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The artificial layering of metals can change both physical and structural characteristics from the bulk. The stabilization of polymorphic metallic phases can occur on a dimensional scale that ranges from single overgrowth layers to repetitive layering at the nanoscale. The sputter deposition of crystalline titanium on nickel, as both a single layer and in multilayer form, has produced a face-centered cubic phase of titanium. The atomic structure of the face-centered cubic titanium phase is examined using high resolution electron microscopy in combination with electron and x-ray diffraction.

I. INTRODUCTION

The motivation to characterize the structure of Ni/Ti multilayers is found in the need to directly examine and iteratively improve the microstructural morphology.1 Smooth layering and compositionally abrupt interfaces are required for neutron supermirror applications.2'3 The multilayer mirror design improves the reflected intensity of polarized neutrons at grazing incidence in comparison to conventional nickel mirrors. A typical Ni/Ti multilayer mirror design can consist of 15-20 layer pairs of Ni and Ti which are 25-30 nm thick. The Ni/Ti multilayers are metastable structures. In situ electron microscopy observations4 confirm the temperature sensitivity of the Ni/Ti multilayer interface regions to undergo solid-state amorphization.5"7 In fact, heating a single Ni-Ti bilayer furthers amorphization of the interface.8 Presently, several Ni/Ti samples are prepared with layer pairs less than 30 nm thick to study possible layer thickness (i.e., dimensional) effects on the crystalline structure and stability of the Ni and Ti layers. It is not an uncommon occurrence in the nanometric-scaled layering of common metals to stabilize polymorphs.9 For example, the face-centered cubic form of Fe is stabilized during the growth of a Cu/Ni/Fe superlattice.10 A survey of the assessed, equilibrium phase diagram for Ni-Ti reveals the presence of a Ni-rich, face-centered cubic (fee) phase (150 nm) than in the Ni/Ti multilayers (20-50 nm). Two independent pole-projections are now possible from one grain and do in fact confirm the fee Ti structure. The (111) growth direction of the Ti layer is common to both the [011] and [112] pole projections (Figs. 9 and 10). The in-plane spacings of the [112] projection (Fig. 10) correspond to (220). The projections are separated by a 29.8° rotation of the specimen stage, in agreement with the ideal value of 30°. The measured lattice parameter of 0.432 nm is 1.8% less than that measured for the 8.3 and 26.0 nm Ni/Ti multilayers (Table I). FFT's of the

A possible explanation for the discrepancy between the SADP results for Ni/Ti and the apparent lack of an hep Ti is that there may be three or four different sets of Ti texturing. This would be quite unusual. With the greater number of superimposing texture patterns there would be a tendency toward randomization, i.e., the formation of a ring pattern. The Ti(00.2) reflection should then be present. This is not the case. The diffraction evidence fo