Medium Range Ordered Research Articles

Medium-range order endows a bulk metallic glass with enhanced tensile ductility

Developing ductile bulk metallic glasses (BMGs) can benefit from an in-depth understanding of the structure-property relation during plastic deformation. However, endowing BMGs with tensile ductility in BMGs needs to reveal the response of critical structure units during deformation. Here, we report the experimental results of an in-situ synchrotron high-energy X-ray study of a Zr-based BMG under uniaxial tension after preprocessing by canning compression of the three-dimensional compressive stress state. It is revealed that the canning-compressed BMG (CC-BMG) sample has better tensile ductility and higher ultimate strength than the as-cast sample, which possesses heterogeneous and loosely packed local structures on medium-range scales. The experimental results revealed two stages of plastic deformation in the CC-BMGs compared with one stage of plastic deformation in the as-cast BMG. Moreover, the shift in the first sharp diffraction peak along the tension direction for the canning-compressed sample is substantially more pronounced than that of the as-cast sample. Furthermore, the real-space analysis illustrates a competition mechanism between the 2-atom and 3-atom connection modes on medium-range order during the plastic deformation of the CC-BMG. Additionally, the ordering on the medium-range scale decreases in the first plastic deformation stage but increases in the second plastic deformation stage. Therefore, a structural crossover phenomenon occurs in the CC-BMG during plastic deformation. Our results demonstrate a structure-property correlation for the CC-BMGs of heterogeneous medium-range ordered structures, which may be beneficial for endowing BMGs with ductility based on medium-range order engineering techniques.

Model for Medium-Range Order in Metallic Glasses

A new model of the medium-range ordered structure in metallic glasses is proposed. The molecular dynamics simulations revealed that there are two different types of basic clusters formed in metallic glasses. One is the icosahedral cluster (I-cluster) formed around the smaller-sized elements, and the other is the Z14, Z15, and Z16 Frank-Kasper clusters (Z-clusters) formed around the bigger-sized elements. The I- and Z-clusters are mainly connected by volume-sharing type connection and form a network extended to the whole glassy phase. Among them, the connection between Z-clusters corresponds to the dicsclination line proposed by Nelson. The topological nature of the network formed by the Z-clusters can be understood by the continuous random network model.

Effect of medium-range ordered nanoclusters on compression deformation of Mg65Cu25Y10 metallic glasses

Establishing the relationship between microstructures and mechanical properties of metallic glasses (MGs) is of great significance for guiding its application. Therefore, four groups of Mg65Cu25Y10 MGs with the same size were obtained by rapid solidification at four pressures by molecular dynamics (MD) simulation in this paper, and uniaxial compression were performed. The results show that the volumes of shear transformation zones (STZ) of four models increase sequentially. The average atomic potential energy increases, and the plasticity increases. The smaller the non-affine displacement (NAD), the higher Top-10 Topologically close-packed (TCP) and defective TCP clusters, which are the main structural units of MGs. Medium-range ordered (MRO) nanoclusters composed of their interconnections, are broken into small-sized nanoclusters during compression. Regions with large-sized MRO nanoclusters tend to be "stiffer", making these regions difficult for large-scale structural rearrangement and plastic flow. The deformation resistance of these regions is thus improved. Chain-like nanoclusters are easier broken during compression, and MRO nanoclusters linked with larger bond numbers can withstand greater strain for enhancing their plasticity. These findings have excellent theoretical guidance for promoting plasticity in MGs.

