Processing and Characterizing RuAl Eutectics Todd Reynolds - Purdue Graduate Student
There is a need for high temperature materials that have good mechanical properties. The intermetallic coupound RuAl has a melting temperature above 2000 C, and good room temperature toughness. RuAl is difficult to process, so eutectics are of interest do the their lower melting temperature. Eutectic alloys in the Ru-Al-Mo system were directionally solidified by a cold crucible Czochralski technique. The technique involves heating with three arcs (using tungsten electrodes) and extracting the solidifying casting at a 500 mm/hr rate.
The eutectic was found to exist at about 51 Wt% Mo and had a Vickers hardness of 725HV (slightly below the "rule of mixtures" value). A platelike microstucture was evident in the transverse, and fan-like distribution in the cross section.
Hardness was evaluated as a function of Ru concentration and also Re consentration, but the effects were small over the concentrations of interest. The Ru-rich alloys in which (b2) RuAl is in equilibrium with the (hcp) (Ru,Mo) solid solution were found to have the lowest hardness values.
Some three-phase compositions were also studied involving the formation of a "sigma" phase near the peritectoid. Using the data gathered it was possible to develop a processing curve on the ternary phase diagram (near liquidus).
Further work was conducted substituting some Chromium for Molybdenum in the hope of reducing the extremely fast oxidation. There was some encouragement in this phase of the study.
More study will be necessary as most resulting material is hard and very brittle.