Effect of niobium on microstructure and precipitated phase of zirconium alloy

For zirconium-niobium binary alloys, niobium content controls the formation of martensite. When niobium content is lower than 8.0wt%, Ms decreases with the increase of niobium content, and the driving force of martensite formation increases. Under the same quenching condition, the width of martensite decreases, while when Nb content is higher than 8.0wt%, martensite cannot be formed during quenching.

Nb content affects the phase transition and recrystallization temperature. When Nb content is 0.1-0.8 at.%, the transformation temperature between α and β phase is 550 ~ 940℃, and with the increase of Nb content, the transformation temperature between α and β phase decreases, and when Nb content is 0.2at.%, the transformation temperature is 910 ~ 940℃. When Nb content is 0.8at.%, the value is lower than 850℃. The aging time of zirconium alloy with niobium content of 0.2at.% is 60min at 575~600℃, and only about 10min at 625~650℃. When the niobium content exceeds 0.5at.%, the aging time required for recrystallization is longer than that for zirconium alloy with niobium content of 0.2at.%. The lower the Nb content is, the shorter the annealing time for recrystallization is, because the low Nb grain growth resistance is less, while the high Nb grain growth is inhibited by the existence of precipitates. According to the observation of transmission electron microscope, only dislocation exists in Zr-0.2Nb, while both dislocation and twin exist in Zr-1.5Nb.

Niobium content also controls the precipitates in the alloy. The Zr₃Fe type segregation (orthogonal structure) was precipitated from zirconium alloys containing 0.2-0.3wt % Nb at 590℃ after final annealing at 0 ~ 0.6wt% Nb. This was due to the low solid solubility (about 200ppm at room temperature) of the residual iron in the alloys. When the Nb content reaches 0.4 ~ 0.5wt%, Zr(Nb, Fe)² phase with dense hexagonal structure appears and decreases with the increase of Nb content.

After quenching by β phase, the microstructure of zirconium alloy is semi-strip or granular and refined with the increase of Nb content. Due to the presence of β -phase stable element Fe, the equilibrium concentration of Nb is lower than 0.3 wt%. When niobium content exceeds 1wt%, β-Nb phase appears. β phase is an important high temperature stable phase in Zr-Nb binary alloy system. The decomposition of β phase includes three processes :ω phase formation, α phase formation and hydride segregation. ω phase is formed in Zr-Nb binary alloy by electron irradiation and heat treatment. The morphology and internal structure of α phase depend on the type of heat treatment. The amorphous α phase can be obtained by fractional quenching from β phase region, while twin weidenite α phase can be obtained by quenching and aging. Hydride segregation exists with α phase, β phase and α+β phase. When silver content is lower than 8.0wt%, martensite transformation occurs in β phase. In the range of 8.0 ~ 17wt%, the β phase with poor thermal conductivity and the β phase with rich solute can be obtained by quenching in β phase region. When the silver content continues to increase, the β phase tends to be stable, but the unstable ω phase can still be seen. When niobium content exceeds 20wt%, ω phase disappears and β phase becomes stable. When niobium content exceeds 8at.%, the side length of the equilateral triangle ω phase a. As a constant; When niobium content is less than 8at.%, a. Decreases with the increase of niobium content.

In addition to Zr(Fe,Cr)² phase, niobium forms LAVES phase with Fe and Cr, and the solid solubility of Niobium in a-Zr decreases to about 100μg·g^-1. And with the intermediate annealing temperature, the second phase, and the niobium content increase of niobium in alpha zirconium content and decreased with the increase of annealing temperature, the binary alloy and Zr-Nb alloy elements in the solid solubility of significantly increased with a rise of temperature is different, it boils down to with the increase of annealing temperature niobium diffusion capacity, resulting in increased levels in the second phase.