Formation and mechanical properties of Ni-free Zr-based bulk metallic glasses

Glass formation and mechanical properties of Zr–Al–Co–Cu–Ag bulk metallic glasses (BMGs) were investigated. The glass-forming ability (GFA) of Zr₅₅Al₂₀Co₂₀Cu₅ alloy is significantly improved with minor addition of Ag, indicating by the impressive increase of the critical diameter of glass formation from 5 mm for Zr₅₅Al₂₀Co₂₀Cu₅ to 16 mm for (Zr_0.55Al_0.20Co_0.20Cu_0.05)₉₇Ag₃ and (Zr_0.55Al_0.20Co_0.20Cu_0.05)₉₅Ag₅ alloys. The Zr–Al–Co–Cu–Ag BMGs exhibit high compressive strength of 2160–2280 MPa and distinct plasticity of 0.6–2.5%. The Zr-based BMGs with outstanding GFA and mechanical properties as well as low-level cytotoxicity elements are expectative for industrial and biological applications.     

A Ni-free Zr-based bulk metallic glass with remarkable plasticity

The effect of Nb and Pd combination on the glass forming ability (GFA) and mechanical properties of Zr₅₃Cu₃₀Nb_xPd_9?xAl₈ (x = 3.5–6.0) bulk metallic glasses (BMGs) were systematically investigated by X-ray diffractometry (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and compression test. TEM observation revealed that a nanocrystalline phase embeds in the amorphous matrix of the as-cast Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ alloy. A tiny nano-crystalline phase (with size about 5–20 nm) embedded uniformly in the amorphous matrix of the Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ alloy was observed and identified to be the tetragonal structured NbPd₃ phase based on the analyses of nano beam electron diffraction. According to the results of thermal analyses, the composition of Zr₅₃Cu₃₀Nb₅Pd₄Al₈ and Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ present the optimum GFA as well as thermal stability in the Zr₅₃Cu₃₀Nb_xPd_9?xAl₈ (x = 3.5–6.0) alloy system. In addition, the result of compression test shows that the yield strength significantly increases from 1700 MPa (Zr₅₃Cu₃₀Nb₅Pd₄Al₈) to 1900 MPa (Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈). A remarkable compression plastic strain (11.2%) occurs at Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ BMG rod with 2 mm in diameter. This significant increase in plasticity is presumably due to the restriction on shear banding by the nano-size second phase.     

Embrittlement of Zr-based bulk metallic glasses

Embrittlement of Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 and Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glasses (BMGs) is studied after annealing at temperatures below and above the glass transition temperature T_g for time scales comparable with structural relaxation and crystallization. The effect of annealing on the bending ductility, the isoconfigurational elastic constants, the structure and the thermal stability is examined. The embrittlement during sub-T_g annealing originates from structural relaxation and can be reversed by subsequently annealing for a short duration above T_g. The embrittlement kinetics correlate with the structural relaxation. However, only a fraction of relaxation time at a given temperature (<T_g) is sufficient to embrittle the BMG significantly. Above T_g, plasticity is retained for annealing far beyond the relaxation time but, instead, embrittlement is caused by crystallization. The magnitude of the decrease in Poisson’s ratio is insufficient to explain the severe embrittlement within the framework of a critical value as previously suggested.     

Cytotoxicity of Zr-based bulk metallic glasses

Recently we developed a family of Ni-free Zr-based bulk metallic glasses in the Zr–Cu–Fe–Al system. X-ray diffraction and differential thermal analysis measurements demonstrate its good glass-forming ability, and amorphous rods of up to 13 mm in diameter can be produced for the alloy Zr₅₈Cu₂₂Fe₈Al₁₂. This new glassy system is potentially very interesting for biomedical applications. Thus we have investigated the effect of surface modification (treatment with nitric acid and oxygen plasma) on the alloy's cytotoxicity and compared it with the results for a Ni-bearing Zr-based bulk metallic glass. The surfaces were analyzed by X-ray photoelectron spectroscopy, and cytotoxicity was tested by measuring the viability and metabolic activity of mouse fibroblasts. Our results show that the surfaces of the as-cast glasses consist almost exclusively of zirconium oxide, which yields good biocompatibility. With nitric acid treatment this oxide layer can be stabilized further, to the extent that the cytotoxicity becomes as good as that of the non-toxic negative control (polystyrene).     

