In-Situ TEM Phase Formation in Cold Rolled Aluminum-Nickel Multilayers
- PDF / 2,134,786 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 29 Downloads / 140 Views
ABSTRACT Multilayer samples of Nickel and Aluminum with an overall composition of Al-20Ni were prepared by cold rolling of elemental foils. The sample microstructures and phases were characterized by XRD, SEM and TEM/SAED, and the reactive phase formation was then examined by DSC measurements. XRD, SEM and TEM measurements show that the rolling procedure results in a decrease of the Al and Ni layer thicknesses (down to 100 nm in average) and a decrease of the grain size (down to less than 50 nm). No phase formation is observed during the cold rolling procedure. In isochronal DSC scans of the Al-Ni multilayers, the formation of the Al 3Ni phase was found to be a two step reaction process due to 2-dimensional nucleation and lateral growth and a 3-dimensional phase thickening. While XRD measurements showed Al3Ni as the only phase that forms, more detailed TEM investigations of the samples after DSC treatment also showed a small amount of an amorphous Al-Ni phase, formed by a thermally activated solid state amorphization reaction (SSAR). In-situ TEM heating of the amorphous areas under the electron beam in the microscope yielded the crystallization of the amorphous phase to a B2 structure and a growth of the B2 grains up to 100 nm in size.
INTRODUCTION The reactive phase formation in the Al-Ni system has been characterized intensively in the past (for a review see [1]). Recently, the reaction kinetics were studied in multilayer systems and ball milled powders, and the formation of several metastable phases (amorphous phases and nanocrystalline B2 phases, outside the equilibrium B2 composition of AlNi) were reported. Barmak et al. [2] observed an amorphous phase and a B2 phase in a codeposited lNi/3Al film. Here, the amorphous to B2 transition had already started during the deposition. Edelstein et al. [3] and Michaelsen et al. [4] reported the observation of a B2 phase with a metastable concentration of Al-37Ni as the first phase to grow upon annealing of a multilayer film (10 nm multilayer wavelength). Cardellini et al. [5] observed the formation of an amorphous Al-rich Al-Ni phase containing a few at.% oxygen in powder samples ball milled for a few hours. This phase transformed into the B2 structure by-passing the nucleation of Al3Ni as the initial phase. Similarly, Atzmon [6] found AlNi formation in ball milled powder in an explosive exothermic reaction where the Al 3Ni phase was by-passed. Schaeublin and Gotthardt [7] investigated the formation of an amorphous phase in aluminum induced by high-energy nickel implantation with a composition of the amorphous phase close to Al-25Ni. Their results suggest that the formation of precipitates with the Al3Ni phase composition is a necessary condition for the successful amorphization. The present paper deals with the observation of thermally activated formation of an amorphous A1-25Ni phase together with the formation of the intermetallic Al3Ni phase in multilayer samples prepared by cold rolling of elemental foils at ambient temperature. During TEM experiments, the amorpho
Data Loading...
