MSE 200 Unit 2

σw = σy/ N

plastic tensile strain at failure

maximum strength. point of fracture. metals- when necking starts, polymers- when backbone chains are about to break. max point on a graph

by disl motion, increased by obstacles due to disl motion

disl motion difficult, tends to fracture before disl motion

yes yield, no disl motion

energy to break a unit volume of material, approx by area under the stress-strain curve

ability to store energy, the integral of stress

place where noticeable plastic def occurs, σy

a 2-way arrow, not permanent

directly proportional to Eo

localized plastic def, resistance to permanently indenting the surface

resistance to plastic deformation or cracking in compression, better wear properties

disl motion easier due to non-directional bonds, close packing, and ion cores

Si, Diamond. disl motion harder due to directional bonding

NaCl. motion hard due to avoidance of ++/-- neighbors

make disl motion harder by adding slip barriers

crystal structure, class of material, temp of material, presence of barriers to slip (grain boundaries)

combination of slip plane and direction

{0001} only close packed plane, more brittle

for edge disl, disl motion is in the same direction as b

Tr, why crystals slip, θcosλcosθ

Fs/ As

Tr > Tcrss

all reduce ductility. grain refining, alloying or solid sol strengthening, cold working or strain hardening, precipitation hardening

smaller grains in a greater boundary area. hall-petch eqn

σ yield = σo + Ky d ^-1/2

lattice strains of impurities interact with disl movement, alloys are stronger than pure metals

metal becomes harder and stronger when it accumulates dislocations; dislocations entangle; density increases

volume obstacles, disl cut through or go around large obstacles (precipitates)

recovery, recrystallization, grain growth

revert partially back to pre-cold working values; disl density decreases, no microstructural changes. some relief of internal strain by disl motion

deformed grains are replaced by new grains, properties fully returned to pre-cold work.

new set of strain-free grains forms, and the material becomes softer and more ductile.

temp 100% strain free grains in 1 hr. temp up time to anneal down. increased % cold working decreases temp

small grains disappear, unit volume decreases.

d^n - do ^n = kt

greater than 0.5, density increases. less, decreases and void forms

ceramics then metals then polymers

.002

low atomic density plane

stronger than single crystals

decreased yield strength and TS, increased %EL

recrystallization temperature = point of highest rate of property change

occurs with plastic deformation, not crack sensitive, desirable, larhe %AR %EL

little to no plastic def, catastrophic, crack sensitive, small %AR %EL

breaking atomic bonds

along grain boundaries

due to microscopic flaws or cracks, measured value smaller than predicted

at the crack tip

higher in brittle materials; ductile has more uniform distribution of stress

deform at the tip and blunt at the tip, sharp tips indicate larger stress conc

σm > σys, elastic strain energy is released when crack propogates

resistance to brittle fracture when a crack is present. metals and alloys are the most forgiving

thickness > crack dimensions

directly proportional

inversely proportional

makes material more brittle and decreases toughness

directly to sigma y and Ts, inversely to %EL

sharpest slope indicates; crystal structure impacts

increase yield strength, decrease temp, increase strain rate

qualitative

failure under cyclic stress

number ot cycles to cause failure

no fatigue if S (stress amp) < S fatigue

increase stress levels decrease fatigue life. remove stress concentrations (sharp corners) to improve life

higher temperature test

slop decresaes with time, increase in strain hardening

constant (steady-state) slope, competition between recovery and strain hardening

slope increases, accelerates to rupture

Tm up, E up, large grain size prevents sliding

edge dislocation--parallel

screw dislocation--perpendicular

mixed dislocation--neither parallel nor perpendicular

not as much crystallographic misalignment in the grain boundary region for small-angle, and therefore not as much change in slip direction