EFFECT OF Cr ADDITION ON MICROSTRVCTURES AND MECHANICAL PROPERLIES OF β-NiAl ALLOYS

The effect of Cr addition on the microstructures and mechanical properties of NiAl inermetallic alloys containing 4 to 8 mol% Cr has been characterized. It is shown that Cr is not only an efficient soild solution strengthener in NiAl, it is also an efficient precipitation Strengthener by fine dispersion of α-Cr particles. Transmission electron microscope observations have revealed that fine spherical particles of α-Cr appeared homogeneously in the NiAl matrix by aging at temperatures around 1073K after solution annealing at 1563K. The aging behavior was investigated by microhardness measurements and the temperature dependence of the yield stress of NiAl containing α-Cr particles was investigated by compression tests over the tempemture range from 298 to 1173K. NiAl alloys happened appreciably by the precipitation of α-Cr particles.Dislocations bypass the particles during deformation and typical Orowan loops were observed surrounding the α-Cr particles after deformation. Although the dispersion Of α-Cr particles increases the yield strength of NiAl at intermediate temperutre, the strength decreases appreciably at higher temperatures.

MICROSTRUCTURAL CHARACTERISTICS ASSOCIATED WITH HIGH- STRAIN-RATE PLASTIC DEFORMATION IN THE ELECTROFORMED COPPER LINER OF SHAPED CHARGES

The microstructures of electroformed copper liners of shaped charges that had undergone high-strain-rate deformation were observed by optical microscopy （OM） and scanning electron microscopy （SEM）. Meanwhile, the orientation distribution of the grains in the recovered jet was examined by electron backscattering Kikuchi pattern （EBSP） technique. EBSP analysis reveals that the fibrous texture observed in the as-electroformed copper liners disappeared after explosive detonation deformation. OM observation shows that the microstructure evolves system- atically from the jet center to its perimeter during cooling from high temperatures after explosive detonation deformation. This microstructural characteristic is similar to that of solidification, i.e. there exist equiaxed grains in the center of the jet and significant columnar grains around the equiaxed grains. The result reveals that there is melting-related phenomenon in the jet center. Corresponding microhardness variations from the jet center to its perimeter is also determined. All the phenomena can be explained by a strong gradient of temperature across the section of the jet during plastic deformation at high-strain-rate.

DISLOCATION PINNING IN HIGH TEMPERATURE DEFORMED Ni_3(Al,Ti) SINGLE CRYSTALS CONTAINING DISORDERED γ PRECIPITATES

A transmission electron microscopy （TEM） investigation has been performed on the dislocation pinning in Lie-ordered Ni3（Al, Ti） containing disordered γ precipitates.The morphology of deformation induced dislocations in the γ base alloys containing fine dispersion of disordered γ was investigated by means of weak-beam electron microscopy. The superdislocations are strongly attracted to the disordered particles and dissociate on the （111） plane in the γ particles, while they dissociate on the （010） plane in the γ＇ matrix. The disordered γ precipitates play an important role as a pinning point during the cross-slip of superdislocations from （111） to （010） planes in the γ matrix and restrain the cross-slip of superdislocations. The interaction of superdislocations with disordered particles causes the formation of superkinks, jogs and closed loops.

PREPARATION OF TEM THIN FOIL CONTAINING POWDER PARTICLE BY ELECTRODEPOSITION METHOD

A practical technique to prepare transmission electron microscopy （TEM） thin foil containing powder particle was described and the data for the codeposition of two type particles with copper in the electroplating were presented. By depositing the particles which were distributed in CuSO4 electrolyte on cathode together with Cu^2＋ in electrodeposition bath, composite coating with suitable thickness could be formed. The thin coating was separated from the substrate and cut into a disc with diameter of 3mm for electropolishing. When the Cu matrix was thinned during electropolishing, the particles contained in the coating plate were also thinned to meet the suitable thickness for TEM observation. Various experimental results revealed that during electrodepositing the current density, pH-value of electrolyte and stirring mode all have significant effects on the distribution of particles in composite coating and the surface quality of the composite coating. The proper parameters used during electrodepositing to prepare TEM thin foil containing powder particle were discussed.

LITHIUM ORDERING IN CHEMICALLY DELITHIATED Li_xCoO_2

Chemical delithiated LixCoO2 （x=1, 0.7, 0.66 and 0.57） has been synthesized by using a strong oxidant, NaS2O8 .The structure investigations indicate that all the samples remain with hexagonal cells, which have expansion of c axis during Li extraction. Transmission electron microscopy （TEM） observations revealed the presence of superstructures arising from intercalated U-ordering corresponding to first-principle calculations. The measurements of the physical properties are also influenced by Li concentration.