C.D. Cao

Liquid-liquid phase separation in undercooled Co-Cu alloys

Abstract The liquid–liquid phase separation in undercooled Co–Cu alloy melts has been investigated by differential thermal analysis in combination with glass fluxing technique over a composition range of 16.0–87.2 at.% Cu. The DTA signals, obtained during isochronous cooling, indicate that this separation process is exothermic and proceeds till the rapid solidification of Co-rich liquid phase occurs. The metastable miscibility gap that was determined directly and reproducibly from the onset temperatures of this process is slightly shifted to the Cu-rich side and roughly symmetrical about a Cu concentration of 53 at.%. The measured critical temperature of phase separation is 1547±1.5 K and is about 108 K below the corresponding liquidus temperature. In the present measurements the separated Co-rich liquid always solidified prior to the Cu-rich phase, which crystallized near the peritectic temperature. Lower surface tension and better wetting properties of the Cu-rich liquid phase with glass flux are responsible for the Co-rich phase to be always encased by the Cu-rich phase. In addition, thermodynamic calculations have been accomplished leading to a binodal line, which is in sufficient agreement with the experimental results.

Rapid solidification of Cu84Co16 alloy undercooled into the metastable miscibility gap under different conditions

Abstract The Cu 84 Co 16 alloy melt processed by differential thermal analysis, electromagnetic levitation and drop tube experiences a liquid phase separation when it is undercooled into the metastable miscibility gap. The phase separation and coagulation processes are mainly controlled by the degree of undercooling, cooling rate and convection level in the containerless states. Disperse structures have been formed in droplets solidified during free fall in the drop tube.

PERITECTIC SOLIDIFICATION UNDER HIGH UNDERCOOLING CONDITIONS

The solidification characteristics of highly undercooled Cu-7.77% Co peritectic alloy has been examined by glass fluxing technique. The obtained undercoolings vary from 93 to 203 K(0.14 TL). It is found that the α(Co) phase always nucleates and grows preferentially, which is followed by peritectic transformation. This means that the peritectic phase cannot form directly, even though the alloy melt is undercooled to a temperature far below its peritectic point. The maximum recalescence temperature measured experimentally decreases as undercooling increases, which is lower than the thermodynamic calculation result owing to the actual non-adia-batic nature of recalescence process. The dendritic fragmentation of primary α (Co) phase induced by high undercooling is found to enhance the completion of peritectic transformation. In addition, the LKT/BCT dendrite growth model is modified in order to make it appllcable to those binary alloy systems with seriously curved liquidus and solidus lines. The dendrite growth velocities of primary α (Co) phase are subsequently calculated as a function of undercooling on the basis of this model.

Rapid monotectic solidification during free fall in a drop tube

Droplets of Ni-31.4%Pb monotectic alloy with different sizes are rapidly solidified during free fall in a drop tube. The theoretical calculations indicate that the undercooling was achieved before solidification exponentially depends on droplet diameter. The maximum undercooling of 241 K (0.15Tm) is obtained in the experiments. With the increase of undercooling, the volume fraction of monotectic cells increases, and the L2(Pb) grains are refined. Calculations of the nucleation rates of L2(Pb) and α-Ni phases indicate that L2(Pb) phase acts as the leading nucleation phase during the monotectic transformation.

Peritectic solidification of highly undercooled Cu-Co alloy

Abstract Peritectic solidification has been investigated on Cu 91.7 Co 8.3 alloy by containerless processing in a 3m drop tube. Droplets ranging from 100 to 900μm in diameter have been obtained. The microstructure of the droplets larger than 150μm is characterized by primary α-Co dendrites embedded in Cu-rich matrix, and the completion of peritectic transformation is facilitated as droplet size decreases. It is also found that α-Co dendrites and peritectic cells are conspicuously refined with the decrease of droplet diameter due to the fragmentation of α-Co dendrites by remelting during recalescence. In those droplets finer than 150μm, the precipitation of α-Co dendrites and the peritectic transformation have been suppressed completely, and a coarse metastable supersaturated Cu-rich dendritic microstructure is formed directly from the undercooled melt. In addition, the dendritic growth of primary α-Co phase has been calculated on the basis of the modified LKT/BCT model.

Solidification behaviour of silver-copper alloys in a drop tube

Abstract Ag-15wt%Cu hypoeutectic and Ag-28.1wt%Cu eutectic alloys were solidified in a containerless environment using a 3m drop tube. It is found that for Ag-15wt%Cu hypoeutectic alloy, with the decreasing of the droplet size, a “dendritic-equiaxed” morphology transition takes place. Meanwhile, for Ag-28.1wt%Cu eutectic alloy, a “regular lamellar eutectic-irregular anomalous eutectic” microstructure change occurs. The dendrite fragmentation mechanism and TMK model of eutectic growth were used to analyze the experimental results.

Ternary Eutectic Growth in Microgravity Environment

Rapid growth of Ag38.5Cu33.4Ge28.1 ternary eutectic alloy was accomplished in a 3 m drop tube and its phase selection and growth mechanism were investigated. The experimental results revealed that the semiconductor phase (Ge) was the primary nucleating phase during solidification, which agrees with the calculated results of nucleation rate. The solid solution phase (Ag) and intermetallic compound phase ?(Cu5Ge2) grew cooperatively and lamellar structures similar to binary eutectic formed. Moreover, with the decreasing of droplet size, the growth morphology of primary (Ge) phase transformed from plate-like to granular shape and a kind of anomalous ternary eutectic formed. The microgravity environment has a significant effect on the crystal growth process, which makes the (Ge) phase distribute homogeneously and the anomalous eutectic grains show good geometrical symmetry. The calculation of cooling rate versus droplet diameter showed that it was the high cooling rate and large undercooling that brought about the eutectic growth morphology transition.

Rapid solidification mechanism of Ag60Sb34Cu6 ternary alloy in drop tube

Droplets of Ag60Sb34Cu6 ternary alloy within the diameter range of 60–800 μm were rapidly solidified by means of drop tube containerless processing, and the solidification mechanism is analyzed. With a decrease in droplet size, the cooling rate increases from 57 to 5.8×104 K/s. The maximum undercooling is determined to be 180 K (0.23TL) and the microstructure of primary ε(Ag3Sb) dendrite refines drastically until homogenous equiaxed dendrite forms. Experimental results indicate that (ε+Ag) pseudobinary eutectic cannot form under high undercooling conditions and the solubility of Ag in primary ε phase increases as undercooling increases. Based on thermal analysis and crystal growth morphology, it is found that this alloy is solidified in two ways corresponding to different undercooling levels.

Solute Diffusion Controlled Dendritic Growth Under High Undercooling Conditions

Liquid Cu70Co30 alloy is undercooled up to 363 K (0.22TL) by using glass-fluxing technique. The LKT/BCT dendritic growth theory is modified and made applicable to predict the kinetic characteristics of dendritic growth in those alloy systems which have extremely curved liquidus and solidus lines. Actual calculations reveal that the dendritic growth of primary α-Co phase in Cu70Co30 alloy is constantly controlled by solute diffusion. The growth velocity of α-Co dendrite has a maximum value of 54.5 mm/s at an undercooling of 333 K. The calculated solubility of Cu in α-Co dendrite decreases with the increase of undercooling, which agrees well with the experimental data. Prior to peritectic transformation, the growth of α-Co dendrite is close to equilibrium solidification.