Chapter 7 - Dislocations and Strengthening Mechanisms Flashcards
fundamental dislocation types (2)
- edge: extra half-plane of atoms is inserted, motion is
2. screw: extends alonga line that passes through the center of a spiral
slip
the process by which plastic deformation is produced by dislocation motion, along the plane with the highest density
dislocation density
the number of dislocations, total dislocations per unit volume
lattice strains
compressive, tesile, shear, imposed by dislocations, in screw dislocations there are only shear strains
slip plan
the plane that the dislocations prefer to move on
slip direction
the direction of dislocation movement
slip system
the slip plane and slip direction, depends on the crystal structure of the metal, wants the minimum atomic distortion
b(FCC)
a/2<110>
b(BCC)
a/2<111>
b(HCP)
a/3<11-20>
resolved shear stresses
the shear components that exist at all but parallel or perpendicular alignments
critical resolved shear stress
the minimum shear stress required to initiate slip, property of the material that determines when yielding occurs
gross plastic deformation
corresponds to the comparable distortion of the individual grains by means of slip
grain boundary reaction
mechanical integrity and coherency are maintained along the grain boundaries, do not usually come apart or open up
grain size reduction
the smaller the grains, the harder/stronger the material
solid-solution strengthening
technique to strengthen and harden materials, alloying metals with impurity atoms, resistance to slip is greater where impurity atoms are present
smaller substitutional impurity atom
exerts tensile strain, located where the tensile strain will partially nullify some of the dislocation’s compressive strain
larger substitutional impurity atom
imposes compressive strains
strain hardening
where ductile metals become harder and stronger as they are plastically deformed, aka work hardening or cold working
percent cold work trends
increased yield and tensile strengths, decreased ductility
strain hardening exponent
n, measure of the ability of a metal to strain harden, the larger the magnitude the greater the strain hardening for a given amount of plastic strain
mechanisms to strengthen and harden single-phase metal alloys (3)
- strengthening by grain size reduction
- solid-solution strengthening
- strain hardening
annealing treatment
heat treatment that reverts properties to precold-worked states, recovery or recrystallization (followed by grain growth)
recovery
some of the stored internal strain energy is relieved by virtue of the dislocation motion, electrical and thermal conductivities are recovered to their precold-worked states
recrystallization
the formation of a new set of strain-free and equilaxed grains that have low dislocation densities, material becomes more ductile but softer and weaker
recrystallization temperature
the temperature at which recrystallization just reaches completion in one hour, typically it is between one-third and one-half of the absolute melting temperature
