(2016-09-07) Thompson, Gregory B.; Wan, Li; University of Alabama Tuscaloosa
A series of 40–2 nm bilayer spacing Ti/Fe multilayers were sputter-deposited. As the length scale
of individual Ti layers equaled to 2 nm, Ti phase transforms from a hexagonal close packed (hcp)-
to-body centered cubic (bcc) crystal structures for equal layer thicknesses in Ti/Fe multilayers.
Further equal reductions in bilayer spacing to less than 1 nm resulted in an additional transformation
from a crystalline to amorphous structure. Atom probe tomography reveals significant intermixing
between layers which contributes to the observed phase transformations. Real-time,
intrinsic growth stress measurements were also performed to relate the adatom mobility to these
phase transformations. For the hcp Ti/bcc Fe multilayers of equivalent volume fractions, the multilayers
undergo an overall tensile stress state to a compressive stress state with decreasing bilayer
thickness for the multilayers. When the above phase transformations occurred, a modest reduction
in the overall compressive stress of the multilayer was noted. Depending on the Fe thickness, the Ti
growth was observed to be a tensile to compressive growth change to a purely compressive growth
for thinner bilayer spacing. Fe retained a tensile growth stress regardless of the bilayer spacing
studied.