Grain Nucleation and Growth During Phase Transformations

Abstract

The mechanical properties of polycrystalline materials are largely determined by the kinetics of the phase transformations during the production process. Progress in x-ray diffraction instrumentation at synchrotron sources has created an opportunity to study the transformation kinetics at the level of individual grains. Our measurements show that the activation energy for grain nucleation is at least two orders of magnitude smaller than that predicted by thermodynamic models. The observed growth curves of the newly formed grains confirm the parabolic growth model but also show three fundamentally different types of growth. Insight into the grain nucleation and growth mechanisms during phase transformations contributes to the development of materials with optimal mechanical properties.

Deformation-induced orientation spread in individual bulk grains of an interstitial-free steel

“…Several experimental set-ups are possible, each with specific advantages, e.g. high temporal resolution [12,15], high spatial resolution of grain maps [11,16] or high resolution of crystallographic orientations [17,18].…”

In-Situ Synchrotron X-ray Diffraction Studies on Effects of Plastic and Elastic Loading on bcc Phase Transformations of a 3rd Generation 1 GPa Advanced High Strength Steel

“…In-situ synchrotron diffraction is a suitable technique to study these solid-state phase transformations. [11][12][13][14][15][16][17] Dutta et al observed that loading conditions below the yield strength of austenite lead to elastic strains upon martensitic transformation and M s increases with an increase in the applied external tensile load. [18] From a thermodynamic point of view, a reduction in Gibbs free energy of a system indicates whether it is favorable for phase transformations to take place.…”

Analysis of the Grain Size Evolution for Ferrite Formation in Fe-C-Mn Steels Using a 3D Model Under a Mixed-Mode Interface Condition

“…A value of W % 5 9 10 À8 J 3 m À6 [14] is used in the present work. DG V is the difference in Gibbs free energy per unit volume between ferrite and austenite, which is calculated with Thermo-Calc under para-equilibrium conditions.…”

“…Traditional in situ experimental techniques, like dilatometry, only determine the total fraction transformed, but yield no information on the evolution of the ferrite grain size. Such complementary information can however be obtained by micro-beam X-ray diffraction [14,15] and three-dimensional neutron depolarization. [16,17] These in situ techniques provide a more detailed view on the microstructure development, but cannot monitor the evolution in chemical composition profile.…”