Correlation between microstructures and oxidation resistance in zirconium-niobium-titanium alloys

Oxidation behavior of Zr-10Nb-10Ti and Zr-10Nb-20Ti (compositions are in atomic percent) alloys has been investigated in air between 300 and 700°C. Higher Ti content in the alloy enhances the oxidation resistance. The calculated isotherms by Pandat(TM) [1,2] show that 20Ti enters a 3 phase (alphaZr-HCP, betaNb-BCC, and betaZr-BCC) region at 500°C while 10Ti alloy continues to be a 2 phase (alphaZr and betaNb) alloy until 550°C and then enters the 3 phase (alphaZr, betaNb, and betaZr) region. Both alloys have a single phase beta Zr solid solution at 700°C which is detrimental for the oxidation resistance. beta Nb-phase greatly contributes to the oxidation resistance in these two alloys. The common oxidation products have been identified as TiO2, ZrO2, and Nb 2O5. Formation of substoichimoetric structures has been observed at lower temperatures. Both alloys suffer from pest oxidation at temperatures between 500 and 550°C, respectively (20Ti and 10Ti), up to 700°C. XRD indicates strong peaks for monoclinic structure of ZrO2 at temperatures above 600°C. Vickers micro-hardness measurements for the as-cast alloys and Rockwell Hardness measurements were conducted on the as-cast and after experimental exposure alloys, in order to obtain some sense of the mechanical properties of the alloys. Increased hardness is related to the formation of beta Zr phase in the microstructure.