Mechanical properties of Pd–Cu–Si bulk metallic glass

In present work, Pd_77.5Cu₆Si_16.5 bulk metallic glass balls with diameter up to 6 mm have been prepared by fluxing and water quenching method. It has been found that the Pd–Cu–Si glassy alloy exhibits a compressive plastic strain of about 11.4%, together with strain hardening characteristics. A large localized shear band, accompanied by a shear step of about 220 μm in size, has been clearly observed on the deformed specimen. The related shear plane is estimated to have an angle of 42 degrees with respect to the loading axis. The good ductility of the glassy alloy is believed to be partially attributed to strain hardening and the higher resistance of the glassy alloy to crack nucleation and propagation due to its large Poisson's ratio.     

Deformation behavior of cadmium telluride stressed at elevated temperatures

CdTe crystals were uniaxially compressed along several crystallographic axes at temperatures from 773 to 1353 K. The applied stress ranged from 14 to 74% of the critical resolved shear stress (CRSS) measured in the authors’ laboratory. The deformed specimens were annealed without applying stress at temperatures from 573 to 1073 K. No twins were observed after the above operations. Dense slip bands were observed on most of the compressed specimens. Secondary slip systems were activated in some experiments. CdTe crystals were sheared along {111}<112> at 1073 K with a load of 40% CRSS. Slip bands, but no twins, were observed. Synchrotron x-ray topography was used to study in situ the effect of stress on crystal deformation. CdTe specimens were uniaxially stressed in tension along <112> at 293 to 673 K. When the load reached ~50% of the CRSS, the topograph began to distort, indicating the beginning of plastic deformation. No twins were observed on the stressed specimens.     

ZrCuNiAl块体非晶合金的低温压缩变形行为

采用Instron 5500万能试验机对Zr55.7Cu23Ni9Al12.3块体非晶合金进行了应变速率为4×10-4 s-1、环境温度为143 K的低温压缩试验,研究了其低温压缩变形行为。结果表明,当测试环境温度从298 K（室温）降低到143 K时,该合金的强度和塑性均呈增加的趋势,且未出现传统晶态材料的冷脆性现象。分析表明,其屈服强度的增加源于低温下剪切带的形核需要更高的载荷,而低温下断口侧面高密度剪切带的形成导致了其塑性的增加。     

Deformation behavior of Zr-based bulk metallic glass in an undercooled liquid state under compressive loading

High temperature deformation behavior of a Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk amorphous alloy has been studied in a temperature range between 355 and 460°C under compressive loading after rapid heating. A transition of flow behavior, viz. from, a Newtonian to a non-Newtonian flow, has been reported by many researchers as the temperature is decreased at a given strain rate. In the present study, two different theoretical relations based on a viscous flow model and a transition state theory have been applied to analyze the transition behavior of deformation in terms of viscosity and flow stress. An experimental deformation map was then constructed to specify the boundaries between Newtonian and non-Newtonian flow, based on the relationship between the flow stress and strain rate in an undercooled liquid state. It has further been confirmed that the stress overshoot phenomena can be observed mostly in a non-Newtonian flow regime appearing in an intermediate temperature and strain rate region in this deformation map.     

Corrosion behaviors of coatings fabricated using bulk metallic glass powders with the composition of Fe68.5C7.1Si3.3B5.5P8.7Cr2.3Mo2.5Al2.0

Two different types of coatings were prepared, by a high velocity oxy-fuel spraying method and a laser spraying method, respectively, using bulk metallic glass powders with the nominal composition of Fe_68.5C_7.1Si_3.3B_5.5P_8.7Cr_2.3Mo_2.5Al_2.0. The corrosion behaviors of the two coatings in 1M HCl, H₂SO₄, NaCl and NaOH solutions were investigated based upon the microstructural differences originating from the different coating methods. The amorphous coating layer formed by the high velocity oxy-fuel spraying method exhibited higher, excellent corrosion resistance in the 1M HCl solution. The coating layer formed by the laser spraying method exhibited a high pitting tendency attributed to the dendritic microstructure with various borides and carbides. Due to a great number of pores, the HVOF coating exhibits slightly lower corrosion resistance than the LS coating in alkaline solution.     

块体Ti40Zr25Ni8Cu9Be18非晶合金的低温压缩行为

研究块体Ti40Zr25Ni8Cu9Be18非晶合金在液氮温度下的准静态压缩力学行为,并对不同应变速率时的低温力学性能和室温性能进行对比.结果表明:块体Ti40Zr25Ni8Cu9Be18非晶合金在低温时的压缩强度明显高于室温时的压缩强度;且随着应变速率的增大,低温压缩强度增加的幅度较大,即正应变速率敏感因子增大;在液氮温度和低应变速率条件下,光滑断面的出现说明低温对降低粘度起着阻碍作用,粘度降低的量级没有达到形成脉状花样所需要的要求;随着应变速率的增加,剪切断口脉状花样的出现说明高应变速率能够明显降低剪切面粘度.     

