An alternative method for the measurement of precipitate volume fractions in microalloyed steels by the means of atom probe tomography

Knowledge of the volume fraction of precipitates is crucial for estimating the impact of precipitation on microstructure evolution or mechanical properties. However, its experimental determination is often a difficult task. In this work, atom probe tomography was applied to an industrial TiNb microalloyed steel, to follow the evolution of austenite solute composition in titanium and niobium as a function of temperature in the austenitic domain. These composition measurements were used to calculate the volume fraction of (Tix, Nb1-x)C carbides in austenite, using mass balance. This type of measurement is made possible by the considerable evolutions experienced by tomographic atom probes over the past 20 years in terms of volume analyzed and mass resolution. Atom probe tomography is nowadays not only able to help determining volume fractions below 0.1%, but also provides unique information related to solubility limits as low as a few tens of ppm, most useful for developing phase diagrams, or assessing existing ones. •Knowing the precipitate and matrix compositions, the volume fraction of precipitates is estimated by a simple mass balance.•APT is sufficiently precise to measure very low solute contents of the order of a few tens of ppm in microalloyed steels.•The technique can also be relevant for the determination of solubility products.