W.H. Li

Binary Ni–Nb bulk metallic glasses

We studied the glass forming ability of Ni–Nb binary alloys and found that some of the alloys can be prepared into bulk metallic glasses by a conventional Cu-mold casting. The best glass former within the compositional range studied is off-eutectic Ni62Nb38 alloy, which is markedly different from those predicted by the multicomponent and deep eutectic rules. The glass formation mechanism for binary Ni–Nb alloys was studied from the thermodynamic point of view and a parameter γ* was proposed to approach the ability of glass formation against crystallization.

Plasticity-improved Zr-Cu-Al bulk metallic glass matrix composites containing martensite phase

Zr48.5Cu46.5Al5 bulk metallic glass matrix composites with diameters of 3 and 4mm were produced through water-cooled copper mold casting. Micrometer-sized bcc based B2 structured CuZr phase containing martensite plate, together with some densely distributed nanocrystalline Zr2Cu and plate-like Cu10Zr7 compound, was found embedded in a glassy matrix. The microstructure formation strongly depends on the composition and cooling rate. Room temperature compression tests reveal significant strain hardening and plastic strains of 7.7% and 6.4% before failure are obtained for the 3-mm- and 4-mm-diam samples, respectively. The formation of the martensite phase is proposed to contribute to the strain hardening and plastic deformation of the materials.

Parameters investigation during simultaneous synthesis and densification WC–Ni composites by field-activated combustion

Abstract The field-activated, and pressure-assisted combustion synthesis (FAPACS) process, which combines the simultaneous synthesis and densification of materials, was utilized to produce WC–Ni composites from powdered reactants, mixtures of tungsten, carbon and nickel. These reactants were subjected to high DC currents and uniaxial pressures. Under these conditions, a reaction is initiated by field activation and completed within a short period of time. Several experimental parameters, such as pulse current, power-controlled mode, temperature-increasing rate, maximum temperature and pressure during FAPACS on the relative densities of products were studied. Finally, the material with nearly complete density was fabricated. The percentage of the total shrinkage occurring before and during the synthesis reaction and addition densification was measured. The relative density of the end product and Vickers microhardness measurement (at 50 kg force) on the dense sample is 99.2% and 1424 kg mm −2 , respectively.

Effects of gravity on the microstructure of AlN/glass composites

AlN-borosilicate glass composites were fabricated on the parabolic flight plane by using a self-propagating high-temperature synthesis (SHS) chemical oven. The effects of different gravity levels on the material transition and the microstructure of composites were investigated. The results show that AlN crystal and glass segregated in elevated gravity; on the contrary, the composites with uniform distribution of multiphases can be obtained in microgravity, which proves the microgravity environment is good condition to prepare improved AlN/glass composites.

Mechanistic investigation of the field-activated combustion synthesis of tungsten carbide with or without cobalt added

Abstract The activation of self-propagating combustion reactions in the system of tungsten and carbon and its composites with cobalt as additive was achieved using an electric field. The reaction mechanism of Field-Activated Combustion Synthesis (FACS) of tungsten carbide and its composites has been investigated using the quenching sample method. By turning off the electric field during FACS, a series of combustion products with different phase compositions has been obtained. Layer to layer X-ray and microscopic analyses of these combustion products across the quenched combustion front suggest that the synthesis of WC involves the solid-phase diffusion of carbon into a carbide layer. W 2 C is the intermediate phase between WC and the reactants (W and C). A metallic additive produces liquid phase and accelerates the diffusion of the solid reactants (W and C); this facilitates the formation of W 2 C and the transformation of W 2 C to WC. Moreover, molten Co reacts with W and W 2 C to form mixed compounds of type W x C y Co z .