Solidification Processing and Fracture Behavior of RuAl-Based Alloys
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Solidification Processing and Fracture Behavior of RuAl-Based Alloys Todd Reynolds and David Johnson Materials Engineering, Purdue University West Lafayette, IN 47907-1289 ABSTRACT Alloys of RuAl-Ru were processed using various solidification methods, and the fracture behavior was examined. The fracture toughness values for RuAl-hcp(Ru,Mo) and RuAlhcp(Ru,Cr) alloys ranged from 23 to 38 MPa√m, while the volume fraction of RuAl ranged from 22 to 56 percent. Increasing the volume fraction of RuAl resulted in a decrease in fracture toughness. The hcp solid solution was shown to be the more ductile phase with a fracture toughness approaching 68 MPa√m, while the B2 solid solution (RuAl) was found to have a fracture toughness less than 13 MPa√m. An alloy of Ru-7Al-38Cr (at.%) that consisted of a hcp matrix with RuAl precipitates had the highest room temperature toughness and the greatest hardness. INTRODUCTION Ruthenium aluminide with a CsCl (B2) crystal structure has been identified as a possible high temperature aluminide (Tmp>2000 °C) for structural applications [1]. Processing RuAl alloys from the melt is difficult due to the high vapor pressure of Al. The resulting Al loss results in the occurrence of an intergranular eutectic film (RuAl-Ru). Wolff et al. reported that such an intergranular eutectic film may enhance the fracture toughness by acting as a ductile compliant layer between the more brittle intermetallic phase [2-4]. Thus, eutectic alloys consisting of RuAl may potentially have good fracture toughness. However, due to the cost of Ru, producing ingots of the RuAl-Ru eutectic with a composition of Ru-24Al (at.%) [5] is prohibitively expensive. To lower the cost of RuAl alloys, ruthenium must be substituted with a less expensive element. Additions of Mo to the RuAl-Ru eutectic have been investigated and good room temperature fracture toughness values were measured for these alloys [6]. However, the oxidation resistance of these Ru-Al-Mo alloys is poor [6,7]. Additions of chromium instead of molybdenum may improve oxidation resistance while retaining the good fracture toughness. In this paper, the fracture toughness of RuAl-Ru(Mo) and RuAl-Ru(Cr) alloys will be reported. EXPERIMENTAL DETAILS Alloys containing a combination of Ru, Al, Mo, and Cr with compositions as listed in Table I were arc-melted with a non-consumable tungsten electrode into approximately 5 g buttons. Buttons of same composition were then welded together to produce an ingot for directional solidification. These ingots were then processed using one of five different processing techniques. Ingot-1 was directionally arc-melted with a growth speed of 45 mm/h, while ingot-2 remained in an as arc-melted condition. Ingot-3 was directionally solidified at 500 mm/h by using a cold crucible Czochralski tri-arc melter, and ingot-4 was zone melted in an optical floating zone furnace at 20 mm/h. Lastly, ingots 5-7 were directionally solidified at 20 mm/h in a modified Bridgman furnace using MgO crucibles. A flowing Ar/5% H2 atmosphere was used in
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