Papers by Keyword: Bulk Metallic Glass

Abstract: Ca-Mg-Zn bulk metallic glasses (BMGs) are promising biomaterials for orthopaedic applications because when they get reabsorbed, a retrieval surgery is not needed. In this study, Ca-Mg-Zn metallic glasses with different compositions, Ca_56.02Mg_20.26Zn_23.72 and Zn_50.72Mg_23.44Ca_25.84, were fabricated by induction melting followed by copper mould casting. Their degree of crystallinity was modified by annealing, obtaining exemplar specimens of fully amorphous, partially amorphous (i.e., a BMG composite (BMGC)) and fully crystalline alloys. The microstructure, thermodynamic and corrosion performance of these alloys were evaluated as well as their electrochemical behaviour. The results of polarisation tests demonstrate that the corrosion resistance of the Zn-rich alloy is markedly better than the Ca-rich BMG. Corrosion rates of these Ca-and Zn-rich alloys with different degrees of crystallinity illustrate that the corrosion behaviours of alloys strongly depend on their microstructure, which shows a positive correlation between the corrosion current density and the crystallised volume fraction of the alloy. This study aims to shed light on the impact of the amorphicity-to-crystallinity ratio on the multifunctional properties of BMGs/BMGCs, and to assess how feasible it is to fine-tune those properties by controlling the percentage of crystallinity.

Abstract: Since the first synthesis of bulk metallic glasses (BMGs) by copper mold casting in 1990, much effort has been devoted to the searching of new BMG composition, the clarification of fundamental and engineering properties for BMGs and their industrialization. At present, BMGs have been formed in a large number of multicomponent alloy systems where the empirical three component rule is satisfied. Nowadays, commercialized BMGs are classified to Zr-based and Fe-based alloy groups. When we look at the industrialization of Zr-Al-Ni-Cu-based BMGs, the first commercialization was made for golf clubs in Japan in 1998, followed by watch parts etc. Since then, Zr-based BMGs have been used continuously up to 2013, though their application scale was in a limited state. Since 2014, the application scale was significantly extended in collaboration with the rapid developments of smartphones and electric vehicles. At present, the mass production facilities for Zr-based BMGs have been significantly developed and variety of BMG products have been produced. On the other hand, Fe-based soft magnetic BMGs were found in 1995. Their BMGs have also been used on a huge number of pieces in various kinds of electronic-magnetic instruments. These recent application states for Zr- and Fe-based BMGs are introduced together with new nanocrystalline Fe-based soft magnetic alloys developed through the derivation of alloy composition from Fe-based BMGs.

Abstract: Amorphous alloys are promising structural materials because of their high mechanical strength. Their drawbacks are low ductility and severe size restrictions for components obtained by casting. Additive manufacturing by selective laser melting (SLM) consists in successive fusion of small portions of material to add to a growing part. Each portion is subjected to a short thermal cycle favorable for amorphization. Thus, one can build an amorphous part as large as necessary. We study the microstructure of cast samples of a zirconium-based bulk metallic glass after laser processing with the parameters typical for SLM. Scanning electronic microscopy has shown partial crystallization in the heat affected zone of the laser beam. The spatial distribution and the volume fraction of the crystalline phase can be controlled by laser processing parameters. The obtained amorphous-crystalline structures are promising to increase the ductility of bulk metallic glasses.

Abstract: Bulk metallic glasses (BMGs) are an important class of materials with unique set of properties. A bulk metallic glass with composition of (Fe_0.6Co_0.4)₇₁Nb₄Si₅B₂₀ was cast in the form of a 1 mm thick strip in a water cooled copper mold. The BMG produced was characterized for structure, thermal and mechanical properties. The X-ray diffraction performed on the as cast alloy has shown completely amorphous structure. The glass transition and crystallization peak temperatures obtained through differential scanning calorimetry scan were 542 °C and 588.4 °C, respectively. Some cast amorphous alloy sample was annealed below glass transition (450 °C for 30 mi93nutes) and others above glass transition (580 °C for 5 minutes) temperatures. Nano- indentation hardness of 13.3 GPa was obtained for as cast alloy while a hardness values of 12.8 and 15.84 GPa were measured for heat treated alloys at temperature of 450 °C and 580 °C, respectively. Increase in hardness was attributed to formation of crystals in an amorphous matrix whereas decrease in hardness was due to relaxation of quenching residual stresses. The maximum value of elastic modulus obtained through indentation was 255 GPa for 580 °C heat treated sample.

Abstract: In this study, quasi-static and dynamic compression tests were performed on a ZrCuNiAl bulk metallic glass. The results demonstrated that the ZrCuNiAl bulk metallic glass changed from plastic deformation without strain rate effect to brittle fracture with negative strain rate sensitivity. The fracture surface morphology was related to the strain rate and temperature rise. The modified cooperative-shear model was determined to describe the effect of strain rate and temperature rise on the yield stress of ZrCuNiAl bulk metallic glass.

