Search

The liquid local structure of Au50Cu50 solid solution was detected by high-temperature X-ray diffraction experiment and Reverse Monte Carlo (RMC) simulation. The clusters in the liquid Au50Cu50 alloy comprise the 12-coordinated polyhedron with Au center, which was the same as the clusters in the liquid pure Au. In the case of alloying, there was a high population of Au-Au bonds, and the local structure around Cu atoms was changed. In the case of solidification, the 12-coordinated clusters around Au atoms were preserved into the AuCu alloy, forming the disordered solid solution structure. The strong tendency for Cu-Cu bonds was weakened from 2.35 Å in the liquid to 2.81 Å in the solid solution, and the local structure around Cu atoms rearranges. It is shown that the liquid structure of the Au50Cu50 alloy plays a crucial role in the solid solution. Our findings elucidate that the disordered solid solution structure in AuCu alloy stems from the highly dominated 12-coordinated clusters associated with centered Au atom in the melt.

A refractory ReTaWNbMo high-entropy alloy (HEA) with an equal atomic ratio prepared by rapid solidification displays a BCC crystal structure. After annealing the as-quenched samples at 673, 873, 1073, and 1273 K for 12 h, respectively, the crystalline phase still keeps its BCC structure, but the crystalline morphology transforms from coarse dendrites to fine equiaxed crystals together with gradual grain refinement. The intragranular regions are enriched in W and Ta, while the crystals in the intergranular regions are enriched in Nb, Mo, and Re. With the increase in annealing temperature, the yield strength increases, and the compressive plasticity and fracture strength are obviously enhanced. The failure mode gradually changes from transgranular to intergranular fracture. Furthermore, the corrosion behaviors of the as-cast ReTaWNbMo HEA and the annealed states in a 3.5 wt.% NaCl solution were also studied. The samples annealed at 1273 K exhibit the best corrosion resistance due to the elemental re-distributions within grains.

The structural order in metallic supercooled liquids and glasses is a long-standing issue. Unlike the medium-range order (MRO) which involves at least hundreds of atoms, short-range order (SRO) only concerns the central atom and its neighbors, which can be described more easily. Here, we use the molecular dynamics simulation to investigate the SRO in supercooled liquids and glasses of two pure metals Ta and Ni respectively with the body-centered cubic (BCC) and the face-centered cubic (FCC) crystalline lattice and two binary alloys Cu50Zr50 and Ni50Al50 both with a B2 crystalline phase. To compare, a ternary alloy of Cu47.5Zr47.5Al5 is also investigated. Hexagonal closed-packed (HCP) and icosahedron (ICO) clusters are found to dominate the SRO in both supercooled liquids and glasses. As expected, the glass is found more structurally ordered than the supercooled liquid. The glass transition is found to be linked to the kink of the temperature dependence of SRO. The binary system, as the relatively good glass-former, is less structurally ordered than the monatomic system as a poor glass-former, but it is more structurally ordered than the ternary one. These findings have profound implications for understanding the glass transition and designing new metallic glasses.

The dynamic mechanical properties of the AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) fabricated by drop-casting (DC) and suction-assisted casting (SC) are investigated under different strain rates and temperatures using Split Hopkinson Pressure Bar (SHPB) system. Microstructure analysis is conducted through SEM, EBSD and TEM to reveal the deformation and fracture mechanisms of the present EHEA. The yield strength and flow stresses for both the DC alloy and SC alloy increase with increasing strain rate, but decrease with increasing temperature. A mixture of ductile and brittle fracture morphologies exists in both the DC and SC EHEAs. Moreover, the SC alloy displays more shear dimples on the fracture surface, while brittle fracture features become dominant for the DC alloy. Abundant accumulation of dislocations in the quasi-static test produces slip traces in the FCC lamellar phase which further enhances the plasticity of the SC alloy. Meanwhile, the dislocation cells formed by high-density dislocation tangles at high strain rates, leading to apparent work hardening and strain rate sensitivity of the SC alloy. The adiabatic shear band (ASB) forms in the SC alloy before shear failure and the adjacent twisted and curved lamellar structures play a significant role in preventing shear localization. Overall, the SC alloy exhibits a stronger strain rate effect and better ductility than the DC alloy.

