Materials ch 8
plastically deforming polycrystalline metals at low temperatures produces microstructural property changes including what 3 things
1 change in grain shape 2 strain hardening 3 increase in dislocation density
dislocation densities typically found in carefully solidified metal crystals
10^3 mm^-2
dislocation densities typically found in heavily deformed metals
10^9 to 10^10 mm^-2
for a face-centered cubic, there are ____ slip systems
12: four unique planes in each of which there are three independent directions
twinning occurs on ____ and in ____ direction
a definite crystallographic plane, a specific direction
ordinarily, dislocations move along _______
a preferred plane in specific directions
the minimum stress necessary to introduce yielding occurs when ____
a single crystal is oriented such that ф = γ = 45º (σy = 2τcrss)
during the 30s, it was theorized that the discrepancy measured in mechanical strengths of perfect crystals could be explained by _______
a type of linear crystalline defect now called dislocation
during plastic deformation in a polycrystalline metal, slip or dislocation must take place _____
across the common grain boundary
the effects of strain hardening may be removed by ____
an annealing heat treatment
magnitude of resolved shear stress depends on ____ as well as _____
applied stress, the orientation of both the slip plane and the direction within that plane
two dislocations of opposite sign and having the same slip plane will _____
be attracted to one another, and dislocation annihilation will occur when they meet
HCP structure metals are normally ____ because of their low number of active slip systems
brittle
another term for strain hardening since the temperature at which deformation takes place is "cold" relative to the absolute melting temperature of the metal
cold working
many dislocations in crystalline materials are _____
comprised of both edge and screw components; mixed dislocations
the critical value of resolved shear stress that causes slip to commence in a single crystal on the most favorably oriented slip system
critical resolved shear stress, τcrss
the minimum shear stress required to initiate slip and is a property of the material that determines when yielding occurs
critical resolved shear stress, τcrss
heat treating a deformed metal specimen can ___ the dislocation density
diminish
it was not until the 50s that the existence of dislocation defects was established by _____
direct observation with the electron microscope
slip planes are ___ and ____ across a grain boundary
discontinuous, change directions
_____ motion is analogous to the mode of locomotion employed by a caterpillar
dislocation
when two dislocations of opposite sign and have the same slip plane come together, and the two extra half-planes of atoms align to become a complete plane
dislocation annihilation
number of dislocations
dislocation density
the total dislocation length per unit volume; the number of dislocations that intersect a unit area of a random section
dislocation density
restricting or hindering _____ renders a material harder and stronger
dislocation motion
metals with large numbers of slip systems are quite _____ because extensive plastic deformation is normally possible along the various systems
ductile
_____ dislocations move in response to shear stress applied in a direction perpendicular to its line
edge
the two fundamental dislocation types
edge and screw dislocations
in this dislocation, localized lattice distortion exists along the end of an extra half-plane of atoms, which also defines the dislocation line
edge dislocation
dislocation interactions are possible between ______
edge, screw, and/or mixed dislocations, and for a variety of orientations
for metals having BCC and HCP crystal structures, some slip systems are often operable only at _____
elevated temperatures
before deformation, grains are ____ or have approximately the same dimension in all directions
equiaxed
during plastic deformation, an important source for new dislocations is ____, which multiply
existing dislocations
other sources for new dislocations during plastic deformation
existing dislocations grain boundaries internal defects surface irregularities (scratches, nicks)
larger strain-hardening exponent, the ___ the strain hardening for a given amount of plastic strain
greater
_____ of a polycrystalline specimen corresponds to the comparable distortion of the individual grains by means of slip
gross plastic deformation
____ metals are almost always softer and weaker than alloys composed of _____
high-purity, the same base metal
during plastic deformation, the number of dislocations ____dramatically
increases
the dislocation density in a metal ___ with cold work
increases
a fine-grained material (one with small grains) is or is not harder and stronger than one that is course grained
is - small grains have a greater total grain boundary area to impede dislocation motion
when metals are plastically deformed, some fraction of the deformation energy ____ while the remainder ________
is retained internally (5%) dissipated as heat
metals with FCC or BCC crystal structures have a relatively large or small number of slip systems
large (at least 12)
a given slip plane may or may not contain more than a single slip direction
may
during deformation, _____ and ____ are maintained along grain boundaries - that is, the grain boundaries usually do not come apart or open up
mechanical integrity, coherency
several characteristics of dislocations are important with regard to ______ of metals
mechanical properties
by reducing the mobility of dislocations, the _____ may be enhanced
mechanical strength
dislocation densities in order of magnitude for metals, ceramics, single crystals
metals>ceramics>single crystals
units of dislocation density
millimeters of dislocation per cubic millimeter or just per square millimeter
the strain-hardening exponent
n
the direction of motion of mixed dislocation line
neither perpendicular nor parallel to the applied stress (somewhere in between)
