The Strengthening Mechanism in Consolidated Rapidly Solidified Alloys
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THE STRENGTHENING MECHANISM IN CONSOLIDATED RAPIDLY SOLIDIFIED ALLOYS MONDE A. OTOONI U.S. Army Research Office, Physics Division, Research Triangle Park, NC 27709-2211 ABSTRACT Attempts have been made to study mechanisms of strengthening in the Ni6 8 W2 2 C8 B? and the Ni4 5CO 2 0 Cr1 oMO 4 Fe5 B1 6 alloy systems during their amorphous crysialline transitions. In the Ni68 W2 C B2 system where amorphous particulates of varying sizes have been cons Ti a ed at high pressure and low temperature (3.6 MPa, and 673 0 K respectively), the initial stage of crystallization is marked by transformation of the localized regions of the specimen. These crystallized regions contain microcracks and voids. Microhardness measurements from the consolidated specimens indicate an increasing trend in the microhardness with decreasing particulate sizes. Premature failures of the consolidated specimen during tensile stress measurements have been attributed to the presence of microcracks and voids in these specimens. In the Ni4 5 Co2OCrlOMo 4 Fe 5 BI6 alloy system isothermal annealing of an initially amorphous alloy has been allowed to produce grains of varying sizes. The tensile stress measurements from these thermally annealed ribbons indicate two distinctly different functional relationships between the strength, a , and the grain size parameter, X. In the early stage of transformation where grain reach a maximum growth of up to 400 A, the functional form of the strength, a , with the grain parameter, X , is a = a + K Log X , where a and K are constants. During latter stages of transformation, where g~ains larger than 400 R have been formed, the strength, a , varies with the inverse square root of the grain sizes. This latter relationship is analogous with the well known Hall-Petch relationships, which describes the strength, a , as a function of the grain sizes in conventionally processed alloys. INTRODUCTION Noncrystalline alloys are produced by rapidly quenching molten alloys at a rate of nearly 10 K/sec. Although the main reason for this high quench rate is to suppress crystallization during the solidification process, it has the disadvantage of restricting dimensions of the resulting products so that presently only thin ribbons can be formed. This inability to directly produce amorphous specimens of large dimensions has limited the technical application of rapidly solidified materials despite their advanced thermomechanical properties. As a result, much research has been directed towards a technology whereby consolidation of the rapidly solidified processed powder into larger sections is possible [1]. There are several, potentially feasible, techniques of consolidation. These include explosive compaction, dynamic compaction, warm extrusion, and warm die pressing. Despite extensive research, however, thus far the relative merits of each of these techniques has remained questionable. The strength in a well-consolidated mass, having undergone either a purely solid-state compaction (mechanical interlocking of particulates) or a compact
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