Dynamic mechanical behavior of a Zr-based bulk metallic glass during glass transition and crystallization

The dynamic mechanical behaviors of the Zr41Ti14Cu12.5Ni8Be22.5Fe2 bulk metallic glass (BMG) during continuous heating at a constant rate were investigated. The glass transition and crystallization of the Zr-based BMG were thus characterized by the measurements of storage modulus E and internal friction Q-1. It was found that the variations of these dynamic mechanical quantifies with temperature were interre-lated and were well in agreement with the DSC trace obtained at the same heating rate. The origin of the first peak in the internal friction curve was closely related to the dynamic glass transition and subsequent primary crystallization. Moreover, it can be well described by a physical model, which can characterize atomic mobility and mechanical response of disordered condense materials. In comparison with the DSC trace, the relative position of the first internal friction peak of the BMG was found to be dependent on its thermal stability against crys-tallization.     

The impact of fragility on the calorimetric glass transition in bulk metallic glasses

The glass transition of bulk metallic glasses with various fragilities as well as strong oxide glasses is studied using differential scanning calorimetry (DSC). It is found that the liquid fragility determined from equilibrium viscosity measurements is very well correlated with the scaled maximum slope of the DSC heat flow during the glass transition. We compare the correlation found in this work and those correlations with fragility from previous studies on other classes of glass-formers and find that the slope, which describes the curvature of the enthalpy on a reduced temperature scale, is a quantity better correlated with fragility, as it reflects the timescale of the non-equilibrium relaxation and the distribution of relaxation times in the glassy state. The present findings are supported by a recent theoretical report for calculated enthalpy curves with different fragilities from a model of selenium using the enthalpy landscape approach.     

Synthesis of LaCe-based bulk metallic glasses with low glass transition temperature

LaCe-based bulk metallic glasses (BMGs) with diameter up to 10 mm have been synthesized in copper mold. The BMG alloys have low glass transition temperatures, about 403 K and large supercooled liquid regions up to 58 K. The hardness of the BMG alloys is around 1.9 GPa. They also exhibit large plasticity and imprintability feature in the supercooled liquid region, which make the alloy a potential imprinting material. In addition, the BMG alloys might offer an ideal model system for the study of glass transition with a large experimentally accessible time and low temperature windows.     

Shear punch tests for a bulk metallic glass

A shear punch test technique was used for characterization of the mechanical properties of Zr–5Ti–17.9Cu–14.6Ni–10Al bulk metallic glass. The ultimate shear stress values matched very closely with values derived from uniaxial compression tests reported in the literature. This is consistent with a lack of pressure sensitivity in compression reported for this particular metallic glass. Deformation response was strain rate insensitive up to a critical rate, beyond which softening occurred. The latter was attributed to thermal heating effects.     

Boss formability of a Zr-based bulk metallic glass

Macroscopic solid-to-solid hot formability of a Zr-based bulk metallic glass has been investigated in this study by applying a boss forming process within supercooled liquid region (SLR). The morphology and microstructures after boss forming were first examined by using an optical microscopy (OM), a field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM). The variation of thermal properties before and after boss forming was also analyzed by using a differential scanning calorimeter (DSC). The glass transition temperature, T_g and the crystallization onset temperature, T_{x, onset} were found to decrease with increasing test time and temperature. Macroscopic extrusion formability was found to match well with the results predicted through a processing map based on a dynamic materials model (DMM). The specimens were found to form well under the conditions of high temperature and slow punch jig speed. A FEM simulation study has also been carried out to understand the causes of piping problem. A sound boss without a pipe could be formed by reducing the flow rate difference in the contact and the core sites.     

Tribological studies of a Zr-based bulk metallic glass

In the present study, the tribological behavior of a Zr_52.5Cu_17.9Ni_14.6Ti₅Al₁₀ bulk metallic glass (BMG) was investigated using pin-on-disk sliding measurements under an argon atmosphere, rubbing against a type 303 stainless steel counterface. The tested pins and disk were examined using X-ray diffractometry, optical microscopy, profilometry, scanning electron microscopy and transmission electron microscopy. The results showed that the wear of the BMG pins was substantially larger compared with previous tests performed against a zirconia counterface. Strain softening was found in the near-surface region of the glassy pin due to the highly localized shearing. Frictional heating contributed to the occurrence of viscous flow and material transfer on the worn surface of the wear pin and the disk, respectively. Thus, the pin exhibited a severe adhesive-dominated sliding wear.     

