Crystallization studies of amorphous melt-spun Ti50\mathsf{_{50}}Ni25\mathsf{_{25}}Cu25\mathsf{_{25}}

Amorphous ribbons of the ternary shape memory alloy Ti$_{50}$Ni$_{25}$Cu$_{25}$ crystallize polymorphously upon heating. The subsequent growth proceeds isotropic resulting in the formation of perfectly spherical grains in an amorphous matrix at intermediate stages of the transformation. These experimental results match exactly with the idealized preconditions that are employed in various models describing nucleation- and growth kinetics at deep undercooling. Therefore, amorphous Ti$_{50}$Ni$_{25}$Cu$_{25}$ is an ideal candidate to study the crystallization by calorimetry in conjunction with microstructure analyses and to compare the results with theoretical predictions. It was found, that the isothermal crystallization process of Ti$_{50}$Ni$_{25}$Cu$_{25}$ near the glass transition can be adequately described by Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetics using an Avrami exponent of $\rm n =3$. Kissinger analysis of nonisothermal crystallization using different heating rates revealed an activation energy of $\rm Q_a = 374$ kJ/mol

Fabrication of Zirconia Based Targets for Transmutation.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

In situ transmission electron microscopic observations of deformation and fracture processes in nanocrystalline palladium and Pd<SUB>90</SUB>Au<SUB>10</SUB>

In situ tensile tests were performed in a transmission electron microscope (TEM) so that the deformation and fracture processes in nanocrystalline Pd and Pd90Au10 could be observed directly. High-resolution electron microscopy was used to characterise the events. Material with average grain sizes of 10 and 65 nm, prepared by inert-gas condensation and repeated cold rolling and folding (RCR), respectively, were used in this investigation. Intergranular fracture was the materials’ main response obtained in tensile testing and it was observed for both grain sizes, which indicates an early onset of the limit to plastic flow in the samples. This, in our opinion, is because dislocation-based deformation processes are not able to operate sufficiently within such small grains to relax the stress concentration prior to fracture. Moreover, it was found that deformation twins had formed due to the stress field of the advancing crack tip during the in situ tensile test and subsequently provided favourable intragranular propagation paths for the crack tip. However, it is also emphasised that such deformation twins could be observed only in very few grains (about 1–2% of the total number of grains) next to the crack