Microstructural Development of Mo(ss) + T2 Two-Phase Alloys
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In addition, due to the slow diffusion rates involved in the high-temperature ternary system, an extended annealing treatment at high temperatures is necessary to dissolve these phases [4-5]. It is therefore of great interest to examine the feasibility of utilizing alternative solidification pathways which may allow for a much shorter diffusion path during solid-state annealing. In this study, the microstructure evolution in both arc-melted and rapidly quenched alloys with compositions located in the Mo(ss)-T2 quasi-binary section have been examined. The SQ method allows for the attainment of high cooling rates (up to 106 K/sec) which may yield highly undercooled melts. The DTP may also produce highly undercooled powders due to potential nucleant isolation achievable during solidification. Thus, fast solidification rates (at deep undercooling) can be obtained resulting in refined microstructures. The central objective of this study is to identify the important characteristics of microstructural modifications achieved by these RSP techniques to attain a uniform Mo(ss) + T2 two-phase microstructure. EXPERIMENTAL PROCEDURE Alloys with compositions in the two-phase field at 1600'C were arc-melted from pure elements
of Mo (99.97%), B(99.5%) and Si(99.995%) under Ar atmosphere (99.998%). Multiple melting cycles
were performed to assure homogeneity of the ingots. Some of the alloys were rapidly solidified by means of two-piston splat quenching and drop-tube processing under Ar or He-2% H2 atmosphere. Subsequent annealing treatment at 1600'C and 1200'C up to 150 hours were carried out in a MoSi 2 element furnace under Ar flow. The as-solidified as well as the annealed microstructures were characterized by scanning electron microscopy (JEOL-6100), transmission electron microscopy (JEOL 200CX and Philips CM200), differential thermal analysis (Perkin-Elmer DTA) and XRD (STOE-XRD). RESULT AND DISCUSSION The two-phase field at 1600'C has been previously reassessed [5] and is used in this study to analyze the solidification pathways within the region. Within the Mo(ss) -T2 quasi-binary section (Figure 1), four different types of primary phases has been observed in the arc-melted alloys depending on the composition (Mo(ss), Mo2B, T2 and MoB). In the Mo-rich portion on the two-phase field (79-100 at. % Mo with 0
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