Materials Science Forum Vols. 561-565

Abstract: We investigated the crystallization behavior of the melt-spun Cu50Zr45Ti5 alloy ribbon on thermal annealing and electron irradiation using a transmission electron microscopy (TEM). The structure composed of a monoclinic CuZr phase with an average grain size of about 8 nm and a glassy matrix was formed by electron irradiation induced crystallization, while by thermal annealing the equilibrium Cu10Zr7 and CuZr2 crystalline phases directly precipitated in the glassy phase of the Cu50Zr45Ti5 glassy alloy.

Abstract: Rod-shaped precipitates in Al -1.1 mass% Mg2Ge (balanced) and Al -1.0 mass% Mg2Ge – 0.5 mass% Mg (excess Mg) alloys aged at 523 K were observed by high-resolution transmission electron microscope (HRTEM) to understand their crystal lattices and chemical compositions. Rod-shaped precipitates were parallel to <100> directions of the matrix. There were 2 groups for rod-shaped precipitates in the base alloy, namely, small cross sections about 10 nm and large ones over 20nm in diameter. Small precipitates showed a hexagonal network of bright dots in their HRTEM images, and its crystal lattice was estimated as a hexagonal having a= 0.72 and c= 0.405 nm based on analysis of HRTEM images and selected area electron diffraction (SAED) patterns. This lattice parameter was slight larger than that of the β’-phase in Al-Mg-Si alloy.

Abstract: Intragranular bainite is known to be effective to develop toughness greatly in steels. Effects of aging below eutectoid temperature before austenitization, the deformation before solution treatment and the duration of solution treatment on nucleation of intragranular bainite have been examined. In the austenite (γ) region where carbide pre-existed, the stress due to inverse transformation, ferrite (α) to γ, would be introduced into fine austenitized region with lower transus temperature to γ, resulting in the introduction of stable small-angle boundary at high temperatures acting as the nucleation site for intragranular bainite lath. Deformation of α by cold rolling before solution treatment enhances the nucleation of intragranular bainite, suggesting that the above γ region through the inverse transformation is easily deformed by the hardened neighboring α. In the deformed γ region, small-angle boundary would be more easily introduced, acting as nucleation site for intragranular bainite. Intragranular bainite lath grew and hardness decreased by prolonging the duration of solution treatment, indicating that transformation stress introduced by the pre-treatment was influenced by solution treatment. The effect of sulfur addition on formation of intragranular bainite was also analyzed.

Abstract: Relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) crystals were grown at low temperature by a single-step hydrothermal process, using the mixture of PbO, MgO and Nb2O5 as precursors. The precursors were just heated to 150 oC for 24 h. The effect of the KOH concentration on the morphology and microstructure of PMN crystals was systematically investigated. The optimum KOH concentration was found to be 11M, which led to the stabilization of the perovskite crystals against the pyrochlore phase, and to the growth of high-quality PMN crystals suitable for subsequent characterization. According to morphological evolution during the reaction, two possible growth mechanisms are proposed. When the KOH concentration is 11M, at the early stage of the reaction, the starting materials dissolve, and then promote the formation of spherical intermediate pyrochlore phase followed by the formation of primary cubic PMN crystals. At the same time, a platelet-shaped intermediate pyrochlore phase also forms and then grows into larger platelet-shaped PMN crystals.

Abstract: Microstructure evolutions and hardness variation during aging in metastable β titanium alloy TIMETAL®LCB have been examined. In as-quenched specimen after solution treatment, athermal ω phase formed. Isothermal ω phase formed during aging at 623 K, and α phase formed after precipitation of isothermal ω phase during aging at 673 and 773 K. Alpha phase nucleated at isothermal ω phase, and had the orientation relationship with β and ω, (111)β//(0001)ω//(1120 )α and [10 1 ]β//[1120 ]ω//[0001]α . During aging at 873 K, α phase formed without isothermal ω phase. Hardness increased with increasing aging time and decreasing aging temperature, partly because number density of isothermal ω or α precipitates increased with increasing aging time and decreasing aging temperature.

Abstract: The charging effects on non-conductive materials due to electrons irradiation were investigated by electron holography. The phenomena that the charging effects were more enhanced with an increase in the incident electron density were visualized through the direct observations in the electric potential distribution around the specimens. In addition, through the comparison between the electron holography results and the simulations, we were able to obtain the quantitative results indicating the amount of charges accumulated during electron irradiation.

Abstract: Superconductivity in two Li-containing compounds of Li2Pd3B and Li2Pt3B was recently discovered. These materials, showing the superconducting transition at 7.2 K and 2.6 K, respectively, have the same cubic structure (P4332) composed of distorted octahedrons without mirror or inversion symmetry along any directions. This is a very interesting feature of those materials in relation to the symmetry of superconductivity. Resistivity measurements in magnetic fields gave their upper critical fields, Hc2(0) = 45 kOe and 19 kOe, respectively. Their specific heat was measured using a heat-pulse relaxation method. The Sommerfeld coefficient (γ) and Debye temperature (θD) terms of Li2Pd3B were given as γ=9.5 mJmol-1K-2 and θD=228 K . The value of C/γT at Tc was calculated to be 1.7. In the same manner, those parameters were described for Li2Pt3B as γ=9.6 mJmol-1K-2, θD=240 K, and C/γTc =0.75, respectively. Since C/γTC in the weakcoupling limit by the BCS theory is 1.43, the value of 1.7 for Li2Pd3B is slightly higher. The electronic specific heat of Li2Pd3B at a zero magnetic field follows the typical exponetial behavior discribed in the BCS theory, while that of Li2Pt3B shows quadratic behavior. This result suggests the line nodes exist in the superconducting gap of Li2Pt3B driven by the spin-orbit interaction.

Abstract: The contribution of nano-scale particles observed using Atom Probe Tomography in an increase of yield strength of conventional and advanced HSLA steels was studied. The advanced HSLA steel showed higher yield strength than conventional HSLA steel. There were two types of carbides, which primarily contribute to an increase in yield strength of conventional HSLA steel: (i) coarse TiC with average size of 25±5nm and (ii) fine TiC with average radius of 3±1.2nm. The presence of two types of carbides was found in the microstructure of advanced HSLA steel: (i) nano-scale Ti0.98Mo0.02C0.6 carbides with average radius of 2.2±0.5nm, and (ii) C19Cr7Mo24 particles with an average radius of 1.5±0.3nm. The contribution of precipitation hardening in the yield strength of advanced HSLA steel due to the nano-scale particles was 174MPa, while this value in the conventional HSLA steel was 128MPa.