Materials Science Forum Vols. 706-709

Abstract: In this study, commercial AM60B magnesium alloy was studied under different solidification conditions to understand the influence of cooling rate, thermal gradient, growth velocity, Niyama criterion, solidification time and mold dimensions on microstructural features such as secondary and tertiary dendrite arm spacing, grain size, porosity, pore shape and size, local morphological and phase variations. Porosity, grain size and dendrite arm spacing were measured as functions of the process variables. It was realized that the process of mold filling and solidification are simultaneous in nature and they significantly affect the microstructure development trends and its dependency on the process parameters. A significant effect, of the above mentioned, was found on the obtained porosity values and their variation along the casting. The results clearly indicate that rate of filling, nature of flow of liquid and shape of the mold greatly affect the solidification process and thereby the microstructure.

Abstract: The artificial ageing treatments have been performed in the commercial magnesium alloys AZ80 (Mg-8.2Al-0.5Zn) with different grain size in the temperature interval 150-250 °C. The comprehensive microstructure observations through ageing were carried out by light microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction analysis. Solid solution decomposition proceeds in non-deformed materials by two modes of precipitated γ – Mg₁₇Al₁₂ phase: discontinuous (grain boundary) and continuous (intragranular) precipitation. A competitive formation of precipitation is influenced by either initial grain size or applied cold deformation before ageing. The electrical resistivity measurements have allowed evaluating some kinetic parameters of precipitated γ - Mg₁₇Al₁₂ phase. The good agreements of the obtained volume fractions of γ - Mg₁₇Al₁₂ phase after different ageing time by electrical resistivity and a XRD technique were observed.

Abstract: Microstructural evolution during the annealing of cold rolled Mg, Mg-1.5Nd and Mg-3Y sheets has been examined. The experimental results show a significant difference in recrystallization kinetics and grain growth between pure Mg and Mg-RE alloy sheets. Pure Mg sheet shows rapid recrystallization and grain growth, whereas recrystallization is considerably retarded in the Mg-RE alloys. Although recrystallized grains which are triggered at shear bands in the cold rolled pure Mg sheet show a relatively weak texture with a basal pole split into the sheet rolling direction, rapid grain growth is accompanied by re-strengthening of the basal-type texture. In contrast, a weak texture appears in the early recrystallization stage in Mg-RE alloys and is retained during annealing due to retarded recrystallization and grain growth.

Abstract: Pre-cold rolling with the reduction of 15% was employed on Mg-8Gd-3Y-0.5Zr (wt.%) (GW83K) alloy in different initial states: as-extruded (state 1) and extruded followed by annealing (state 2) with the aim to investigate the effects on microstructure and mechanical properties. Microstructure observation revealed that there are more amounts of mechanical twins in the alloy in state 2 than that of the alloy in state 1 after cold rolling, which indicates the different deformation mechanisms. Further investigation through EBSD has elucidated the grain boundary structure and types of twins in the alloys. Pre-cold deformation greatly promotes the age hardening response and the peak aging time at 200°C was found to be nearly 12h for the alloy in both state 1 and state 2, which were about 24h and 80h less than that of their non-deformed counterparts, respectively. Tensile tests at temperatures lower than 250°C showed that the alloy in state 1 has a predominant mechanical property than that of the alloy in state 2, while at 300°C, it displayed a reverse tendency.

Abstract: One of the fundamental physical quantities necessary to describe the mechanical properties of the materials is the bulk modulus. In the present report, a simple method to estimate the values of the bulk modulus and its pressure derivative of metallic glasses is presented. The method which is based on a jellium model of metals provides a good agreement with measured data. The estimated values of the elastic constants have been used to determine the equation of state of bulk metallic glasses. It is found that the usual Murnaghan equation of state deviates considerably from the experimental results at high pressures. The deviation has been interpreted to arise from the structural relaxations. The effect of pressure on the fragility of bulk metallic glasses is discussed briefly.

