Papers by Author: Akira Takeuchi

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.

Abstract: Molecular dynamics (MD) simulations based on a plastic crystal model (PCM) were performed for a Pd0.4Ni0.4P0.2 alloy in Metal-Metalloid (M-MLD) type of bulk metallic glass (BMG). Two kinds of clusters of cubeoctahedron capped with four half-octahedra and trigonal prism were used as initial atomic arrangements of the Pd0.4Ni0.4P0.2 alloy. Random rotations of clusters around their centers of gravity and subsequent structural relaxation vitrified the alloy. The high glass-forming ability of the Pd0.4Ni0.4P0.2 alloy is due to the critically-percolated, cluster-packed structure that is a universal feature for both M-MLD and M-M types of BMGs.

Abstract: The relationships of optimal compositions of bulk metallic glasses (BMGs) have been analyzed with a set of thermodynamic quantities of mismatch entropy normalized with respect to Boltzmann's constant (Sσ!/kB) and mixing enthalpy (Hmix) and with assistance of classification result of BMGs. The analysis shows clear dependence of the location of Sσ!/kB and Hmix values for optimal compositions of ternary BMGs in Sσ!/kB-Hmix chart in each alloy system. The ternary BMGs are summarized into four types: (1) Cu-based, (2) La-, Zr- and Ca-based, (3) Pd-based and (4) Fe- and Mg-based BMGs according to their maximum value of Sσ!/kB ((Sσ!/kB)max) and the largest and negative value of Hmix ((Hmix)L.N.) or their half values which are obtained for each system. These types of BMGs are related with the local arrangements of BMGs, which are widely accepted as an icosahedral structure for Metal-Metal type, a transformed tetragonal dodecahedrons and triacontrahedrons structure for Pd-Metalloid type, and a network-like structure for Metal-Metalloid type.

Abstract: This paper reviews our recent results of the formation, fundamental properties, workability and applications of late transition metal (LTM) base bulk glassy alloys (BGAs) developed since 1995. The BGAs were obtained in Fe-(Al,Ga)-(P,C,B,Si), Fe-(Cr,Mo)-(C,B), Fe-(Zr,Hf,Nb,Ta)-B, Fe-Ln-B(Ln=lanthanide metal), Fe-B-Si-Nb and Fe-Nd-Al for Fe-based alloys, Co-(Ta,Mo)-B and Co-B-Si-Nb for Co-based alloys, Ni-Nb-(Ti,Zr)-(Co,Ni) for Ni-based alloys, and Cu-Ti-(Zr,Hf), Cu-Al-(Zr,Hf), Cu-Ti-(Zr,Hf)-(Ni,Co) and Cu-Al-(Zr,Hf)-(Ag,Pd) for Cu-based alloys. These BGAs exhibit useful properties of high mechanical strength, large elastic elongation and high corrosion resistance. In addition, Fe- and Co-based glassy alloys have good soft magnetic properties which cannot be obtained for amorphous and crystalline type magnetic alloys. The Feand Ni-based BGAs have already been used in some application fields. These LTM base BGAs are promising as new metallic engineering materials.