Microstructural Aspects of the Zircaloy-4/SS-304L Interface Obtained by Diffusion Bonding Technique

Abstract

A dissimilar metal joining method based on diffusion bonding was developed to join 304L stainless steel (SS) and Zr alloy (Zy-4). This was done at 820°C and 950°C under argon and dynamic pressure for 45 minutes.The metallurgical structure of the interface and the evolution of its texture during the treatment were studied by evaluating the distribution of the constituent chemical elements and by identifying the crystalline phases formed. Chemical exchanges through the interface are favored by diffusion phenomena. The junction was characterized by: microscopic observations and chemical analyzes (ESEM-EDS, EPMA), X-RD and mechanical tests (HV and Shear test). Treatment at 820°C does not form a bond because the reciprocal solubilities of the chemical elements of SS and Zy-4 are very low. The junction obtained at 950°C has a reaction zone (RZ) formed at the SS/Zy-4 interface, composed of three layers. The first layer (LI = α-(Fe,Cr) on the SS side and the third layer (LIII=Zr2(Fe, Ni)) on the Zy-4 side are single-phased. The middle layer LII is biphasic (LII= e-Zr(Cr,Fe)2+Zr2(Fe, Ni)). The maximum hardness measured in the RZ is ~ 1120 HV. It is due to the formation of the intermetallic compounds of type e-Zr(Cr,Fe)2 in LII. Examination of fracture facies obtained from the joints reveals that the fracture is localized in the LIII layer and it is fragile in nature of the trans-granular type.