Institute of Metals Division - Hardening by Internal Oxidation as a Function of Velocity of the Oxidation Boundary

Oxidation hardening of cylindrical and spherical specimens first decreases with depth below the surface, but then increases again as the center is approached. This is in agreement with the view that this change in hardness is mainly determined by the change in velocity of the oxidation boundary, which affects the dispersion of the oxide. WHEN a thick slab of silver containing 0.3 pct Mg by weight is internally oxidized, the hardness measured on a cross section is found to decrease considerably with distance from the surface.&apos; This phenomenon has been attributed to the diminution of the velocity of the oxidation boundary. The smaller this velocity is, the more slowly the free-oxygen concentration rises towards the concentration in equilibrium with the oxygen gas. And a small 0 concentration favors dissociation and thus conglomeration of the oxide. For instance, the hardness decreases more rapidly during long annealings in nitrogen than in air.&apos; Measurements by Gregory and Smith3 on sections through internally oxidized Al-silver wire showed a smaller variation in hardness, probably because the wire diameter was only 0.9 mm. Wood4 found a drop in hardness with depth in internally oxidized Al-copper strip, and also—by electron microscopy—the concomitant increase in Al2O3 particle size. Under certain circumstances internal oxidation leads to formation of rather irregular inner oxide films in the midst of dispersed oxide. This special sort of conglomeration—discussed in Ref. 5—is also favored by greater depth below the surface.3,5 In this paper hardness measurements in large diameter cylindrical and spherical specimens are described. Here the oxidation velocity* goes through ness initially decreases with depth but increases again towards the center. Normally, the diffusion coefficient D1 of oxygen is very large compared to D2, that of the dissolved metal. If, furthermore, the atomic fraction c2 of the latter is much larger than the O-solubility cl- but c2D2 << c1D1- the velocities in question are easily calculated2 to be: for the cylinder and for the sphere. Here p is the radial coordinate of the oxidation boundary and R the specimen radius, which in dilute silver alloys is constant, as no external oxidation occurs. These equations have been derived for a divalent solute; for nondivalent solutes c2 has to be multiplied by half the valency. The equations are not valid near the center. For which can be considered as a standard case,2 the Eqs [I] and [2] begin to yield U-values which are appreciably too high for p/R = 0.05 and 0.15, respectively. But at the v-minima they are still very good approximations. These minima lie at p = emlR= 0.37R in the cylinder and p = 1/2 R in the sphere. Other factors besides the velocity v will also influence the hardness. The oxide concentration cox is constant in a flat strip, apart from a narrow region in the middle,2 but in cylindrical and spherical specimens cox decreases steadily with depth. When C1 << c2 and c1D1 >> c2D2 one can calculate