Synergistic effects of microalloying and pre-straining on enhanced nanoprecipitation and creep property of alumina-forming austenitic stainless steels

Non-shearable metal carbide (MC) nanoparticles are one of the most effective strengthening media for improving creep resistance of alloys at elevated temperatures. However, it is difficult to achieve sufficient homogeneous dispersion due to their large crystallographic difference with the matrix. Herein, we report a feasible method to form massive NbC nanoprecipitation in an aluminum-forming stainless steel via the combination of microalloying and pre-straining. The employment of pre-straining generated dislocations uniformly distributed inside the alloy matrix, which provides high-density heterogeneous nucleation sites and enables fast pipe diffusion channels for involved solutes, thus substantially enhancing the nucleation rate of the secondary NbC. Moreover, due to its stronger affinity with C and larger diffusivity as compared with Nb, Ti atoms were found to occupy the Nb sublattice of the NbC at the onset of precipitation. The formation of (Ti,Nb)C nuclei with decreased lattice misfit (from 20.8% to 19.1%) effectively reduced the critical nucleation energy barrier and thus enhanced the nucleation rate accordingly. With the synergistic effect of 0.1 wt% Ti microalloying and 10% pre-straining, massive precipitation of refined secondary (Ti,Nb)C nanosized particles (4.4 +/- 0.7 nm) with a much increased volume fraction (almost 6.8%) was practically realized, remarkably reducing the steady-state creep rate and prolonging the creep-to-rupture lifetime to 2044 h at 1023 K and 120 MPa. The current findings provide a feasible strategy to disperse strong yet massive nanoparticles within metallic metrics, which may shed new insights into the development of advanced high-temperature alloys with much enhanced creep properties at no expense of oxidation resistance.

Automated summary

This study reports a method to achieve massive NbC nanoprecipitation in an aluminum-forming stainless steel through microalloying and pre-straining. The generation of dislocations through pre-straining provides high-density nucleation sites for secondary NbC, while the addition of titanium atoms reduces the nucleation energy barrier. Experimental results show that with 0.1 wt% titanium microalloying and 10% pre-straining, massive precipitation of refined secondary (Ti,Nb)C nanosized particles can be achieved, significantly enhancing the creep resistance of the alloy.