Effect of Fe2Zr phase on the mechanical properties and fracture behavior of Fe-Cr-W-Zr ferritic alloy

Two Fe-9Cr-2W-Zr ferritic alloys with Zr contents of ~7 and ~10 wt% were designed to investigate the effect of Fe2Zr phase on the mechanical properties and fracture behavior. It is found that after hot deformation and annealing, the as-cast lamellar eutectic structures are destroyed, showing a microstructure consisting of dispersed micrometer-sized Fe2Zr phases and equiaxed α-Fe grains. The area fraction of Fe2Zr phase increases with the increase of Zr content, and the strengthening effect of dispersed Fe2Zr phases leads to a remarkable increase in the strength and a decrease in the ductility at room and elevated temperature. Compared with 9Cr2WVTa steel, Fe-Cr-W-Zr ferritic alloy with the Zr content of ~10 wt% presents a higher strength-ductility synergy at elevated temperature. Investigation on the fracture process shows that the critical event controlling fracture at room temperature is the nucleation of cracks within Fe2Zr phases due to the plastic strain incompatibility between the soft α-Fe and hard Fe2Zr phases, which further leads to the cleavage fracture of α-Fe matrix. When tensile at 973 K, ductile α-Fe matrix could retard the nucleation and propagation of cracks, resulting in the enhanced high-temperature ductility. Besides, the excellent thermal stability of α-Fe grain boundaries and Fe2Zr/matrix interfaces in the Fe-Cr-W-Zr ferritic alloy ensures better creep-rupture properties than 9Cr2WVTa steel.