Phase transformation and precipitation of Zr-Sn-Nb-Fe alloys at different cooling rates

Abstract

The phase transformation mechanisms and elemental precipitation behaviors in Zr−1.0Sn−1.0Nb−0.3Fe (N1) and Zr−1.0Sn−0.3Nb−0.3Fe (N2) alloys under varying cooling rates were systematically investigated. As the cooling rates decrease from water cooling (WC) to liquid nitrogen cooling (LNC), air cooling (AC), and finally furnace cooling (FC), the phase transformation progressively shifts from a non-diffusional martensitic mechanism to a diffusional mechanism, accompanied by increasing Nb and Fe segregation. Rapid cooling (WC) retains metastable (β+ω)-Zr phases with limited Fe enrichment. Intermediate cooling rates (LNC/AC) promote semi-diffusional transformations, leading to the formation of binary Zr-Fe precipitates (Zr₂Fe, Zr₃Fe). Slow furnace cooling (FC) allows for complete elemental diffusion, resulting in residual β-Zr and equilibrium Zr(Nb,Fe)₂ phases. Notably, Zr₂Fe persists in the low-Nb N2 alloy under these conditions. Phase transformations and precipitate phases nucleate and grow along the low-index habit planes of the parent phase. In addition to the Burgers orientation relationship between α-Zr phase and β-Zr phase, the orientation relationships between Zr₂Fe and Zr(Nb,Fe)₂ and the matrix, designated as $<0001>\alpha //<001>\text{Zr}2\text{Fe}$, $\left\{\bar{1}100\right\}\alpha //\left\{110\right\}\text{Zr}2\text{Fe}$ and $<011>\beta //<\bar{2}110>\text{Zr}(\text{Nb},\text{Fe})2$, $\{21\bar{1}\}\beta //\{0002\left\}\text{Zr}\right(\text{Nb},\text{Fe})2$, have also been characterized.