Highlights

Phases, Structure and Strength of RuAlCr Alloys

Scientific Achievement

Alloys of the Ru-Al-Cr system were investigated to find candidate compositions to generate precipitation strengthened HCP+B2 microstructures and to obtain a better description of the ternary isotherms at 1100°C and 1500°C.  In order to construct partial experimental phase field isotherms at 1100°C and 1500°C, selected compositions were annealed at 1100°C for 100 and 200h and at 1500°C for 2.5 and 6h.  Phase compositions were measured by standardized energy dispersive spectrometry (EDS) using the ZAF matrix correction method.

The partial experimental isotherms constructed in this investigation, display a large (RuAl) region at 1100°C and 1500°C with considerable solubility towards the Ru-Cr binary.  As previously observed by Reynolds, the (Ru) phase region was observed to have low solubility in Al (~1% at 1100°C and ~3% at 1500°C).  From the group of alloys investigated, two compositions, induction melted Ru-6Al-38Cr and Ru-4Al-41Cr (at%), exhibited a uniform distribution of B2 precipitates throughout a HCP matrix upon heat treatment at 1100°C for 200h.  The hardness of these two alloys increased from ~370 HV in the as-cast condition to 637.2±12.4 HV and 527.8±16.0HV, respectively, after aging at 1100°C for 200h.  However, an aging heat treatment at 1500°C for 2.5 and 6h did not significantly increase the hardness.  In addition, TEM observations were made on the alloy of composition Ru-6Al-38Cr.  From double tilt experimentation, it was found that matrix (HCP) and precipitate (B2) have a Burger’s orientation relationship (,).

The B2 precipitates are of semicoherent nature with a cigar type shape surrounded by dislocations that run along the long  axis of the precipitate.  A series of compositional profiles taken from the HCP matrix and across different B2 precipitates in the heat treated Ru-6Al-38Cr (at%) alloy using scanning transmission electron microscopy (STEM) indicated that the Al concentration is uniform inside the precipitate and very low in the circumvent regions of the matrix; meanwhile, the Cr composition showed to be lower inside the precipitated phase.  The steep changes of the compositional ratios at the interface between the B2/HCP phases and the shape of the profile suggest that both phases have reached compositional equilibrium after the mentioned heat treatment.

Significance

RuAlCr alloys display a set of properties that make them a strong candidate for high temperature structural applications. Further description of their microstructural features allows understanding the phenomena involved on their macroscopical behavior.  The research described above generated two high temperature isotherms which would allow for better  understanding of the relationship between the phases that could be generated in alloys of specific composition and important chemical and thermomechanical properties.  Furthermore, alloys with low Al content showed microstructural features that would allow to engineer specific RuAlCr compositions that could outperform commercial Co and Ni-based alloys.

Performers

M. Acosta, D. R. Johnson, E. P. Kvam, (Purdue-MSE); N. J. Zaluzec (Argonne-MSD)