Regenerator performance below 4 K in Tm-based bulk metallic glasses

The main obstacle for reaching low temperatures below 4 K using cryocoolers was the inefficiency of the regenerator materials. We report that large volumetric specific heat below 4 K in a wide temperature range has been obtained in Tm-based bulk metallic glasses, and the peak temperature of specific heat peak is tunable. We show that the amorphization of rare-earth based crystalline alloys and the lower magnetic transition temperature of spin glass behavior in the glasses result in the specific heat anomaly below 4 K. The large and tunable specific heat anomaly below 4 K indicates the potential regenerator performance of the glassy materials for sub-4 K cryocoolers.     

Stability of Ni-based bulk metallic glasses

Several ternary (Ni_xNb_ySn_z) refractory alloy glasses (RAGs) were studied at elevated temperatures in order to assess the stability of the amorphous state, i.e. devitrification, and to identify subsequent phase transformations in these materials. differential scanning calorimetry (DSC) experiments indicated a complex phase transformation sequence with several distinct crystallization and melting events being recorded above the glass transition temperature, T_g. Below T_g the RAG samples were studied with an in situ environmental X-ray furnace facility, which allowed step-wise isothermal ramping experiments commencing at a temperature below the reduced temperature of T/T_g ≈ 0.80. Distinct crystalline phases were observed when T/T_g ≈ 0.84 for ternary RAG alloys, while similar experiments on Zr-based Vit 106 glass alloys did not reveal any apparent phase separation until T/T_g ≈ 0.96. The phase separation kinetics followed an Arrhenius type of relationship with Ni₃Sn, and Nb₂O₅ being the principle crystalline precipitates.     

Elastic properties of some bulk metallic glasses

The relationships between the elastic moduli, glass forming ability and response to deformation of bulk metallic glasses are investigated. Five bulk metallic glasses are prepared from high purity elements via suction casting. The results confirm that there exists a correlation between energy absorbed to failure during compression testing and the bulk to shear modulus ratio. This finding is developed such that it corresponds only to the elastic component of energy absorption, and that the bulk modulus dominates this. Plastic deformation appears to be favored by a reduced shear modulus, although it shows greater dependence on structural features that are frozen in during the glass transition, and so may well be dependent on the liquid fragility.     

Formation and thermal stability of Pd-based bulk metallic glasses

The glass-forming ability and thermal stability of bulk glassy Pd₇₉Cu₆Si₁₀P₅ alloy were studied by substitution of Cu with Ag and with Au + Ag from 0 to 6 at.%. The results indicated that the small addition of Ag strongly affects the thermal stability and glass-forming ability of the Pd₇₉Cu₆Si₁₀P₅ alloy. The alloy doped with 4 at.% Ag (Pd₇₉Cu₂Ag₄Si₁₀P₅) exhibits the largest glass-forming ability among the Pd₇₉Cu_6−xAg_xSi₁₀P₅ (x = 0–6 at.%) alloys. The critical diameter for glass formation of this alloy reaches as large as 7 mm by copper mold casting. On the other hand, the multi-addition of Au + Ag does not increase the glass forming ability though the Ag and Au are similar in atomic size. The largest glass forming ability is obtained at 1 at.% Au + 2 at.% Ag among the Pd₇₉Cu_6−x−yAu_xAg_ySi₁₀P₅ (x = 1–4 at.%, y = 1–3 at.%) alloys. The critical diameter of this alloy is 5 mm by copper mold casting.     

Properties of as-cast and structurally relaxed Zr-based bulk metallic glasses

We have studied the influence of the arc melting procedure of Nb-containing Zr-based bulk metallic glasses (BMGs) on their thermal and mechanical properties. We found that the strength and plastic strain to failure, determined at room temperature, increases by the addition of a few percentage of Nb into the Zr-based BMGs. High-resolution transmission-electron microscopy results show that the addition of 2 at.% Nb introduces the formation of as-cast nanocrystals. At the same time, thermal analyses indicate an increase in glass forming ability with the small addition of Nb. Contrary to the reported results in other amorphous alloys, we found that the plastic strain increases further after heat-induced structural relaxation in the Zr-based BMGs.     

Improved plasticity of iron-based high-strength bulk metallic glasses by copper-induced nanocrystallization

A significant enhancement in plasticity was achieved by the micro-addition of Cu in Fe-based bulk metallic glasses (BMGs) which have high strength at room-temperature. It was found that the nanocrystallization caused by the formation of the α-Fe phase is responsible for the improvement in plasticity of the Fe-based BMGs. Our results suggest that the micro-addition of Cu is a promising approach to enhancing the plasticity of Fe-based BMGs through compositional and structural design by microalloying.     

