Effect of Ti/Nb ratio on the microstructure and 650 °C stress-rupture properties of 06Cr15Ni20Mo2TiNb austenitic stainless steel for fuel cladding

Austenitic stainless steels are promising cladding materials for sodium-cooled fast reactors, yet their long-term stress-rupture life at elevated temperatures remains a challenge. In this study, with total Ti+Nb fixed at 0.7 wt.%, we systematically investigated the influence of Ti/Nb mass ratios (5/9, 1, 9/5) on the microstructure and 650 °C stress-rupture properties under applied stresses of 245, 225, and 195 MPa. In all alloys, both primary and secondary MX phases are identified as (Ti,Nb)C. After long-term rupture, Mo-rich Laves phases precipitate along grain boundaries and around secondary (Ti,Nb)C, with no significant differences in amount or size among the alloys. As the Ti/Nb ratio increases, the fraction of coarse primary (Ti,Nb)C rises while that of fine secondary (Ti,Nb)C decreases. This is attributed to the reduced formation energy of (Ti,Nb)C at higher Ti/Nb atomic ratios, which enhances primary precipitation during solidification, competitively consuming Ti and Nb and thereby suppressing secondary precipitation in the solid state. The alloy with Ti/Nb=5/9 exhibits the fewest primary precipitates—minimizing crack initiation—and the most abundant secondary precipitates, which provide the strongest dislocation pinning and dispersion strengthening, leading to the longest rupture life across all stress levels. Fracture surfaces show dimples, with cracks initiating at coarse grain-boundary primary (Ti,Nb)C and propagating along the brittle Laves-phase network. This work elucidates the competitive precipitation mechanism governing stress-rupture properties and provides a theoretical basis for optimizing cladding alloy composition.