Neutron Radiography Studies of Spinodal Decomposition in NbH_x

Abstract

The phase diagram of hydrogen dissolved in niobium shows great similarity to that of a one-component fluid. However, there are also striking differences, and unusual features due to the long-ranged elastic interaction which is responsible for the lattice gas (α’-phase) — lattice liquid (α-phase) phase transition [1]. This elastic interaction originates from the strain field a proton produces whilst acting as a dilatation center in the host lattice. This strain is then felt by the other interstitials. Concentration fluctuations are accompanied by coherency stresses and therefore critical fluctuations of short wavelength are strongly suppressed compared to fluctuations on a macroscopic scale. The long-ranged strain field leads to spinodal decomposition with smoothly varying density profiles. By means of neutron radiography we have mapped the projections of the hydrogen distribution in a niobium sample, and hence reconstructed the distribution in arbitrary planes of the crystal by computerized tomography. These were then compared to the predictions of the well-established theory of Wagner and Horner [2].