements, and transmission electron microscopy. The mechanical behavior was evaluated by uniaxial tensile tests with an InstronTM universal testing machine. Fracture surfaces of the metallic glass were observed using scanning electron microscopy and site specific transmission electron microscopy. Fe-Si-B metallic glass samples treated with lower laser fluence (<0.49 J/mm2) underwent structural relaxation while higher laser flounces led to partial crystallization. The crystallization temperature experienced an upward shift due to rapid heating rates of the order of 104 K/s during laser treatments. The heating cycle was followed by termination of laser upon treatment attainment of peak temperature and rapid cooling of the similar order. Such dynamic effects resulted in premature arrest of the crystallite growth leading to formation of fine crystallites/grain (~32 nm) of α-(Fe,Si) as the major component and Fe2B as the minor component. The structural relaxation, crystallization fractions of 5.6–8.6 Vol% with α-(Fe,Si) as the main component, and crystallite/grain size of the order of 12 nm obtained in laser fluence range of 0.39-0.49 J/mm2 had minimal/no influence on tensile behavior of the laser treated Fe-Si-B metallic glass foils. An increase in laser fluence led to progressive increase in crystallization fractions with considerable amounts of Fe2B (2-6 Vol%) and increase in grain size to ~30 nm. Such a microstructural evolution severely reduced the strength of Fe-Si-B metallic glass. Moreover, there was a transition in fracture surface morphology of laser treated Fe-Si-B metallic glass from vein pattern to chevron pattern. Tensile loading lacked any marked influence on the crystallization behavior of as-cast and structurally relaxed laser-treated metallic glass foils. However, a significant crystallite/grain growth/coarsening of the order of two and half times was observed in the fractured region compared to the region around it for the laser-treated partially crystallized metallic glass foils. The simultaneous effects of stress generation and temperature rise during tensile loading were considered to play a key role in crystallite/grain growth/coarsening.

High-strength Mg-Cu-Y amorphous alloys with significant supercooled liquid region have attracted attention as a structural light metal. The future application of the new alloys seems to be dependent on the development of a technique of consolidating the amorphous powders to an amorphous bulk. The effects of applied pressure, pressing temperature and flow strength of an alloy on the densification of amorphous powders by uniaxial warm pressing were examined for an amorphous Mg65Cu25Y10 alloy with a wide supercooled liquid region of 67K.The amorphous alloy with a distinct glass transition phenomenon is easily consolidated near Tg, especially above Tg, and the minimum consolidation pressure required to obtain an amorphous bulk with true density is about four times as high as the flow strength of the alloy at the consolidation temperature. As the pressing temperature approaches Tg the effect of applied pressure on the densification decreases. The amorphous Mg65Cu25Y10 bulk with true density was obtained even at a small pressure of 0.2GPa, when the pressing temperature is 10K higher than Tg. The significant temperature dependence is attributed to the drastic decrease in the flow strength of the amorphous alloy by the glass transition.

Glass-to-liquid transition in zirconium and palladium based metallic glasses

Shear localization in metallic materials at high strain rates

Toward atomic-scale understanding of structure-dynamics-properties relations for metallic glasses

Owing to the unique properties, such as excellent mechanical performance, and nano-scale surface roughness, metallic glass can be applied in various novel fields. Meanwhile, the improvement of thermal stability and hardness is the common target in these applications. Minor alloying is an effective method to enhance the thermal and mechanical properties of metallic glasses. Different from elements used to be doped into metallic glass, the role nitrogen atoms play in metallic glass is quite distinct and critically important, owing to its strong electronegativity and small atomic radius. In this work, an alternative class of metallic glass is developed. By adding nitrogen into Zr-Cu-Al-Ag TFMG, the configuration energy of short range structure is greatly modified. From the viewpoint of thermal behavior, the evolution of Tg and elastic modulus with nitrogen could be well correlated, implying significant effect of nitrogen atoms on the potential energy landscape (PEL) of the TFMGs. Besides, more amounts of nitrogen addition lead to the increase of short range order structure in the amorphous matrix. That is, most metallic atoms are strongly attracted by nitrogen, forming the nitrogen-centered cluster, in which more energy is required to cause plastic deformation. On the other hand, a meta-stable state with lower energy is attained as indicated by the much higher Tx than normal Zr-based metallic glasses. However, the amorphicity region of nitrogen in Zr-Cu-Al-Ag metallic glass is not wide enough, restricting the enhancement amount of thermal and mechanical performances. By incorporating Ta firstly, the competing ZrN crystalline phase is destabilized, leading to the wider amorphicity region of nitrogen in Ta-Zr-Cu-Al-Ag TFMG. As a result, hardness over 10 GPa, Tg near 800 K, and supercool liquid region as wide as 112K is achieved in the nitrogen-doped Ta-Zr-Cu-Al-Ag TFMG.

