Powder Alloy System and Homogenization Variables

Characteristics of the powder particles that compose the mixture exert a significant effect on homogenization kinetics (Ref 10). The sizes of the particles in the mixture establish the distance over which interdiffUsion must occur to achieve homogeneity. In alloy mixtures where the elemental solvent particles form a continuous matrix around the dispersed solute-rich particles, particle size of the dispersed particles becomes the important parameter in establishing the interdiffusion distance (assuming that the proportion of the powders in the mixture is already established by the alloy composition desired). Very small powders, such as those produced by reduction of submicron mixtures of oxides or alternatively by mechanical alloying, result in diffusion distances that are small enough to preclude the consideration of homogenization.

The composition of the dispersed particles also can influence homogenization kinetics; for example, solvent alloyed into the solute-rich particles reduces the extent of inhomogeneity in the mixture and can shorten the interdiffusion distance. If the dispersed copper particles in a nickel matrix was 50% Cu, the amount of nickel needed in the mixture to achieve the same overall composition would be less, thus shortening interdiffusion distance.

Alloy systems with more than two components that contain more than a single phase are more difficult to analyze in terms of homogenization kinetics. Interdiffusion in single-phase ternary systems must be described by four diffusion coefficients to consider the diffusion of both solutes. Each is influenced by its own concentration gradient and that of the other solute.

Multiphase binary systems must be described by the solute solubilities and interdiffusion coefficients of each of the phases present (Ref 10, 11, 12). In such complexities, the parameters required to analyze the homogenization process are not available, and reasonable simplifications (assumption of binary and/or single-phase behavior) are made. Similarly, the presence of a transient liquid phase at the outset of the homogenization process also is difficult to analyze, and reasonable approximations are necessary.

The duration of sintering required to achieve a desired degree of homogeneity is critically dependent on temperature, because interdiffUsion coefficients ( D) (units of length squared time) are exponentially dependent on temperature:

where D0 is the preexponential factor, Q is the activation energy, R is the universal gas constant, and T is the absolute temperature.

For example, the interdiffusion coefficient for most solid-solution elements in nickel doubles for every 50 K increase in temperature near the liquidus. The total duration (t) of elevated-temperature treatment to achieve essentially complete homogenization is inversely proportional to D.

where f is the average diameter of the dispersed particles and k is a constant, depending on alloy system parameters, powder particle compositions, and overall alloy composition. For example, k is approximately 0.2 for a 4-to-1 mixture of elemental powders that exhibits complete solid solubility (Ref 10). Thus, it can be seen that both the homogenization temperature and the size of the dispersed particles are the most significant variables in controlling homogenization kinetics.

Mixing and mechanical working during powder fabrication of an alloy also can influence homogenization kinetics (Ref 10, 13). Ideal mixing is assumed in Eq 4 to predict homogenization time. Inadequate blending technique or incompatible powder particles (significantly different sizes and/or densities) result in poor mixing. The greatly enlarged interdiffusion distances dictate longer homogenization times. The serious problems of poor mixing stem from homogenization time being proportional to the square of the interdiffusion distance.

Effective interdiffusion distance may be reduced and homogenization kinetics increased by mechanical working (Ref 10). Such working has the same effect as a reduction in particle size of the dispersed particles. However, the effect of a given mechanical working reduction is less than that indicated by the same reduction in fin Eq 4, because the spherical diffusion flux geometry in a compacted mixture of powders is changed to the less efficient unidirectional flux (by rolling) or cylindrical flux (by extrusion). Any enhancement of homogenization kinetics will result only if mechanical working reduces the thicknesses of the solute-rich regions, little, if any, increase in homogenization kinetics is exhibited during subsequent thermal processing.

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