Tribological behaviors of Zr-based bulk metallic glass versus Zr-based bulk metallic glass under relative heavy loads

As a novel engineering material, bulk metallic glass (BMG) has received much attention. However, the knowledge concerning the tribological behavior of BMG versus BMG under relatively heavy loads is still insufficient. In this study, Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 metallic glass pins and discs were prepared by copper-mold suction casting. The dry sliding friction and the wear characteristics of the as-cast Zr-based BMG versus Zr-based BMG were tested under loads of 100, 125 and 150 N, respectively, using a pin-on-disc tribological apparatus at room temperature. The worn surfaces were studied by scanning electron microscopy in order to identify the wear mechanisms. The results showed that both the coefficient of friction and the wear rate increased with both the normal load and the rotational sliding velocity. X-ray diffraction patterns recorded after the tribological experiments indicated that no sliding-induced crystallization occurred. Transmission electron microscopy was also used to confirm the amorphous of the BMGs after sliding tests. In addition, the wear mechanisms changed with the experimental conditions. For a normal load of 100 N, the main mechanisms were abrasive wear, slight grooves and micro-cracks. For higher loads, adhesive wear was predominant, accompanied by abrasive wear and deeper grooves and more micro-cracks. When the rotational sliding velocity was increased, the dominant wear changed from slight grooves to viscous flow and adhesive wear.     

Biocompatibility of Ni-free Zr-based bulk metallic glasses

Three Ni-free Zr-based BMGs with composition of Zr₆₀Nb₅Cu₂₀Fe₅Al₁₀, Zr₆₀Nb₅Cu_22.5Pd₅Al_7.5, Zr₆₀Ti₆Cu₁₉Fe₅Al₁₀ were fabricated by suck copper-mould casting. All the BMGs prepared exhibit good glassy forming ability and wide supercooled liquid region ranging from 38 to 99 K. These BMGs also show good mechanical properties under static compression with yield strength of over 1350 MPa, Young's modulus of 70–80 GPa, and plastic strain of 3.6–9.5%. Friction and wear tests revealed that the BMGs exhibit much better wear resistance than the medical alloy Ti6Al4V, although BMGs have a higher friction coefficient. In addition, the in vitro test indicated that the BMGs have a similar or even better cell viability and proliferation activity as compared with Ti6Al4V. Finally, the in vivo evaluation of the BMGs was carried out by the implantation of BMG samples into white rabbits. It is shown that the BMG implants performed as well as the Ti alloy, demonstrating that the Ni-free Zr-based BMGs developed in this work are promising in medical applications.     

Effect of Copper and Zirconium Addition on Properties of Fe-Co-Si-B-Nb Bulk Metallic Glasses

In this research work, iron-based bulk metallic glasses (BMGs) have been fabricated, characterized and compared with Fe-Si alloy. BMG alloys of composition ((Fe_0.6Co_0.4)_0.75B_0.20Si_0.05)₉₆Nb₄) were synthesized by suction casting technique using chilled copper die. Effect of copper and zirconium addition on magnetic, mechanical, thermal and electrochemical behavior of ((Fe_0.6Co_0.4)_0.75B_0.20Si_0.05)₉₆Nb₄ BMGs was investigated. Furthermore, effect of annealing on nano-crystallization and subsequently on magnetic and mechanical behavior was also analyzed. Amorphousness of structure was evidenced by XRD analysis and microscopic visualization, whereas nano-crystallization behavior was identified by peak broadening of XRD patterns. Magnetic properties, measured by vibrating sample magnetometer, were found to be improved for as-cast BMG alloys by copper addition and further enhanced by nano-crystallization after annealing. Mechanical properties were observed to be increased by zirconium addition while slightly declined by copper addition. Potentiodynamic polarization analysis manifested the positive role of zirconium in enhancing corrosion resistance of BMGs in acidic, basic and brine mediums. Moreover, mechanical properties and corrosion analysis results affirmed the superiority of BMG alloys over Fe-Si alloy.     