In this work, we investigated the effect of boron addition on glass-forming ability and mechanical properties of Co-Cr-Mo-C alloys. The starting alloy was (Co0.65Cr0.13Mo0.22)80C20 derived from the Co65Cr13Mo22 ternary eutectic composition. This alloy is almost fully crystalline and exhibits brittle mechanical properties. Replacing carbon with boron allows obtaining bulk metallic glasses (BMGs) and bulk metallic glass composites. The designed alloys show very high strength (∼4100 MPa), wide super-cooled liquid region (∼100 K) and large endothermic enthalpy of the supercooled liquid region (∼35 J/g), indicating high thermal stability. The electronegativity difference and supercooled liquid region of the present alloys are comparable to rare-earth elements containing BMGs. The physical properties of the present alloys corresponded well with the alloy design strategy based on the unified parameter using the glass transition temperature, fracture strength, and molar volume. Furthermore, we propose the optimum compositional condition for glass formation by controlling the carbon-to-boron ratio and the influence of carbon on phase formation in this alloy system is discussed.

In this work, the microstructure and mechanical properties of rapidly solidified Ti50−x/2Ni50−x/2Hf x (x = 0, 2, 4, 6, 8, 10, and 12 at.%) and Ti50−y/2Ni50−y/2Si y (y = 1, 2, 3, 5, 7, and 10 at.%) shape memory alloys (SMAs) were investigated. The sequence of the phase formation and transformations in dependence on the chemical composition is established. Rapidly solidified Ti-Ni-Hf or Ti-Ni-Si SMAs are found to show relatively high yield strength and large ductility for specific Hf or Si concentrations, which is due to the gradual disappearance of the phase transformation from austenite to twinned martensite and the predominance of the phase transformation from twinned martensite to detwinned martensite during deformation as well as to the refinement of dendrites and the precipitation of brittle intermetallic compounds.

Transformation-induced plasticity in CuZr-based bulk metallic glass (BMG) composites opens up a new perspective for the design of ductile BMG composites. In this work, rapid and partial crystallization of BMGs is proposed to tailor microstructures of ductile BMG composites. During the early crystallization stage, the precipitation of B2 CuZr crystals in the Cu42Zr42Al10Co6 BMGs is growth-controlled at low or high heating rates after having passed through a nucleation-dominated temperature region. Abundant nucleation is beneficial for the uniform precipitation of B2 crystals in the glassy matrix. Compared with annealing processes at slow heating, the degree of the free volume annihilation could be reduced to some content by employing a high heating rate. The microstructures of CuZr-based BMG composites can be effectively tuned by changing rapid annealing parameters, which enhances the ductility of the resulting BMG composites. This approach proves to be efficient for the development of ductile metastable alloys and composites. Keywords: Metallic glasses, Crystallization, Annealing, Composites, Deformation

Dynamic mechanical relaxation processes are very important to understand physical and mechanical properties of metallic glasses. In the current work, dynamic mechanical behaviors of La43.6Ce18.4Ni24Al and Cu46Zr46Al8 metallic glasses were investigated by mechanical spectroscopy. Unlike Cu46Zr46Al8 metallic glass, La43.6Ce18.4Ni24Al14 glassy system shows an evident slow β relaxation process. In addition, physical aging below the glass transition temperature Tg induces a reduction of intensity of the β relaxation. Evolution of the concentration of “defects” of metallic glasses was analyzed in the framework of Interstitialcy theory. Our research provides a new insight on understanding of the correlation between the structural heterogeneity and β relaxation of metallic glasses.

Microstructural heterogeneity is an important issue in understanding the mechanical and physical properties of metallic glasses (MGs). Theoretically, physical aging below the glass transition temperature relax the MGs systems to a lower potential energy state on the potential landscape (PEL) while the cold rolling rejuvenates MGs to a higher potential energy state. In the current work, we systemically turned the structural heterogeneity through dynamic mechanical relaxation and rejuvenation. Correlations among the structural heterogeneity, internal friction, and shear transformation zones (STZs) of the model alloy was connected. Based on the cooperative shear model and statistical analysis, the scale variation of the STZs was revealed. This study demonstrated that the spatial structural heterogeneity can be enhanced by severe plastic deformation which can reduce the energy barrier for STZs activation. And we proposed the barriers decrease and more nucleation sites of STZs from physical aging to rejuvenation, which may be potential mechanism for improving the plasticity in MGs.

The Arrhenius to non-Arrhenius crossover of the structural relaxation dynamics, happened at TA when cooling the metallic melts, has been widely studied, but the underlying structural evolution is still an open issue. Through molecular dynamics (MD) simulations combined with the topological order analysis of atomic structure, we investigated the evolution of the hidden structural topological features associated with the non-Arrhenius crossover. It was found that the average spatial connectivity degree k of the characteristic slow atoms approaches 3 at TA in various metallic glass-forming liquids (MGFLs), corresponding to the constant dynamic heterogeneity. The non-Arrhenius behavior below TA is attributed to the cooperative motions of atoms with k > 3 by developing a stable topological network. Our findings shed light on the structural origin of the relaxation dynamics in MGFLs.

