Papers by Author: J.R. Gao

Abstract: The glass fluxing technique was used to undercool melts of pure Cu and Ge in the presence of a static magnetic field generated by a superconducting magnet. It was found that the mean undercooling of liquid Cu increased with increasing magnetic field, whereas the mean undercooling of liquid Ge did not show any significant changes with increasing magnetic field. Such a difference in the undercooling behavior can be related to the Lorentz force imposed by the magnetic field, which was larger for liquid Cu because of a larger electrical conductivity than that of liquid Ge.

Abstract: Phase selection in undercooled melts of Cu-14.8Ge and Cu-18Ge compositions was investigated using the electromagnetic levitation technique in combination with substrate quenching and a drop tube. The results showed that the levitated and gas-cooled samples were all solidified into a microstructure consisting of primary -Cu plus peritectic -Cu5Ge. However, the samples quenched onto a copper substrate showed a segregation-free microstructure in the chilled zone, suggesting direct crystallization of the peritectic -Cu5Ge phase from a highly undercooled liquid. The Cu-18Ge samples quenched onto a glass substrate as well as those solidified in the drop tube also showed a segregation-free microstructure. An analysis of the nucleation kinetics revealed that the -Cu5Ge phase had a larger nucleation barrier than that of -Cu for all accessible undercoolings. It was suggested that phase selection in the undercooled Cu-Ge liquid might be controlled by transient nucleation kinetics.

Abstract: Liquid Cu-Co shows a metastable miscibility gap where the homogeneous melt separates into the Co-rich L1-liquid and the Cu-rich L2-liquid. The required undercooling of the melt of > 120 K can be achieved by containerless methods as electromagnetic levitation, laser melting or drop tube processing. Due to the large undercooling, rapid solidification of the melt is favoured and preserves microstructure features of the metastable (liquid) phases. In Co-84.0 at% Cu alloy the L1- phase nucleates in the Cu-rich majority phase L2 as a dispersion of spherical droplets. Convective flow in the liquid influences largely the time evolution and the nature of the droplet dispersion and makes a theoretical description of the droplet growth extremely difficult. In the present work droplet dispersions are compared which formed under processing methods with different levels of convection: (i) (Terrestrial) electromagnetic levitation (EML), (ii) processing in the TEMPUS facility under parabolic flight conditions and (iii) processing in an 8 m drop tube. The distributions of droplet radii of the L1-phase has been measured in the solidified samples. EML processing leads to significant convection in the melt which causes coagulation of droplets. Reduced gravity conditions in the TEMPUS facility during parabolic flight or in a drop tube can decrease convection, but effects of the convective flow on the dispersion of droplets are still present. The need for experiments under micro-gravity conditions is evident from the results.

Abstract: Diffraction experiments on electromagnetically levitated Nd-Fe-B alloys during solidification of the undercooled melt have been performed at the European Synchrotron Radiation Facility (ESRF). By using high intensity synchrotron radiation complete diffraction spectra could be detected within a short period of some seconds thus enabling the observation of metastable solidification products that exhibits a limited lifetime. A metastable phase that crystallizes in wide composition range and that initiates the solidification of the stable Nd2Fe14B1-phase (φ-phase) have been observed.