Compositional design and microstructure analysis of Zr-based bulk metallic glasses

The systematical studies of Zr-based BMGs were summarized in terms of their compositional design and their structural characterization. In particular, several key issues of BMG materials were focused, including initial alloy design and subsequent composition optimization, solidification microstructure characterization and crystallization process specification. The results show that a compositional designing approach is successfully developed and, through extensive microstructure characterization using transmission electron microscopy, several new crystalline phases are discovered in these newly developed Zr-based BMG alloys. Crystallization behavior ofZr-based BMG is also determined based on the microstructure analysis.     

Zr基大块非晶合金的微区变形及力学性能

利用纳米压痕仪、扫描电镜等研究了Zr基大块非晶合金在纳米压痕条件下的变形及力学性能。Zr基大块非晶合金在纳米压头作用下以弹性-塑性方式变形，载荷-位移曲线及压痕周边多重剪切带(堆起或波纹状)的特征证明了塑性变形的存在。冷却速度、第二相及退火等因素影响非晶合金的压痕硬度HV和弹性模量E，冷却速度小的试样或部位(如试样中心)的HV，E值略高；离第二相(W丝)越近，HV，E值越高；退火处理提高非晶的HV，E值，同时退火与第二相还明显改变压痕周边的变形状态及塑性变形量的大小，退火显著减小塑性变形量，使压痕周边凹陷，而第二相使压痕堆起消失。对塑性变形机理进行了初步分析。     

Role of addition in formation and properties of Zr-based bulk metallic glasses

A various multicomponent bulk metallic glasses (BMGs) were prepared at a low cooling rates of 1–100 K/s. The effects of various additions on the glass forming ability (GFA), properties and thermal stability of the alloy systems were investigated. The structural and properties changes of the BMGs upon addition were studies using X-ray diffraction, differential scanning calorimetry, density measurement, and acoustic measurement. It is found that the proper elemental addition can significantly improve the GFA and properties of the bulk glass-forming alloys. The addition is an effective way for improving GFA, and properties of the bulky glass-forming alloys. The roles of the additions in the glass formation, properties and crystallization are discussed.     

Enhancement of plasticity in Zr-based bulk metallic glasses electroplated with copper coatings

Electrodeposition was used to coat copper films on the surface of the BMG pillars (bulk metallic glasses) of Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (Vit. 105) with the film thicknesses of 71.5 and 161.1 μm. The experimental results of the compression tests of the bare Vit. 105 pillars and the coated Vit. 105 pillars revealed that the copper costing increased the density of shear bands in the Vit. 105 pillars formed during the tests, resulting in the improvement of plasticity. The plastic strain was 6.1% for the coated pillars with a coating thickness of 161.1 μm, which is 3.59 times of 1.7% of the bare Vit. 105 pillars. The deformation of the copper films dissipated the strain energy and limited the propagation of shear bands, which led to the initiation and formation of multiple shear bands. The technique developed in this work provides an effective way to enhance the plasticity of BMGs at room temperature.     

Shear striations and deformation kinetics in highly deformed Zr-based bulk metallic glasses

Detailed microscopic observations of the shear surfaces of a deformed, but unfractured, bulk metallic glass sample reveal a wealth of information on the deformation characteristics, kinetics and influence of temperature during serrated flow. The shear surfaces exhibit shear striations, which are similar to those resulting from viscous-like flow in rock-forming minerals. On the shear surface only a few areas show typical vein patterns, the thicknesses of which are less than those known from fracture surfaces. Combined with estimates for adiabatic heating, this indicates that sufficiently high temperatures are already present during shear banding before fracture, though instigated by non-purely adiabatic effects. A kinetic model based on an energy variable which reflects the structural relaxation ability is proposed that accounts for the occurrence of serrated flow combined with negative strain rate sensitivity, and the transition to non-serrated flow, i.e. positive strain rate sensitivity, below a critical temperature and strain rate.     

