Paper TF-ThA2
Direct Visualization of Atomic Diffusion Distances after Self-Propagating Reactions in Zr/Al Multilayers

Thursday, October 31, 2013, 2:20 pm, Room 102 C

It is generally assumed that the diffusion distance for heterogeneous reactive materials is defined simply as half of the characteristic dimension of the constituent materials. In a powder system, this assumption results in the diffusion distance being equivalent to the particle radius, and, in a lamellar system, the diffusion distance would be half of the thickness of a single layer of material. This assumption is important as it is used to estimate overall reaction times and heat release rates. In this study, the validity of this assumption was investigated in sputter deposited multilayers of Zr/2Al composition with a single Zr layer replaced by a Hf marker layer. Hafnium was selected as the marker material because Zr and Hf have similar chemical behavior. This is due to lanthanide contraction, which causes Hf and Zr to have very nearly identical electronegativities and atomic radii. Hafnium and zirconium also react with Al to form intermetallic compounds with similar structures, including di-aluminide line compounds. These similarities between Zr and Hf allow the reaction to progress with little detrimental effect from the Hf inclusion. Multilayers containing a single marker layer were converted to product phases (nominally ZrAl₂/HfAl₂) in the self-propagating reaction mode. Both unreacted and reacted foils were then cross-sectioned and imaged in an aberration-corrected transmission electron microscope. Energy dispersive spectroscopy (EDS) was performed to quantitatively identify the elemental distribution in both reacted and unreacted multilayers. EDS results clearly showed that the Hf atoms spread to over ten times the assumed diffusion distance. The results are in strong disagreement with the general assumption of diffusion distance and indicate that the atoms can remain mobile, even after complete mixing has occurred.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.