Large crystal growth of Cr-doped h-YMnO3 has been investigated by using a high pressure optical floating-zone method. The size of the grown crystals is typically 60–70 mm in length and 4–5 mm in diameter. The structure of the grown crystals is analyzed by powder X-ray diffraction and scanning electron microscopy. The defects in the as-grown crystals, including low-angle grain boundary and inclusions are studied. An off-stoichiometric phenomenon is found with a slight Cr deficiency in different parts. The relationship between defects and growth conditions during crystal growth is also discussed. The magnetic properties show spin-glass phase features with weak ferromagnetic behavior below 30 K.

The influence of Nb substitution for Fe on the precipitation of α -Fe, the glass forming ability (GFA) and the magnetic properties of the (Fe 85− x B 10 P 5 Nb x ) ( x = 0, 1, 3, 4 and 5 at.%) alloys are investigated. Based on the XRD measurement of the melt-spun ribbons, it is found that the dominant crystal plane in α -Fe precipitates gradually changes from (200) to (110) and then only broad peaks without any appreciable crystalline peaks can be observed with increasing Nb content from 0 at.% to 5 at.%. Such microstructural evolution are clarified with other elements M (M = Ni, Mo, Y, Cr and Si) addition, and can be well explained based on the Bravais-Friedel law and the negative mixing enthalpy between Nb(or M) and Fe (or B). Furthermore, the strong bonding between Nb and Fe atoms results in the formation of FeNb clusters and then enhances short-range-order clusters, leading to the enhance of GFA and decrease of coercive force from 13.82 A/m to 2.3 A/m with the addition of Nb content less than 4 at.%. With further increasing Nb content, the coercive force becomes higher, which is contributed to the formation of NbB clusters and the poor solute-solute avoidance.

We investigate the effect of interfacial anisotropy on the growth direction selection during directional solidification of alloys by using the thin-interface phase-field model. A convergence study with respect to the coupling constant λ is carried out for the tilted growth of dendritic arrays with different values of anisotropic to choose proper λ in simulations. The influence of the artificial noise at the interface on the growth direction selection is discussed. By analyzing the data from two-dimensional phase-field simulations, we discuss the dependence of the coefficients f and g in DGP law (Deschamps et al., 2008) on anisotropic strength e 4 for a wide range of misorientation angle Θ 0 in order to extend the DGP law. Results confirm that the coefficient f can be expressed as f ( Θ 0 , e 4 ) ≡ α ( e 4 ) χ ( Θ 0 ) , where α ( e 4 ) is an increasing function of e 4 and χ ( Θ 0 ) solely depends on Θ 0 with a constant coefficient β . Meanwhile, g ( e 4 ) is a decreasing function of e 4 , which can be modeled by a power-law function. Moreover, we comment on the influence of the pulling velocity on the growth direction selection for a wider range of the pulling velocity.

A nanometer-scale second phase in metallic glass (MG) heterostructures is effective to improve mechanical properties. In this work, molecular dynamics simulations are conducted to investigate the influence of various critical structural aspects such as the size/volume fraction, distribution of a nanoscale secondary phase and different combinations of the matrix and the secondary phase on the deformation behavior of dual-phase MGs. We find an obvious change in deformation mode with varying the size/fraction and the chemical composition of the secondary phase. When the yield stress of the dual-phase MGs is lower than critical shear stresses required for forming a mature shear band (SB), the MGs show homogeneous deformation. Otherwise, those dual-phase MGs with inclusions smaller than width of the SB or that cannot confine plastic zones between the larger inclusions have high tendency for shear instability and brittle failure. By systematically varying the characteristics of the secondary phase one can design MGs with improved mechanical properties. Keywords: Metallic glasses, Molecular dynamics, Dual-phase metallic glasses, Deformation, Ductility

In this paper, the effect of severe plastic deformation (SPD) induced by high-pressure torsion (HPT) and isothermal heat treatment (HT) below the glass transition on the structure-property relationships of Cu46Zr46Al8 bulk metallic glass (BMG) is assessed separately and alternately in reference to the as-cast state. Pronounced modifications are confirmed by X-ray diffraction, differential scanning calorimetry, dynamic mechanical analysis (DMA), dilatometry analysis, and scanning electron microscopy. Diffusion-controlled crystallization is achieved via HT with optimized parameters, and HPT is used afterwards to disperse crystalline phases relatively homogenous throughout the matrix. The dispersion of the second phase improves the thermomechanical stability around the glass transition temperature, as confirmed by DMA. Compared to a single HT process, a sequential combination of HT + HPT leads to a glass showing a linear hardness profile with values higher than the as-cast state. This study provides insights into possible processing routes for enhancing the mechanical and/or thermal properties of CuZr-based BMGs, which can also be used to tailor new design routes for other BMG systems.

