Metastable Phases and Defect Microstructures in Melt-Spun Ribbons of Nb 3 Al
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METASTABLE PHASES AND DEFECT MICROSTRUCTURES IN MELT-SPUN RIBBONS OF Nb 3 AI M. Aindowt, J. Shyue, T.A. Gaspar* and H.L. Fraser Dept. of Materials Science & Engineering, The Ohio State University, Columbus, OH 43210 f"Now at; School of Metallurgy and Materials ,University of Birmingham, Birmingham, B15 2TT, England *Ribbon technology Corporation, Gahanna, OH 43230 ABSTRACT The phases and defects produced in a binary alloy, with nominal composition Nb-18at.% Al, have been studied using transmission electron microscopy (TEM). In addition to the equilibrium A15 phase (Nb3AI) a previously unreported Nb-rich B2 phase is present. A significant number of grow-in dislocations were observed in the A15 phase, these have b= a and are predominantly screw on character. They are dissociated into two partial dislocations which bound a stacking fault according to the reaction;
->
1/2 + SF + 1/2
The habit plane of the fault is shown to be (012). These observations are compared with previous studies performed on the isostructual compound V3Si. The consequences of the observed dissociation for the plastic deformation of Nb3AI are discussed. INTRODUCTION High melting point intermetallic compounds for aerospace applications should have low density, a crystal structure with high symmetry, a small lattice parameter and a high tolerance to deviations from stoichiometry. The compound Nb3Al exhibits the cubic B-W structure (A15 in the Struktbereicht notation) and meets all of the above criteria. It is also of interest because it exhibits a superconducting transition above liquid helium temperatures and could be used for zero-resistance power transmission as it has a high critical current. Furthermore, there has been a recent trend towards the addition to binary Ti-Al alloys to improve their high temperature properties and to stabilize certain phases. In the limit this process will lead to the formation of phases based on Nb3A1, but little is is known about these phases. The purpose of the present research was to elucidate the nature of metastable phases and deformation mechanisms in Nb3AI based alloys. The approach adopted is to study microstructures using TEM. In this paper preliminary results on the phases and dislocations produced in melt-spun ribbons of a binary alloy, with nominal composition Nb-18at.% Al, are presented and the significance of these observations for the plastic deformation of Nb3AI are discussed. EXPERIMENTAL PROCEDURE Master ingots of Nb-18at.% Al were prepared by plasma melting under an Ar plus He atmosphere, using 99.9% Al and 99.99% Nb as starting materials. The ingots were remelted twice to promote homogeneity. Melt-spun ribbons were cast in a plasma melt overflow furnace which is described in detail elsewhere [1,2]. The master ingots were melted under vacuum in a water cooled copper heath using a plasma transferred arc operating at -50V and 700A with a helium flow rate of 2scfm at 30psi. Ribbons were cast on a chill wheel manufactured from Grade H-13 steel at a rate of 0.25 ms-1 . Specimens for TEM were prepa
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