Abstract: Non-isothermal crystallization kinetics of Co₆₇Fe₄Cr₇Si₈B₁₄ amorphous ribbons was studied by differential scanning calorimetry (DSC) technique under 10, 20, 30, 40 and 80 °Cmin^-1 heating rates. It is found that Co₆₇Fe₄Cr₇Si₈B₁₄ amorphous alloy exhibits two-stage crystallization on heating. The two crystallization peaks shift to higher temperatures with increasing heating rate. The apparent activation energies (E_C) for the first stage of crystallization were determined as 443.44 and 434.47 kJmol^-1 by using the Kissinger and Ozawa equations, respectively. Frequency factor (A) estimated to be 1.084×10²⁶ s^-1 using Kissinger equation. Kinetics parameters such as Crystallization exponent (n) and dimensionality of growth (N_dim) were determined using JMA (Johnson-Mehl-Avrami) method. Details of the nucleation and growth behaviours during the non-isothermal crystallization were studied in terms of local activation energy E_C(x) by the OFW (Ozawa, Flynn and Wall) method. Also the activation energy for nucleation (E_n) and growth (E_g) separately estimated.

Abstract: This paper deals with the pressure dependence of plasticity in metallic glasses below glass transition. Recent results indicate that some metallic glasses have such a dependence and that it increases with temperature (Keryvin et al., Phil. Mag., 88, 1773, 2008). We investigate the possibility that such a situation could be a common feature for all metallic glasses by performing a literature review. Results indicate that it is not strightforward to draw decisive conclusions.

Abstract: Metallic glass alloys have been considered as attractive materials due to their excellent mechanical and magnetic properties. However, commercial application of metallic glasses has been limited not only because of high production cost, but because of low market demands. A critical production cost factor was raw material since ferrous metallic glasses were made from high purity electrolytic iron. An idea to reduce the cost was to utilize the raw materials which were taken directly from molten iron and steel manufactured at the existing steel plants. Investigations have been performed to find whether metallic glasses made of molten iron and steel demonstrated appropriate mechanical and magnetic performance. Pilot scale equipments were designed to produce amorphous metallic fibers and strips continuously under the atmospheric condition. Process conditions were optimized by controlling process variables such as alloying and nozzle design, feeding temperature, speed and so on. The glass formability was tested by XRD and DSC analysis. Possibilities of application of Fe-based metallic glasses to reinforced concrete were also evaluated. It was confirmed that a small addition of metallic glass fibers in concrete increased the mechanical performance compared to that of commercial concrete reinforced with steel wires. In addition, amorphous strips were manufactured to apply them to the transformer core. It has been demonstrated that the magnetic properties were equivalent to those of commercial products.

Abstract: Fatigue tests of Zr-based bulk metallic glass plates were conducted under cyclic-torsion, and the results were compared with those obtained under cyclic-bending and axial-loading. The fatigue strength of axial-loading was lower than that under cyclic-bending because the fatigue strength depends on the cooling rate in the casting process of the material, which is different in the thickness direction. The fatigue strength of plane-bending reflects the strength at the specimen surface while that of axial-loading is determined by the weakest strength in the thickness direction. For cyclic torsion of specimens with rectangular cross-section, the fatigue crack initiation site must be determined by the ratio of specimen width to specimen thickness, and fatigue strength depends on the ratio. Therefore, the criteria for fatigue limit under combined stress for metallic glass should be constructed considering the crack initiation site.

Abstract: Molecular dynamics (MD) simulations were performed for a Zr₂Ni alloy by referring to crystallographic features of a metastable Zr₂Ni phase. Simulation method was identical to our previous studies named plastic crystal model (PCM), which includes crystallographic operations for an intermetallic compound in terms of the random rotations of hypothetical clusters around their center of gravity and subsequent annealing at a low temperature. On the basis of MD-PCM, the present study considers an additional refinement named united atom scheme (UAS) on the motions of atoms in the hypothetical clusters. In MD-PCM-UAS, Dreiding potential was assigned for atomic bonds in a cluster whereas Generalized Embedded Atom Method potential for the other atomic pairs. The simulation results by MD-PCM-UAS yield a liquid-like structure. However, annealing did not cause subsequent structural relaxation, which differs from the results by MD-PCM and conventional MD simulations. Further simulations based on MD-PCM-UAS were performed for a nanostructure comprising clusters and glue atoms, leading to the best fit with the experimental data.