The influence of phosphorus in dilute solid solution on the absolute surface and grain boundary energies of iron

Abstract

A technique has been developed for the determination of the absolute surface energies of iron and iron alloys at temperatures of up to the melting point. The technique is based upon the measurement of the contractile forces in thin foils. All measurements are made in situ and chemical equilibrium is maintained throughout the experiment. The results show that phosphorus in dilute solid solution decreases progressively the surface energy of iron at 1450 $^\circ$C from 2100 ergs cm$^{-2}$ to 1200 ergs cm$^{-2}$, for increasing phosphorus contents of up to 0.36%. Results were also obtained for the $\gamma$ phase in which the effect is less pronounced. Absolute grain boundary energies were also determined as a function of phosphorus content. From the strain rate of the foils which move by a diffusion creep mechanism, it was deduced that the self diffusion coefficient increases linearly with phosphorus content. Application of the Gibbs adsorption theorem has permitted the evaluation of the extents of equilibrium segregation of the solute to interfaces. The maximum levels are 2.3 x 10$^{-9}$ g-atom cm$^{-2}$ at surfaces and 1.1 x 10$^{-9}$ g-atom cm$^{-2}$ at grain boundaries. The relevance of these measurements to the problem of the intergranular brittle fracture of iron/phosphorus alloys is discussed.