L.-S. Chang

Thermodynamic aspects of the grain boundary segregation in Cu(Bi) alloys

Abstract The grain boundary segregation of Bi in dilute polycrystalline Cu–Bi alloys was systematically studied as a function of temperature and composition. The temperature dependencies of the Gibbsian excess of Bi at the grain boundaries exhibited discontinuous changes at the temperatures close to, but different from the bulk solidus temperatures. The observed segregational phase transition was interpreted in terms of prewetting model.

Temperature dependence of the grain boundary segregation of Bi in Ni polycrystals

Abstract The grain boundary segregation of Bi in Ni polycrystals doped with Bi and annealed at 400–1000 °C was investigated by Auger electron spectroscopy (AES). In the Ni–50 wt. ppm Bi alloy the amount of Bi segregation at grain boundaries decreases continuously with increasing temperature. Abrupt changes of segregation were observed in Ni–90 wt. ppm Bi and Ni–160 wt. ppm Bi alloys between 400–500 and 900–1000 °C, respectively.

The Solidus Line of the Cu-Bi Phase Diagram

The solid solubility of Bi in Cu single crystals has been experimentally determined. It is shown that the solidus line is a retrograde curve without a monotectic transition. The solid and liquid phases are successfully described with simple thermodynamic models. The experimentally measured maximum solubility of 0.0207 at. % Bi at 975 °C correlates well with that from the model (0.0193 at. % Bi at 968 °C). A linear temperature dependence of the interchange energies is suggested, and the values of the optimized coefficients are in accordance with those estimated from the thermal expansion coefficients. The calculated thermodynamic functions are in good agreement with the assessed experimental data.

Characterization of alumina ceramics by ultrasonic testing

Some material characteristics of alumina ceramics, e.g., porosity, Youngs modulus and Poissons ratio, have been evaluated using ultrasonic sound. Experimental results showed that the velocity of an ultrasonic sound wave in alumina ceramics decreased proportionally with increasing porosity. Youngs modulus and Poissons ratio calculated from the longitudinal and transverse ultrasonic velocities were found to be dependent on porosity. The results from the present program have been compared with data from the literature, and similarities and differences discussed. The deviation of these data from a theoretical prediction may be due to the texture of the porosity introduced by different alumina-forming processes.

Kinetic aspects of the grain boundary segregation in Cu(Bi) alloys

Abstract The kinetics of grain boundary segregation in Cu(Bi) alloys has been studied at temperatures of 600 and 850°C by Auger electron spectroscopy. A substantial acceleration of the grain boundary segregation kinetics in the two-phase area of the Cu–Bi phase diagram is observed. To explain this acceleration, a model of dislocation diffusion is proposed. In the framework of the model, the accelerated kinetics of grain boundary segregation in the two-phase area of the Cu–Bi phase diagram results from the microscopic pipe-like precipitation of a Bi-rich liquid phase along the dislocation cores. The dislocation diffusivities are estimated and two kinds of diagrams of the segregation kinetics are constructed.

On the correlation between grain-boundary segregation, faceting and embrittlement in Bi-doped Cu

Abstract Cu bicrystals with a ∑ = 19a {331} grain boundary, grown by the Bridgman technique, have been doped with 14, 24, 25 and 64 at.ppm Bi. After annealing between 500 and 800°C the segregation was quantified using energy-dispersive X-ray spectroscopy in a dedicated scanning transmission electron microscope. Grain-boundary faceting on low-index planes was observed and the segregation level was significantly higher at the facets than at the non-faceted segments. The fraction of faceted grain boundaries was found to increase with increasing annealing temperature, implying that the faceting transition is thermally activated. Only the completely faceted grain boundary exhibits brittle behaviour. Faceting appears to be a necessary prerequisite for grain-boundary embrittlement of the ∑ = 19a, {331} grain boundary, possibly because it allows more Bi atoms to be incorporated at the boundary. It is suggested that this embrittling mechanism may be a general phenomenon in Bi-doped Cu.

The effect of bismuth segregation on the faceting of Σ3 and Σ9 coincidence boundaries in copper bicrystals

Abstract The faceting of a cylindric tilt grain boundary (GB) in Cu and Cu-160 ppm Bi bicrystals was studied. The crystal lattices of both grains form a superlattice called a coincidence site lattice (CSL) with an inverse density of coincidence sites relating to the density of lattice sites Σ = 3 and Σ = 9. The Σ3 and Σ9 <110> tilt GBs became faceted upon annealing at 0.95 Tm. The ratio of GB energy σGB and surface energy σsur of the specimen was measured, applying atomic force microscopy, from the profile of the GB thermal groove formed upon additional annealing. The 160 ppm Bi concentration in the Bi-containing bicrystal is between GB and bulk solidus lines in the Cu – Bi phase diagram. In this concentration region GBs contain a multilayer Bi-rich film which is supposed to be liquid-like. Wulff – Herring diagrams were constructed using the measured σGB/σsur values, revealing stable and metastable GB facets.

The Grain Boundary Wetting in the Sn– 25 at% In Alloys

The grain boundary (GB) wetting was investigated in the Sn – 25 at.% In alloy. It was found that the portion of GBs wetted by the melt depends on the annealing temperature. No GB completely wetted by melt was observed at 140°C, while all GBs were fully wetted after annealing at 180°C. Between 140°C and 180°C the portion of wetted GBs increases with increasing temperature. The tie-lines of GB wetting phase transition were constructed in the Sn–In bulk phase diagram.

The Influence of Quenching Baths on Grain Boundary Wetting Transition in Sn–25 at% In alloy

The wetting behavior of grain boundaries is affected by temperature, pressure and misorientation of grain boundaries. However, the influence of quenching baths on liquid state grain boundary wetting is rarely reported. In this work, this effect in the Sn−25 at% In alloy was investigated. The Sn−In alloy was prepared by smelting of In and Sn at 300°C for 6 hr in Ar atmosphere. Samples were annealed at temperature between 130 and 165°C and quenched in two kinds of baths: −10°C salt-saturated water and liquid nitrogen. The results from X-ray diffraction show a difference in preferential orientations between samples quenched in these two baths. Metallographic analysis reveals that the percentage of grain boundaries fully wetted in samples quenched in −10°C salt-saturated water is lower than that quenched in liquid nitrogen. It is pointed out that a proper quenching bath is necessary for preserving the initial microstructure of grain boundary wetting.

Evaluation of the Coverage Pattern on the Fracture Surface of Bi-Embrittled Cu Grain Boundaries by Means of Auger Electron Spectroscopy

In studying grain boundary segregation in Cu–Bi alloys by means of Auger electron spectroscopy samples must be broken in-situ. Consequently, the distribution of segregants on both sides of fracture path must be considered quantitatively. This question can be addressed by studying the influence of an adsorbed oxygen layer on the intensity of Auger peaks. This oxygen layer forms on the fracture surface when it is intentionally exposed to air. In this work, the values of Bi coverage have been measured both on the as-fractured Cu fracture surface and on the fracture exposed to air. The coverage values evaluated from the model of a crosstie-like pattern agree better with each other than those from the model of a continuous layer. Our study reveals that the Cu-Bi bond is weaker than the Bi-Bi bond.