The Hardening of Fe–Mo Single Crystals by Non-Deforming Precipitated Particles

Abstract

Single crystals of iron base molybdenum alloys containing a high volume fraction of precipitated particles Fe2Mo noncoherent with the matrix were extended at various temperatures between −110° and 200°C. Critical resolved shear stresses in these single crystals depended on the particle size, mean interparticle spacing, and deformation temperature, but not on the crystal orientation of the single crystals used. The increment of the macroscopic yield stresses by the precipitated particles was obtained by subtracting the solid solution strengthening of the matrix from the observed yield stresses in the aged specimens. It was found that the stress increment by the noncoherently precipitated particles was explained fairly well by the Orowan’s mechanism in this alloy system. At the initial stage of plastic deformation, neither primary nor cross slip lines could be resolved by replica-electron microscopy. On the other hand, at a large strain of about 8%, cross slips were clearly observed to take place over a distance of the order of particle size. The changes in crystal orientation with extension were barely perceptible in the over-aged Fe-10%Mo alloys, probably less than 1 degree even after 10% elongation. The increases in flow stress due to work hardening seemed to be rather independent of the crystal orientation of aged specimens and deformation temperature. This can better be described in terms of a secondary slip theory by Ashby than a model proposed by Fisher et al..