Crystal Structure in Metals

The Table shown here must have raised the question in your mind about the impact of crystal structures on the material property of metals.
In general, crystals fail parallel to the plains containing the smallest Burger’s Vector. These plains are called slip-planes and the directions are known as slip-directions. Each pair of slip-plane and slip-direction is referred as a slip-system.
It is noted that in general FCC crystals have 12 slip systems, whereas BCC crystals have 48 slip systems and HCPs have only 3 slip systems. However, the resolved shear stress at which the slip occurs in a metallic crystal could be obtained by using Schmidt’s Law and is defined by the parameter called Critical Resolved Shear Stress.
The Critical Resolved Shear stress (τc) for the FCC structure varies from 340-680 KPa, for BCC from 35 to 70 MPa, for HCP from 340-680 KPa. This clearly explains why FCC structures are usually more ductile than any other crystal structures.

Another interesting observation is related to the HCP metals. The ‘c/a’ ratio plays a very important role there. From the number of slip-planes and critical resolved shear stress it is expected that HCP metals would behave closely to the FCC metals. However, this is only true for Group B HCPs like Zinc. For group – A HCPs like Titanium or Beryllium, since the planes are too closely packed, they offer high resistance to slipping and shows properties close to the BCC metals. These materials could be made ductile by alloying them with other metals and thereby allowing cross-slips to occur in the system.