The martensitic transformation within B2 phase plays a critical role for the ductility of CuZr-based bulk metallic glass (BMG) composites, yet the underlying mechanism for the martensitic transformation process needs further investigations. Here, we found that the onset temperature of the thermally-activated martensitic transformation for CuZr-based BMG composites was reduced to 162.7 ± 2.5 K, which is far lower than those of polycrystalline samples, due to the stabilization of the austenitic phase induced by crystalline-amorphous interfaces. In situ high-energy X-ray diffraction measurements (HEXRD) further demonstrate that the thermally-activated martensitic transformation is governed by an extremely incomplete martensitic transformation from B2 to B19’ and B33 phases without the decomposition of B2 phase which generally appears in polycrystalline samples. Due to unconfined stress states during the thermally-activated martensitic transformation, B33 phase forms more early during thermal process than deformation process. The present study provides a further understanding on the martensitic transformation behaviors of shape memory type CuZr-based BMG composites.

In this paper, we present a thorough stress analysis of the Cu Zr metallic-glass composite with embedded B2 particles subject to a martensitic transformation. Within the framework of the Eshelby theory, we are able to explain, in a quantitative manner, (1) the formation of three types of shear bands with distinct morphologies as observed experimentally in the severely deformed Cu Zr metallic-glass composite and (2) the work hardening ability of the Cu Zr metallic-glass composite as related to the coupled effects of elastic back stress and elastic mismatch caused by the martensitic transformation. Furthermore, we also discuss the issues about the stress affected zone of the individual B2 phase and the stability of the crystalline–amorphous interface. Given the general agreement between the theoretical and experimental findings, we believe that the outcome of our current work can lead to a deeper understanding of the transformation-induced plasticity in the Cu Zr based metallic glass composites, which should be very useful to the design of the metallic-glass composites with improved ductility.

The glass-forming ability (GFA) of (Cu 0.5 Zr 0.5 ) 100-x Zn x (x = 1.5, 2.5, 4.5, 7, 10, and 14 at %) metallic glasses (MGs) was studied. With increasing Zn content, the GFA of CuZr-based MGs gradually increases, enabling the fabrication of Cu–Zr–Zn bulk metallic glasses (BMGs) and metallic glass matrix composites. The enhancement of GFA with Zn addition to the CuZr binary alloy is well explained by considering topological instability and electronic criteria. Upon annealing of Cu–Zr–Zn MGs Cu 10 Zr 7 and subsequent CuZr 2 crystals precipitate in the glassy matrix, being followed by the nucleation of B2 CuZr crystals. It is shown that Zn addition can effectively stabilize B2 CuZr phase to lower temperatures. Analysis of the crystallization kinetics of Cu–Zr–Zn MGs under isothermal conditions shows that the dominated crystallization mechanism is the interface-controlled two- or three-dimensional growth with different nucleation rates.

This paper investigated the effect of coupling of direct current (DC) and pulling rate on dendrites and cooling behaviours of directionally solidified Pb–50Sn alloy. Experimental results indicated that the secondary dendritic arm spacing (SDAS) decreased and temperature gradient increased as increasing current densities. Moreover, temperature rise and SDAS under positive DC were higher than those under negative DC. It was speculated that Joule heating and electromigration were obviously induced by the present DC. The effect of DC on the microstructure and solidification parameters was weakened as the pulling rate increases. The coarsening rate reduced from tf1/3 toward a value of tf0.29 when DC of ±200 A cm–2 were applied. The refinement mechanism of SDAS was discussed.

The refinement mechanism of alternating current pulse (ACP) on the solidification macrostructures of pure Al and the characterization of refining efficiency were investigated by embedding the wire mesh in the mold. The experiment results showed that ACP treatment during solidification led to the formation of fine equiaxed grain. There were remarkably differences with respect to the area of fine equiaxed zone inside and outside the tube. Lorentz force, induced melt flow and the rest of intrinsic effects of ACP inside and outside the tube were discussed in the present study. It demonstrated that the forced melt flow could lead to the columnar fragmentation and make the crystal nucleus on the mold wall fall off and drift in the liquid, leading to grain refinement. In addition, Reynolds number was suitable to characterize the refining efficiency of pure Al under ACP.

