Hypercooling and the Specific Heat Capacity of Cu-Ni Alloy

The average heat capacities of the undercooled Cu-25%Ni,Cu-50%Ni,Cu-50%Ni and Cu-75%Ni melts were derived by using the glass fluxing technique.The undercoolings of the above alloys were 381,380,349 and 431K,respectively,which exceed the critical undercooling of the classical nucleation theory.A detailed analysis of the heat transfer condition during the solidification process was carried out,which suggested a linear relationship between the time duration of thermal arrest ta and the undercoolingΔT.The hypercooling points of the alloys,derived from the relationship between ta andΔT,were determined to be 457.7,461.1,448.4 and 528.K,respectively.

Role of Al_2O_3 fiber in eutectic Al-Si alloy composites

The effects of Al 2O 3 fiber on wear characteristics of eutectic Al-Si alloy composites were studied using a pin-on-disk tester under dry sliding condition. The results show that the Al 2O 3 fiber can make matrix grain be fine, specially the eutectic Si be finer and prevent the plastic flow of matrix and prohibit the crack propagation in the wear layer, thereby it can remarkably improve the mechanical property and the wear resistance of the MMCs. Since Al 2O 3 fiber plays a role of certain framework in protecting the matrix against crash, it can eliminate the severe wear of MMCs with higher φ f of fiber from the beginning of test. At mild stage, when φ f is in the range of 8%～10%, the wear rates are the lowest. With increasing φ f of Al 2O 3 fiber, the wear mechanism of MMCs can be transformed from adhesive delamination to brittle breakaway.

Rapid growth of nickel dendrite in highly undercooled Ni-Mo alloys

Ni-39.3%Mo, Ni-45%Mo hypoeutectic alloys and Ni-47.7%Mo eutectic alloy have been rapidly solidified with different droplet sizes by containerless processing in a drop tube. For Ni-39.3%Mo hypoeutectic alloy, which corresponds to the maximum solid solubility of nickel phase, the solidification microstructure is characterized by nickel dendrite plus (Ni+NiMo) eutectic struc-ture. The undercooling of this alloy up to 182 K has been realized in the experiments. With an in-crease in undercooling, the dendritic microstructure is refined. The microstructural evolution of primary Ni phase in Ni-45%Mo hypoeutectic alloy evolves from remelted dendrite to equiaxed grains, whereas Ni-47.7%Mo eutectic alloy exhibits a structural transition from lamellar eutectic to anomalous eutectic. Theoretical analyses indicate that, for Ni-39.3%Mo, Ni-45%Mo and Ni-47.7%Mo alloys, the nickel phase shows a transition from solutal-diffusion-controlled growth to thermal-diffusion-controlled growth at undercoolings of 66.6, 81.9 and 85.0 K. The critical transition temperature decreases with a reduction in the nickel content.

Thermophysical properties of Ni-5%Sn alloy melt

The surface tension and specific heat of Ni-5%Sn alloy melt were measured by the oscillating drop method and the drop calorimetric method using electromagnetic levitation, respectively. The temperature coefficient of surface tension is 6.43×10-4 N·m?1K?1 within the temperature regime of 1464-1931 K. The enthalpy change was measured in the temperature range from 1461 to 1986 K, and the average specific heat was obtained as 43.03 J·mol?1K?1. Some other thermophysical properties, such as viscosity, solute dif-fusion coefficient, density, thermal diffusivity and thermal conductivity of this alloy melt, were derived based on the experimentally measured surface tension and specific heat. Using these thermophysical parameters, the relation between solute trapping and under-cooling in rapidly solidified α-Ni was calculated, and the theoretical prediction shows a good agreement with experimental data.

Rapid monotectic solidification under free fall condition

Fe-48.8% Sn monotectic, Fe-40% Sn hypomonotectic and Fe-58% Sn hypermonotectic alloys have been rapidly solidified during free fall processing in drop tube. For droplets of 100–1000 μm, the maximum undercooling for Fe-48.8% Sn, Fe-40% Sn and Fe-58% Sn alloys is 270, 282 and 288 K respectively. For Fe-48.8% Sn monotectic alloy, a homogeneously dispersed microstructure can be obtained when the droplet diameter is small, and the Marangoni migration velocity Vm is 37 times as fast as Stokes velocity Vs when the dispersion sphere radius is 6 μm and undercooling is 30 K. For Fe-40% Sn hypomonotectic alloy, the microstructure undergoes a transition from columnar α-Fe dendrites distributed in Sn-rich matrix to α-Fe particles. The growth velocity of α-Fe dendrite changes from 0.45 to 4.65 m/s when the droplet diameter varies from 1000 to 100 μm. For Fe-58.8% Sn hypermonotectic alloy, the grain size of primary α-Fe dendrites decreases remarkably when undercooling increases. Keywords monotectic solidification - phase separation - undercooling - containerless processing