(688b) Direct Observation of Effective Atomic Diffusion Distances In Zr/2Al Multilayers Due To Self-Propagating Reactions

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 and that the heat release associated with reaction is finished when mass diffusion is complete.  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, actual diffusions distances of self-propagating synthesis reactions were 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 very similar chemical behavior.  This is due to lanthanide contraction, which causes Hf and Zr to have very similar electronegativities and atomic radii.  Hf and Zr also react with Al to form intermetallic compounds with similar structures, including di-aluminide line compounds.  These similarities between Zr and Hf allow the overall 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 also performed to quantitatively identify the elemental distribution in both reacted and unreacted multilayer.  EDS results clearly showed that the Hf atoms spread to over ten times the assumed diffusion distance.   The results are disparate to the general assumption of diffusion distance and indicate that the atoms can remain mobile, even after complete mixing has occurred.  A time scale of heat release as related to diffusion distance is also discussed.

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.