High temperature neutron diffraction study of the order-disorder transition in TiC 1− x

Abstract

High temperature neutron diffraction experiments on TiC 1− x samples have allowed to determine from the thermal dependence of the (1/2 1/2 1/2), (3/2 1/2 1/2) and (3/2 3/2 1/2) superlattice reflexions the transition temperatures due to long-range ordering of carbon vacancies: 765° ± 5° C for TiC 0.58. 785° ± 5° C for TiC 0.63 and 770° ± 5° C for TiC 0.67. For the first composition, the transition is practically of second order type with a critical coefficient β = 0.26 ± 0.03, suggesting predominant short-range interactions between carbons and vacancies. A preliminary study of the kinetics of longrange ordering was performed between 650° and 750°C for TiC 0.63: at 735°C, the average ordered domain size D increases with time t as D 3.24 − D 3024 0 ∼- Ct ; the process is thermally activated with an apparent activation energy of 3.7± 0.2 eV. The kinetics of ordering decrease rapidly with increasing carbon content. The results are discussed in terms of recent theories of order-disorder and antiphase domain coarsening in alloys.