In-Situ Neutron Diffraction Study of Non-Equilibrium Phase Transformation in Advanced High-Strength Steels

Advanced high-strength steels (AHSS) are an integral part of the materials solution for automotive industry. Phase transformations under far-from-equilibrium conditions play an important role in the making of AHSS. They are also commonly associated with welding and other thermo-mechanical manufacturing processing of AHSS, which can lead to unexpected property degradation of the final fabricated structures/components. However, our knowledge of non-equilibrium phase transformation is very limited in a complex alloy system during fast heating and cooling conditions. This is largely due to the lack of direct experimental measurement techniques to identify and quantify the transformation products. In this work, a novel asynchronous stroboscopic neutron diffraction experimental approach was used at VULCAN beam line of Oak Ridge National Laboratory. It is capable of probing in-situ the non-equilibrium allotropic phase transformations of AHSS during fast heating and cooling conditions. In our study, the effect of heating rate (ranging from 1 to 30C/s) on the phase transformation kinetics from ferrite (α) to austenite (γ) in different AHSS was investigated. Figure 1 shows the heating-cooling process (peak temperature was 1050C) in DP980 with a heating rate of 3C/s. With the one-of-a-kind neutron diffraction technique, a time-resolution of 0.1 second or less was achieved, which corresponds to a time increment of 3C or less.  A trend was clearly observed indicating that higher heating rate causes a wider temperature range for the α to γ transformation. An unusual lattice spacing change associated with the austenite formation on heating was obtained as well. The kinetics governing such observed non-equilibrium phase transformation phenomena is discussed. The information obtained from the in-situ measurement is essential to both the current generation AHSS and the development of the third generation AHSS for lightweight automotive applications.

Figure 1 Neutron diffraction pattern of non-equilibrium phase transformation in DP980.