Question

Which of the following statements regarding mechanical twinning is FALSE?

• A. Twinning occurs more frequently at high strain rates and low temperatures.
• B. The atomic displacement in twinning is a fraction of the interatomic distance.
• C. Twinning changes the orientation of the crystal lattice in the deformed region.
• D. Twinning produces the same total strain as slip for the same amount of atomic movement.

Hint:

Twinning is common in hexagonal close-packed (HCP) metals (like Zn, Mg) because they have fewer independent slip systems than FCC or BCC metals.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to identify the incorrect (FALSE) statement about mechanical twinning, which is a key mechanism of plastic deformation in metals alongside dislocation slip.

Step 2: Key Formula or Approach:
Compare the mechanisms of slip and twinning:
- Slip: Involves sliding of blocks of crystal over one another along a slip plane. The displacement is an integer multiple of the lattice spacing (Burgers vector).
- Twinning: Involves cooperative, homogeneous shear of lattice planes where every atom moves a fraction of the interatomic distance relative to its neighbor.

Step 3: Detailed Explanation:

• Statement A is True: Since twinning requires less localized thermal activation than slip, it is favored at low temperatures and high strain rates where slip is restricted.

• Statement B is True: In twinning, the displacement of atoms within the twinned region is a fraction of the interatomic spacing.

• Statement C is True: Twinning creates a region with a mirror-image orientation relative to the parent lattice, thus changing the crystal orientation.

• Statement D is False: Because atomic displacement in twinning is limited to a small fraction of the lattice parameter, the total plastic strain produced by twinning is much smaller than that typically achieved by dislocation slip for a given amount of global atomic movement. Hence, they do not produce the same strain.

Step 4: Final Answer:
Statement (D) is the false statement.