Diffusion controlled and diffusionless growth of new phases

A fascinating topic in materials science!

Diffusion-controlled and diffusionless growth of new phases refer to two different mechanisms by which new phases can form in a material. The key difference between the two is the role of diffusion in the growth process.

Diffusion-Controlled Growth

In diffusion-controlled growth, the formation of a new phase is driven by the diffusion of atoms or ions through the material. This process occurs when the concentration of the new phase is lower than the surrounding material, and the atoms or ions need to diffuse to the growing interface to form the new phase.

Here's a step-by-step explanation of the process:

1. Nucleation: A nucleus of the new phase forms, which can be a result of thermal fluctuations, impurities, or defects in the material.
2. Diffusion: Atoms or ions from the surrounding material diffuse towards the growing interface, driven by concentration gradients.
3. Growth: The diffusing atoms or ions accumulate at the interface and form the new phase, increasing its size and volume.
4. Coarsening: As the new phase grows, it can undergo coarsening, where smaller particles merge to form larger ones, reducing the total surface area.

Examples of diffusion-controlled growth include:

• Precipitation hardening in alloys, where precipitates form through the diffusion of solute atoms.
• Grain growth in polycrystalline materials, where grain boundaries diffuse and merge to form larger grains.

Diffusionless Growth

In diffusionless growth, the formation of a new phase occurs without the need for atomic or ionic diffusion. This process is also known as "martensitic" or "displacive" transformation.

Here's a step-by-step explanation of the process:

1. Nucleation: A nucleus of the new phase forms, which can be a result of thermal fluctuations, impurities, or defects in the material.
2. Strain accommodation: The new phase accommodates the strain generated by the transformation by creating defects, such as dislocations or vacancies.
3. Growth: The new phase grows by a process of "displacement" of the surrounding material, without the need for atomic or ionic diffusion.
4. Coarsening: As the new phase grows, it can undergo coarsening, where smaller particles merge to form larger ones, reducing the total surface area.

Examples of diffusionless growth include:

• Martensitic transformations in steels, where a high-temperature austenitic phase transforms into a low-temperature martensitic phase without diffusion.
• Shape memory alloys, where a parent phase transforms into a martensitic phase upon cooling, without the need for diffusion.

Key differences between diffusion-controlled and diffusionless growth:

• Diffusion-controlled growth requires the diffusion of atoms or ions, while diffusionless growth does not.
• Diffusion-controlled growth is typically slower and more gradual, while diffusionless growth can occur rapidly and catastrophically.
• Diffusion-controlled growth often leads to the formation of precipitates or particles, while diffusionless growth can result in the formation of a single, continuous phase.

Understanding the mechanisms of diffusion-controlled and diffusionless growth is crucial in designing and optimizing materials with specific properties, such as strength, toughness, and shape memory behavior.