QUASI-CRYSTALLINE AND CRYSTALLINE PRECIPITATION DURING ISOTHERMAL TEMPERING IN A 12CR-9NI-4MO MARAGING STAINLESS-STEEL

A thorough microstructural investigation has been performed on a high strength maraging steel of the type 12%Cr-9%Ni-4%Mo-2%Cu-1%Ti. The major precipitate formed during isothermal aging at 475 degrees C is a quasicrystalline phase possessing icosahedral symmetry termed R'-phase with a typical chemical composition of 48%Mo-33%Fe-13%Cr-2%Ni-4%Si. At 550 degrees C the major precipitate is trigonal R-phase with a typical composition of 45%Mo-31%Fe-18%Cr-4%Ni-2%Si. At 550 degrees C also Laves phase with a composition of 48%Mo-35%Fe-13%Cr-2%Ni-2%Si could be observed. At both 475 and 550 degrees C an ordered phase termed L-phase precipitated. This minority phase has an ordered face centred cubic (f.c.c.) structure of type Ll(0),. Its composition is typically 9%Fe-4%Cr-52%Ni-15%Mo-16%Ti-4%Al. R'-phase formed at 475 degrees C transformed to R-phase and Laves phase during aging at 550 degrees C. In an analogous manner, R-phase and Laves phase formed at 550 degrees C transformed to R'-phase during subsequent aging at 475 degrees C. This transformation was rationalized by a strong similarity in crystal structure between quasicrystalline R'-phase of icosahedral symmetry and Frank-Kasper phases such as R-phase and Laves phase.