Amorphous Al(Ni) Alloy Formation by Pulsed Electron Beam Quenching
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AMORPHOUS Al(Ni) ALLOY FORMATION BY PULSED ELECTRON BEAM QUENCHING
S. T. PICRAUX AND D. M. FOLLSTAEDT Sandia National Laboratories, Albuquerque, NM 87185
ABSTRACT The microstructural transition from the epitaxial crystalline to amorphous state as a function of Ni concentration is examined in Al(Ni) alloys formed by ion implantation and e-beam melt quenching. The evolution from epitaxial crystalline Al at _ 3.5 at.%Ni to amorphous phase formation at 15-30 at.% Ni is characterized in detail for 8, 10 and 12% Ni uniform concentration samples by ion channeling and transmission electron microscopy. The transition occurs via complete loss of epitaxial regrowth at 9 at.% and the formation of a finegrained polycrystalline Al, with grain size decreasing with increasing Ni from 20-100 A at 10% Ni concentration to _ 10-50 1 at 12% Ni.
INTRODUCTION In this paper we discuss the crystalline to amorphous transition in Al(Ni) formed by Ni implantation and electron beam (e-beam) pulsed melting. The microstructural evolution of this transition with increasing Ni concentration is examined by ion channeling/backscattering and transmission electron microscopy (TEM). We have previously shown that at low implanted Ni concentrations (_ 3.5 at.%) epitaxial regrowth of crystalline Al occurs [1], whereas at sufficiently high concentrations (-15-30 at.%) an amorphous alloy is formed [2]. This defines the composition regime in which to examine this crystalline to amorphous transformation in detail. There are several special aspects to the use of ion implantation and e-beam melt quenching for studying rapid solidification. First, by multiple energy implantation of Ni we form surface layers (-0.1 im thick) of uniform concentration with the Ni microscopically dispersed. Second, the heat is deposited uniformly into a la er a few microns deep on a thick substrate, so that the quench rates (108 - 10 K/sec) and interface velocities (1-10 m/sec) are among the highest of those studied in rapid solidification. Also, the sample geometry is better controlled than in splat quenching and the thermal history of the sample is readily calculated. Finally, by melting into the crystalline Al substrate, which always acts as a seed for planar epitaxial regrowth back to the surface, we exclude nucleation-limited transitions to the amorphous phase, and thus are able to focus just on the growth-limited case. The rapid solidification of Al alloys by splat cooling and related techniques has been extensively studied, and greatly improved mechanical properties have been observed in many cases due to the microstructural refinement of the crystalline state [3]. In addition, evidence for amorphous phase formation has been seen in several binary aluminum alloys as a limiting case of high solidification rates [4]. In the case of the Al(Ni) system evidence has been presented for the enhanced solid solubility of Ni within crystalline Al to _ 0.95 isec for Al 3Ni 2. These lower limits to nucleation times contrast to the recently measured value of 15 + 10 nsec for the precipit
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