C. R. Feng

Alternative approach to electroless Cu metallization of AlN by a nonaqueous polyol process

Cu metallization of AlN substrates was performed using a nonaqueous, electroless, alcohol based approach known as the polyol method. The concentration of Cu2O in the film depended on the orientation of the substrates during deposition. This approach is very attractive for metallizing electronic substrates which are susceptible to hydrolytic degradation.

Characteristics of plasma processed SiC nanocrystallites and nanorods

Nanoparticles and nanorods of SiC were synthesized using arc-plasma processing of coarse particles. X-ray diffraction and Raman spectroscopic studies showed the presence of β-SiC and carbon nanotubes in the starting coarse particles and SiC nanorods in the ultrafine particles produced by plasma processing. Scanning electron microscopy and transmission electron microscopy confirmed the presence of carbon nanotubes in the starting material and nanorods of SiC in the plasma-processed samples.

High temperature structural studies of HgS and HgSe quantum dots

We report the structural investigations of β-HgS and HgSe quantum dots as a function of temperature between 300 and 600 K using x-ray diffraction. For both the chalcogenides, the zinc-blende structure remains stable up to 600 K without undergoing any phase transformation. The crystallite size increases as a function of temperature. However, for nanocrystallite ∼5.0 nm, lattice parameters show reduction in comparison to their bulk values. With increase in temperature, the lattice parameter increases and approaches the equilibrium value as the crystallite sizes grow to more than 10.0 nm. We attribute the temperature induced increase in crystallite size primarily to normal grain growth, a phenomenon observed in crystalline solids when the crystallite size undergoes gradual increase as function of time at elevated temperatures with accompanying recrystallization of new crystallite nuclei, and we rule out the possibility of size-dependent melting.

Tensile Deformation and Fatigue Crack Growth in Bulk Nanocrystalline Al-7.5Mg

Abstract : The fatigue crack growth kinetics and tensile deformation of bulk nanocrystalline Al-7.5Mg were investigated. Nanocrystalline particulates were first prepared by mechanically ball milling spray atomized Al-7.5Mg powders in liquid nitrogen. These particulates were then degassed, consolidated by hot isostatic pressing and extruded into rods. Bulk nanocrystalline Al-7.5Mg has significantly higher fatigue crack growth rates and lower fatigue crack growth thresholds than those of ingot-processed 7050-T7451. The fatigue crack growth thresholds exhibit only a weak stress ratio dependency and can be identified as having a Class I behavior when using the fatigue classification proposed by Vasudevan and Sadananda. In 3.5% NaCl solution, fatigue crack growth rates of bulk nanocrystalline Al-7.5Mg are as much as three times higher than those obtained in air. Tensile fracture of bulk nanocrystalline Al-7.5Mg is preceded by the formation of a localized shear band. In contrast to the low dislocation density in the as-extruded material, the gage section and the shear band region both exhibited a high dislocation density and dislocation cell structure.

Deformation Band Velocities and Local Strain Rates in a Ni-Cr-Fe Alloy (Inconel)

Nickel based alloys with nominal compositions similar to 78Ni -15Cr -7Fe, commonly referred to as “Inconel”, exhibit serrated flow (Portevin-LeChatelier effect) in the temperature interval of 230-730°C. Within this temperature range a series of thermally activated processes can also be observed when a wire sample of the alloy is heated with the direct resistance method under dead-weight loading while stressed above the room temperature yield. These processes include the expected initial period of plastic deformation at the start of heating followed by its complete arrest at a higher temperature, a behavior that is completely at odds with models for the thermal activation of plastic flow in metals. As the temperature is increased after this first arrest a cascade of two or three large plastic instabilities involving the high velocity propagation of narrow deformation bands is observed. Measurements of the band velocities using the time of flight within a 50.8 mm gage length extensometer indicate that they can exceed 2 m/s in some cases. Estimates of the maximum local strain rate attained within the deformation bands, obtained with a diametral extensometer, approach 15-18 s −1 . The localization of plastic flow into narrow, high velocity bands in this material is the result of the collective behavior of dislocations interacting at a high density. As demonstrated by TEM examination of the complex dislocation structures associated with these various events, however, it is difficult to rationalize a specific mechanism for these effects. If one assumes that both serrated flow and the thermally activated strain bursts are manifestations of the same basic mechanism these observations pose a challenging problem for interpretation with models for the Portevin-LeChatelier effect in this material.