Abstract
Overall kinetics of lamellar overaging reactions in U-5.5Nb and U-7.5Nb were analyzed by Avrami-Arrhenius analyses of volume fractions measured from an extensive temperature-time (T-t) matrix of specimens. The cellular initiation site (grain boundaries, inclusions) and regimes of lamellar divergency-cum-slowing growth rate were explicitly accounted for. Avrami exponents n from T-t regimes of constant-growth rate were consistent with theory (1<n<3); those from divergent T-t regimes were smaller, n~0.7, which is not surprising given their different growth rate behavior. The apparent activation energies Q were similar for grain-boundary and inclusion-nucleated discontinuous precipitation, indicating that their nucleation site does not alter their overall kinetics. Avrami Analysis of Isothermal Aging Kinetics
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