Small-angle neutron scattering measurements of deuteride formation in Nb

Small-angle neutron-scattering (SANS) measurements of deuteride precipitation in pure Nb have been performed to investigate the effect of deformation, grain boundaries, and thermal history on the phase transformation temperature hysteresis. SANS is a very useful bulk probe of deuteride formation in metals for several reasons. First, the low interaction probability of thermal neutrons in matter results in low attenuation over several millimeters of sample thickness. This is opposed to transmission electron microscopy, which is limited to a few thousand angstroms and X-ray-based techniques that are limited to tens of microns. In addition, neutron-based techniques have superior sensitivity to hydrogen in metals. This is because the neutron-scattering lengths of the elements do vary systematically with atomic number. Thus, significant neutron-scattering contrast can exist between neighboring elements on the periodic table. Even more important with respect to hydrogen in metals is the fact that the scattering length of the deuteron is as large as that of any element. The formation of hydrogen-rich phases, or hydrides, is easily observed when deuterium is used in place of hydrogen. (The use of hydrogen is generally detrimental to the ability to resolve coherent scattering signals; the incoherent background is a factor of 40 larger and the coherent foreground a factor of 3 lower than that of deuterium.) SANS is an accurate probe of the early stages of deuteride formation because of this excellent sensitivity. Finally, the technique can yield particle morphological information, such as the shape, size, and orientation to host lattice (with the use of single crystal material)