The effects of minor Ti addition on the thermal stability of B2 CuZr phase, the microstructure and the martensitic transformation (MT) in Cu 50 Zr 50−x Ti x (x = 0, 2.5, 7.5 and 10 at%) alloys were investigated. It was found that the crystallization products, i.e. Cu 10 Zr 7 and Cu(Ti, Zr) 2 , of Cu–Zr–Ti amorphous alloys transform to B2 CuZr phase at high temperatures. The corresponding eutectoid transformation temperature gradually increases with increasing Ti content, implying the decrease of the thermodynamic stability of the B2 CuZr phase. The microstructures of Cu–Zr–Ti martensitic alloys were proven to contain B2 CuZr, CuZr martensite, Cu 10 Zr 7 , Cu(Ti, Zr) 2 , and ZrTiCu 2 crystals. Dilatometric measurements reveal that the MT temperature reduces with increasing Ti content, which would be of electronic nature. With increasing thermal cycles, the MT temperature gradually decreases while the reverse MT temperature increases, which results from the enhancement of the dislocation density, the partial decomposition of the equiatomic CuZr crystals and the partially reversible MT.

The mechanical properties of high-entropy alloys (HEAs) strongly depend on their microstructural features. In this work, the phase formation and morphologies of Fe26.7Co26.7Ni26.7Si8.9B11.0 HEAs fabricated under different undercooling levels by the drop tube technique together with subsequent quenching were investigated in detail. It was found when the droplet size decreases (i.e. the increase of the undercooling level), the grain refinement is easily achieved without any phase transformation for the present hypoeutectic HEA which is composed of the fcc dendrites and eutectic structure. After quenching under large undercooling levels, the liquid should be deeply undercooled into an asymmetric couple zone, leading to the primary formation of eutectic structure and the subsequent precipitation of the fcc dendrites. The direct product of rapid solidification after deep undercooling and subsequent recalescence may induce the decoupled eutectic growth, resulting in a transition from the regular into anomalous eutectic structures. Furthermore, the volume fractions of the eutectic structures and the fcc dendrites increase and decrease with decreasing droplet size, respectively. As a result, the Vickers hardness gradually increases with decreasing droplet size, which exhibits a linear relationship with the volume fraction of eutectic matrix. The present study may provide a simple way to tailor microstructures and mechanical properties of HEAs.

Co–Cr based alloys are recognized as one of the most widely used category of alloys amongst the biomedical materials. This is because these alloys show a high strength, corrosion resistance and good biocompatibility suitable for biomedical applications. Many researchers have been focused on the processing of these alloys using a selective laser melting process. The effects of different processing parameters on the obtained microstructural and mechanical properties of the processed parts have been investigated in these studies. However, the effect of powder distribution size (powder granulometry), as a primary processing parameter, on the microstructural and mechanical properties of Co–Cr alloys is not well investigated and studied so far. This study investigated the influence of this parameter, by using powders with the same composition and morphology but a different particle size of coarse and fine, on the microstructural and mechanical properties of as-built, homogenizing heat treated and homogenizing heat treated followed by subsequent aging treatment SLM fabricated Co28Cr9W1.5Si (wt.%) samples. X-ray diffraction, scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy were used to characterize and identify the microstructure, phase composition and crystallographic information of the manufactured samples. Besides, the mechanical properties of samples, processed using powders having different size distributions, were measured using a tensile test machine and results were correlated to the microstructure of the samples. It was concluded that the powder granulometry has a negligible effect on the microstructural and mechanical properties of the as-built SLM Co–Cr–W–Si parts but a remarkable influence on those of the heat-treated ones.

Development of the two-phase bulk metallic glasses (BMGs) is essentially retarded due to difficulties in finding of phase-separated (immiscible) alloys with high glass-forming ability (GFA) of coexistent phases. Referring to the concept of solute partitioning and minimization of free energy, in this work we present an idea that a metallic liquid system containing two liquids with individual self-assembled eutectic composition may yield two-phase BMGs upon casting. The formation of the two-glass structure is discussed and a strategy of partial substitution of chemically similar elements for overcoming the drawback of low GFA is proposed. A family of two-phase zirconium–lanthanide based metallic glasses in bulk form is developed. The mechanical behavior of the two-phase BMGs with different ratio of zirconium-rich to lanthanide-rich glassy phase is studied for the first time. This work provides a new concept for fabrication of two-phase BMGs and reveals the role of constituent phases in determining the mechanical properties of the whole glass.

