GRAIN NUCLEATION AND GROWTH DURING PHASE TRANSFORMATIONS

GRAIN NUCLEATION AND GROWTH DURING PHASE TRANSFORMATIONS

Grain nucleation and growth in polycrystalline materials, such as metals and most ceramics, are important phenomena. We control the kinetics of many storage step transitions and processes of recrystallation. After processing, the ultimate average grain size is directly linked to the product strength. Typically, the average grain size is smaller and the material is heavier. The cinemas of these phase transformations are still poorly understood, despite the different models of transformation that were proposed in the recent 60 years. Many these models are based on the Classical Nucleation Theory (CNT), which describes the actions of individual grains in the bulk of the material, and the law of parabolic grain growth derived from Zener.

The experimental methods used to check these nucleation and growth models are either restricted to surface observations or to average grain growth behavior in the bulk. The 3DXRD microscope development of beamline ID11 has led to the study of individual cereals for the majority of a material. Such measurements provide unique information on the nucleation and growth of grains during the transformations process. The phase transitions in steel were studied more extensively than in other materials due to a combination of fundamental scientific interest and technical significance.

Carbon steel consists of iron and carbon (up to 2 wt.%) with small quantities of alloying elements, and exists in three stable crystalline phases: austenite (above 822°C), ferrite (below 822°C), and cementite Fe3C (below 685°C). In the experiments we continuously cooled the steel from 900°C to 600°C in 1 hour. In order to study the time evolution of individual grains during the phase transformations, a relatively small volume of steel is illuminated with a monochromatic beam of hard X-rays (80 keV). Several grains cause diffraction spots on a 2D detector. From the standard diffraction theory, the intensity of each spot can be shown to be proportional to the amount of grain from which it comes. The nucleation and growth of the individual kernels was studied in a typical time resolution of 10 seconds by repeated acquisition of images.

Fig. 79: Nucleation as a function of temperature: (a) the total number of valid ferrite reflections; (b) the normalised experimental nucleation rate (bars) compared to the classical nucleation theory (line). The different stages during the phase transformations in steel are schematically drawn at the top of the figure, which shows the three phases: Austenite (), ferrite (), and cementite ().

Our measurements show that the grain nucleation energy is of at least two magnitude orders lower than the CNT's expected activation energy (Figure 79). The newly formed grains have been shown to validate the parabolatic growth model, but they also display three essentially different growth forms. We distinguish four types of grain growth on the level of individual grains, as illustrated in Figure 80. 

Some grains do not interact with neighboring grains. Some grains continue to grow with the same crystallographic orientation into another stage. Some grains interact indirectly.

Fig. 80: Particle radius of individual ferrite grains as a function of temperature: (a) no interaction with neighbouring ferrite grains; (b) ferrite grains, which continue to grow with the same crystallographic orientation during the pearlite formation as part of a pearlite colony; (c) ferrite grains that indirectly interact; (d) ferrite grains that directly interact with neighbouring grains. The solid line indicates the classical Zener theory.

We have concluded that current models do not predict the phase kinetics in polycrystalline materials correctly. These new insights are technological important for the modern process of manufacturing materials, which relies heavy on grain nucleation and development modeling for the manufacture of custom made materials.