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I.

INTRODUCTION

RAPID solidification (RS) has been shown to improve the properties of many alloys: tensile strength, ductility, fatigue resistance, crack propagation rates, to name a few. ~-4 Improvements in properties are associated with reduced segregation, suppression or elimination of coarse segregation phases, refinement of grain size, and changes in secondphase particle size, shape, and distribution. In addition, RS can extend the solubility of most alloying elements, some quite dramatically, and improve the homogeneity, minimizing the temperature or time for solution and aging treatments. Durand et al.S and Lebo and Grant 6 have demonstrated these phenomena for aluminum alloys 7075 and 2024, respectively. For the aircraft industry, 7000 series alloys have been used broadly for high-strength, low-density applications over many years. Recently, a rapidly solidified P/M X7091 alloy was shown to have improved room-temperature tensile properties compared to those of I/M 7075 (for example, 15 to 20 pet increase in yield strength) while maintaining high fracture toughness.7 The alloy chemistry of X7091 is similar to that of 7075 except for the substitution of 0.4 wt pet cobalt for chromium. The cobalt forms A19Co2 whose particle size varies from about 0.05 ~m to 2 ~m. The majority of the particles are randomly dispersed within the grains but also decorate the grain boundaries. The cobalt addition acts as a grain refiner and stabilizer during thermomechanical treatments, and imparts improved stress corrosion cracking resistance. The objective of this study was to improve further the mechanical properties compared to those of I/M 7075-T651 and P/M X7091 by larger alloying additions of several transition metals while preserving the highly refined strucPETER K. DOMALAVAGE, Research Associate, and NICHOLAS J. GRANT, Professor, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. YIGAL GEFEN is Professor of Metallurgy, Ben Gurion University of the Negev, Beer Sheva, Israel. Manuscript submitted September 29, 1982. METALLURGICALTRANSACTIONS A

ture of the RS P/M alloy. For this purpose approximately 1 wt pet additions of Ni and Zr were selected because they form fine submicron precipitates of A13Ni and AI3Zr, respectively. In particular, finer, more stable dispersions of strengthening phases were sought which would not be found preferentially on grain boundaries after consolidation. Durand et al. followed similar reasoning for additions of iron and nickel to 7075; excellent tensile properties were reported (634 MPa YS, 717 MPa UTS, and 9 pet elong.)/ With slow solidification rates associated with ingot casting, both the Ni and Zr would form coarse complex intermetallies which would be detrimental to properties, particularly to fatigue crack growth rates; this does not happen with RS alloys, as was demonstrated for Fe plus Ni additions/

II.

EXPERIMENTAL PROCEDURES

Reynolds Metals Company provided master remelt alloys using high purity materials. A ch