Flattening of isothermal magnetic entropy change versus temperature curve in amorphous Fe–Mn–Mo–B ribbons

Microstructure, Curie temperature and isothermal magnetic entropy change of the amorphous Fe70Mn10Mo5B15 alloy in the as-quenched state and after annealing within the amorphous state at Ta1=723K and after the accumulative annealing at Ta1=723K and then at Ta2=753K for 0.5 h have been studied. The transmission Mössbauer spectra recorded at 300K are typical of a paramagnetic amorphous alloy and do not reveal distinct changes on annealing. The spectra in all cases are decomposed into a set of symmetric quadrupole doublets ascribed to the amorphous phase and a single line stemming from medium range ordered (MRO) regions which are paramagnetic at room temperature. The volume fraction of such MRO regions is small and increases only slightly on annealing. The Curie temperature of the amorphous phase decreases from 298 K in the as-quenched state to 268 K after the annealing at Ta1 and then increases to 282 K after the accumulative heat treatment at Ta1 and Ta2 due to invar effect. The isothermal magnetic entropy change of the alloy exhibits the “caret like” behavior with the peak values near the Curie points of the amorphous phase. The composites consisted of two adherent ribbons with different mass fraction (α) subjected to the annealing and made of two ribbons in the as-quenched state and after the heat treatment at Ta1 show the flat maxima of the magnetic entropy change in the temperature spans 14K and 34K for α=0.7 and α=0.5 in the former and latter composite, respectively. Parameter α denotes the mass fraction of the ribbon annealed at Ta1 in the composites. The flat maximum of the magnetic entropy change is obtained for the composite made of two ribbons with the same chemical composition but different thermal history. The refrigerant capacity (RC) of the amorphous Fe70Mn10Mo5B15 alloy is about 115 J/kg in the as-quenched state and decreases slightly on annealing. RC is enhanced for the composites in comparison with their components.

Microstructure and some thermomagnetic properties of amorphous Fe-(Co)-Mn-Mo-B alloys

X-ray diffractometry, high resolution transmission electron microscopy (HREM) and transmission Mössbauer spectroscopy are used to study the microstructure and magnetic ordering in rapidly quenched amorphous Fe70-xCoxMn10Mo5B15 alloys with x = 0, 0.25, 0.5, 1 and 2. HREM images reveal the existence of medium range ordered (MRO) regions 2–4 nm in size in the as-cast alloys. The best fitting of the experimental, transmission Mössbauer spectra at 300 K is achieved applying the spectra decomposition into a set of symmetric quadrupole doublets and a single line for the paramagnetic at room temperature alloy (x = 0) or into a set of sextets and the single line for ferromagnetic alloys (x = 0.25, 0.5,1 and 2). The single line is described to the MRO regions which remain in paramagnetic state in the 77–300 K temperature range with small volume fraction in the ribbons. Within the experimental error the isomer shift of the single line for all alloys is the same indicating that MRO regions form similar paramagnetic phase independently of Co content. The magnetic properties of the studied alloys are determined mainly by the amorphous phases presented in the specimens. The Curie temperature rises with the increase of Co content and exhibits almost linear dependence on the average value of the hyperfine magnetic field induction. The maximum values of the magnetic entropy change occur near the ferromagnetic-paramagnetic transition temperature of the amorphous phase and are related to the Curie temperatures. An attempt to explain such feature is undertaken.

Influence of the Ag concentration on the medium-range order in a CuZrAlAg bulk metallic glass

Fluctuation electron microscopy of bulk metallic glasses of CuZrAl(Ag) demonstrates that medium-range order is sensitive to minor compositional changes. By analyzing nanodiffraction patterns medium-range order is detected with crystal-like motifs based on the B2 CuZr structure and its distorted structures resembling the martensitic ones. This result demonstrates some structural homology between the metallic glass and its high temperature crystalline phase. The amount of medium-range order seems slightly affected with increasing Ag concentration (0, 2, 5 at.%) but the structural motifs of the medium-range ordered clusters become more diverse at the highest Ag concentration. The decrease of dominant clusters is consistent with the destabilization of the B2 structure measured by calorimetry and accounts for the increased glass-forming ability.