Elevating the fracture toughness of Cu49Hf42Al9 bulk metallic glass: Effects of cooling rate and frozen-in excess volume

A major challenge for the structural applications of bulk metallic glasses (BMGs) is to improve their fracture toughness. Here we demonstrate that by increasing the cooling rate during the casting of liquid Cu₄₉Hf₄₂Al₉ into BMG, using a mixed argon and helium atmosphere, the notch toughness of the resultant BMG can be tripled relative to that obtained at slower cooling rates. The much elevated toughness is attributed to a ten-fold increase in the size of the plastic zone at crack tip, due to the proliferation of shear banding facilitated by enhanced propensity for shear transformations. The latter propensity is explained by the reduced shear modulus and microhardness, as well as increased enthalpy recovery, all of which are rooted in structural disorder as reflected by the lowered density and increased frozen-in excess volume. Such a structure-property correlation is systematically demonstrated by monitoring all these properties over a range of diameters of the as-cast BMG rods that correspond to cooling rate levels from 40 K/s to 10³ K/s.     

Understanding the mechanism for the embrittlement of a monolithic Zr-based bulk metallic glass by oxygen

Monolithic bulk metallic glasses (BMGs) with a nominal composition of Zr₆₅Cu_27.5Al_7.5 and the oxygen concentrations of 0.06 at.% and 0.68 at.% were prepared respectively. Oxygen effects on the deformation behavior and as-cast structural state were investigated. Although no crystalline phases were found in the BMG with higher oxygen concentration, the compressive plasticity was dramatically decreased. It was found that there is no direct correlation between the notable embrittlement and the initial free-volume content in the BMG. Geometrically-constrained compression tests for the BMGs shown that the processes of free-volume accumulation and shear band propagation during plastic deformation were obviously retarded by oxygen. It appears that the mechanism for the oxygen-induced embrittlement was closely related with the decreased atomic mobility by oxygen. As an indicator of atomic mobility, activation energy may be used as a parameter to evaluate the plasticity of monolithic Zr-based BMGs with different dissolved oxygen concentrations.     

Influence of Superheat on Microstructure and Mechanical Properties of Ductile Cu47.5Zr47.5Al5 Bulk Metallic Glass-Matrix Composite

Generally bulk metallic glasses (BMGs) posses very less ductility and toughness at room temperature. Over the recent past years to improve up on these properties in many alloy system BMG composites have been developed. It was also reported that Cu_47.5Zr_47.5Al₅ BMG composite shows a very high strength together with an extensive work hardening-like behavior of large ductility around 18%. In this study, the influence of superheat on microstructure and the resulting mechanical properties in Cu_47.5Zr_47.5Al₅ bulk metallic glass-matrix composite alloy has been studied. The Cu_47.5Zr_47.5Al₅ melt solidifies into a composite microstructure consisting of crystalline precipitates embedded in an amorphous matrix. The crystalline phase consists of B2 CuZr (cubic primitive with CsCl structure) with a small amount of monoclinic CuZr martensitic structure embedded in an amorphous matrix. The volume fraction of crystalline phases varies with melting current as well as position along the length of the as-cast rod, depending on the local cooling condition. The volume fraction and the distribution of the crystalline precipitates are heterogeneous in the amorphous matrix. Room temperature uniaxial compression tests revealed high yield strength ranging from 796 to 1900 MPa depending upon the volume fraction of the crystalline phases present. The presence of the dendritic B2 CuZr significantly improved the ductility. The BMG composites show a pronounced plastic strain up to 14% for the higher volume fraction of crystalline phase.     

A Dy-based bulk metallic glass with high thermal stability and excellent magnetocaloric properties

A new heavy rare-earth-based Dy₃₆Ho₂₀Al₂₄Co₂₀ bulk metallic glass (BMG) with high thermal stability and excellent magnetocaloric properties has been prepared by a copper mold casting. Compared with the other known rare-earth-based BMGs, this BMG alloy possess higher glass transition temperature and crystallization temperature, larger effective activation energy for glass transition and crystallization. Under a modest magnetic field, this BMG alloy exhibits a comparable or even larger magnetocaloric effect than that the previously reported RE-based BMGs and crystalline compounds. The high thermal stability and the excellent magnetocaloric effect together with other merits of the BMGs make this BMG alloy suitable candidate for use as magnetic refrigerant in a temperature range below 50 K.     

