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

Abstract

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™ ^[1,2] show that 20Ti enters a 3 phase (αZr-HCP, βNb-BCC, and βZr-BCC) region at 500°C while 10Ti alloy continues to be a 2 phase (αZr and βNb) alloy until 550°C and then enters the 3 phase (αZr, βNb, and βZr) region. Both alloys have a single phase β Zr solid solution at 700°C which is detrimental for the oxidation resistance. β 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 β Zr phase in the microstructure.