Thermal stability and glass-forming ability of new Ti-based bulk metallic glasses

New Ti-based metallic glass (Ti₅₃Cu₁₅Ni_18.5Al₇Si₃M₃B_0.5 (M = Sc, Hf, Ta, Nb)) alloys were prepared by melt spinning and copper mold casting. The effects of foreign atoms on the thermal stability and the glass-forming ability were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and differential thermal analysis (DTA). The results show that the values of T_rgs are 0.619, 0.609, 0.606, 0.597 K and ΔT_xs are 58, 54, 85, 78 K, respectively. The foreign atom’s size factor (∥R_F − R_Ti∣/R_Ti − 0.12∣) and mixing heat factor (∑x_iH_iF/∑x_iW_i) are proposed to predict the glass-forming ability and thermal stability, respectively.     

Crystallization behaviour of Al-based metallic glasses below and above the glass-transition temperature

The present paper aims to report an effect of a supercooled liquid region on crystallization behaviour of the Al₈₅Y_8−xNd_xNi₅Co₂ metallic glasses produced by rapid solidification of the melt. The paper describes the crystallization process at different regimes of heat treatment. It is found that crystallization behaviour of the above-mentioned Al-based metallic glasses above the glass-transition temperature and below it follows different transformation mechanisms. Formation of the primary nanoscale α-Al particles was observed during continuous heating or after isothermal annealing above the glass-transition temperature. During isothermal annealing below the glass-transition temperature an unknown metastable phase is formed conjointly with α-Al. The metastable phase formed in the Nd-free alloy varies from that in the Nd-bearing alloys. Al₈₅Nd₈Ni₅Co₂ amorphous alloy exhibiting no glass transition crystallizes equally during isothermal calorimetry at different temperatures and during continuous heating.     

Evaluation of glass-forming ability for bulk metallic glasses based on characteristic temperatures

A new criterion ω₂, defined as T_g/(2T_x?T_g)?T_g/T_l (wherein T_g is the glass transition temperature, T_x the onset crystallization temperature, and T_l the liquidus temperature), has been proposed to assess the glass-forming ability (GFA) of bulk metallic glasses (BMGs) based on the classical crystallization theory and the crystallization resistance. The analysis indicates that the factors T_g/(2T_x?T_g) and T_g/T_l could reflect the crystallization resistance and liquid phase stability of metallic glasses, respectively. From the available experimental data in literatures, the new criterion ω₂ has a better correlation with the GFA of metallic glasses than all other existing criteria such as T_rg(=T_g/T_l), ΔT_x(=T_x?T_g), γ(=T_x/(T_g+T_l)), ΔT_rg(=(T_x ? T_g)/(T_l ? T_g)), α(=T_x/T_l), β(=T_x/T_g + T_g/T_l), δ(=T_x/(T_l ? T_g)), φ(=T_rg(ΔT_x/T_g)^0.143), γ_m(=(2T_x ? T_g)/T_l), β(=T_x × T_g/(T_l ? T_x)²) and ξ(=ΔT_x/T_x+T_g/T_l). It has also been demonstrated that this ω₂ parameter is a simple and efficient guideline for exploring new BMG formers.     

Tensile properties of ZrCu-based bulk metallic glasses at ambient and cryogenic temperatures

The tensile behaviors of a series of (Zr_47.5Cu_47.5Al₅)_1 ? x(Zr₈₀Nb₂₀)_x (x = 0, 0.05, 0.10, 0.15) bulk metallic glasses were studied at ambient and cryogenic (77 K) temperatures. It is found that the tensile strength of the alloys increases as the temperature decreases from 298 K to 77 K. The maximum enhancement is 15.7%, and the toughness of these alloys does not deteriorate at low temperatures. We demonstrate that the higher energy required to raise the temperature in the shear bands from the cryogenic temperature to glass transition temperature is the origin of the tensile strength enhancement at low temperatures.     

