Turnbull’s transient nucleation theory, when applied to the transformation that takes place when high purity aluminum-copper alloys are quenched directly to the aging temperature, explains the incubation period that precedes steady state nucleation. This incubation period arises from the comparatively slow rate of diffusion of solute atoms in forming the equilibrium concentration of critical size embryos which must be achieved before steady state nucleation can proceed. This rate of diffusion is a function of the temperature, degree of supersaturation, number of critical size embryos and plastic deformation. / Master of Science