Lecture 18: Shape Memory Alloys (Contd.)

Summary

In this lecture, Dr. Jayanta Das from IIT Kharagpur explores the intricate process of martensitic transformation from face-centered cubic (FCC) to body-centered tetragonal (BCT) structures. He highlights the role of various defects such as dislocations and twins in accommodating strain during transformation, emphasizing their importance in alloy systems. The discussion covers both ferrous and non-ferrous alloys, illustrating how compositional variations affect the martensitic structure. Key concepts like the Bain correspondence, habit planes, and the significance of invariant shear are presented, setting the stage for future exploration of the shape memory effect.

Takeaways

• 😀 The speaker, Dr. Jayanta Das from IIT Kharagpur, introduces the topic of advanced materials and processes, focusing on the transformation from face-centered cubic (FCC) to body-centered tetragonal (BCT) structures.
• 🔄 The transformation discussed is the martensitic transformation, where the FCC austenite phase transforms into the BCT martensite phase without any diffusional movement of atoms.
• 🧊 Carbon atoms occupy octahedral sites in the FCC lattice, playing a crucial role in the martensitic transformation process.
• 🌀 Various invariant shear mechanisms, such as twinning, dislocations, and stacking faults, are essential in accommodating strain during martensitic transformations.
• 🔧 Dislocations allow for accommodation of shear strain within the martensite phase, differentiating them from those in the austenite phase.
• 🪞 Twinning involves the formation of mirror-like planes within the lattice, which can help accommodate the transformational strain during the phase change.
• 🔄 The transformation can involve multiple invariant deformation processes simultaneously, not just one.
• 🧪 Different ferrous alloys demonstrate varied habit planes, and factors like carbon content can influence the structural outcome of the martensitic transformation.
• 📊 The presentation highlights that in non-ferrous systems, austenite can exist in various structures (e.g., BCC, HCP), and martensite may take different forms depending on the alloy composition.
• 🔍 The next class will cover the shape memory effect, which relates to the behaviors and properties exhibited during martensitic transformations.

Q & A

What is the main topic discussed in the video?

The video discusses advanced materials and processes, specifically focusing on the Bain correspondence for the transformation of face-centered cubic (FCC) austenite to body-centered tetragonal (BCT) martensite.

What are the key phases mentioned in the transformation process?

The key phases mentioned are austenite, which is the face-centered cubic (FCC) structure, and martensite, which is the body-centered tetragonal (BCT) product phase.

How are iron and carbon atoms represented in the lattice diagrams?

Iron atoms are represented as circular shapes, while carbon atoms are represented as square shapes in the lattice diagrams.

What role do carbon atoms play in the FCC structure?

Carbon atoms occupy octahedral sites within the FCC structure, specifically at the center of the edges, which plays a crucial role in the martensitic transformation.

What is the significance of dislocations in martensitic transformation?

Dislocations are important because they accommodate the strain during the transformation from austenite to martensite, allowing for invariant deformation within the lattice.

What are the different types of invariant shear involved in the transformation?

The different types of invariant shear involved in the transformation include dislocation slip, twinning, and lattice rotation.

How does the habit plane relate to martensitic transformation?

The habit plane is an invariant plane that remains unrotated and undistorted during martensitic transformation, helping to accommodate transformational strain.

What happens to the crystal structure when carbon content is increased in ferrous alloys?

When carbon content increases in ferrous alloys, the crystal structure remains the same for both austenite and martensite, but the habit plane changes.

What type of structure does martensite assume in non-ferrous systems like cobalt or titanium?

In non-ferrous systems such as cobalt and titanium, martensite assumes a hexagonal close-packed structure, while austenite can be either body-centered cubic (BCC) or face-centered cubic (FCC).

What is the next topic to be covered in the following class?

The next topic to be covered in the following class is the shape memory effect, which is related to the concepts discussed in martensitic transformations.