Structural transition in sputter-deposited amorphous germanium films by aging at ambient temperature

The structure of amorphous Ge (a-Ge) films prepared by sputter-deposition and the effects of aging at ambient temperature and pressure were studied by pair-distribution-function (PDF) analysis from electron scattering and molecular dynamics simulations. The PDFs of the as-deposited and aged samples for 3–13 months showed that the major peaks for Ge-Ge bonds decrease in intensity and broaden with aging for up to 7 months. In the PDFs of a-Ge of molecular dynamics simulation obtained by quenching liquid at different rates, the major peak intensities of a slowly cooled model are higher than those of a rapidly cooled model. Analyses on short- and medium-range configurations show that the slowly cooled model includes a certain amount of medium-range ordered (MRO) clusters, while the rapidly cooled model includes liquid-like configurations rather than MRO clusters. The similarity between experimental and computational PDFs implies that as-deposited films are similar in structure to the slowly cooled model, whereas the fully aged films are similar to the rapidly cooled model. It is assumed that as they undergo room-temperature aging, the MRO clusters disintegrate and transform into liquid-like regions in the same matrix. This transition in local configurations is discussed in terms of instability and the non-equilibrium of nanoclusters produced by a vapor-deposition process.

Magnetic entropy change in amorphous and partially crystallized Fe–Mo–Cu–B alloy

Microstructure of the amorphous Fe76Mo10Cu1B13 ribbons in the as-quenched state and after the annealing at 723K for 0.5h is studied by transmission electron microscopy and Mössbauer spectroscopy. In the as-cast state α-Fe medium range ordered (MRO) regions are revealed and become the nuclei of crystalline grains. Nanograins 6nm in the average diameter are observed in the sample subjected to the heat treatment. Mössbauer spectrum at 300K of the annealed sample is decomposed into three subspectra ascribed to the amorphous paramagnetic and ferromagnetic phases and interface. At room temperature nanograins do not contribute to the spectra in the form of a single sextet due to magnetic relaxations. At 77K the single sextet can be evidently introduced and its hyperfine parameters indicate the existence of the α-Fe(Mo) crystalline phase. The Curie point (TC) of the amorphous phase in the annealed samples shifts from 277K in the as-cast state to 320K after the annealing. The maximum of the magnetic entropy change (−ΔSM) in the as-quenched state occurs at temperature around TC of the amorphous phase and distinctly decreases after the annealing and shifts towards higher temperature but lower than TC of the amorphous remainder. −ΔSM in the superparamagnetic temperature range, i.e. above the Curie temperature of the amorphous phase obeys the phenomenological relation: −ΔSM=a(T)Bm/T+bB.m2/(T−Θ)2. The excellent |ΔSM|×Bm−2=f(Bm−1) linear dependences for both, as-quenched and annealed samples are observed.

Superparamagnetic behavior and magnetic entropy change in partially crystallized Fe–Mo–Cu–B alloy

The microstructure and some thermomagnetic properties of the amorphous Fe76Mo10Cu1B13 alloy in the as‐quenched state and after annealing at 713 K for 0.5 h are studied. High‐resolution electron microscopy reveals the existence of medium‐range ordered (MRO) regions 1–2 nm in size within the amorphous phase that become the nuclei of the grains during primary crystallization at 713 K for 0.5 h. The α‐Fe grains with average diameter of 6 nm and mean distance between them of 26 nm behave like an assembly of noninteracting, single‐domain superparamagnetic particles. This is confirmed by the Mössbauer spectroscopy, magnetization versus temperature and magnetizing field measurements above the Curie point of the amorphous remainder. The partial crystallization of the Fe76Mo10Cu1B13 alloy is accompanied by giant magnetic hardening. The maximum of the magnetic entropy change () in the as‐quenched state occurs at temperatures around the Curie point of the amorphous phase and equals to 0.88 J kg−1 K−1. After the annealing a threefold decrease of is observed with the flat maximum in the temperature range of 280–310 K and this may be attributed to the magnetic hardening. The magnetic entropy change depends on both the temperature and the maximum of magnetizing field induction according to . For the annealed samples the exponent n is close to 1 below the Curie point of the amorphous phase and does not reach 2 above these temperatures. Such behavior is ascribed to the α‐Fe granular phase embedded in the amorphous remainder after annealing at 713 K for 0.5 h. In this case, the material can be treated at least as biphasic one.

