Precipitation Sequence and Kinetics in an Fe-Si-Ti Alloy

The Fe-Si-Ti system is known to show nanoscale precipitation of the Fe2SiTi Heusler phase with potentially high volume fraction (~4%), very high density and a size ranging from 1 to 20nm after artificial aging. The strong hardening potential of these precipitates make these steels candidates for automotive applications; however no understanding of the precipitation sequence (competition with other phases) nor the precipitation kinetics are available. The present study presents a quantitative study of the precipitation kinetics (size, volume fraction and number density) in a wide temperature range (450-800°C), realised by coupling systematically Small Angle Neutron Scattering (SANS), Transmission Electron microscopy (TEM) and Tomographic Atom Probe (TAP). Tensile tests were also carried out so as to determine the microstructure/properties relationships. Along the complete temperature range, it is shown that a compromise between time for precipitation and small precipitate sizes can be reached around 550°C. At this intermediate temperature, precipitation is shown to occur in two steps, linked with a second nucleation process after nucleation & growth of the first family of Fe2SiTi has been completed. This second precipitation step results in a temporary decrease in precipitate size and an increase in hardness. The nature of these precipitates is discussed in view of the TEM and TAP observations.