Abstract: Bulk metallic glass matrix composites have emerged as new potential material for structural engineering applications owing to their superior strength, hardness and high elastic strain limit. However, their behaviour is dubious. They manifest brittleness and inferior ductility which limit their applications. Various methods have been proposed to overcome this problem. Out of these, introduction of foreign particles (inoculants) during solidification have been proposed as most effective. In this study, an effort has been made to delimit this drawback. A systematic tale has been presented which explain the evolution of microstructure in Zr_47.5Cu_45.5Al₅Co₂ and Zr₆₅Cu₁₅Al₁₀Ni₁₀ bulk metallic glass matrix composites with varying percentage of ZrC inoculant as analysed by secondary electron and back scatter electron imaging of as cast unetched samples. A support is provided to hypothesis that inoculation remain successful in promoting phase formation and crystallinity and improve toughness.

Abstract: In this study, the effects of Zr content on the bending property, non–isothermal and isothermal crystallization kinetics of high–Zr–based BMGs were investigated. The BMGs exhibit high bending strength and their bending plasticity enhances with increasing Zr content, which is due to more free volume with high–Zr–content. During continuous heating, the crystallization phases for Zr66 and Zr70 BMGs are Zr₂Cu and Zr₂Ni phases. Zr70 alloy exhibits the highest activation energies for glass transition and crystallization because of the sluggish diffusion of large Zr atoms. Under isothermal condition, the average Avrami exponent of Zr66 alloy modeled by the JMA equation is about 2.6, implying a diffusion–controlled three dimensional crystallization growth with an increasing nucleation rate. The average Avrami exponent of 2.0 for Zr70 alloy indicates a diffusion–controlled three dimensional crystallization growth with a decreasing nucleation rate, which can be attributed to its higher activation energy for crystallization.

Abstract: Determining mechanical properties of Bulk Metallic Glasses (BMGs) requires synthesizing of the alloys in bulk form. However obtaining metallic glass in bulk form is quite challenging due to its tendency towards crystallization. In such circumstances it is beneficial to determine the mechanical properties of materials using finite elemental analysis of microstructures. Thus, in the present investigation, using Object Oriented Finite Element Analysis (OOF2) software package, Stress-Strain analysis has been carried out on Zr₆₀Cu₁₀Al₁₅Ni₁₅ BMG to determine such mechanical properties. Specimen of Zr₆₀Cu₁₀Al₁₅Ni₁₅ BMG exhibiting three microstructurally distinct regions amorphous, partial crystalline and crystalline regions was used for this analysis. The Stress-Strain relationship have been estimated for each of the three distinct phases and the results are validated by determining the Modulus of Elasticity for all the phases and comparing it with the available experimental results from Nano-indentation test.

Abstract: Glassy overspray powders of Ni₅₉Nb₃₅Sn₆ (at%) bulk metallic glass (BMG) obtained by spray forming were used in order to produce coatings on AISI 1020 mild steel substrate by laser cladding of the pre-placed powders. Different laser parameters, resulting in a variation of the power density, PD (J/mm2), were tested with a Yb fiber laser (up to 500 W). Gas atomized powders, suction cast sample trough copper mold casting and the laser clad tracks were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and coatings were subjected to measurements of Vickers microhardness. Atomized powder obtained showed no crystalline phases formation up to 425 μm, indicating good glass forming ability (GFA) of Ni₅₉Nb₃₅Sn₆ (at%) alloy. Microstructure characterization confirmed maximum glassy dimension of t_c =1mm for the Ni₅₉Nb₃₅Sn₆ (at%). Laser cladding track showed nanocrystalline phases embedded in a glassy matrix with Vickers microhardness ranging from 336 to 1184 HV.

Abstract: Electrical resistance is always related to the electronic structure of metallic glass and sensitive to structural changes, which provides a more intuitive approach to investigate structure evolution of metallic glasses upon structural relaxation. Electrical resistance relaxation of the La₅₅Al₂₅Ni₁₀Cu₁₀ bulk metallic glass was studied using the standard four-probe method. The electrical resistance of La₅₅Al₂₅Ni₁₀Cu₁₀ bulk metallic glass decreases significantly with the structural relaxation below the glass transition temperature at 445 K. During the subsequent continuous heating, the relaxed specimen shows a reduction in the resistivity decrease at the glass transition. The relaxed electrical resistance caused by the structural relaxation during the isothermal measurement equals the changes of the electrical resistance reduction in the glass transition region in the subsequent isochronal measurements. The calculated relaxed electrical resistance as a function of the annealing time can be fitted by Kohlrausche-Williams-Watts (KWW) equation. The equilibrium value, time constant and the stretched exponent for the isochronal electrical resistance measurements at 445 K are 0.0384, 2390s and 0.66, respectively. The in-situ electrical resistance data recorded in the isothermal annealing process show the same relaxation behavior with fitting parameters 0.0362, 2033 s, and 0.74, respectively.