Phase separation in titanium hydrides studied by small-angle neutron scattering

Various phases of titanium hydrides with differing hydrogen and deuterium content are studied by small-angle neutron scattering. The low temperature metastable x phase produced by quenching under high pressure shows small, but pronounced scattering at liquid nitrogen temperature. At 140 K a very abrupt increase in the scattered intensity is detected, more than one order of magnitude for less than 30 s. The scattering from the room temperature (a+-/) phase is analysed by the fractal as well as the random two-phase model. The fit does not prove fractal behaviour of the &-Ti precipitates in the Y-T~H(D)~ matrix. A good representation is obtained in a random two-phase model, giving about 50 and 60 A correlation lengths, independently of the deuterium or hydrogen content. For the S phase, only the asymptotic behaviour of the scattering curves is observed in the scattering vector range covered in the experiments. Introduction A sequence of phase transitions can be observed on heating the TiH(D). (x=0.7-0.9) low temperature metastable phase prepared by fast cooling under high pressure[l]. Wide angle neutron diffraction experiments showed that these transformations change the distribution of H(D) atoms in the metal sublattice[2]. In the low temperature metastable x phase, H(D) randomly occupies the octahedral interstitial sites of the fco metal sublattice. Above 140 K the system transforms to TiH(D)I ( ~ ~ h a s e ) containing finely dispersed cr-Ti. The 7 phase involves H(D) atoms occupying ordered tetrahedral interstices of the subsequent (110) planes of the fco metal sublattice. If the 7 phase is annealed above 480 K, it undergoes transformation into TiH(D)2-v hydrogen(deuterium) deficient S phase with fcc metal sublattice, in which H(D) is randomly distributed in the tetrahedral interstices. In the present work small-angle neutron scattering has been used to investigate the short and medium range order in TiH(D), samples of various phase composition.