A critical analysis of the glass-forming ability of alloys

Several empirical rules have been proposed during the past few years to synthesize bulk metallic glasses. But, the real reasons for the improved glass-forming ability of these alloys are still not clear and the ability to design alloy compositions to enable synthesis of larger diameter rods has not improved. The present work conducts a critical analysis of the existing data in terms of the different glass-forming criteria and concludes that the available parameters cannot satisfactorily predict the GFA and explain all the observed data. Reasons for this failure have been suggested.     

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.     

Selection of good glass former compositions in Ni-Ti system using a combination of topological instability and thermodynamic criteria

A combination of two criteria, which have been recently reported in the literature to predict compositions with high glass-forming ability, has been applied in the present work for the Ni-Ti system. The first criterion is an extension of the topological instability ‘λ criterion’, used here to indicate (and avoid) the composition fields associated with the crystallization of the first phase (solid solution or intermetallic compound) to appear during crystallization. The second, the thermodynamic criterion, is based on the calculation of a parameter (γ∗) that relates the enthalpies of all the competing phases including the amorphous one. A combination of these two criteria for the binary Ni-Ti system indicated the existence of only a narrow composition interval for glass formation. Alloy compositions in this interval were prepared by melt-spinning and the glass-forming ability (GFA) of the as-cast ribbons was evaluated by X-ray diffraction and differential scanning calorimetry. The results indicated that the two criteria are complementary, since compositions with high GFA are expected to have a high γ∗ parameter and also correspond to intervals where the λ parameter is greater than 0.1.     

Effect of Ti addition on the crystallization behavior and glass-forming ability of Zr-Al-Cu alloys

The crystallization behavior of melt-spun (Zr₆₅Al_7.5Cu_27.5)_100−xTi_x (x = 0-15; in at.%) metallic glasses has been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The DSC traces showed an altered crystallization mode in the vicinity of 3 at.% Ti addition. A metastable icosahedral quasicrystal precipitated at the first crystallization stage of the Ti-bearing metallic glasses, which subsequently transformed to the stable Zr₂Cu-type phase in the followed exothermic reaction. The glass-forming abilities (GFAs) of these metallic glasses were assessed by the recognized GFA indicators T_rg, ΔT_x and γ. BMGs were easily made in the compositions containing 3-7 at.% Ti by means of copper mold casting. The validity of these parameters was clarified using the critical BMG forming diameter evidence.     

Crystallization of Fe75Zr25 metallic glass: a two‐step process involving metastable bcc‐Fe and polymorphic transformation

The crystallization process of mechanically alloyed Fe₇₅Zr₂₅ metallic glasses is investigated by means of both thermo‐magnetization and in situ neutron powder thermo‐diffraction experiments in the temperature range 300–1073 K. It was found that the crystallization takes place in a two‐step process, involving firstly the appearance of metastable Fe and Fe₂Zr crystalline phases between 880 K and 980 K, and a subsequent polymorphic transformation into Fe₃Zr above 980 K. These findings explain the anomalous magnetization vs. temperature behaviour on heating–cooling cycles.

     

弹丸侵彻混凝土的SPH算法

给出了弹丸侵彻混凝土的数值计算,其中弹体作为刚体处理并划分成Lagrangian标准有限元网格,而混凝土划分成光滑粒子并经历大应变、高应变率和高压作用。为了描述混凝土的非线性变形及断裂特性,在计算中引入了Holmquist Johnson Cook本构模型及损伤模型。计算结果与实验结果的对比表明,利用光滑粒子流体动力学方法对混凝土材料进行大应变、高应变率的变形计算是有效的,并可避免网格重分或网格消蚀。此外,将光滑粒子流体动力学方法和有限元方法结合可以保持计算过程中材料界面的清晰。     

Theoretical prediction of bulk glass forming ability (BGFA) of Ti-Cu based multicomponent alloys

The bulk glass forming ability (BGFA) of Ti-Cu based multicomponent alloys has been evaluated via theoretical modeling and computer simulation studies based on a combination of electronic theory of alloys in the pseudopotential approximation and the statistical thermodynamical theory of liquid alloys. The magnitude of atomic ordering energies, calculated by means of the electronic theory of alloys in the pseudopotential approximation, was subsequently used for calculation of the key thermodynamic parameters such as enthalpy, entropy and Gibbs free energy of mixing, viscosity, and critical cooling rate of the binary Ti-Cu and ternary Ti-Cu-X alloys. The potential alloying elements (X) can be divided in two groups defined by their effect on the variation of the negative heat of mixing and their influence on the critical cooling rate. Most of the predicted candidate alloying elements from either X_I (Al, Si, Ag) or X_II (Co, Ni, Fe, Sn, Be) and/or both groups have already been used successfully for the fabrication of new Ti-Cu based bulk metallic glasses. It was also shown that the critical cooling rate appears to be a more important parameter rather than the change in the negative heat of mixing for the prediction of candidate alloying elements improving BGFA.     

Kinetic analysis of the isochronal crystallization of Ti40Zr25Ni8Cu9Be18 metallic glass

The isochronal crystallization kinetics of the Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ metallic glass has been investigated by differential scanning calorimetry (DSC). Results indicate that the two crystallization events of this metallic glass cannot be well-described by the classic Johnson-Mehl-Avrami (JMA) kinetic equation. The kinetic equation considering the impingement effect has been found more applicable for describing the isochronal crystallization kinetics of this amorphous alloy. Accurate values of kinetic parameters were determined by fitting the theoretical DSC data to experimental curves. The kinetic parameters change in different crystallization stages and show strong heating rate dependence. Reasons of the deviation from the JMA kinetics for the isochronal crystallization of Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ metallic glass were discussed.     

Estimation of shear-banding resistance in metallic glass containing nano-crystalline particles

Bulk metallic glasses (BMGs) have a variety of excellent properties compared with the majority of conventional crystalline alloys. However, they exhibit limited global plasticity at room temperature because of shear banding. Several methods have been proposed to improve the limited ductility of BMG; one method is the homogeneous distribution of crystalline particles. However, our understanding of the interaction between the crystalline particles and shear bands (SB) is not sufficient. Here, we performed molecular dynamics (MD) simulations of mode II deformation of a notched BMG plate and BMG plates containing one nano-crystalline particle ahead of the notch bottom. To compare the effect of crystalline particle size on the resistance to SB propagation, we used the J-integral. By comparing J-R curves and the deformation behavior of the BMG plates with and without nano-crystalline particles, we found that the resistance to shear banding is efficiently improved by introducing crystalline particles with sufficient size, compared to the SB width.     

Estimation of Kauzmann temperature and isenthalpic temperature of metallic glasses

We propose expressions for the estimation of the isenthalpic temperature T ₀ (T ₀ = αT _m , α is a semi-empirical parameter and 0 ⩽ α < 1, T _m is the solidus temperature) and the Kauzmann temperature T _k (T _k = T _m exp(α−1)) for glass forming alloys. It is found that T _k estimated by T _k = T _m exp(α−1) is in agreement with that directly calculated from the heat capacity data, indicating that T _k = T _m exp(α − 1) can be used to estimate T _k of glass forming alloys. T ₀ estimated by T ₀ = αT _m , on the other hand, widely deviates from that of directly calculated from the heat capacity data. This suggests that the enthalpy difference of the under-cooled liquid and the crystal might be a nonlinear function of the temperature below T _k . Moreover, the Gibbs free energy difference ΔG is not sensitive to the deviation of α.