How big is the difference between precipitation at twin boundary and normal grain boundary in an alumina-forming austenitic steel during creep at 700 °C?

Abstract

Twin boundaries are supposed to affect mechanical properties in many alloys systems. Our study demonstrates details of difference in precipitation at coherent twin boundary (CTB), incoherent twin boundary (ITB) and normal grain boundary (NGB) in an alumina-forming austenitic steel after creeping for 50 h at 700 °C/150 MPa. The CTBs were free of precipitates while continuous M23C6 and NiAl precipitates covered the ITB (~40 nm in size) and NGB (~150 nm in size). This is attributed to the different interfacial energy and diffusion rate of the boundaries while the grain boundaries act as preferential nucleation sites and fast diffusion channels. However, the particle coarsening was governed by the lattice diffusion when the grain boundaries were fully covered by the precipitates. Hence, the precipitation at CTB, ITB and NGB eventually became similar after long term creep testing (>500 h).