Question

does the transformation of austenite into ferrite begin at the austenite grain boundaries and propagate inward, or does it begin inside the grains and propagate outward toward the boundaries?

Answer 1

The transformation of austenite into ferrite starts at the grain boundaries and moves inward as these are the sites with higher energy that are more favorable for nucleation. The ferrite particles then grow into the grains. This process is influenced by cooling rates and alloying elements.

Step-by-step explanation:

The transformation of austenite into ferrite typically begins at the austenite grain boundaries and propagates inward. This transformation is a critical aspect of the heat treatment of steel and involves changes in the microstructure of the material. During cooling of austenite, ferrite starts to form at the grain boundaries because they serve as sites with higher energy and are more favorable for nucleation. This process is called nucleation and growth. The nucleation of new phases like ferrite generally occurs at grain boundaries, impurities, or other defects within the crystal lattice, since these locations reduce the energy barrier for phase transformation.

Once nucleation has occurred, these small particles of ferrite grow into the austenite grains inwards. This growth is driven by the reduction in free energy as the system tries to move towards a more stable state. The transformation rate can be influenced by the cooling rate and the presence of alloying elements in the steel.