The effect of quench rate on the microstructure,mechanical properties,and corrosion behavior of U-6 wt pct Nb

The effect of cooling rate on the microstructure, mechanical behavior, corrosion resistance, and subsequent age hardenability of U-6 wt pct Nb is described and discussed. Cooling rates in excess of 20 Ks^-1 cause the parent γ-phase to transform martensitically to a niobium supersaturated variant of the α-phase. This martensitic phase exhibits low hardness and strength, high ductility, good corrosion resistance, and substantial age hardenability. As cooling rate decreases from 10 Ks^-1 to 0.2 Ks^-1, fine scale microstructural changes (consistent with spinodal decomposition) occur to an increasing extent. These changes produce large increases in hardness and strength and large decreases in ductility, slight decreases in corrosion resistance, and slight changes in age hardenability. At cooling rates less than 0.2 Ks^-1 the parent phase undergoes cellular decomposition to a coarse two-phase lamellar microstructure. This lamellar microstructure exhibits intermediate strength and ductility, substantially reduced corrosion resistance, and no age hardenability. An analysis of the cooling rates at the centers of water quenched plates indicates that fully martensitic microstructures can be obtained in plates as thick as 50 mm.