In this paper, the thermal stability, the crystallization kinetics and magnetic properties of the melt-spun Co40Fe22Ta8B30 glassy alloy were investigated. The results show that the present alloy has a wide supercooled liquid region of 74 K and a high incubation time prior to crystallization upon isothermal annealing above the glass transition temperature compared to the well-known Co43Fe20Ta5.5B31.5 glassy alloy. The crystallization kinetics was systematically investigated by the Kissinger, Ozawa and isoconversional approaches under the isochronal condition, and Johnson–Mehl–Avrami (JMA) model under the isothermal situation, respectively. It was found that the Co40Fe22Ta8B30 metallic glass indicated an apparent activation energy of 520 ± 5 kJ/mol corresponding to the first crystallization event, which is higher than many other Fe-based and Co-based glassy alloys. The Avrami exponent calculated based on the JMA approach lies in the range of 2.13–1.58 for different isothermal annealing temperatures, revealing that the crystallization is governed by the three-dimensional diffusion-controlled growth with a decreasing nucleation rate. In addition, Co40Fe22Ta8B30 metallic glass exhibits good soft magnetic properties, i.e. a very low coercivity of 0.8 A/m and a magnetization of 42 Am2/kg. The influence of the devitrification on the evolution of the coercivity, magnetization and thermomagnetic behavior of Co40Fe22Ta8B30 glassy alloy was investigated.

Three different Cu–Zr–Co alloys, namely Cu 40 Zr 37.5 Co 22.5 , Cu 42.5 Zr 45 Co 12.5 and Cu 49 Zr 49 Co 2 , were obtained by rapid cooling. The microstructure and phase formation of as-cast rods with diameters of 2 mm are compared with those of the respective ingots. An increasing Co content stabilises the B2 CuZr phase and leads to the precipitation of a ternary Cu–Zr–Co phase. The variation of the cooling rate affects the size of the B2 dendrites as well as the volume fraction and the morphology of the interdendritic phases. The mechanical properties were determined in compression and all alloys show a certain plastic deformability despite the presence of several binary and ternary intermetallic phases. The deformation mechanisms are discussed on the basis of the microstructures and the constituent phases.

cited By 0; Pronounced slow β-relaxation in metallic glasses (MGs) has been proven to be closely related to macroscopic plasticity especially around 0.6–0.8Tg. However, the micro-scale phase-separating (ZrxLa1-x)57Co27Al16 (x = 0.2, 0.3, and 0.4) MGs obviously display brittleness around 0.7Tg even though an obvious slow β-relaxation peak is observed based on the dynamic mechanical analysis (DMA). In order to probe their intrinsic brittleness, the slow β-relaxation processes of the (ZrxLa1-x)57Co27Al16 (x = 0, 0.2, 0.3, 0.4, and 1) MGs were observed together with their mechanical performances. The results demonstrated that the La57Co27Al16 and Zr57Co27Al16 MGs show obvious ductility around 0.7Tg. Based on DMA results, the La57Co27Al16 MG displays an evident slow β-relaxation peak while the Zr57Co27Al16 MG presents an excess wing. When the phase-separating Zr–La–Co–Al MGs consisting of La-rich and Zr-rich components were heated up to 0.7Tg, only the β-relaxation behavior originating from La-rich components rather than from Zr-rich components is preferentially thermal-activated. In addition, the slow β-relaxation is well described by the Cole-Cole (CC) model, which becomes more obvious with increasing phase-separating degree. Then only partial but not all of potential structural units in whole samples can be activated, leading to the whole intrinsic brittleness of the micro-scale phase-separating MGs. These findings suggests that the large compositional difference between components is harmful to design ductile phase-separating MGs. © 2019 Elsevier B.V.

The structure of the Guinier–Preston (GP) zones precipitated in the Al–Si matrix composites after natural aging is investigated. The results show that, after natural aging, spherical GP zones with a diamond crystal structure form in the supersaturated α-Al matrix produced by high pressure solidification. The lattice parameter of the GP zones is 1.62 nm and their structure consists of Si 3 atomic clusters. The GP zones disappear and Si crystals are formed in the α-Al matrix after heating to 673 K. The precipitation sequences in the supersaturated α-Al matrix can be summarized as: α-Al (SSSS)→GP zones→equilibrium Si phase.

The glass-forming ability and the thermal stability of Cu 45 Zr 45 Al 5 Ni 5 metallic glass were systematically investigated by the differential scanning calorimetry. The activation energies for the glass transition and the onset crystallization and the crystallization peak were calculated to be 255.1, 308.7 and 311.5 kJ/mol, respectively, while the corresponding liquid fragility was determined to be 23.4. By controlling the casting process, Cu 45 Zr 45 Al 5 Ni 5 bulk metallic glass composite with the presence of ductile B2 CuZr phase was fabricated. The composite shows a pronounced plastic strain of 6.8 ± 0.05% with obvious work hardening, which results from the formation of multiple shear bands and the deformation-induced martensitic transformation.

