Microstructure evolution and mechanical property of high temperature solid-state diffusion bonded Cr-Zry4 with and without a 316 SS interlayer

Abstract

Solid-state diffusion bonding between pure Cr and Zircaloy-4 (Zry4) was studied with and without 316 stainless steel (SS) as an interlayer at temperatures ranging from 1203 to 1573 K, followed by the detailed microstructural analysis and mechanical property evaluation at the bonding interfaces. For the samples directly bonded between Cr and Zry4, two regions were confirmed at the diffusion interface. One was a continuous block of Zr2Cr intermetallic compound formed by the inter-diffusion of Cr and Zr, and the other was a mixture of Zr2Cr and Zrα phases as the consequence of Zr-Cr eutectic reaction. For the samples with SS as an interlayer, three regions were mainly observed at the SS-Zry4 interface, consisting of (FeCr)α-, Zr(Fe, Cr)2-, and Zr2(Fe, Ni)-phase dominated layers, with (FeCr)α-phase observed at Cr-SS interface. The microstructure of the phases at diffusion interfaces were closely related to the bonding temperature. Nano-indentation results confirmed a rapid increase in hardness at Cr-Zry4 and SS-Zry4 interfaces for Cr-Zry4 and Cr-SS-Zry4 bonded specimens, respectively, due to formation of the ZrCr2/Zr(Fe, Cr)2 phase. The presence of the hard brittle intermetallic compounds at the interface infers the possible brittle fracture risk of the Cr-Zry4 interface during long-term operation under normal condition and high temperature under accident conditions.