Current understanding of the origin of icosahedral clusters or icosahedral short-range ordering in undercooled metallic liquids or glasses is based on Frank’s consideration of an isolated icosahedron whose core has lower potential energy than the shell. Using large scale atomistic simulations and statistical analysis of several bcc (body-centered-cubic) and fcc (face-centered-cubic) metals, here we show that the shells of icosahedrons spontaneously formed inside deeply undercooled metallic liquids or glasses in fact have lower (averaged) potential energy than the cores. The shell potential energy deficiency occurs only to the icosahedral clusters but not to the equilibrium-crystal clusters, and, for icosahedral clusters, this deficiency grows with decreasing temperature. Compared with fcc metals, bcc metals exhibit greater potential energy deficiency on the icosahedral shells and produce significantly more icosahedral clusters upon liquid quenching, which explains the higher tendency of bcc metals to be vitrified observed in ultrafast cooling experiments. Inspecting the potential energy deficiency on the icosahedral shells through computation provides a new avenue to the search for amorphous metals (i.e. metallic glasses) with high glass forming ability and processability.

We studied thermal stability and its relationship to the glass-forming ability (GFA) of the Ni62Nb38–xTax (x=5, 10, 15, 20, 25) bulk metallic glasses (BMG) from a kinetic point of view. By fitting the heating-rate dependence of glass transition temperature (Tgonset) and crystallization temperatures (Txonset and Txpeak) of the Ni62Nb38–xTax BMG using the Vogel-Fulcher-Tammann (VFT) equation, we obtained the ideal glass transition and crystallization temperatures (Tg0 and Tx0) and the fragility parameter (m), and also constructed continuous heating transition (CHT) diagrams for crystallization of the BMG. The CHT diagrams of the BMG indicate enhanced thermal stability by Ta addition; the Tg0 as well as the Tx0 also illustrates this improved stability limit. The compositional dependence of m, which agrees well with that of the reduced glass-transition temperature, indicates a strong correlation between liquid fragility and glass-forming ability in the present alloy system. These results provide new evidence for understanding thermal stability, liquid fragility, and GFA in BMG.

Amorphous metallic alloys, also called metallic glasses, are of considerable technological importance. The metastability of these systems, which gives rise to various rearrangement processes at elevated temperatures, calls for an understanding of their diffusional behavior. From the fundamental point of view, these metallic glasses are the paradigm of dense random packing. Since the recent discovery of bulk metallic glasses it has become possible to measure atomic diffusion in the supercooled liquid state and to study the dynamics of the liquid-to-glass transition in metallic systems. In the present article the authors review experimental results and computer simulations on diffusion in metallic glasses and supercooled melts. They consider in detail the experimental techniques, the temperature dependence of diffusion, effects of structural relaxation, the atom-size dependence, the pressure dependence, the isotope effect, diffusion under irradiation, and molecular-dynamics simulations. It is shown that diffusion in metallic glasses is significantly different from diffusion in crystalline metals and involves thermally activated, highly collective atomic processes. These processes appear to be closely related to low-frequency excitations. Similar thermally activated collective processes were also found to mediate diffusion in the supercooled liquid state well above the caloric glass transition temperature. This strongly supports the mode-coupling scenario of the glass transition, which predicts an arrest of liquidlike flow already at a critical temperature well above the caloric glass transition temperature.