6: Deformation Characteristics by Lattice Type

High Symmetry: FCC has the highest symmetry of the major lattice types.
Slip Systems: Contains 12 different closed pack slip systems, allowing multiple slip directions and planes.
- Result: Easier dislocation movement due to low critical resolved shear stress.
Dislocation Types: Both edge and screw partial dislocations are in plane, eliminating the need for recombination during slip.
- Consequence: Facilitates easy dislocation movement.
Strain Hardening: FCC metals start with low yield stresses (when annealed) but experience significant increases in flow stress due to active dislocation interactions.
- Effect of Temperature and Strain Rate: Temperature and strain rate notably affect the strain hardening rate more than yield stress itself.

Aluminum: Highly annealed aluminum shows difficulty in identifying a clear yield stress; it indicates a gradual transition.
- Temperature Impact: Strain hardening rate decreases with rising temperature, highlighting the pronounced influence of thermal activation.
Copper: Oxygen-free high-conductivity copper displays similar behavior with minimal yield stress variation across temperatures, while strain hardening is significantly affected.
Key FCC Metals: Include aluminum, copper, nickel, silver, gold, platinum, and austenitic stainless steels. Notably, the malleability in metals like silver and gold is due to rapid dislocation movement in the FCC structure.

Lower Symmetry: BCC has lower symmetry and lacks closed packed planes, presenting approximately 48 slip systems.
Dislocation Movement: More challenging to move dislocations in BCC due to screw dislocation partials being out of plane.
- Requirement: Partial dislocations must recombine for slip, enhancing lattice friction.
Yield Stress Behavior: BCC metals typically exhibit high yield stresses with clear yield points that may also show yield point phenomena. Strain hardening is less pronounced due to difficulty in dislocation movement.
- Thermal Activation Effects: Strong impact on yield stress but lesser on the strain hardening rate.

Carbon Steel (1025): Yield stress increases with strain rate, while strain hardening rates remain relatively constant across rates at high temperatures.
Vanadium & Tungsten: Show large yield stress variation with minimal changes in strain hardening under high strain rate conditions across temperatures.
Key BCC Metals: Include iron, chromium, molybdenum, tungsten, and tantalum.
- Noteworthy: Ferrite as the BCC phase within steel and beta brass as a BCC phase resulting from copper and zinc alloying.

Closed Pack Planes: HCP has close packed planes parallel to each other with relatively low symmetry.
- Result: Limited intersecting slip systems, leading to potential strain accommodation challenges.
Dislocation Slip Behavior: Slip within basal planes resembles FCC, while out-of-plane screw dislocations mirror BCC challenges.
Twinning: Important mechanism for accommodating strain in HCP metals by orienting the lattice to increase available slip systems.
- Thermal Activation: Strongly influences both yield stress and strain hardening rate in HCP metals.

Magnesium Alloy: Shows dramatic yield stress changes with increasing temperature, along with decreasing strain hardening rates as temperatures rise.
Titanium: Commercially pure titanium exhibits significant changes in both yield stress and strain hardening rate with temperature elevation.
Key HCP Metals: Include titanium, magnesium, zinc, cadmium, and high-temperature materials like rhenium, hafnium, zirconium, and osmium.

Understanding the characteristics of FCC, BCC, and HCP lattice structures is crucial for their engineering applications, particularly regarding plastic deformation, strain hardening, and thermal response.