Strain-induced Z-phase formation in a 9% Cr-3% Co martensitic steel during creep at elevated temperature

The size and distribution of Cr(V,Nb)N (Z-phase) particles in a 9Cr-3Co-2W-0.6Mo-0.1Ni-0.2V-0.06Nb-0.1C-0.05N-0.005B steel subjected to creep rupture test during 11,151 h at 650 °C under an applied stress of 100 MPa were studied. The replacement of V-rich (V,Nb)(C,N) carbonitrides by Z-phase was accelerated by plastic flow as suggested by a comparative analysis of these particles in different portions of crept specimen, namely, the grip section, the portion of uniform elongation, and the necked portions. Coarse “hybrid” Z-phase particles evolved throughout the crept specimen by an in situ transformation mechanism. The strain-induced Z-phase nucleated on the V-rich (V,Nb)(C,N)/ferrite interfaces, leading to the formation of numerous Z-phase particles with dimensions less than 50 nm in the necked portion.