at the end of an edge dislocation, the extra half-plane may emerge from the right surface of the crystal, forming an edge that is _____
one atomic distance wide
before and after the movement of a dislocation through some particular region of the crystal, the atomic arrangement is ______
ordered and perfect; it is only during the passage of the extra half-plane that the lattice structure is disrupted
the direction of movement in an edge dislocation in response to the applied shear stress
parallel to the shear stress
the direction of movement in a screw dislocation in response to the applied shear stress
perpendicular to the stress direction
_____ corresponds to the motion of large numbers of dislocations
plastic deformation
for screw dislocations, lattice strains are _____ only
pure shear
the strains from a screw dislocation extend into the surrounding atoms, and their magnitudes decrease with ____ from the dislocation
radial distance
some of the stored internal strain energy is relieved by virtue of dislocation motion (in the absence of an externally applied stress), as a result of enhanced atomic diffusion at the elevated temperature
recovery
two dislocations of the same side and having the same slip plane will have a ____ force that ____
repulsive, tends to move them apart
an applied tensile or compressive stress resolved into a shear component along a specific plane and direction within that plane
resolved shear stress
τR
resolved shear stress
most metals strain harden at ____
room temperature
this dislocation may be thought of as resulting from shear distortion; its dislocation line passes through the center of a spiral, atomic plane ramp
screw dislocation
when τR(max)=τcrss
single crystal plastic deformation or yielding
____ occurs in metals that have BCC and HCP crystal structures, at low temperatures, and at high rates of loading (shock loading), conditions under which the slip process is restricted
slip
for ____, the crystallographic orientation above and below the slip plane is the same both before and after the deformation
slip
the process by which plastic deformation is produced by dislocation motion
slip
___ occurs in distinct atomic spacing multiples, whereas the atomic displacement for ____ is less than the interatomic separation
slip twinning
the specific direction along which dislocation motion occurs
slip direction
with continued extension of a single crystal, both the number of ___ and the ____ increases
slip lines, slip step width
the preferred plane along which dislocations occur
slip plane
the amount of bulk plastic deformation from twinning is normally ___ relative to that resulting from slip
small
a technique used to strengthen and harden metals by alloying them with impurity atoms that go into either substitutional or interstitial solid solution
solid-solution strengthening
which of the following is NOT affected by heat treatment grain size reduction solid-solution strengthening strain hardening
solid-solution strengthening
when were dislocations introduced in metals and alloys
solidification, plastic deformation, and as a consequence of thermal stresses that result from rapid cooling
____ exist around dislocations and are influential in determining the mobility of the dislocation as well as their ability to multiply
strain fields
the phenomenon by which a ductile metal becomes harder and stronger as it is plastically deformed
strain hardening
increasing percent cold work results in an increase in ___ and a decrease in ____
strength, ductility
grain size reduction improves ___ and ____ of many alloys
strength, toughness
three mechanisms that may be used to strengthen and harden single-phase metal alloys
strengthening by grain size reduction solid-solution strengthening strain hardening
strain fields and associated forces are important in the _____ for metals
strengthening mechanisms
the ability of a metal to deform plastically depends on _____
the ability of dislocations to move
the real importance of twinning lies with _____; twinning may place new slip systems in orientations that are favorable relative to the stress axis such that the slip process can now take place
the accompanying crystallographic reorientations
the slip system depends on ____
the crystal structure of the metal and is such that the atomic distortion that accompanies the motion of a dislocation is a minimum
the slip direction corresponds to the direction in the slip plane having ______
the highest linear density/the direction most closely packed with atoms
the slip system that is oriented most favorably will have ____ resolved shear stress
the largest, τR(max)
for a particular crystal structure, the slip plane is that plane having _____
the most dense atomic packing/the greatest planar density
macroscopic plastic deformation simply corresponds to permanent deformation that results from _____
the movement of dislocations, or slip, in response to an applied shear stress
the resistance to slip is greater when impurity atoms are present because ____
the overall lattice strain must increase if a dislocation is torn away from them
the net plastic deformation for the motion of edge dislocations compared to screw dislocations
the same
the crystallographic plane along which the dislocation line traverses
the slip plane
the combination of the slip plane and the slip direction
the slip system
a measure of the ability of a metal to strain harden
the strain-hardening exponent
all metals and alloys contain some dislocations (true or false)
true
___ effectively block slip and increase the strength of a material
twin boundaries
for ____, there is a re-orientation across the twin plane
twinning
in addition to slip, plastic deformation in some metallic materials can occur by the formation of mechanical twins or _____
twinning
for polycrystalline metals, the direction of slip _____
varies from one grain to another
slip system for FCC
{111}<110>
for high-angle grain boundaries, dislocations tend to _____
"pile up" (or back up) at grain boundaries