Local Strain Development in High Temperature RuAl Intermetallic Alloys

Abstract

The limited ductility in many high temperature B2 aluminides has significantly hindered their integration into structural components in bulk form in areas such as aircraft engines and commercial power generators. Compared to other high temperature B2 aluminides, RuAl displays a very high melting temperature (Tm ~2068˚C) and unusually high compression ductility, which is thought to arise from its diverse slip behavior and the two phase microstructure. The objective of this study is to examine the plastic behavior of this compound in greater detail, with emphasis on developing a quantitative understanding of the straining processes at the scale of the microstructure. Several advanced experimental techniques, including a newly developed surface displacement mapping technique, orientation imaging microscopy, nanoindentation, focused ion beam and transmission electron microscopy are utilized to investigate the strain development behavior at the local microstructural scale of three different RuAl alloys, each with varying volume fraction of a secondary δ-Ru phase. This research is unique in that it directly connects the local straining behavior to the microstructure as well as to the underlying dislocation activity. It is found that a significant degree of strain heterogeneity developed in RuAl alloys after a few percent nominal deformation, with strains varying by a factor of 10～300％ from the mean imposed strains within the neighborhood of several grains. The characteristics of such heterogeneity vary with the amount of δ-Ru phase present. This δ phase serves as a compliant layer by deforming preferentially during deformation and redistributing strain in the local microstructural areas. In single phase RuAl, the straining of grains is dominated by the <110>{110} slip system in comparison to the <100>{110} system under the local microstructural conditions studied. By examining the details of strain heterogeneity and local lattice distortion, it is found that large strain gradients in the vicinity of grain boundaries are not associated with variations in the density of geometrically necessary dislocations.