Materials Science Forum Vol. 985

Abstract: The thermal conductivity of metals and alloys is an important physical property. The thermal conductivity of metals and alloys in the liquid state is usually hard to be measured, because it is difficult to exclude the effect of convection. As such, as a convenient approach, the Wiedemann-Franz law has been used for the estimation of thermal conductivity of metals and alloys in the liquid state. However, some reports show the deviation from Wiedemann-Franz law of pure Sn and Sn-based metals in the liquid state. Measuring the thermal conductivity of various Sn-based alloys in the liquid state is of significant importance to identify its heat transfer in terms of whether the deviation from Wiedemann-Franz law is observed. In this study, Field’s metal is focused on as one of the Sn-based alloys. Then, its thermal conductivity in the solid and liquid states with various temperature ranges by employing the transient hot wire method. This method is appropriate due to its nature of dealing with the convection-including problems. Finally, the adaptability of Wiedemann-Franz law is analyzed by using the obtained thermal conductivity and electric conductivity data. In this study, the deviation from Wiedemann-Franz law was observed in the liquid state.

Abstract: Zero-field muon spin relaxation experiments were carried out for Al-1.6%Mg₂Si and a pure aluminum in isothermal conditions between 260 and 300 K. Observed relaxation spectra were analyzed to extract the dipole width (D) values which were found to decrease with time after solution heat treatment and quenching. Time variations of D appeared to take place two stages in both samples. The stage transition times (t_II) deduced for Al-1.6%Mg₂Si were comparable to those for the Si-rich clustering stage reported for Al-Mg-Si alloys. The estimated activation energy of Si-rich clustering was 0.62 (±0.04) eV. The stage transition times (t_M) for the pure aluminum were 255, 110 and 82 min after quenching at the measuring temperatures of 260, 280 and 300 K, respectively. An Arrhenius plot of logarithmic t_M against reciprocal temperature resulted in an activation energy of 0.19 (±0.06) eV.

Abstract: This study investigates a brazing method for manufacturing PbTe thermoelectric modules using a Ag-based filler metal with a melting point of about 650 °C. To improve the bonding strength between the Ag-based brazing layer and the PbTe thermoelectric module, an electroless Ni-P plating layer is formed on the surface of the thermoelectric module as a diffusion barrier layer. The bonding strength of the PbTe thermoelectric module manufactured by the electroless Ni-P plating and Ag-based brazing has a high value of approximately 8.3 MPa. No defects such as pores or cracks were observed at the bonding interface between the thermoelectric element and the brazing layer. Furthermore, because of the high bonding strength of the manufactured thermoelectric module, fractures occur inside the thermoelectric element rather than at the bonding interface. Accordingly, the electroless Ni-P plating and Ag-based brazing method proposed in this study is found to be effective in manufacturing PbTe-based thermoelectric modules with high bonding strength.

Abstract: Misfit precipitates greatly contribute to precipitation hardening in wrought aluminum alloys, where attractive and repulsive interactions are expected by stress-strain field of fine misfit precipitates. There are two types of dislocation cutting manner of {001} GP-zone and θ’ phase in Al-Cu alloys; one is dislocation burgers vector intersects (001) variant by 0 deg. (Type A), the other is dislocation Burgers vector intersects (001) variant by 60 deg. (Type B). In order to simulate the interaction of dislocation and fine misfit precipitates, internal stress fields by dislocation and precipitate are computed by Micromechanics based Green’s function method. The elastic field inside and outside a precipitate is deduced from Eshelby’s inclusion theory, where misfit strain of a (001) precipitate is assumed by unidirectional eigenstrain across the disk shaped inclusion. Dislocation motion under three different kinds of dislocation Burgers vector is tested by computing interaction force acted on the discretized dislocation line elements. The interaction force caused by (001) misfit precipitate is varied with types of dislocation cutting manner, magnitude of the interaction force associated with dislocation glide is increased by Type B variant (60 deg.), whereas that is minutely zero for Type A variant (parallel).

Abstract: We report on very accurate magnetic measurements on large rare earth orthoferrites single crystals of ErFeO₃ and NdFeO₃. Our results show that the interaction between rare earth and iron spin system does not change during the spin-flip process. This implies that the coupling between the iron and rare earth spin systems is robust enough to withstand the effects of spin flipping against the magnetic anisotropy energy. This is despite rare eath ions, polarized by the ordered iron ions, being in partly metastable state and their magnetic moment decays with time.

Abstract: Effect of precipitation strengthening on metal is generally attributed to the dislocation interaction with the precipitate which acts as the barrier to the dislocation motion on the slip plane. In order to achieve better understanding of critical events of dislocation motion and evolution of dislocation microstructure, we have developed numerical simulation method of dislocation-dislocation and dislocation-particle interactions by means of discrete dislocation dynamics at mesoscopic scale. In this work, Green’s function method is utilized for the computation of the stress fields of dislocation and misfitting particle, and the interaction forces acting on the dislocation. We also proposed the efficient algorithm of the connectivity vector for the dislocation line elements, linked-list data structure, to deal with the flexible interaction of dislocation line elements. The geometrical effect of dislocation slip planes on the dislocation bypassing behaviors is tested by changing the relative height of dislocation slip plane against the center plane of spherical particle, where cross slip event is also taken into account for the dislocation motion. Simulation results show a wide variety of topological changes of dislocation during motion on the slip planes around the particle, which results from the stress field of the particle varied with the relative height between the dislocation slip plane and center plane of particle. The full analysis of the mechanisms of dislocation line bypassing misfitting particle has been explained in this study.

Abstract: The reverse transformation of austenite from pearlite was observed in-situ by using a sophisticated EBSD system. Quantitative information on the nucleation sites and orientation selection of the austenite was obtained. Initially, the nucleation sites were restricted to high-angle grain boundary (HAB) and all the austenite orientations were selected by Kurdjumov-Sachs (K-S) orientation relationship. Latterly, the constraints were relaxed as new nucleation sites were involved. The γ growth was preferentially into the pearlite grains without K-S relation. Nucleation and growth at HAB edge may contribute to γ coarse grains, whereas nucleation at pre-existing γ boundary is important to grain refinement of γ structure.

Abstract: The geometric structure and electronic properties of the adsorption of organic carboxylic acids on the closed-packed Cu(111) surface have been addressed by periodic density functional theory (DFT) calculations. We also have taken into account van der Waals (vdW) interaction by the VdW-DF method. The optimized structures show that formic and acetic acids have stable structures of molecular adsorption in clean copper surfaces. We find that the adsorption energies at 1/16 ML coverage are -0.27, -0.30 and -0.10 eV for formic, acetic, and propionic acid, respectively. On the other hand, in the case of vdW-DF, their adsorption energies increased to -0.63, -0.70 and -0.73 eV.

Abstract: We use the Fe-Cu-C sintered material as a representative material of powder metallurgy (PM). Cu and C which are additive elements of the Fe-Cu-C sintered material contribute to the improvement of mechanical strength and Cu additionally has a role of volumetric control. Graphite powder is used for C, which is diffused into Fe during sintering.