A newly developed molecular dynamics code was used to study the effect of free surfaces, grain boundaries and voids in the process of melting. It was found that conventional “thermodynamic melting” occurs via nucleation of the liquid at the extended defects with subsequent growth into the crystal. In the absence of interfaces, or when this transition is kinetically hindered, however, a second type of melting transition can be triggered by an elastic instability first described by Born (“mechanical melting”). It is suggested that the distinct characteristic features associated with the two types of melting are actually observed in solid-state amorphization experiments. A unified thermodynamics-based description, in the form of an extended phase diagram, of melting and solid-state amorphization is proposed which brings out the parallels between these two phenomena and suggests that their underlying causes are apparently the same. By investigating the effect of surface stresses on the structure and elastic behavior of free-standing thin films, we discuss how these concepts need to be modified in thin-film and small-grained materials.

The amorphous phase formation and initial crystalline reactions at Pt/GaAs interfaces has been investigated via high-resolution electron microscopy (HREM), microdiffraction, and energy dispersive spectroscopy (EDS). A thin amorphous intermixed layer consisting of three elements, platinum, gallium, and arsenic was observed at Pt/GaAs interface in an as-deposited sample. This interlayer grew to 4.5nm in an amorphous state upon low temperature(e.g. 200°C) annealing by a solid-state amorphization reaction. Following the growth of the amorphous interlayer, subsequently, Pt3Ga and PtAs2 phases nucleated within the amorphous layer and grew at the Pt and GaAs sides, respectively. We also observed the same reaction processes with in-situ annealing HRTEM.

We have studied the kinetics of the solid-state amorphizing reaction in thin film multilayers of Ni and Zr. Crystalline Ni and Zr films were deposited in ultra-high vacuum onto platinum resistance thermometers embedded in alumina. An electronic feedback circuit controls the temperature of the substrata by adjusting the power dissipated by the platinum resistors. We find that structural relaxation in the asdeposited Ni and Zr films affects the initial stages of the reaction. For long reaction times there is a discontinuous change in the reaction rate. The time to reach this transition increases with film thickness and depends exponentially on 1/T, with an apparent activation energy of 3 eV atom−1.

We determine the glass forming range (GFR) of co-deposited Ni1−xZrx (0 < x < 1) thin films by measuring their electrical resistance during in situ constant-heating-rate anneals. The measured GFR is continuous for 0.10 < x < 0.87. We calculate the GFR of Ni-Zr melts as a function of composition and cooling rate using homogeneous nucleation theory and a published CALPHAD-type thermodynamic modeling of the equilibrium phase diagram. Assuming that the main competition to the retention of the amorphous structure during the cooling of the liquid comes from the partitionless crystallization of the terminal solid solutions, we calculate that for dT/dt = 1012 K s−1, the GFR extends to x = 0.05 and x = 0.96. Better agreement with the measured values is obtained assuming a lower ‘effective’ cooling rate during the condensation of the films.

Solid state reactions at the interface of bulk Ni-Zr couples have been induced at several temperatures by compressive plastic deformation. The reaction product is an amorphous phase whose thickness increases with applied load and sample temperature. In addition, similar samples have been thermally reacted in order to measure the thermal interdiffusion coefficent in the same conditions. Measurements on stressed couples show that the interdiffusion coefficent (several orders of magnitude larger than the corresponding thermal value) follows a dual regime Arrhenius behaviour. The activation energy is independent of load and of the order of 0.2 eV in the low temperature regime extending up to 550 K. A different behaviour characterized by a single value of the activation energy in the whole temperature range has been observed in thermally treated samples.

Phase equilibria along the CoGa-CoAs join were determined by DTA and metallography. On the basis of these data and the phase diagram of Co-Ga-As at 600 °C, thermodynamic values for the various phases along the GaAs-Co join were estimated. The Gibbs energy of the amorphous phase is approximated to be that of the supercooled liquid phase. These data were used to rationalize the amorphization process.

Solid-state amorphization reaction between GaAs and Co thin-films was investigated by transmission electron micorscopy and Auger electron spectroscopy. Upon annealing of GaAs/Co thin-film couples at 260–300 °C, an amorphous phase was observed to form. The amorphization was attributed to the openness of the GaAs structure relative to the size of the Co atoms. This allows rapid diffusion of Co into the GaAs lattice and promotes the occurrence of SSAR. Annealing at higher temperatures or for longer times led to the formation of a crystalline phase, designated as the μ-phase which was determined to be a metastable supersaturated solid solution of CoAs exhibiting the B31 structure of the approximate composition of Co(Ga. .48As.52). The growth kinetics of both the amorphous phase and the μ-phase are parabolic in nature. The parabolic rate constant is higher for the μ-phase than for the amorphous phase. The activation energies are 1.47 and 1.35 eV, respectively.

