Processing and Characterizing RuAl Eutectics
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Processing and Characterizing RuAl Eutectics Todd Reynolds and David Johnson Materials Engineering, Purdue University, West Lafayette, IN 47907-1289 ABSTRACT Eutectic alloys in the Ru-Al-Mo system were directionally solidified by a cold crucible Czochralski technique. The solidification microstructures and composition of the phases were examined and used to estimate a partial liquidus projection. Preliminary results from compression and hardness testing indicate that the hardness of RuAl varies significantly with composition. The Ru-rich alloys in which RuAl is in equilibrium with the hcp (Ru,Mo) solid solution were found to have the lowest hardness values. INTRODUCTION The intermetallic compound RuAl has a melting temperature above 2000°C, and a good room temperature toughness has been reported from limited and qualitative tests [1]. The deformation behavior of this alloy is also unusual. Pollock et al. [2] have shown that the deformation substructure consists of a high density of and dislocations even for specimens tested at liquid nitrogen temperatures. The laboratory growth of RuAl single crystals is a challenging task due to the high melting temperature and loss of Al during melt processing. However, processing RuAl alloys from the melt is significantly easier with eutectic alloys due to the lower melting temperature. For example, polycrystalline specimens produced by arc-melting typically have an intergranular eutectic film (RuAl+Ru) [3]. Directional solidification can be used to align eutectic phases parallel to the growth direction to produce ‘in-situ’ or ‘natural’ composites. In addition to the RuAl-Ru eutectic having a composition of Ru-24Al (at.%) [4], an eutectic consisting of RuAl and a bcc-Mo solid solution will be considered in this paper. The RuAl-Mo eutectic has a composition of Ru-29.5Al-41Mo (at.%) with nearly equal volume fractions of RuAl and Mo [5]. With the greater ease in processing of the eutectic alloys, bulk sized samples of controlled microstructure can be produced for fracture toughness testing. These results can then be compared to the different toughening mechanisms used to describe other B2base alloys [6,7]. This paper will report upon the range of microstructures that can be produced in the Ru-Al-Mo system. Preliminary mechanical property data from microhardness and compression tests will also be reported. EXPERIMENTAL DETAILS Alloys containing Ru, Al, and Mo with compositions as listed in Table 1 were arc-melted with a non-consumable tungsten electrode into approximately 15 g buttons. These buttons were then remelted and directionally solidified in a cold crucible Czochralski tri-arc melter using a pull rate of 500 mm/h. The microharndess of all the samples was measured with a Vickers indentor using a load of 5 kg. At least 10 readings were taken for each sample. In addition to the alloys listed in Table 1, a series of alloys near the RuAl-Mo eutectic (composition-E, Table 1) were produced by arc-melting in which the Ru and Al concentrations were varied. The microhardness
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