Assessment of rheological and thermodynamic properties of the Pd40Ni40P20 bulk metallic glass around glass transition using an indentation creep technique

The thermodynamic and rheological properties of the Pd₄₀Ni₄₀P₂₀ bulk metallic glass are explored by means of an indentation creep technique around the glass transition. We have developed a dedicated instrumented indentation apparatus allowing to assess the mechanical properties at elevated temperatures. The analysis of results is made possible by using the viscoelastic solutions of contact mechanics. We also analyse the thermodynamics of creep around glass transition to estimate the activation free energy changes from the activation free enthalpy changes via the shear modulus – temperature data. The shear viscosity values extracted using this technique allow for the derivation of activation energies (free enthalpy 210 kJ/mol, enthalpy 456 kJ/mol, entropy 410 J/mol/K) for the flow process. All these properties were found to closely match with those obtained using conventional techniques for viscosity measurements. Compared to the latter, the indentation creep technique requires small volumes and samples are easy to prepare. It is therefore expected that such a technique might be employed for the study of glass transition in metallic glasses.     

In situ tensile deformation of Fe-rich metallic glass at elevated temperatures using hard X-ray diffraction

The present work focuses on the study of the mechanical properties of the Fe_72.5Cu₁Nb₂Mo₂Si_15.5B₇ metallic glass using in situ hard X-ray diffraction techniques. In situ tensile deformation tests provided detailed information about the mechanical properties of the investigated Fe-based metallic glass. Analyzing series of two-dimensional XRD patterns in reciprocal space yields strain tensor components of the amorphous alloy providing insight about its deformation mechanisms. Comparing tensile tests performed at different temperatures indicates that the deformation mechanism gradually changes from purely elastic to completely plastic regime. The Poisson ratio ν of the investigated alloy increases with increasing deformation temperature however the fracture strength σ_f shows opposite behavior.     

Pd40Ni40Si5P15 bulk metallic glass properties variation as a function of sample thickness

This work presents a comparative study of different properties variation of Pd₄₀Ni₄₀Si₅P₁₅ bulk metallic glass (BMG), namely: density, hardness, relaxation enthalpy and mechanical properties as a function of the sample thickness. The samples were produced in the form of a long pyramid by conventional Cu-mold casting in an Ar atmosphere. The results illustrate significantly different sensitivity of these properties of the glassy phase to the cooling rate. The difference in crystallization behavior is also discussed.     

Deformation behavior of Zr55.9Cu18.6Ta8Al7.5Ni10 bulk metallic glass matrix composite in the supercooled liquid region

A Zr_55.9Cu_18.6Ta₈Al_7.5Ni₁₀ bulk metallic glass (BMG) composite with an amorphous matrix reinforced by micro-scale particles of Ta-rich solid solution was prepared by copper-mold casting. Isothermal compression tests of the BMG composite were carried out in the range from glass transition temperature (∼673 K) to onset crystallization temperature (∼769 K) determined by differential scanning calorimetry (DSC). The compressive deformation behavior of the BMG composite in the supercooled region was investigated at strain rates ranging from 1 × 10⁻³ s⁻¹ to 8 × 10⁻² s⁻¹. It was found that both the strain rate and test temperature significantly affect the stress–strain behavior of the BMG composite in the supercooled liquid region. The alloy exhibited Newtonian behavior at low strain rates but became non-Newtonian at high strain rates. The largest compressive strain of 0.8 was achieved at a strain rate of 1 × 10⁻³ s⁻¹ at 713 K. The strain rate change method was employed to obtain the strain rate sensitivity (m). The deformation mechanism was discussed in terms of the transition state theory based on the free volume.     

Indentation size effect and shear transformation zone size in a bulk metallic glass in two different structural states

The existence of an indentation size effect (ISE) in the onset of yield in a Zr-based bulk metallic glass (BMG) is investigated by employing spherical-tip nanoindentation experiments. Statistically significant data on the load at which the first pop-in in the displacement occurs were obtained for three different tip radii and in two different structural states (as-cast and structurally relaxed) of the BMG. Hertzian contact mechanics were employed to convert the pop-in loads to the maximum shear stress underneath the indenter. Results establish the existence of an ISE in the BMG of both structural states, with shear yield stress increasing with decreasing tip radius. Structural relaxation was found to increase the yield stress and decrease the variability in the data, indicating “structural homogenization” with annealing. Statistical analysis of the data was employed to estimate the shear transformation zone (STZ) size. Results of this analysis indicate an STZ size of ～25 atoms, which increases to ～34 atoms upon annealing. These observations are discussed in terms of internal structure changes that occur during structural relaxation and their interaction with the stressed volumes in spherical indentation of a metallic glass.     