Microstructure and Magnetic Entropy Change in Amorphous <inline-formula> <tex-math notation="TeX">${\rm Fe}_{76}{\rm Mo}_{10}{\rm Cu}_{1}{\rm B}_{13}$ </tex-math></inline-formula> Alloy

Microstructure and some thermomagnetic properties of the amorphous ${\rm Fe}_{{76}}{\rm Mo}_{{10}}{\rm Cu}_{{1}}{\rm B}_{{13}}$ alloy in the as-quenched state and after annealing at 623 and 673 K are studied. High-resolution electron microscopy, apart from the amorphous phase, reveals the existence of medium range ordered (MRO) regions 1–2 nm in size. Transmission Mossbauer spectroscopy confirms that these regions are $\alpha$ -Fe MRO ones and grow during annealing. The Curie point of the alloy is 277, 272, and 284 K in the as-quenched state and after mentioned above heat treatments, respectively. The maximum of the magnetic entropy change is rather modest and equal to $0.88~{\rm J}\cdot{\rm kg}^{{-1}}\cdot{\rm K}^{{-1}}$ in the as-quenched state and reaches the value of $1.12~{\rm J}\cdot{\rm kg}^{{-1}}\cdot{\rm K}^{{-1}}$ after annealing at 673 K. Magnetic entropy change depends not only on the temperature, but on the maximum of the magnetizing flux density, ${{B_{m}}=\mu_{0}{H_{m}}}$ (according to ${{\Delta}}{S_{M}}={C(T)}\cdot{B_{m}^{{n}}}$ ) as well. The exponent ${n}$ is close to 1 below the Curie point and does not reach 2 in high and equal to 2 in low magnetizing flux density range above the Curie point like in Curie–Weiss paramagnets. Such behavior is ascribed to the presence of MRO regions beside the amorphous phase and the material can be treated as biphasic.

Local chemical and topological order in Al–Tb and its role in controlling nanocrystal formation

How the chemical and topological short- to medium-range order develops in Al–Tb glass and its ultimate effect on the control of the high number density of face-centered-cubic-Al (fcc-Al) nuclei during devitrification are described. A combined study using high-energy X-ray diffraction (HEXRD), atom probe tomography (APT), transmission electron microscopy and fluctuation electron microscopy (FEM) was conducted in order to resolve the local structure in amorphous Al90Tb10. Reverse Monte Carlo simulations and Voronoi tessellation analysis based on HEXRD experiments revealed a high coordination of Al around Tb atoms in both liquid and amorphous states. APT results show Al-rich and Al-depleted regions within the as-quenched alloy. A network structure of Tb-rich clusters divides the matrix into nanoscale regions where Al-rich clusters are isolated. It is this finely divided network which allows the amorphous structure to form. Al-rich regions are the locus for fcc-Al crystallization, which occurs before the intermetallic crystallization. FEM reveals medium-range ordered regions ∼2nm in diameter, consistent with fcc-Al and trigonal-like Al3Tb crystal structures. We propose that the high coordination of Al around Tb limits diffusion in the intermetallic network, allowing for the isolated Al-rich regions to form at high density. These regions are responsible for the extremely high density of Al nanocrystal nuclei.

Medium range ordering and its effect on plasticity of Fe–Mn–B–Y–Nb bulk metallic glass

Glass formation, thermal properties and mechanical behavior were investigated in a series of Fe65− x Mn13B17Y5Nb x (x = 0, 3, 5 and 7) alloys. Appropriate partial substitution of Fe by Nb in a Fe65Mn13B17Y5 bulk glassy alloy simultaneously enhances the glass-forming ability and the mechanical properties of the alloys. The Nb-containing glassy alloys exhibit an additional exothermic event in the supercooled liquid region. This exothermic reaction is correlated with the formation of medium range ordered (MRO) clusters originated from the large difference in mixing enthalpy between constituent elements such as Nb–Y (+30 kJ/mol) and Nb–B (−39 kJ/mol). The presence of MRO clusters in the as-cast state is dependent on the cooling condition from the liquid state, and plays a crucial role in providing plasticity in as-cast Fe–Mn–B–Y–Nb bulk metallic glass.