The corrosion behaviour of Zr-based bulk metallic glasses in 0.5 M NaCl solution

The corrosion behaviour of eutectic Zr₅₀Cu₄₀Al₁₀ and hypoeutectic Zr₇₀Cu₆Al₈Ni₁₆ bulk metallic glasses (BMGs) was studied by electrochemical measurements, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Zr₅₀Cu₄₀Al₁₀ BMG was highly susceptible to pitting corrosion in naturally aerated 0.5 M NaCl solution at 30 °C. In contrast, Zr₇₀Cu₆Al₈Ni₁₆ BMG passivated spontaneously under the same condition. EDX results for Zr₅₀Cu₄₀Al₁₀ indicated that enrichment of Cu, Cl and O occurred in the pitted region, while for Zr₇₀Cu₆Al₈Ni₁₆ BMG, no significant difference was found in the surface composition from the specimens before and after immersion in the solution. XPS analysis including angle-resolved measurements for Zr₇₀Cu₆Al₈Ni₁₆ BMG revealed that zirconium cation (Zr⁴⁺) was highly concentrated in both air-formed and passive films. Furthermore, the concentration of Zr⁴⁺ ions after immersion for 24 h or more showed tendency to increase with decreasing take-off angle, indicating that the exterior part of the passive film consisted exclusively of zirconium oxyhydroxide. The high corrosion resistance of Zr₇₀Cu₆Al₈Ni₁₆ BMG was attributed to the formation of homogeneous and stable passive film enriched with zirconium.     

Interactions between mechanically generated defects and corrosion phenomena of Zr-based bulk metallic glasses

Using the example of a Zr-Ti-Al-Cu-Ni bulk metallic glass with an ex situ approach, two aspects were studied: (i) the effects of pre-formed corrosion pits on the room temperature deformation behaviour, and (ii) the effect of mechanically generated defects on the local corrosion susceptibility. For the latter, Vickers micro-indents generated with different loads were also employed as model defects. It is revealed that pre-formed corrosion pits act as favourable surface sites to which shear bands are linked and most likely nucleate, which leads to a slight reduction in the maximum compression stress. Further, mechanically generated defects have to be considered as critical weak points at a glassy sample surface which can act as preferential sites for local corrosion initiation and propagation. Local chemical and structural changes due to the formation of shear bands as a response to mechanical load are discussed as possible reasons for this. For potential high-load applications of bulk metallic glasses this interaction must be considered as an important lifetime limiting aspect.     

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.     

Zr基块体非晶合金的微塑性成形性能

采用自行研制的微成形系统进行热压缩实验,分别研究成形温度、成形时间和冲头速度等对尺寸为d1 mm×1.5 mm的Zr41.2Ti13.8Cu12.5Ni10Be22.5块体非晶合金(Vit.1)在过冷液相区微塑性成形性能的影响规律。进一步研究了不同坯料尺寸对Vit.1块体非晶合金在过冷液相区超塑性成形性能的影响程度,结果表明流动应力随坯料尺寸的减小而降低。在此基础上,利用闭式模锻方法成形了分度圆直径为d1 mm的微型齿轮,采用SEM观察成形件的表面形貌,结果表明采用微成形方法可以获得尺寸精度较高的Vit.1块体非晶合金微型齿轮。     

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.     

不均匀性抑制Zr基块状金属玻璃的脆化

研究了Zr_47.25Cu_47.25Al_5.5块状金属玻璃(BMG)在铸态和不同退火时间下的室温压缩塑性与微观结构之间的关系。结果表明,Zr_47.25Cu_47.25Al_5.5BMG具有3.96%的压缩塑性,经过0.5、1.5、3和6 h退火后,该合金的压缩塑性均有所提升。在0.5 h退火后,压缩塑性达到最高(5.84%)。铸态Zr_47.25Cu_47.25Al_5.5BMG的微观结构呈现出5 nm左右网状分布。经过6 h退火后,富Cu的网状区域尺寸为50 nm左右,并伴有少量晶化。这种微观结构的不均匀性使该铸态合金在室温具有一定的塑性,同时抑制了其在退火中因自由体积减小而导致的脆化。     

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.