Effect of strain rates on the fracture morphologies of Zr-based bulk metallic glasses

The effect of strain rates from 1 × 10⁻⁴ s⁻¹ to 2 × 10³ s⁻¹ on tensile fracture morphologies of Zr_52.5Al₁₀Ni₁₀Cu₁₅Be_12.5, Zr₆₅Al₁₀Ni₁₀Cu₁₅, and Zr_52.5Al₁₀Ni_14.6Cu_17.9Ti₅ bulk amorphous alloys was investigated by scanning electron microscopy. The results show that the tensile fracture morphologies of three compositions of bulk amorphous alloys are dependent on strain rate. At low strain rates, the tensile fracture surface morphology of Zr-based bulk metallic glasses presents cleavage veins. However, the morphology will become microvoid-coalescence dimples when the strain rate is high enough.     

Direct imaging of phase separation in Pd41.25Ni41.25P17.5 bulk metallic glasses

The alloy system, Pd_41.25Ni_41.25P_17.5, has a negative heat of mixing among its constituent elements. By using high resolution transmission electron microscopy, high angle annular dark field in scanning transmission mode, and energy dispersive X-ray spectroscopy, it was found that phase separation occurs in Pd_41.25Ni_41.25P_17.5 glassy alloys. For a clear exhibition of the amorphous phase separation reaction, it is desirable to introduce intermediate thermal annealing before an undercooled Pd_41.25Ni_41.25P_17.5 melt is cooled down to become a solid amorphous specimen. The results suggest that there may be unique short range orders in amorphous/liquid Pd_41.25Ni_41.25P_17.5, which are responsible for the phase separation.     

Effect of microalloying of Nb on corrosion resistance and thermal stability of ZrCu-based bulk metallic glasses

Bulk metallic glasses (BMGs) of Zr_46−xNb_xCu_37.6Ag_8.4Al₈ with x = 0, 0.5, 1, 2 and 4 at.% were prepared by copper mould casting. The corrosion resistance of the ZrCu-based BMGs with different Nb contents was carefully examined by weight loss measurements and potentiodynamic polarization tests in 3 mass% NaCl, 1 N HCl and 1 N H₂SO₄ solutions, respectively. Nb addition improves the newly developed BMGs’ corrosion resistance in chloride-containing solutions and the alloys all exhibit excellent corrosion resistance in 1 N H₂SO₄. The corrosion behavior of the alloy containing 0 and 4 at.% Nb in phosphate-buffered solution was examined by electrochemical polarization tests. The influence of Nb addition on glass forming ability (GFA), thermal stability and mechanical property was investigated by X-ray diffraction, differential scanning calorimetry and compression tests, respectively. It is found that the addition of Nb can deteriorate the GFA and thermal stability of the base system, but little effect is observed on the mechanical properties, e.g., yielding strength and plasticity, of the ZrCu-based BMG alloys.     

Effects of Zn on the glass forming ability and mechanical properties of MgLi-based bulk metallic glasses

Through the addition of Zn element, Mg-Li-Cu-Zn-(Y, Gd) bulk metallic glasses (BMGs) with diameter of 2 mm have been successfully fabricated by conventional copper mold injection casting method. The X-ray diffractometer (XRD) patterns and differential scanning calorimeter (DSC) traces demonstrated that the as-cast Mg₆₀Li₅Cu₂₀Zn₅(Y, Gd)₁₀ BMGs were fully amorphous phase. By the minor addition of 5 at.% Zn, the supercooled liquid regions (SLR) of Mg₆₀Li₅Cu₂₀Zn₅(Y, Gd)₁₀ BMGs increased 6 K and 3 K respectively compared with that of Zn-free samples, which implied that their glass forming ability (GFA) and thermal stability were improved. It was also proved that the addition of Zn effectively enhanced the alloys’ fracture strength (453 MPa and 456 MPa) and elastic strain (0.75% and 0.92%), which were much higher than that of Mg₆₀Li₅Cu₂₅Y₁₀ BMG (403 MPa, 0.62%) and Mg₆₀Li₅Cu₂₅Gd₁₀ BMG (412 MPa, 0.82%).     

The influence of Ag substitution for Cu on glass-forming ability and thermal properties of Mg-based bulk metallic glasses

A group of pseudo-ternary Mg–(Cu–Ag)–Dy bulk metallic glasses (BMGs) was developed by copper mold casting. The glass-forming ability (GFA) is significantly improved by the coexistence of similar elements of Ag and Cu. The critical diameter for glass formation increases from 10 mm for ternary Mg_56.5Cu₃₂Dy_11.5 alloy to 18 mm for pseudo-ternary Mg_56.5Cu₂₇Ag₅Dy_11.5 alloy. Thermal stability, crystallization and melting behaviors of the Mg-based BMGs were evaluated. The decrease of Gibbs free energy difference between undercooled liquid and crystalline phases caused by similar element substitution with optimal amount can be responsible for the increase in GFA of the resulting alloys.     

