Diffusion reaction between Zr–2.5 wt% Nb alloy and martensitic grade 403 stainless steel

Diffusion reaction between Zr–2.5 wt% Nb alloy and 403 stainless steel has been investigated by employing miniature type diffusion couples in the temperature range between 750°C and 940°C for 1–240 h. An electron probe microanalyser (EPMA) has been used to establish the concentration penetration profiles across the diffusion zone and a transmission electron microscope (TEM) has been used to identify various phases formed close to the interface. The microstructure of the bonded region on the 403 steel side is essentially a martensitic structure and remains nearly unchanged during annealing. However, the microstructure of the Zr–2.5 wt% Nb alloy changes substantially, leading to the formation of coarser α phase. The diffusion reaction is extremely sluggish. Localised melting occurs in the specimens annealed at and above 940°C. This is essentially due to the eutectic reaction between zirconium and iron. The experiments confirm that diffusion bonding of 403 steel to Zr–2.5 wt% Nb could be carried out at a pressure of 10 MPa at 900°C for 1 h. The formation of various phases in this multi-phase and multi-component system along with change in their composition with annealing temperature and the nature of reaction products is discussed.