Tribological properties improvement of conventionally-cast Al-8.5Fe-1.3V-1.7Si alloy by multi-pass friction stir processing

The effect of multi-pass friction stir processing (FSP) on the tribological properties of conventionally-cast Al-8.5Fe-1.3V-1.7Si (FVS0812) alloy was investigated. The pin-on-disk dry sliding wear tests were conducted at room temperature under the applied pressures of 0.25, 0.50, and 0.75 MPa. The results showed that FSP substantially refined and improved the distribution of coarse θ-Al₁₃Fe₄ platelets and α-Al₁₂(Fe,V)₃Si intermetallics in the microstructure of alloys and eliminated the intermetallic-related defects. Consequently, the mechanical properties of the alloys, especially their ductility, were improved, which enhanced the stability of the protective tribolayer formed on their worn surfaces. According to the wear test results, the FSPed samples showed improved tribological properties especially at the higher applied pressures. For instance, at the applied pressure of 0.75 MPa, the wear rate and average friction coefficient of four-pass FSPed sample were lower than those of the base as-cast sample by 97% and 52%, respectively. SEM examination of the worn surfaces and wear debris also demonstrated that the wear mechanism changed from severe delamination/abrasion and microcracking of the tribolayer in the as-cast samples to mild delamination/abrasion and minor plastic wear in the FSPed samples.     

Synthesis,properties and XPS analysis of Cu-,Ni-,Ti-based metallic glasses

Metallic glasses（MG）represent an interesting group of materials as they possess outstanding physical,chemical and mechanical properties compared to their crystalline counterparts.This paper reviews the synergistic influence of Ni and Nb elements on thermal stability of supercooled liquid and corrosion resistance of as-cast Cu-Zr（Hf）-Ti-Ni-Nb bulk metallic glasses（BMGs）.Additionally,in-situ second phase reinforced Cu-based BMG composites with high corrosion resistance and excellent mechanical properties are investigated.On the other hand,this paper reports the development of ultra-high corrosion resistant Ni-based metallic glasses at high temperatures for their potential applications.Corrosion resistance and XPS analysis of the Nifree Ti-based BMG are also introduced.     

Effects of pre-compression on the microstructure,mechanical properties and corrosion resistance of RE65Co25Al10 (RE = Ce,La, Pr,Sm and Gd) bulk metallic glasses

The effects of the pre-compression treatment at room temperature on the microstructure, mechanical properties, and corrosion resistance of the RE₆₅Co₂₅Al₁₀ (RE = Ce, La, Pr, Sm and Gd) bulk metallic glasses (BMGs) in the seawater solution have been investigated. The results indicate that the pre-compression treatment induces the microstructure changes or partial crystallization, which can significantly improve the thermal stability and the mechanical properties of the RE₆₅Co₂₅Al₁₀ BMGs. The BMG-based composite with nanocrystals under the pre-compression stress of 450 MPa shows the excellent mechanical properties with the compressive fracture strength of 832 MPa and microhardness of 261 HV, while the BMG pre-compressed by the stress of 250 MPa exhibits the enhanced corrosion resistance in the seawater solution, as indicated by the low corrosion current density and high corrosion potential. However, the corrosion resistance of the RE₆₅Co₂₅Al₁₀ BMG composite pre-compressed under the stress of 450 MPa does not perform as well as this of the specimen pre-compressed under the stress of 250 MPa. Moreover, the discussion has been made based on the short-range order (SRO), free-volume content, and nanocrystal precipitation under the pre-compression treatment. The proper pre-compression design can be beneficial for improving the practical application of the RE-based BMGs.     

Study on corrosion behavior of continuous bulk metallic glass-coated steel wire composite

Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 (atomic percent) (Vit1) Bulk metallic glass (BMG)-coated steel wire composite was produced by using continuous process. The existence of amorphous structure in the BMG coating was proved by differential scanning calorimeter (DSC) measurement and transmission electron microscope (TEM). The corrosion behaviors of the bare steel wire and the wire composite in 3.5 wt% NaCl solution corrosive media were investigated by immersion tests and electrochemical polarization measurements at room temperature. Scanning electron microscopy (SEM) was utilized to examine morphology of the corroded surface. By comparing the corrosion behaviors of the wire composite and the bare steel wire as well as the Vit1 BMGs reported in the literatures, the results of the present investigation reveal (1) it is important to choose sound technical parameters such as processing temperature to avoid the crystallization for the BMG coating of steel wire (2) the Vit1 BMG-coated steel wire composite has better corrosion resistance than the bare steel wire from perspective of either mass loss or polarization behavior (3) strong protective layers grow on the surface of the coated steel wire by anodization, of which the barrier effect to initiate pitting is slightly lower than that of the monolithic Vit1 BMG, but the general polarization behavior is quite similar to that of the monolithic Vit1 BMG.     

