Chapter 7. Effects of Plastic Deformation and Heat. Plastic Deformation and Structure • Plastic Deformation and Mechanical Properties • Heat, Structure, and Mechanical Properties.
Effects of Plastic Deformationand Heat
Plastic Deformation and Structure • Plastic Deformation and Mechanical Properties • Heat, Structure, and Mechanical Properties
During slip, one part of a metal crystal undergoes a shear displacement relative to another that preserves the crystal structure of the metal.
When a metal undergoes plastic deformation, certain parallel crystallographic planes undergo slip. The bulk of the material between the planes retains its original crystallographic orientation.
Slip bands change direction at the grain boundary because each grain has a different crystallographic orientation.
To calculate the resolved shear stress acting on a set of crystallographic planes, the force applied is resolved into two components (parallel and perpendicular to the slip planes).
FCC metals have 12 slip systems, consisting of four (111) slip planes multiplied by three 〈110〉 slip directions.
Individual crystals tend to rotate under an applied force, so that the crystallographic planes move into the most favorable orientation for slip.
Slip originates in the grains with slip systems most favorably oriented (45°) to the direction of the applied force.
The crystal structure of a metal is preserved during mechanical twinning.
Slip appears as slip bands and mechanical twinning appears as bands in the microstructure.
Point defects include vacancies and interstitial atoms.
The movement of each atom during slip can be illustrated by describing the resisting force associated with sliding a heavy rug across a floor.
Edge dislocation is characterized by the Burgers vector, which is obtained by enclosing the dislocation in an atom-by-atom path and measuring the change in distance and direction from the starting position.
The mechanism of slip caused by movement of an edge dislocation greatly lowers the stress required for slip.
The path taken around the axis of the screw dislocation is a helix.
The boundary between the slip region and the region without slip is perpendicular to the axis of an edge dislocation and parallel to the axis of a screw dislocation.
Stacking faults are most common in close-packed planes of atoms.
With increased cold working or reduction of cross-sectional area, the grains elongate in the direction of cold working.
The increase in strain hardening is the result of the increase of cold working.
Annealing refers to a variety of specific heat treatment processes.
Stress-relieving temperature ranges vary for different types of alloys.
The percent residual stress removed is related to the stress-relieving temperature and the time at temperature.
Sharp changes in mechanical properties are exhibited during recrystallization.
Recrystallization requires an incubation period to initiate the process.
Recrystallization temperature and recrystallized grain size fall to minimum values with increasing amounts of cold working.
A stacking fault formed after annealing of certain cold-worked FCC metals leads to the formation of an annealing twin (mirror images).
Amounts of grain growth increase with increasing temperature and time at temperature. As both increase, grain growth will eventually stabilize.
Critical strain produces abnormal grain growth in a material.
Hot working is plastic deformation by controlled mechanical operations performed above the recrystallization temperature of a material.