Materials Section 2

what does strengthening in metals do?

reduces disclocation motion

dislocation motion is easier in , difficult in __.

metals, ceramics

elastic deformation

moves just slightly out of energetically favored equilibrium position

what do dislocations do?

allow deformation at a lower stress than in a perfect crystal

what ist he slip plane?

crystallographic plane of dislocation motion

direction

small stress has to be applied to lead to plastic deformation
the movement of edge and screw dislocations ca lead to the formation of steps at the end of a crystal

location of edge dislocs

parallel to the applied stress

location of slip dislocs

perpendicular to the applied stress

what is a strain field?

the strain on the surrounding lattice done by dislocaitons

slip systems

certain planes that are preferred paths of dislocation movement.

stress causes planar slip.

shear

atomic distorition is minimized by?

movement on planes with high atomic density

and xtals have more slip systems than HCP.

BCC, FCC

an applied stress is only pure if?

a slip plane is oriented perpendicular or parallel to the applied stress

an xtal will start to yield when?

resolved shear stress becomes sufficiently large

minimum shear stress needed to initiate slip

critical resolved shear stress

slip will occur first in the slip system oriented close to ____ degrees

45

conditions for dislocation motion:

Tr > Tcrss

for deformation to occur, the applied stress must be _ the yield stess.

greater than or equal to

when there is random xtallographic orientation of the grains, the ___ varies between grains.

direction of slip

the crystal with the largest _ will yield first.

R???

mechanical strength

the ability to withstand plastic deformation

what is the primary principle of strengthening?

the restriction and hindrance of dislocation motion
increased strength = loss in ductility

four main methods of strengthening

grain size reduction, solid soln strengthening, precip hardening, strain hardening

hall petch relation describes:

yield strength as a function of grain size

grain size can be controlled by:

the rate of solidification by plastic deformation and appropriate heat treatment

aloys of a metal are _ than the pure metal.

stronger

impurities cause:

lattice strain and impede dislocation motion

impurities tend to and segregate around dislocation _ to find atomic sites more suited to their __.

diffuse, cone, radii
this pins the dislocation

precipitation hardening arises from:

heat treatment, T-C behavior of elements

precips are difficult to shear

hard

hardness of a ductile metal can be increased by

repeated plastic deformation at temperatures far below the melting point. this process is called cold-working

plastic deformation causes:

increased dislocation, increased strain field, decreased motion, decreased ductility

plastic deformation enhances:

mechanical properties

result of cold working:

disloc D = total disloc length/unit vol

change cross sectional area by

forging, rolling, drawing, extrusion

percent CW

Ao - Ad/ Ao x 100

restoration to the state before cold work can be done by:

heat treatment and involves two processes: recover and recrystallization, followed by grain growth

recovery:

heating, increases diffusion
decreased in dislocation
relief of internal strain energy
enhanced atomic diffusion leads to healing of defects

What is the result of rextallization?

Leads to formation of strain free grains.

What does rextallization return materials to?

Prior state.

What drives the process of rextallization?

Difference in internal energy between strained and unstrained grains.

rextallization temp

temp at which rextallizaiton is completed in one hour

impossible to initiate cold work when

under the critical limit of cold work

heating (annealing) can reduce ___ and increase _____, leading to ______

disloc desnity, grain size, softening

fracture

separation of a body into pieces in response to static stress below the melting pt

two types of fracture:

ductile and brittle

ductile fracture

metals, plastic deformation, stable crack growth, can be prevented

brittle fracture

ceramics/glass/ice/cold metals, no plastic def, unstable crack, catastrophic failure

process of cup cone fracture

1. formation of small cavities
2. cracks coalesce and grow perpendicular to applied stress
3. cracks propagate and remaining bridge can fracture due to shear stress and tensile loading

process of brittle fracture

1. fast crack propagation, nearly perpendicular to direction
2. propagate with cleavage
3. breaking of atomic bonds along cleavage planes
4. smooth and shiny fracture surface

transangular fracture

cracks pass thru grains
faceted surface bc of different orientation of cleavage planes in grains

What is intergranular fracture?