Medium range ordering and some magnetic properties of amorphous Fe 90Zr 7B 3 alloy

High-resolution electron microscopy (HREM) reveals in the as-quenched Fe 90Zr 7B 3 alloy the existence of medium range ordered (MRO) regions 1–2 nm in size. Transmission Mössbauer spectroscopy confirms that these regions are α-Fe MRO ones. Above the Curie point of the amorphous phase ( T C =(257±2)K) they behave like non-interacting superparamagnetic particles with the magnetization decreasing linearly with the temperature. For these particles the average magnetic moment of 390μ B and the average size of 1.7 nm, in excellent agreement with HREM observations, were estimated. The maximum of the isothermal magnetic entropy change at the maximum magnetizing field induction of 2 T occurs at the Curie temperature of the amorphous phase and equals to 1.05 Jkg −1 K −1. The magnetic entropy changes exhibit the linear dependence on the maximum magnetizing field induction in the range 0.5–2 T below, near and above T C . Such correlations are attributed to superparamagnetic behavior of α-Fe MRO regions.

Nanocrystallization of complex Fe 23B 6-type structure in glassy Fe–Co–B–Si–Nb alloy

Local structural development in the bulk metallic glass (Fe 0.5Co 0.5) 72B 20Si 4Nb 4 on annealing has been investigated using transmission electron microscopy. Nanoscale crystalline grains with the Fe 23B 6-type structure were densely formed during annealing around the first crystallization temperature. In the state prior to the crystallization (below T g), we found extended medium range ordered (MRO) regions as small as 2 nm with an extremely high density in the glass matrix. The nanocrystallized microstructure is presumably ascribed to the presence of the dense MRO regions. On the basis of nanobeam diffraction analysis, however, it was found that most of the MRO regions have no clear order of the Fe 23B 6-type structure. The clear Fe 23B 6 order was found in regions extending as large as 5 nm at a higher temperature (around T g). A relationship between the structural change and the glass stability is discussed.

Metastability studies in silicon thin films: from short range ordered to medium and long range ordered materials

The effects of disorder on the kinetics of creation of metastable defects on silicon thin films have been studied through the evolution of the dark conductivity, the photoconductivity and the subgap absorption. Hydrogenated amorphous silicon, representative of short range ordered materials has been compared to polymorphous silicon (medium range order) and to microcrystalline silicon (long range order). Changing from short range ordered to medium range ordered materials results in a faster kinetics of creation of metastable defects as well as to films with better properties in both as-deposited and light-soaked states. Microcrystalline silicon films with crystalline fractions above 60% present stability and an improvement of their properties during light-soaking.

APFIM Studies of Nanocrystallizations of Amorphous Alloys

AbstractThis paper reports recent atom probe analysis results of Fe-Zr-B(-Cu) and Al-Ni-Ce(-Cu) amorphous alloys, in which nanocrystalline microstructures develop by primary crystallization. In these alloy systems, enrichment of slow diffusing solute was found at the interfaces between primary crystals and amorphous matrix during the nucleation and growth stage. In the case of ternary Fe-Zr-B, no evidence for compositional heterogeneities were found prior to the onset of crystallization reaction. On the other hand, clustering of Cu atoms was observed in quaternary Fe-Zr-B-Cu alloy prior to the crystallization reaction. In the ternary Fe-Zr-B alloy, nucleation sites seem to be provided by the quenched-in nuclei which were observed as medium range ordered (MRO) domains by HREM. In the as-quenched Al-Ni-Ce(-Cu) alloy, compositional fluctuations were present from the as-quenched state. These observations suggest that nuclei for primary crystallization are provided in various forms such as MRO domains, solute clusters and compositional heterogeneities.