On the structural relaxation of bulk metallic glass under warm deformation

The influence of both isothermal annealing and warm deformation on the structural relaxation behavior of Zr₄₄Ti₁₁Cu_9.8Ni_10.2Be₂₅ (LM1B) bulk metallic glass (BMG) was investigated. The structurally relaxed samples were obtained from warm extrusion to introduce different amounts of free volume annihilation. We employed density measurements as well as differential scanning calorimetry to study free volume changes in these structurally relaxed samples. The results demonstrated that isothermal annealing within the supercooled liquid region (SLR) caused annihilation of free volume by ～0.1%, but a small amount of in situ deformation promoted the annihilation process. In contrast to the conventional belief that annealing induced embrittlement caused by the reduction of free volume and densification, it is interesting to note that, in this study, when material was relaxed at the mid-temperature within the SLR (673 K for LM1B) plastic strain actually increased as compared to the as-received counterpart. This indicates that proper warm deformation can improve the room temperature plasticity of BMG as a result of subtle atomic-scale structural changes.     

Fracture surface morphology of compressed bulk metallic glass-matrix-composites and bulk metallic glass

The fracture morphology of Zr-based bulk metallic glass-matrix-composites (BMGCs) and Cu-based bulk metallic glass (BMG) after compression testing has been studied. The quasi-static compression fracture surface displays a mixture of three different distinct patterns: vein-like, smooth featureless and river-like features. The last one corresponds to the morphology known from tensile tests of BMGs. Moreover, randomly distributed transversal steps on the fracture plane are also present. This is in contrast to previous studies where a characteristic vein-like pattern is considered a unique feature of the fracture of BMGs under quasi-static uniaxial compression. The presence of different fracture features indicates that the development of the fracture plane occurs in a stepwise mode.     

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.     

Crystallization of Mg-based bulk metallic glass

1 Introduction Since INOUE et al[1,2] reported that amorphousalloy with the composition of Mg65Cu25Y10 could beproduced with thickness up to 4 mm by conventionamold casting technique, Mg-based bulk metallic glasses(BMGs) have been proposed as a new kind …     

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.     

Ball indentation tests for a Zr-based bulk metallic glass

Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk metallic glass was characterized using ball indentation tests. Comparison of the data with the expanding cavity model revealed that the deformation is pressure insensitive for compressive loading. The plastic flow curves obtained from indentation tests showed perfectly plastic response and no strain rate sensitivity up to 15% strain.     

Ductile to brittle transition of fracture of a Zr-based bulk metallic glass: Strain rate effect

Quasi-static and dynamic tensile experiments were conducted on a Zr-based bulk metallic glass at room temperature. A significant ductile-to-brittle transition was identified with increasing strain rate, based on the changes in the macroscopic fracture mode from shear to normal tension and in the microscopic fracture feature from vein patterns to fine dimples and/or nanoscale periodic corrugations. According to the Mohr-Coulomb criterion, it is revealed that such a transition is due to the competition between the intrinsic critical shear and tensile strengths at different strain rates. Microscopically, the strain-rate-induced transition is attributed to the change in the motion of local atomic groups from shear transformation zone to tension transformation zone, in which the characteristic volume of shear transformation zone is a key parameter.     

ZrCuAlNb大块非晶合金的热稳定性

利用电弧炉+铜模吸铸的方法制备了Zr46.3Cu43.4Al8.3Nb2(at%)块体非晶合金。利用示差扫描量热仪(DSC)研究了合金的热稳定性,利用Kissinger方法计算了其特征温度表观激活能,利用Doyle方法计算了其局域激活能。结果表明,Zr46.3Cu43.4Al8.3Nb2块体非晶合金具有良好的热稳定性。各特征温度的表观激活能分别为:玻璃转变激活能(Eg)为302.7 kJ/mol、晶化起始激活能(Ex)为445.853 kJ/mol、晶化峰值激活能(Ep)为389.20 kJ/mol。局域激活能随着晶化体积分数的增加而显著减小。