The dynamic viscosity of Al-Yb and Al-Ni-Yb superheated melts was measured using a torsional oscillation viscometer. The results show that the temperature dependence of viscosity fits the Arrhenius law well and the fitting factors are calculated. The amorphous ribbons of these alloys were produced by the melt spinning technique and the thermal properties were characterized by using a differential scanning calorimetry (DSC). E (the activation energy for viscous flow), which reflects the change rate of viscosity, has a good negative relation with the GFA in both Al-Yb and Al-Ni-Yb systems. However, there is no direct relation between liquidus viscosity (ηL) and GFA. The superheated fragility M can predict GFA in Al-Yb or Al-Ni-Yb alloy system.
JIA Ran, BIAN XiuFang, Lü XiaoQian, SONG KaiKai & LI XueLian Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China
The atomic structures of liquid Ag-based binary alloys have been investigated in the solidification process by means of X-ray diffraction. The results of liquid structure show that there is a break point in the mean nearest neighbor distance r1 and the coordination number Nmin for glass-forming liquid, while the correlation radius rc and the coordination number Nmin display a monotone variational trend above the break point. It means glass-forming liquids have a steady changing in structure above liquidus and more inhomogeneous state at liquidus. We conclude that there is a strong correlation between liquid structure and glass forming ability in Ag-based binary alloys.
Lü XiaoQian, BIAN XiuFang, XIANG Nan & JIA Ran Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China
采用第一性原理分子动力学(Ab Initio Molecular Dynamics,AIMD)方法研究了Al和Zr液固转变过程中的能量、偶关联函数、结构因子和键对分布的变化规律,获得了不同温度下两种金属液体的扩散系数和黏度.结果表明,AIMD计算得到的液态金属偶关联函数、结构因子和扩散系数与实验测量数据符合得很好.在冷速为5.0×1013和2.5×1013K/s时,液态Al分别在730K附近发生玻璃化转变或者形成有一定缺陷的fcc晶体结构.在平均冷却速率为4.3×1013和2.0×1014K/s的条件下,液态Zr在1200K时分别开始转变为热力学上亚稳定的bcc结构和玻璃相.Zr的液态和玻璃态结构中二十面体和bcc类型短程序是其主要拓扑短程序.
The fragility of superheated melts, M, for 13 kinds of metallic alloys has been evaluated from the data of the dynamic viscosity above their liquidus temperatures. The authors find that the glass forming ability of metallic melts depends on the fragility of superheated melts rather than on the value of viscosity. In the present work the value of fragility is less than 1 for good glass-forming melts but more than 1 for the other melts. The variation rate of atomic coordination number with temperature indicates clearly the relaxation rate of molten structures. The fragility of superheated melts is found in good agreement with the variation rate of the atomic coordination number with temperature.
BIAN XiuFang, QIN JingYu & QIN XuBo Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China
A series of Er-Al-Co bulk metallic glasses (BMGs) have been prepared by the copper mold casting method. The glass forming ability and magnetocaloric effect (MCE) for these alloys have been investigated. The second-order magnetic transition from para-magnetic to ferromagnetic states takes place at about 9 K. These BMGs exhibit excellent MCE because of their large effective magneton number; Er56Al24Co20 BMG has a maximum entropy change and refrigeration capacity of 16.06 J kg-1 K-1 and 546 J kg-1,respectively,under the field of 50 kOe (10 kOe=795.775 kA/m) indicating that these BMGs are potential candidate magnetic materials for hydrogen liquefaction.
HUI XiDong XU ZhiYi WU Yuan CHEN XiaoHua LIU XiongJun LU ZhaoPing
The microstructures and thermal properties of Fe61Co9-xZr8Mo5WxB17 (x=0 and 2) formed under different vacuum conditions were investigated by scanning electron microscopy(SEM), X-ray diffractometry(XRD), transmission electron microscopy(TEM), and conventional dilatometry(DIL). The variation of the non-monotonic effects of tungsten content and vacuum conditions on the glass forming ability(GFA) of Fe-based alloys can be drawn in a schematic diagram. The higher the GFA of alloys, the higher the difference between the thermal expansion coefficients of glassy state and crystalline state(-α), which can be described by the free volume model during dilatometric measurements. Under low and high vacuum conditions, the viscosity and microhardness are improved and the fragility of the Fe-based alloys are decreased by adding tungsten.
