Grain Boundary Self-diffusion Research Articles

Self-Diffusivity Along Edge-Dislocation Singular Lines in Silver

Use was made of a recently developed surface-accumulation diffusion technique to measure the self-diffusivity of edge-type dislocation singular lines (Burgers vector along ) in a bent and polygonized single crystal of silver. Two factors are involved in obtaining a number for the diffusivity. One factor, the dislocation-line density, was measured by a microscopic technique recently developed, and the value of the other parameter, the effective “diameter” of a dislocation pipe, was estimated. With these parameters equal to 7.3×106 dislocations per sq cm and 10−7 cm, respectively, it was found that the self-diffusivity along edge-type dislocation lines was a factor of about two times the average grain-boundary self-diffusivity. The dislocation-pipe diffusivity is about 7×10−7 sq cm per sec at 450°C.

The effect of relative crystal and boundary orientations on grain boundary diffusion rates

The rate of self-diffusion of silver in the grain boundaries of bicrystals, misoriented by rotations of θ around a [100] axis, has been measured for 9° ⪕ θ ⪕ 28°. Within the experimental uncertainty, the results can be described, using the dislocation model of grain boundaries, with the assumption that the coefficient of self-diffusion, D p , in grain boundary dislocation pipes is independent of θ (for θ ≲ 28°) and that D p is several orders of magnitude larger than the lattice self-diffusion coefficient, D L (at 500°C, D p ~ 4 × 10 6 D L ). The dependence of D p on T is given by: D p = 0.14 exp [ −19,700 RT ] cm 2 sec −1 . The activation energy (19.7 ± 2 kcal/gm atom) for D p is in good agreement with that (20 ± 2 kcal/gm atom) reported earlier for self-diffusion in grain boundaries separating randomly oriented crystals. A marked effect of grain boundary orientation on the diffusion penetration was observed for θ = 9°.

On the Mechanism of Grain Growth in Metals, with Special Reference to Steel

The rate of grain growth in the initial stages of isothermal austenitizing has been studied by a thermal etching method in plain, eutectoid, carbon steel in the range 840-970°c. An equation relating the mean initial and instantaneous grain diameters, D0 and D, to the austenitizing time t, of the form D2-D02=Kt exp (-H/kT) is developed on the assumption that the growth rate is controlled by interfacial tension at grain boundaries. An estimate of the constant K is made, and by the use of experimentally obtained values of D, D0 and t the value of H is calculated to be 30 kcal/g atom, close to the activation energy estimated for grain-boundary self-diffusion and to that determined for the damping of low amplitude torsional oscillations.