On factors influencing the ductile-to-brittle transition in a bulk metallic glass

An experimental study to ascertain the ductile-to-brittle transition (DBT) in a bulk metallic glass (BMG) was conducted. Results of the impact toughness tests conducted at various temperatures on as-cast and structurally relaxed Zr-based BMG show a sharp DBT. The DBT temperature was found to be sensitive to the free-volume content in the alloy. Possible factors that result in the DBT were critically examined. It was found that the postulate of a critical free volume required for the amorphous alloy to exhibit good toughness cannot rationalize the experimental trends. Likewise, the Poisson’s ratio–toughness correlations, which suggest a critical Poisson’s ratio above which all glasses are tough, were found not to hold good. Viscoplasticity theories, developed using the concept of shear transformation zones and which describe the temperature and strain rate dependence of the crack-tip plasticity in BMGs, appear to be capable of capturing the essence of the experiments. Our results highlight the need for a more generalized theory to understand the origins of toughness in BMGs.     

Fe-based glassy alloys with high iron content and high saturation magnetization

Iron-based bulk metallic glasses (BMGs) are attractive due to their excellent magnetic properties. It is known that increasing the iron content in alloys would enhance the magnetic property of glassy alloys, but would reduce their glass forming ability. Despite researchers are keen to develop Fe-based BMGs with high Fe content, BMG with Fe content higher than 79 at.% has not been reported. Here we report that Fe-based BMGs with Fe content as high as 78 ～ 81 at.% have been successfully prepared. The as-prepared Fe₈₁Mo₁P_7.5C_5.5B₂Si₃ BMG possesses a saturation magnetization of 1.64 T, which is significantly higher than the reported value in BMGs.     

Glass formation and properties of Ti-based bulk metallic glasses as potential biomaterials with Nb additions

Ti_(47)Cu_(38-x)Zr_(7.5)Fe_(2.5)Sn_2Si_1Ag_2Nb_x (x=0, 1,2; at%) bulk metallic glasses (BMGs) with superior biocorrosion resistance were synthesized by copper mold casting. Although the minor addition of Nb to the Ti-CuZr-Fe-Sn-Si-Ag BMG slightly decreases the glass-forming ability (GF A), the Nb-bearing Ti-based alloys could be casted in a bulk glassy rod form with diameters up to3 mm. It is found that partial substitution of Cu with Nb is effective on enhancing the bio-corrosion resistance of the Ti-based BMG. Potentiodynamic polarization measurements show that Nb addition to Ti-based BMG leads to higher open-circuit potential and pitting potential as well as lower passive current density in Hank's solution. Electrochemical impedance spectroscopy (EIS) results indicate that with Nb content increasing, the charge transfer resistance values of the Ti-based BMGs become larger,demonstrating that the surface oxide films are more protective. The Nb-bearing Ti-based BMGs also exhibit good in vitro biocompatibility comparable to that of Ti-6Al-4V alloy. The enhanced bio-corrosion resistance, excellent in vitro biocompatibility and good mechanical properties of the Nb-bearing Ti-based BMGs are favorable for biomedical applications.     

Characterization of free volume in cold-rolled Zr55Cu30Ni5Al10 bulk metallic glasses

Cold-rolled Zr₅₅Cu₃₀Ni₅Al₁₀ bulk metallic glasses (BMGs) exhibited volume dilatation across the entire range of thickness reduction fractions from 5.7% to 63%. The dilatation was dominantly attributed to the free volume generated during cold-rolling, together with any open volumes, like the voids or microcracks, or both, which survived even after sufficiently annealing the rolled sample at the temperature in the supercooled liquid region. The relaxation of free volume formed during cold-rolling resulted in more heat evolution per unit volume than that of as-cast glass. The structural change during cold-rolling could be ascribed to the increased fluctuation of atomic-level hydrostatic stress, 〈p²〉, in the glass.     