A type of fracture where crack propagation occurs along grain boundaries (GBs).

How are grain boundaries (GBs) affected in intergranular fracture?

GBs are weakened or embrittled by impurities.

What can the fracture pattern in intergranular fracture indicate?

It can locate the crack initiation area.

ductility is _ with decreasing temperature

reduced

fracture is initiated by ___ at microscopic material defects called ______.

stress concentration, stress raisers

_ cause premature failure

flaws

___ samples will contain more flaws

larger

in ductile materials, there can be a of stress, resulting in plastic deformation

distribution, local

fast fracture occurs when reaches

stress (K), intensity factor/fracture toughness (Kc)

, _ stressed cracks grow first

larger, more

increased loading rate increases and , and is decreased by _

σy, TS, %EL

increased gives less time for _ to move past obstacles

rate, dislocations

impact loading

severe testing case
more brittle
smaller toughness

alloying in general _ the ductile to brittle transition temperature

increases

fracture surface appearance

shiny for brittle,
dull for shear,
distortion of x-section

failure can occur at loads ____ than tensile or yield strengths of a material under a ______ .

lower, static load

3 stages of fatigue failure

1. crack initiation in areas of stress
2. incremental crack propagation
3. final catastrophic failure

fatigue

failure from dynamic or fluctuating stresses from lengthy periods of repeated stress or strain cycles

factors that affect fatigue life

magnitude of stress,
quality of the surface,
optimized geometry

thermal fatigue

thermal cycling causes expansion and contraction,
eliminate restrain by design,
use metals with low thermal expansion coefficients (?)

What is corrosion fatigue?

Chemical reactions induce pits which act as stress raisers, enhancing crack propagation.

How can the corrosiveness of a medium be decreased?

By decreasing the corrosiveness of the medium.

What is one method to protect against corrosion fatigue?

Add a protective surface coating.

What is a method to improve resistance to corrosion fatigue?

Add residual compressive stresses.

fatigue mechanism

crack grows incrementally
faster if delta σ increases, crack gets longer, or loading frequency increases

creep

time dependent and permanent deformation of materials when subjected to a constant load at high temperature

What is instantaneous creep?

Mainly elastic deformation.

What characterizes primary/transient creep?

The slope of strain vs. time decreases with time and involves work hardening.

What is secondary/steady-state creep?

The rate of straining is constant, balancing work hardening and recovery.

What is tertiary creep?

It involves a rapidly accelerating strain rate to failure and the formation of internal cracks, voids, grain boundaries, separations, and necking.

rupture time

caused by defects, seen in short life creep

when stress/temperature increases

instantaneous strain increases, steady-state creep rate increases, time to rupture decreases

components

the elements or compounds that are mixed together initially

phases

physically and chemically distinct material regions that result from changes in environment

the solubility limit

maximum concentration for which only a solution occurs
increases with T

change T and Co can change _

the number of phases

microstructure

properties of an alloy depend on the proportion of phases and how they are arranged structurally on microscopic level

a element/compound is at equilibrium if

at a constant temperature, pressure and composition, the system is stable and there is no change with time

metastable state

a state that appears to be stable along path to equilibrium

minimum of thermodynamic sys

free energy of the system

cored phase

fast rate of cooling, cored structure. diffusion too slow to keep up.

equilibrium phase

slow rate of cooling, equilibrium structure

eutectic reaction

transition between liquid and mixture of two solid phases at eutectic composition

at most phases can be in equilibrium within a phase field. phases may be in equilibrium at only a few points. _ phase regions are separated by 2-phase regions.

two, three, single

solvus line

separates one solid solution from a mixture of solid solns
shows limit of solubility

eutectic or invariant point

liquid and two solid phases co-exist in equilibrium at the eutectic composition and the eutectic temperature Te

eutectic isotherm

the horizontal solidus line at Te

binary-eutectic

2 components - has a special composition with a minimum melting temperature