The ϵ → η → θ transition in 100Cr6 and its effect on mechanical properties

Low-temperature precipitation reactions in 100Cr6 are characterized using transmission electron microscopy and X-ray diffraction, and modelled using thermokinetic methods. Martensitically transformed 100Cr6 is shown to display a complex microstructure composed of plate martensite, primary carbides, retained austenite and one or more of the ϵ-, η- and θ-phases. It is demonstrated that the maximum tensile strength (in excess of 2 GPa) and ductility is achieved by the θ-phase and the maximum yield strength is found during the α′ + η → α′ + θ transition. The interplay between the amount of carbon in solid solution, the martensite tetragonality and its morphology are related to the precipitate/matrix strain energy, the precipitate species present and their morphology. The progress in precipitate volume fraction, average radius, particle number and matrix composition can be quantitatively described by performing multicomponent precipitation kinetics calculations in paraequilibrium incorporating: (i) the effects of precipitate/matrix lattice misfit strain and particle aspect ratio, (ii) nucleation at plate boundaries and dislocations and (iii) an appropriate value for the precipitate/matrix interfacial energy, which is the only parameter fitted in the calculation.