Phase Formation kinetics in the reaction of Nb/Al multilaycred thin films were investigated using scanning calorimetry, x-ray diffraction, and transmission electron microscopy. The first phase to form upon annealing the Nb/Al layered structure is the NbAl3 intermetallic. Its formation is clearly identified by the calorimetry to be a two stage process, which has been modeled as the nucleation and three-dimensional growth to coalescence of the product phase in the plane of the initial interface, followed by the thickening of the product layer by one-dimensional growth perpendicular to the interface plane. For the initial reaction stage the reaction front velocity is higher than can be supported by diffusional transport within the lattice adjacent to the moving interface. Thus diffusion along nonequilibrium interfaces must be the growth mechanism. The large volume fraction consumed during the initial reaction stage indicates a lower nucleation site density than expected at a Nb/Al interface at local equilibrium, suggesting that the interface transport is reducing the driving force for nucleation.

High temperature gradient induced fast diffusion effect during pulsed laser melt processes is reported for the first time in the Si semiconductor. We use both the oxidationinduced stacking faults and dislocation loops as markers to determine the degree of enhanced diffusion in the unmelted Si substrate by tracing their movement after laser melt. The crosssectional transmission electron microscope is employed to investigate the defect structures before and after laser melt. The expanded dislocation loops exhibit a significant diffusion and climb occurring during the nanosecond-scale processing duration. We also formulate and solve the governed diffusion equation, based on a high temperature gradient induced electric field, to simulate the dislocation movement. The agreement between the experimental and simulated results verifies that the fast diffusion effect indeed occurs in the Si semiconductor during the pulsed laser melt period.

We investigated the phase formation sequence in the reaction of multilayer thin films of Nb/Al with overall compositions of 25 and 33 at.% AI. We report novel phenomena which distinguish thin-film reactions unequivocally from those in bulk systems. For sufficiently thin layers composition and stability of product phases are found to deviate significantly from that predicted from the equilibrium phase diagram. We demonstrate that in the Nb/Al system the length scales below which such deviations occur is about 150 nm. We believe that these phenomena occur due to the importance of grain boundary diffusion and hence microstructure in these thin films.

Θ phase CuAl2 precipitate size evolution during coarsening at 310°C in 0.5 μm thick Al-2% (wt) Cu thin films was characterized by transmission electron microscopy. Films were sputter deposited onto oxidized Si substrates by standard techniques. The coarsening process preferred the growth of blocky Θ morphologies at Al triple points. Coarsening was via solute Cu diffusion along Al grain boundaries during annealing. The average Θ size dependence on annealing time (t) is approximately (t)1/4 in general agreement with models for particle coarsening along grain boundaries. Concurrent Al grain growth was shown to initially enhance the Θ coarsening rate above (t)1/4 behavior. This boundary coarsening process leads to a grain size dependence of the coarsening rate which has been observed in related and other previous work in thin films. These results are shown to be relevant for effects produced during accelerated electromigration testing, such as previous ‘curious’ 0 morphologies at triple points observed by others, the enhanced flux of Cu during testing, and possible mechanisms affecting electromigration failure processes.

Ion-beam mixing and isothermal annealing techniques were applied to induce phase transformations in Co/Nb and Co/Zr multilayer film stacks. Transmission electron microscopy(TEM) and x-ray diffraction(XRD) were used to characterize the microstructure of the films. Interfacial morphology and chemistry of the films were examined by Rutherford backscattering spectrometry(RBS) and cross-section transmission electron microscopy(XTEM). The formation of amorphous phases was found in both systems by either technique. A comparison of the phase transformation mechanisms induced by ion-beam mixing and isothermal annealing is given. The thermodynamic and kinetic factors controlling the phase formation and stability are discussed. For the isothermal annealing, the final stable configuration is predicted by equilibrium phase diagrams.

The atom probe field-ion microscope has been used to study the diffusion at interfaces in metallic multilayers deposited directly onto field-ion specimens and to develop models for the solid state reactions occuring at the atomic-scale in multilayer systems. Results are presented for the low temperature annealing of a Co-Ni multilayer. Intermixing over about 2 atomic planes is found even in as-deposited samples, extending to mnm after heating at 300°C for 1 hour. Using atom probe results from bulk alloys, a Monte Carlo simulation has been developed for the Fe-Cr system, in which a miscibility gap exists, and is being used in an attempt to model the behaviour of interfaces in Fe-Cr multilayers. Preliminary results are presented, showing that interfaces which are initially mixed over 10 atomic planes become sharper by an ‘interface spinodal’ reaction.

