TEM and 3DAP of investigation of microstructural evolution during tempering of an ultra high strength high toughness steel

Ultra high strength high toughness (UHSHT) steels form a technologically important class of materials with niche applications such as aircraft landing gear components. In this class of highly alloyed Co-Ni containing secondary hardening steels, the demanding combination of strength (YS: 1900MPa, UTS: >2000MPa) and toughness (K_IC: 100Mpa.m^1/2) properties is achieved primarily through a control of the morphology, volume fraction, interparticle spacing and composition of the strengthening nano-sized carbide precipitates. The 3D atom probe (3DAP) is a unique tool capable of yielding chemical information at the atomic level, thus enabling a study of the composition of precipitates that are smaller than 10nm. In conjunction with the well-known techniques of TEM, the 3DAP offers a powerful way of studying the microstructure at the submicroscopic scale