Correlations between the relaxed excess free volume and the plasticity in Zr-based bulk metallic glasses

Zr-based bulk metallic glasses (BMGs) with Nb minor alloying have been fabricated with different free volume (FV) trapped in. FV is evaluated by the relaxed excess free volume (REFV) after annealing just below T_g through loop thermal expansion tests. The results show that there is a qualitative correlation between the plasticity and REFV in Zr-based BMGs. The larger amount of excess FV the BMGs relax, the better plasticity they exhibit. With 1.5% Nb addition, the brittle Zr₆₅Cu₁₅Ni₁₀Al₁₀ BMGs possess REFV up to about 0.428% and exhibit the relatively good plasticity up to 25.6%. This provides a promising way to estimate the plasticity of BMGs and design new ductile BMGs through the minor alloying.     

Forming of bulk metallic glass microcomponents

The present article considers forward extrusion, closed-die forging and backward extrusion processes for fabrication of individual microcomponents from two bulk metallic glass (BMG) compositions: Mg₆₀Cu₃₀Y₁₀ and Zr₄₄Cu₄₀Ag₈Al₈. Two types of tooling were used in the present work: relatively massive die sets characteristic of cold forming operations for crystalline metals and lightweight die sets adapted to the special characteristics of BMGs. In addition to demonstrating that microcomponents of several geometries can be readily fabricated from BMGs, rheological properties are combined with crystallization kinetics to formulate a generally applicable method that can guide selection of optimal forming parameters. Finally, the use of particulate-based lubricants for BMG forming is shown to result in individual lubricant particles becoming mechanically locked into the BMG surface.     

The effect of oxidation on the corrosion resistance and mechanical properties of a Zr-based metallic glass

The oxidation kinetics of the Zr₆₄Cu₁₆Ni₁₀Al₁₀ bulk metallic glass (BMG) roughly follows a two-stage rate law at both 433 and 593 K in air. An oxide film of 940 nm can be formed by oxidation at 593 K, which is ZrO₂-enriched but Cu-depleted on the outer surface. The oxide film leads to a superior passivity in 0.5 M NaCl and great corrosion resistance improvements in other solutions. The oxidation effect on mechanical properties were characterized by nanoindentation, wedge indentation and compression tests. The Zr-based BMG still keeps the amorphous nature and its good mechanical properties are retained after oxidation.     

Ternary La–Al–C bulk metallic glasses

Glass formation, thermal stability, elastic moduli and mechanical properties of La–Al–C bulk metallic glasses (BMGs) were investigated. The BMGs of La_57.5Al_32.5C₁₀, La₆₀Al₃₀C₁₀, and La_62.5Al_27.5C₁₀ with the maximal diameters up to 3 mm can be synthesized. Compared with other La-based glassy systems, like La–Al–(Co, Ni, Cu), the La–Al–C BMGs with the similar La concentration exhibit higher glass transition temperatures (507–577 K) and elastic constants. The La–Al–C BMGs with the unusual T_g and elastic moduli fill a gap in thermal and elastic properties between light rare-earth-based BMGs and heavy rare-earth-based ones. The compressive yield strength of La_57.5Al_32.5C₁₀ BMG reaches to ∼1.1 GPa, higher than that of other reported La-based BMGs. The ternary BMGs with simple element constituent, high glass-forming ability and superior mechanical properties are significant for fundamental research as model materials.     

Microstructure and wear performance of Al5083/CeO2/SiC mono and hybrid surface composites fabricated by friction stir processing

Friction stir processing (FSP) was utilized to produce surface composites by incorporating nano-sized cerium oxide (CeO₂) and silicon carbide (SiC) particles individually and in combined form into the Al5083 alloy matrix. The study signified the role of these reinforcements on microstructure and wear behavior of the resultant surface composite layers. The wear characteristics of the resultant mono and hybrid surface composite layers were investigated using a pin-on-disc wear tester at room temperature. The microstructural observations of FSPed regions and the worn out surfaces were performed by optical and scanning electron microscopy. Considerable grain refinement and uniform distribution of reinforcement particles were achieved inside the nugget zone. All the composite samples showed higher hardness and wear resistance compared to the base metal. Among the composite samples, the hybrid composite (Al5083/CeO₂/SiC) revealed the highest wear resistance and the lowest friction coefficient, whereas the Al5083/SiC composite exhibited the highest hardness, i.e., 1.5 times as hard as that of the Al5083 base metal. The enhancement in wear behavior of the hybrid composites was attributed to the solid lubrication effect provided by CeO₂ particles. The predominant wear mechanism was identified as severe adhesive in non-composite samples, which changed to abrasive wear and delamination in the presence of reinforcing particles.