Compared with FCC and BCC crystals, how do HCP metals typically differ in plastic deformability and why?

• A. More slip systems, leading to greater deformability
• B. Fewer slip systems, leading to reduced deformability
• C. Similar slip systems; deformability is independent
• D. Slippage is controlled by diffusion, not slip systems

Answer: (B)

Plastic deformation in metals mainly happens when dislocations glide along specific slip systems, which are particular planes and directions where atoms can slide past one another. For a crystal to deform easily in any direction, it needs several independent slip systems that can be activated under a range of stresses. Hexagonal close-packed metals have far fewer easily activated slip systems than face-centered cubic or body-centered cubic metals. The easiest slip mode in HCP crystals is glide on the basal plane, but slip on non-basal planes requires much higher stresses and is not readily activated at room temperature, so the number of usable slip systems is limited. With fewer available slip systems, the crystal cannot accommodate arbitrary plastic strains as readily, making HCP metals less deformable (less ductile) under typical conditions. Temperature or alloying can activate additional slip systems and improve deformability, but even then HCPs tend to be less ductile than FCC or BCC metals. The idea that diffusion governs slippage would apply to diffusion creep at very high temperatures, not to the ordinary dislocation glide that controls room-temperature plasticity in metals.