The effect of pre-existing shear bands induced by cold-rolling on the crystallization behavior was investigated in the Cu46Zr46Al8 bulk metallic glass. It was found that with increasing degree of pre-deformation, more shear bands are created resulting in the decrease of the crystallization activation energy for the alloy. Our experimental results demonstrate that pre-existing shear bands can promote the nucleation of crystals, which is discussed in terms of nucleation thermodynamics and kinetics aspects.

Starting from the brittle Cu46Zr46Al8 bulk metallic glass (BMG), the effect of substituting Al by Ti on the glass-forming ability (GFA) and the mechanical properties is systematically investigated. The supercooled liquid region (ΔTx), the reduced glass transition temperature (Trg=Tg/Tliq) and the γ parameter (γ=Tx/(Tg+Tliq)) tend to decrease with increasing Ti content in the present Cu46Zr46Al8−xTix (0≤x≤8) alloys. An increasing distance from the eutectic point might be the origin of the reduced GFA after the Ti addition. At the same time, the Ti additions significantly increase the plasticity of the respective alloys. The Cu46Zr46Ti3.2Al4.8 bulk glassy alloy is found to have the largest plastic strain in compression of around 12% and a fracture strength up to 1950 MPa while the Ti-free Cu46Zr46Al8 BMG exhibits values of 0% and 1754 MPa, respectively. The statistics of the serrated flow behavior, which is closely related with the dynamics of shear bands, are also investigated. The macroscopic and microscopic mechanisms, which contribute to the enhancement of malleability, are analyzed.

The glass-forming ability (GFA), the thermal stability and the crystallization behavior of Cu46Zr46Ag8 metallic glass and the thermal stability of the competing metastable B2 CuZr phase were investigated in order to fabricate the corresponding shape memory bulk metallic glass (BMG) composites based on a new prediction strategy. A rather large supercooled liquid region and a relatively low liquid fragility indicate a quite high thermal stability of the present glass. With the addition of Ag to Cu50Zr50 alloy, it becomes more difficult for the metastable B2 phase to decompose into the room-temperature equilibrium phases and to transform into martensitic phases during quenching. Based on these insights, a Cu46Zr46Ag8 BMG and its composites with different volume fractions of in situ precipitated B2 crystals were fabricated by carefully controlling the casting process. The BMG composites exhibit a larger plasticity and obvious work hardening compared to the monolithic BMG, which can be attributed to the formation of multiple shear bands and a deformation-induced martensitic transformation (MT) within the B2 crystals.

Based on thermal stability investigations, a new parameter has been introduced in order to evaluate the formation tendency of different sized CuZr-based metastable alloys with the precipitation of B2 CuZr [26] . To verify our prediction further, a Cu 46.25 Zr 44.25 Al 7.5 Er 2 alloy was prepared and its bulk metallic glass composites (BMGCs) with larger dimensions containing B2 CuZr precipitates were successfully fabricated by rapid quenching method. The composites exhibit macroscopically detectable compressive plastic strain and obvious work hardening, resulting from a deformation-induced martensitic transformation and the multiplication of shear bands. The plastic deformability of Cu 46.25 Zr 44.25 Al 7.5 Er 2 BMGCs is relatively lower than that of Cu 42.5 Zr 42.5 Al 5 BMGCs, most probably due to the intrinsic brittleness of Cu 46.25 Zr 44.25 Al 7.5 Er 2 metastable alloys and/or the inhomogeneous distribution of B2 CuZr in the amorphous matrix.

Significant tensile plasticity up to 0.7 ± 0.1% together with work-hardening and larger fracture strength was obtained in Cu 47.5 Zr 47.5 Al 5 bulk metallic glass (BMG) upon cold rolling with only 2.9 ± 0.3% thickness reduction. The good deformability could be attributed to the multiple pre-existing shear bands and structural inhomogeneity induced by rolling. The distributions of shear bands upon rolling can be predicted by a simplified rolling model. The underlying mechanism for the tensile plasticity was further discussed in the frame of potential energy landscape theory (PEL).

The compositional dependence of phase formation, thermal stability, and mechanical properties of (Cu0.5Zr0.5)100−x(Al0.5Ag0.5)x (x = 2, 4, 6, 8, 10, 12, 14, 16) bulk metallic glasses was studied. The Young’s modulus (85 ± 1 to 95 ± 1 GPa) and Vicker’s hardness (585 ± 7 to 627 ± 8 Hv) increased with increasing Al + Ag content from 8 to 16 at.%, respectively. The liquidus temperature decreased from 1210 ± 2 to 1110 ± 2 K with increasing Al + Ag content from 2 to 16 at.%. The starting temperature of the endothermic event related with transformation of the low-temperature equilibrium phases to CuZr parent phase increased from 997 ± 2 to 1043 ± 2 K, whereas the electronegativity difference for the (Cu0.5Zr0.5)100−x(Al0.5Ag0.5)x (x = 2, 4, 6, 8, 10, 12) alloys decreased from 0.2838 to 0.2713. The martensitic transformation temperatures decreased with increasing Al and Ag content for the (Cu0.5Zr0.5)100−x(Al0.5Ag0.5)x (x = 2, 4, 6, 8) alloys.

