Plasticity, defects of crystal and their influence

state a few processes that use plastic deformation

extrusion, forging, rolling and sheet forming

how is energy stored in plastic deformation vs elastic

lost to heat, compared to stored reversibly in material

what are the two critical stresses?

yield stress (plastic deformation)
fracture stress (fails)

what type of test can be done to test plastic deformation?

tensile test

list brittle materials from tensile test

ceramics, glass, thermosets, thermoplastics (T< glass transition temperature), fiber-reinforced polymer, cast-irons and glass

describe ductile behavior

capacity of a material to undergo significant plastic deformation before rupture, allowing it to absorb energy and deform under tensile stress.

list ductile materials

most metals

explain the behavior of ductile materials (hint: 3 regions graph)

1) Hooke’s Law: σ=Eε

→ rod deforms elastically

2) from critical stress, slope drastically changes; plastic begins

→ cross-section area is reduced

3) force required increases to max (tensile strength) and then decrease until test rod breaks (necking before fracture)

what determines how thermoplastics behave in tensile test?

temperature

what inherent property determines thermoplastic behavior?

glass transition temp - plastic goes from hard brittle to soft rubbery

what happens when temperature for tensile test on thermoplastic is well below glass transition?

polymer behaves like ceramic material: breaks when critical stress reached

what happens when temperature is slightly below glass transition for thermoplastic tensile test?

acts similar to ductile metal

what happens when temperature is around transition temp for thermoplastic tensile test?

stress remains constant for long after local max, at end of test suddenly rises and rod breaks

thermoplastic vs thermoset

thermoplastics soften when heated and can be remolded and recycled

thermosets harden when heated and cannot be remolded or recycled after their initial curing

what happens when temperature is way above glass transition temperature for thermoplastic tensile test?

viscous polymer, extends under constant load until breaks

what is the yield strength of a material

maximum stress at which material still behaves elastically determined by parallel modulus line drawn from point of 0.2% strain on x-axis: point that intersects the curve is 0.2% proof stress

what is the tensile strength of a material

max stress before fracture: maximum load (N) divided by area of original section (mm2) - measure of strength

define upper yield point

stress at which for the first time the stress no longer increases and suddenly decreasesde

define lower yield point

lowest stress reached during yielding

define yield point elongtetion

elongation between beginning and end of yielding

do you use lower yield point for metals with yield strength?

YES

do you use 0.2% proof stress for metals with a yield strength?

NO

how to avoid Luder’s bands

cold rolled then annealed steel in the cold rolling mill is given small rolling reduction - brings metal beyond yield point elongation region

how to indicate at which stress plastic deformation occur

yield strength

define work hardening

material becomes stronger during plastic deformation due to increase in true tensile stress

when does necking begin

tensile strength, where necessary force reaches max

what does ductility of a material refer to

maximum strain

define toughness

ability to absorb energy without breaking; area under tensile curve up to breaking strain

define hardness

resistance to plastic deformation by compression

what is the common grain size of metal

small; polycrystalline

what causes plastic deformation in metals

crystallographic slip (stacked planes)

what are the two ways of closest possible stacking?

HCP and FCC

what is the packing density of ideal HCP and FCC

74%

define vacancies and their influence on metal properties

atoms missing; do NOT influence strength

define substitutional solid solution

foreign atoms in positions where atoms of host metal were

define interstitial solid solution

foreigh atoms between atoms of host metal

is the strength of alloys or pure metals greater

alloys due to interstitial and substitutional

what is the most important defect for plastic deformation

dislocation

how does a dislocation cause plastic deformation

displacement of the dislocation

how do dislocation affect metal properties

make it soft and ductile

what is another defect type

grain boundaries

why would an edge dislocation cause plastic deformation

shear stress causing it to move, which means a single row of bonds being broken and repaired

define critical shear stress

shear stress required to achieve slip according to slip system

in what direction and plane is shear most easily achieved

most densely packed direction and plane

should the critical shear stress be high or low enough to cause plastic deformation

LOW enough; smooth planes

which is the most easily plastically deformed

FCC

which cannot plastically deform wel

HCP due to insufficient slip systems

how can a pure metal with defect free structure be made stronger?

making movement of dislocations more difficultwha

what are four different ways to make dislocation movement difficult?

solution hardening, precipitation hardening, work hardening, and grain-boundary reinforcement

what does increasing stress required for dislocation movement do?

increase yield strength

define solution hardening

alloying - adding other atoms to a metal, causing greater critical shear stress by repressing slip planes over which dislocations move

where do dislocations move

over slip planes

define precipitation hardening

hard particles forming in the alloy through crystal structure so that dislocations must bend between particles

define dispersion hardening

mixing solid particles with high Tm into liquid metal then solidifying; ex/ matrix composites

define grain boundary strengthening

since different grains are inherent, dislocations have that obstacle since slip plane stops at grain boundary contributing to critical shear stress increase