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