The effects of microalloying on thermal stability,mechanical property and corrosion resistance of Mg-based bulk metallic glasses

(Mg₆₅Cu₂₀Y₁₀Zn₅)₉₈ M₂ (M = Ti, Cr) bulk metallic glasses with diameter of 3 mm were prepared by copper mold casting. The effects of Ti or Cr on thermal stability, mechanical property and corrosion resistance were systematically investigated. It's shown that the Glass forming ability of the Mg₆₅Cu₂₀Y₁₀Zn₅ bulk metallic glasses slightly decreases with the addition of the elements. Whereas, the strength and corrosion resistance significantly increase. The superior corrosion resistance of the amorphous sample containing Ti or Cr is presented for the forming of homogeneous passive layer with highly protective oxides.     

Influence of high temperature deformation on the fracture behavior of a bulk Zr-based metallic glass

Nanocrystalline/amorphous matrix composites obtained by isothermal compression at high temperatures and low strain rates were characterized using transmission electron microscopy. To study the influence of high temperature deformation on the fracture behavior and room temperature plasticity, compression tests with a constant strain rate of 1 × 10⁻⁴ s⁻¹ were applied to the deformed samples. Fracture features of as-cast alloy and deformed samples were analyzed using scanning electron microscopy. Compared with the as-cast alloy, the room temperature plasticity of deformed sample is not destroyed both in the range of 370-395 °C at 1 × 10⁻³ s⁻¹ and at 395 °C in 1 × 10⁻² to 1 × 10⁻³ s⁻¹, and deteriorated at higher temperatures and lower strain rates. Corresponding to the TEM images, the homogenously dispersed nanocrystals with small size contribute to the compressive plasticity, and the aggregated large nanoparticles destroy the plasticity of the sample after high temperature deformation.     

The thermal stability and activation energy of crystallization of (Cu61.8Zr38.2)1−xAlx bulk metallic glasses

The present paper reports on the thermal stability and activation energy of crystallization of bulk metallic glasses (BMGs) (Cu_61.8Zr_38.2)_1−xAl_x. The (Cu_61.8Zr_38.2)_1−xAl_x composition series, prepared by copper mould suction casting into bars with a diameter of 3 mm, form BMGs with an e/a range of 1.24–1.3. These BMGs manifest increased thermal stability with increased e/a ratios. The activation energies (ΔE) of crystallization as derived from thermal analysis at different heating rates follow a similar tendency to that of the thermal stability, indicating stronger short-range ordering with increasing e/a ratios. The optimum BMG Cu_58.1Zr_35.9Al₆ exhibits the highest thermal stability and the largest ΔE.     

Formation and properties of RE55Al25Co20 (RE = Y,Ce, La,Pr, Nd,Gd, Tb,Dy, Ho and Er) bulk metallic glasses

We report that a series of ternary RE₅₅Al₂₅Co₂₀ (RE = Y, Ce, La, Pr, Nd, Gd, Tb, Dy, Ho and Er) alloys can be readily cast into bulk glasses by a conventional casting method. The characteristics and properties of these new bulk metallic glasses (BMGs) are studied and compared. Due to the chemical comparability and well-regulated variety in atomic size, properties and elastic constants of these rare earth elements, the RE₅₅Al₂₅Co₂₀ BMGs could be regarded as a model system to investigate the glass-forming ability, thermal stability, glass transition, crystallization behavior, liquid fragility, elastic and mechanical properties as well as their relationships. An attempt is made to highlight commonality and contrasts of the effects of various factors on the metallic glasses formation and properties.     

Enhanced glass-forming ability of a Zr-based bulk metallic glass with yttrium doping

A significant enhancement in glass formation in a newly developed Zr₅₁Cu_20.7Ni₁₂Al_16.3 alloy has been achieved by yttrium doping. With just 0.5 at.% yttrium doping, the critical diameter of the as-cast alloys for glass formation has been increased from 3 mm to at least 10 mm. In the undoped, large-sized alloys, massive oxygen stabilized crystalline phases are observed but disappear in yttrium doped alloys. Very small amounts of stable α-Y₂O₃ phases found in the yttrium doped alloys, and their negligible effect on the metallic glasses’ properties, provide a superior solution to achieve metallic glasses with a high glass formability.