TiNi alloys have been extensively studied for their shape memory properties, arising from a martensitic transformation between the cubic B2 crystal structure and the monoclinic B19′ structure. However, only recently has the application of TiNi alloys as thin film actuators been considered. Well-controlled thicknesses of TiNi films have been deposited via d.c. magnetron ion sputtering, with as-deposited films exhibiting an amorphous structure. These ductile metallic glass films may be bent into various macroscopic shapes and then heated to crystallize the B2 structure, thereby determining its parent (memory) shape. In situ high voltage electron microscopy (HVEM) heating and cooling experiments are utilized to observe the crystallization of the B2 structure and the martensitic transformation from B2 to B19∼500°C to ∼600°C. Isothermal annealing determines the kinetics of the crystallization process to be nucleation limited. The nucleation of B2 crystallites in thicker regions of TEM specimens, which occurs prior to nucleation in thinner regions, indicates that nucleation does not occur preferentially at surfaces, but rather homogeneously. After crystallization is completed, the existence of random orientations of B2 grains provides films that will exhibit shape memory in any desired orientation. Video-recording of the crystallization processes has been acquired.

This research investigated the influence of heat treatment and precipitation on the phase transformation temperatures of sputter-deposited nickel-titanium films. Films 5 to 10 microns thick were subjected to isochronal and isothermal heat treatments in vacuum. Four-point resistance measutements were made and the TP, TR, Ma, and Mc temperatures identified. The correlation between transformation temperatures and the film's microstructure was studied using transmission electron microscopy. Existence of Ti2Ni and Ti11Ni14 precipitates was seen to adversely influence the phase transformation temperatures. It was concluded that precipitate-free film is preferable for mechanical actuator applications.

Thin films of the shape memory alloy NiTi have been sputter-deposited onto ptype silicon substrates. Films that are initially amorphous may be crystallized by vacuum annealing. The crystalline films exhibit the B2->B19′ phase change associated with the shape memory effect while remaining in contact with the silicon substrate. Transition temperatures were determined by resistance measurements and x-ray diffraction. The NiTi - Si contacts are diodes, as evidenced by their current-voltage characteristics; however, the effect of the phase change on the barrier height could not be determined.

Stress relaxation in polycrystalline layers can be explained by processes, in which the microstructure plays a dominant role. The microstructure itself may also be subjected to changes. With X-ray diffraction information about both the stress and the microstructure can be obtained without destroying the specimen and without disturbing the stress relaxation process.

In this paper a model system is studied: Au on Si<100>. The specimens showed a simultaneous decrease of macrostress and dislocation density with time at room temperature. This could be interpreted on the basis of a model founded on thermally activated dislocation motion. It followed that the grain size is an important parameter for the change of the dislocation density.

The evolution of nanometer Ru films sandwiched between various C layer thicknesses with thermal treatments was studied by plan-view and cross-sectional Transmission Electron Microscopy. Plan-view observation provides information on the Ru grain size, while crosssectional studies allow examination of the multilayer morphology. After annealing at 800°C for 30 minutes, the grain size in the 2 and 4 nm Ru layers show little difference from each other, while that in the I nm Ru layers depends strongly on the thickness of the C layers in the multilayers. It increases with decreasing C layer thickness. Agglomeration of the Ru layers is observed in Inmn Ru / 1nm C multilayers after annealing at 600°C for 30 minutes. The evolution of the microstructures and layered structure stability of the Ru/C system is compared to that of W/C and Ru/B4C systems.

Using a master equation approach, we derive a general expression for the diffusion coefficient as a function of concentration-dependent jump rates. When this approach is applied to diffusion in a binary solid, Darken's equation for intrinsic diffusion coefficients is derived together with an expression for self diffusion coefficients which satisfies the semi-empirical Ugaste relationship. This analysis suggests that the Darken term and the self diffusion coefficients are in general related.

The conditions for the equilibrium shape of a second-phase particle nucleating either at a planar interface or at a spherically curved interface are reviewed under the assumption that the interfacial structure of the nucleation site has a torque of an appropriate strength. The results are then incorporated into the modified Gibbs-Wulff construction in order to demonstrate a graphical representation of both the equilibrium shape and the free energy barrier for heterogeneous nucleation.