What are common slip systems in BCC metals, and how do they affect ductility at room temperature?

• A. {111} planes with <110> directions; high ductility at room temperature
• B. {110} planes with <111> directions; fewer active systems at room temperature reduce ductility
• C. {100} planes with <010> directions; very high ductility
• D. {112} planes with <111> directions; ductility is unaffected by temperature

Answer: (B)

In body-centered cubic metals, plastic deformation mainly happens by dislocation glide on the {110} planes in the <111> directions. These are the primary slip systems for BCC lattices. However, at room temperature the crystal lattice presents a higher barrier to dislocation motion, especially for screw dislocations with non-planar cores, so only a limited number of slip systems are readily activated. That means fewer ways for the crystal to accommodate plastic strain, which reduces ductility at room temperature. As temperature rises, thermal energy helps dislocations overcome barriers and additional slip systems become active (such as {112}<111>), increasing the available deformation paths and raising ductility.