Icosahedral related precipitation at a nanometric scale in a superferritic steel

Frank-Kasper related phases are found in grain boundary precipitates in a 29Cr4MoTi superferritic stainless steel annealed for one hour at 700°C. Electron diffraction patterns show an approximate five-fold symmetry and high resolution electron microscopy reveals that samples consist of a mixture of nanodomains of various pentagonal Frank-Kasper phases. The five-fold appearance of the diffraction patterns is induced by the similar orientation of icosahedral columns forming these phases. An alloy with composition similar to that of the precipitates hs been prepared by rapid solidification. Intergrowth of micro- and nonodomains of Frank-Kasper related phases is also observed and image processing allows analysis of the image periodicities at the nanometric scale.