The Al–Ni–Ce-based amorphous alloys are presented to study compositional dependence of glass-forming ability (GFA), crystallization behavior, thermal stability, medium-range order (MRO) and liquid fragility. The GFA of Al87Ni8Ce5 and Al84Ni8Ce5X3 amorphous alloys ranks as follows: Al84Ni8Ce5Co3 > Al84Ni8Ce5Fe3 > Al84Ni8Ce5Mg3 > Al84Ni8Ce5Cu3 > Al87Ni8Ce5 > Al84Ni8Ce5Si3, which have the same tendency with the ratio of the prepeak area to the total area of the main peak in the X-ray intensity curve (Sprepeak/Stotal). That implies that the GFA has a good relation with the stable MRO structure. Better thermal stability (Ep1) and larger activation enthalpy for the glass transition (ΔH*) corresponds to larger fragility parameter (m). That reflects that the thermal stability of metallic glasses and the stability of supercooled liquids are attributed to the strong–fragile characteristics of glass-forming liquids.

The effect of Si substitution for Al on the glass forming ability (GFA) and thermal stability of melt-quenched Al 88− x Fe 6 La 6 Si x ( x = 0, 0.5, 1–4) alloys has been investigated systematically by using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and viscometer. Results show that the addition of Si enhances the thermal stability and GFA of Al–Fe–La alloys, and the alloy containing 1 at.% Si has the best thermal ability, but the GFA of which is not best, the alloy containing 2 at.% Si has the best GFA. However, when Si content is beyond 2 at.%, the addition of Si reduces the GFA and thermal stability of the alloys. It is also found that the addition of Si influences the crystallization behaviors of the alloys, transforming from two-step crystallization process to three-step crystallization process.

The primary crystallization kinetics of amorphous Al84Ni12Zr1Pr3 alloy has been studied using non-isothermal differential scanning calorimetry (DSC) measurements. The apparent activation energies for the two crystallization stages determined by the Kissinger and Ozawa methods are in good agreement. The local activation energies Ec(α) are determined by an isoconversional method and its average value is about 342.72 kJ/mol. Both DSC traces and X-ray diffractometry (XRD) patterns show that the first crystallization stage with the apparent activation energy of 384.6 kJ/mol, in which fcc-Al precipitates, is a rapid two- and three-dimensional diffusion-controlled nucleation and growth mechanism for bulk crystallization initially. Afterwards, it is mainly dominated by one-dimensional growth of Al crystallites at a near-zero nucleation rate for surface crystallization. The nucleation activation energy (En) and growth activation energy (Eg) of fcc-Al are about 200–230 kJ/mol and 360–388 kJ/mol, respectively. For the second transformation stage, the residual amorphous phase transforms into several intermetallic compounds with the apparent activation energy of 234.3 kJ/mol.

The effects of Ce and Mm contents on the glass forming ability (GFA) of melt-quenched Al 89− x Ni 8 Ce x Si 3 and Al 89− x Ni 8 Mm x Si 3 ( x = 0, 1, 3, 5, 7 at.%) alloys have been systematically investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). According to the XRD and DSC results, both Ce and Mm elements can enhance the GFA and thermal stability of the Al–Ni–Si alloys. Moreover, only the x = 5 and x = 7 alloys are totally amorphous in both systems quenched at the wheel speed of 36.6 m/s. Compared with amorphous Al 84 Ni 8 Ce 5 Si 3 alloy at different cooling rates, amorphous Al 84 Ni 8 Mm 5 Si 3 alloy has higher GFA which is considered to have relation to the different atomic structure of the amorphous alloy.

Metallic glass alloys of Al 84 Ni 12 Zr 4− x Pr x ( x = 0–3) were prepared by melt-spinning technique. Thermal stability and amorphous nature were investigated in Al 84 Ni 12 Zr 4− x Pr x ( x = 0–4) alloys with X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Results show that appropriate Pr addition promotes the GFA of this alloy system while declines the thermal stability. It also can be seen from the results that the total area of exothermic peaks shows good correlation with GFA. The less total area of exothermic peaks is, the better GFA of Al 84 Ni 12 Zr (4− x ) Pr x ( x = 0–3) system is.