Mechanical behavior of Fe65.5Cr4Mo4Ga4P12C5B5.5 bulk metallic glass

Fe_65.5Cr₄Mo₄Ga₄P₁₂C₅B_5.5 bulk amorphous rectangular bars with a cross-section of 2×2 mm² and a length of 30 mm were produced by copper mold casting. The as-cast bars as well as annealed samples were investigated by compression and Vickers hardness tests. The fracture strength for the as-cast samples σ_f is 2.8 GPa and the fracture strain ε_f is 1.9%. Upon annealing at 715 K for 10 min, i.e. at a temperature below the calorimetric glass transition, the fracture strain drops to 1.6% and no plastic deformation is observed. The Vickers hardness HV for the as-cast samples is about 885, and increases to 902 upon annealing. The fracture behavior of this Fe-based bulk glassy alloy is significantly different in comparison with the well-studied Zr-, Cu- or Ti-based good glass-formers. The fracture is not propagating along a well-defined direction and the fractured surface looks irregular. Instead of veins, the glassy alloy develops a high number of microcracks.     

Effect of annealing on microstructure and mechanical property of a Ti–Zr–Cu–Pd bulk metallic glass

The crystallization behavior and mechanical properties of Ti₄₀Zr₁₀Cu₃₆Pd₁₄ bulk metallic glass were investigated. Nano-composites consisting of glassy matrix are formed in the alloys annealed at 693 and 723 K. After annealing at 823 K, the residual glass is completely crystallized. High yield strength of over 2100 MPa and distinct plastic strain of 0.8% are obtained in the nano-composite alloy annealed at 693 K. The compressive strength decreases for the alloys annealed at higher temperatures of 723 and 823 K, accompanying with the progress of crystallization. The alloy rods both in as-cast and annealed (at 693 K for 10 min) states show the fracture surfaces consisting of vein pattern. On the other hand, the fracture morphology of the alloys annealed at 723 and 823 K changes to brittle character with flat facets.     

内生枝晶增韧非晶基复合材料的摩擦磨损性能

采用电弧熔炼、铜模吸铸法分别制备Ti42Zr22V14Cu5Be17、Ti46Zr20V12Cu5Be17、Ti48Zr18V12Cu5Be17的Ti基内生枝晶增韧非晶基复合材料,利用球-盘式摩擦磨损试验机进行干摩擦磨损试验,研究了枝晶的体积分数对其耐磨性的影响,利用X射线衍射仪分析了样品的结构,利用扫描电子显微镜观察合金的显微组织和磨损表面形貌,分析了不同成分Ti基内生枝晶增韧非晶基复合材料的磨损机理。结果表明：在相同的试验条件下,不同体积分数晶体相的非晶复合材料的磨损机理不同。枝晶相体积分数较低时,磨损机理主要体现为轻微的磨粒磨损,随枝晶相体积分数的增加,粘着磨损成为主要的磨损机理,同时伴随有硬质颗粒压入软化相构成三体摩擦。材料耐磨性随着晶体相体积分数的增加而逐渐降低。     

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.     

Mg-11.21Gd-2.26Y-0.44Zr稀土镁合金热变形行为

采用差热分析、金相显微镜等手段,分析了Mg-11.21Gd-2.26Y-0.44Zr稀土镁合金的微观组织,结果发现,在温度530℃均匀化热处理4h,可使大部分合金元素固溶。采用Gleeble3800热模拟实验机,在温度为320℃～480℃、应变速率为0.001s-1～0.1s-1、最大变形程度为60%的条件下,对该镁合金进行热压缩实验,结果表现,材料流变应力行为和显微组织受到变形温度和变形速率的严重影响;合金的流动应力可以采用Sellars方程形式描述;计算出的变形激活能为225.67kJ.mol-1。     

Comparison of fatigue behavior of a bulk metallic glass and its composite

The X-ray diffraction results show that LM001 (Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5) is a monolithic bulk-metallic glass (BMG) and LM002 (Zr_56.2Ti_13.8Nb_5.0Cu_6.9Ni_5.6Be_12.5) is a BMG containing crystalline phases. High-cycle-fatigue (HCF) studies were performed on these zirconium (Zr)-based BMGs. The HCF experiments were conducted, using an electrohydraulic machine at a frequency of 10 Hz with an R ratio of 0.1 and under tension–tension loading, where R=σ_min/σ_max, where σ_min and σ_max are the applied minimum and maximum stresses, respectively. The test environment was air at room temperature. The vein pattern and droplets with a melted appearance were observed in the apparent melting region. The fatigue-endurance limit (239 MPa) of LM002 was found to be significantly shorter than that (567 MPa) of LM001, which indicates that the presence of crystalline phase could reduce the resistances to fatigue. The tension–tension fatigue S (applied stress)–N (lifetime) curve of the present composite was found to be comparable with the four-point-bend result of the composite in the literature. However, the tension–tension fatigue lifetime of the present monolithic BMG was much greater than that of the four-point-bend fatigue of the monolithic BMG in the literature. The fracture morphology indicates that fatigue cracks initiate from porosities or inclusions. The whole fracture surface of LM001 is perpendicular to the loading direction. However, a part of the fracture surface of LM002 is not perpendicular to the loading direction. It demonstrates a ductile facture feature. A mechanistic understanding of the fatigue behavior of the Zr-based BMGs is suggested.