Ultra-high speed neutron diffraction studies of combustion synthesis

Abstract

In-situ neutron diffraction at time resolutions in the range 380–900 ms recorded on the instrument D20 at the Institut Laue-Langevin have been used to capture the reaction mechanisms during the furnace ignited combustion synthesis of a range of M n AX n −1 phases (M—early transition metal, A—group IIIA or IVA element, X—either C or N), intermetallic compounds (Ti 5Si 3) and intermetallic compound-refractory carbide composites (Ni–Al–Ti–C). The sequence of diffraction patterns reveals great complexity. Some reactions occur completely in the solid state and others involve a transient liquid phase(s). In all but binary mixtures, transient solid phases that may persist for as little as a few seconds were observed. Data analysis methods are illustrated which allow the determination of: (i) the sequence of phases and reaction mechanism, (ii) the crystal structure, atomic substitutions etc of the phases present, (iii) the ‘trigger’ mechanism for the self-sustaining reaction, (iv) the reaction kinetics and activation energy for growth of the product phase (via quantitative phase analysis using Rietveld refinement scale factors and a non-isothermal Avrami kinetic equation), (v) the average sample temperature (via thermal expansion/contraction of known phases) and the formation enthalpy of the product phase (via excursions in the sample temperature).