Crystallization Behavior and Properties of Rapidly Solidified Ni-Mo-B Alloys
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CRYSTALLIZATION BEHAVIOR AND PROPERTIES OF RAPIDLY SOLIDIFIED Ni-Mo-B ALLOYS C.C. Wan AiResearch Casting Company,
Torrance,
CA'
ABSTRACT The crystallization phenomena of certain ternary glass Ni-Mo-B alloys and their consolidated bulk properties, including age hardening effects after proper heat treatment, are investigated. Within the composition range studied, the glassy alloys crystallize by a two-stage process - first by forming a metastable microcrystalline structure of Ni solid solution at temperature around 490-530°C followed by boride precipitation at temperatures around 700-770'C. The boride, which is identified by x-ray diffraction as Mo 2 NiB2 , grows to submicron size after the alloys have been thermally treated at 1000'C and above for certain periods of time. Microstructural analysis shows borides isolated and well dispersed in Ni-Mo alloy matrix. Age hardening is achieved by precipitating the Ni/Mo intermetallic compound(s) in Ni-Mo matrix at an intermediate temperature of previously solutionized samples. Another characteristic of this group of alloys is the relatively high eutectic temperature, i.e. 12309C and above. This will allow the alloys to be hot consolidated to full density by certain commercially available processes, such as hot isostatic pressing (HIP), while still maintaining the unique microstructure inherited from rapid solidification process. Hardness, hot hardness and hot tensile properties of some consolidated alloys are discussed.
INTRODUCTION Rapid solidification (RS) has generated new classes of materials ranging from metallic glasses [1] to microcrystalline unconventional alloys such as Al-Li [2]. These RS materials have demonstrated exceptionally good mechanical properties as well as other unique physical properties, as in the case of some ferromagnetic alloys [3] and superconductive alloys [4]. Unfortunately, because of process limitations, the rapidly solidified material has been generally made in two-dimensional shapes, such as foils, flakes, fibers and filaments, instead of the more useful three-dimensional shapes. Many attempts have been made to consolidate RS material into bulk form in order to utilize its metallurgical advantages inherited from the RS process. For instance, Morris [5] has explored the possibility of "cold" compacting the amorphous powders into bulk shape by using dynamic (explosive) compacting technique. On the other hand, hot consolidation of both thermodynamically stable and metastable RS powders by processes such as extrusion, hot isostatic pressing, forging, etc., has drawn even greater attention in recent years. The interest generated in the latter are apparently based on the following expectations: 1) the ultra fine grain size and lack of compositional segregation in the final product, and 2) the possibility of designing unconventional alloy compositions from which unique microstructure and properties are achieved. This paper will briefly discuss the development of RS Ni-Mo-B alloys, with particular emphasis on their crystallization behavior
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