Materials Science Forum Vols. 539-543

Abstract: Optimum corrosive density-grading of T6- and solution-treated alloys was studied using NaOH solutions. Rough corroded strut surfaces in aluminum foams were found to transform to smooth metallic surfaces with an increase in pH from 10 to 13. For instance, during immersion in pH13 solutions, pits with large curvatures appeared on strut surfaces, and corrosive mass losses increased. Strut diameters also decreased, and open cell windows became wider, without an increase in visible damage. Processing parameters such as pH, corrosion rate, and the surface state of the struts were examined to optimize final structure. An experimental method for grading of relative density continuously between 10% and 5% is described.

Abstract: CuxZr1-x (x=0.30~0.85) binary amorphous alloy models were constructed using molecular dynamics simulations. In order to characterize the short-ranged local structures and the free volume of the whole cell, Voronoi polyhedral analyses were performed for their initial states and also for the deformed states. Both geometrical and physical factors are intimately related to each other and exhibit a large influence on mechanical properties. The elastic properties of shear modulus and bulk modulus tend to increase as the free volume decreases. The Poisson’s ratio, which is related to the glass fragility, increases as the free volume. However, the relation among three elastic properties obtained from the continuous mechanics of the isotropic body does not hold. Also, the maximum stress in the finite deformation drastically decreases with increasing free volume content.

Abstract: Nano-crystalline-amorphous Al based alloys with minor additions of rare earth elements and transition metals are of technical interest, because of their extraordinary high mechanical strength. This strengthening effect depends strongly on the alloy composition and the pathway of crystallisation. The crystallisation behaviour of Al87Ni6La7 and of Al87Ni5La7Zr metallic glass was studied with complementary methods such as XRD, TEM and 3D-AP. The amorphous matrix of the Zr-containing glass shows fluctuations of all minor elements on the nanometer scale. It is suggested, that these fluctuations act as nucleation zones for the crystallization of the glass during annealing.

Abstract: The flux treatment was applied to increase the glass-forming ability of a glassy [(Fe0.5Co0.5)0.75B0.20Si0.05]96Nb4 alloy. The large bulk glassy specimen with diameter of 7.7 mm was prepared by water quenching the melt immersed in the molten flux of B2O3. The diameter of the specimens is approximately 1.5 times as large as the previous result, even though the cooling rate of copper mold casting should be much higher than that of water quenching. The critical cooling rate for a glassy phase is 150 - 170 K/s, which is much slower than 500 K/s without the flux treatment. The flux treatment improves the glass-forming ability by cleaning the molten metal where heterogeneous nucleation is difficult to take place.

Abstract: The thermal and mechanical characteristics of various Mg-Cu(Ni)-Y(Gd) metallic glassy alloys prepared by melt spinning are examined using differential scanning calorimetry (DSC), thermomechanical analyzer (TMA), and instrumental nanoindenter. The replacement of Y by Gd appears to benefit both the thermal and mechanical properties, while the replacement of Cu by Ni improves only the hardness and modulus, with the sacrifice of thermal characteristics. The amorphous Mg-Cu-Gd based alloys can be fabricated into rods with a diameter greater than 6 mm, with minimum porosity and reasonable toughness.

Abstract: Temperature dependent elastic constants Cij(T) of Cu60Hf30Ti10 bulk metallic glass (BMG) have been investigated in a MHz frequency range using electromagnetic acoustic resonance (EMAR) up to 823 K. At room temperature, the BMG showed high Poisson’s ratio ν arising from low shear modulus G compared with that of constitutive crystalline elements. With increasing temperature, G showed usual linear temperature dependence while it suddenly drops around glass transition temperature, Tg. Within a framework of quasiharmonic approximation, Grüneisen parameter γ around Tg is estimated to be 10. This extremely large γ indicates the high anhramonicity of long wavelength limit acoustic mode phonon in the supercooled liquid state. The unusual elastic behavior can be interpreted on the basis of heterogeneous microstructure.

Abstract: Mechanical alloying (MA), super cooling process, etc. have been used to prepare amorphous phases, metastable solid solutions, nanocrystals, and so on. It is important to consolidate these powders for evaluating the physical properties, and for applications. On the other hand, shock compression can be used as an effective consolidation method for metastable material powders without recrystallization or decomposition. We had prepared metastable transition-metal system bulk alloys and compounds (Fe-Co, Fe-Cu, Fe-W, Co-Cu, Sm-Fe-N systems, etc) by using MA and shock compression. The Fe-Cu and Co-Cu metastable solid solutions showed a fit curve to the Slater-Pauling one. The Co-Cu metastable solid solution bulk alloy showed a magneto-resistance. The Fe-Co fine-grained bulk alloys show the higher coeicivity than that of molten alloy. In this paper, the preparation and magnetic properties of the metastable alloys (Fe-Co, Fe-Cu, Co-Cu systems) are reviewed, and the applications to materials science and engineering are discussed.

Abstract: Mechanical alloying (MA), super cooling process, etc. have been used to prepare amorphous phases, metastable solid solutions, nanocrystals, and so on. It is important to consolidate these powders for evaluating the physical properties, and for applications. On the other hand, shock compression can be used as an effective consolidation method for metastable material powders without recrystallization or decomposition. We had prepared metastable transition-metal system bulk alloys and compounds (Fe-Co, Fe-Cu, Fe-W, Co-Cu, Sm-Fe-N systems, etc) by using MA and shock compression. The Fe-Cu and Co-Cu metastable solid solutions showed a fit curve to the Slater-Pauling one. The Co-Cu metastable solid solution bulk alloy showed a magneto-resistance. The Fe-Co fine-grained bulk alloys show the higher coeicivity than that of molten alloy. In this paper, the preparation and magnetic properties of the metastable alloys (Fe-Co, Fe-Cu, Co-Cu systems) are reviewed, and the applications to materials science and engineering are discussed.

Abstract: Dependence of the femtosecond laser ablation depth on the laser pulse energy was investigated for Zr55Al10Ni5Cu30 bulk metallic glass. Investigation of the femtosecond laser ablation of bulk metallic glasses has not been reported. Femtosecond laser pulses (wavelength of 800 nm, pulse width of 100 fs, pulse energies of 2 – 900 μJ) were focused and irradiated on the polished surface of metals in air. The ablation depth of the metallic glass is deeper than that of its crystallized metal at a pulse energy in the strong ablation region. We suggest that the energy loss at grain boundaries of hot electrons which is accelerated by the laser electric field influence the ablation depth in the strong ablation region.

Abstract: The purpose of this research is to investigate the micro-mechanism of anelastic behavior of metallic glass. The changes of atomic arrangement in Cu50Zr50 metallic glass were observed directly by EXAFS method during tensile deformation. The transmission-mode EXAFS measurements around Zr K-edge and Cu K-edge of the sample in various amounts of tensile deformation conditions were done. The interatomic distance between Cu-Zr increased, and that between Zr-Zr decreased simultaneously, with the amount of tensile deformation increase of the sample at first stage of deformation. When the atomic distances reached to certain values respectively, they recovered to the initial values of no tensile deformation condition at first stage, and then the atomic distances changed as same manner as the first stage of deformation, respectively. This is the first observation on the change of atomic arrangement during tensile deformation of metallic glass.