Many organisms construct structural ceramic (biomineral) composites from seemingly mundane materials; cell-mediated processes control both the nucleation and growth of mineral and the development of composite microarchitecture. Living systems fabricate biocomposites by: (i) confining biomineralization within specific subunit compartments; (ii) producing a specific mineral with defined crystal size and orientation; and (iii) packaging many incremental units together in a moving front process to form fully densified, macroscopic structures. By adapting biological principles, materials scientists are attempting to produce novel materials. To date, neither the elegance of the biomineral assembly mechanisms nor the intricate composite microarchitectures have been duplicated by nonbiological processing. However, substantial progress has been made in the understanding of how biomineralization occurs, and the first steps are now being taken to exploit the basic principles involved.

Irradiation lnduced Densification of UO, Pellet Fuel," J. Nucl. Muter., 62, 138-66 (1976). 5A. C. Damask, "Radiation Sintering," Rudiat. Eff., 1, 95-100 (1969). 6D. C. Rorer, Brookhaven National Laboratory; REFERENCES 'K. A. Berry and M. P. Harmer, "Effect of MgO private communication, 1989.

A treatment of diffusion limited growth of a coherent spherical precipitate into supersaturated solution is presented. It is found that the growth kinetics are affected by dilatational coherency strains and by compositionally induced strains in the matrix phase. Numerical solutions to the time-dependent problem are obtained and are compared to the quasistationary solution. The parabolic growth coefficient is a function of the transformation strain, partial molar volumes of the components, elastic constants in each phase, interfacial compositions and far-field composition while, in contrast, the growth coefficient in the absence of stress is a function only of the reduced supersaturation. Elastic effects shift the interfacial concentration of the matrix in the direction of the far-field concentration, reducing the effective driving force for growth. At the same time, compositionally induced strains increase the diffusive flux, increasing the growth rate.

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