Mechanical behavior of materials in class quiz 3

Strengthening mechanism where plastic deformation is suppressed by elastic ’surrounding’ material

constraint

In constraint strengthening, higher stresses must be applied in constrained area for yielding to occur when _____ criterion is met.

von mises

constraint is dependent on what factor from this problem? does constraint increase or decrease as this factor goes to zero?

t/D, increase

is notch strengthening applicable to ductile or brittle materials?

ductile

Identify the stage on the single crystal shear stress-strain curve:

Easy glide region, work hardening rate is low, single slip system operates, long slip distances without dislocation-barrier interactions.

Stage 1

Identify the stage on the single crystal shear stress-strain curve:

Linear hardening region, work hardening rate is high and constant with increasing strain, primary slip dominant but secondary slip occurs, dislocation multiplication and interactions reduce slip length as density increases.

Stage 2

Identify the stage on the single crystal shear stress-strain curve:

Parabolic hardening region, onset depends on ease of cross-slip and temperature, decreasing work hardening with increasing strain

Stage 3

In work hardening, the stress required for continued material plastic flow _____ with increasing plastic strain, effects can be eliminated by subsequent ____, and resistance to plastic flow has a complex dependence on _____ density.

increases, annealing, dislocation

self-organized arrangements of dislocations that minimize strain energy during deformation

LEDS (Low-energy dislocation structures)

As stage 2 progresses in a single crystal stress-strain curve, the _____ of LEDS does not change, but ____ does.

character, size

At _____ dislocation density, LEDS are not energetically favorable to form.

low

The LEDS structures eventually reaches a lower bound size, which gives _____ hardening behavior

parabolic

____ ____ energy affects the onset of stage 3. LEDS theory explains this phenomenon - cross-slip easier → ____ LEDS formation → lower shear strain onset of stage 3

stacking fault, increased

Is the temperature dependence of the work hardening behavior well captured by the LEDS mechanism?

yes

Does the Hall-Petch relation work for these nano-scale grains? Answer in order of A, B, C

yes, maybe, no

For most metals, yield strength and hardness _____ as grain size decreases. What type of strengthening mechanism is this?

increase, grain boundary

is this increasing or decreasing CW?

increasing

After mechanical shaping and “annealing” of alloy with solute particles in microstructure are large and widely spaced. Ineffective in “pinning” dislocations, so strength is low… what two critical aspects of the precipitates can be changed with aging time/temperature?

distribution/character, nature (how it slows down dislocation)

in precipitate hardening, identify the y axis for graphs A and B.

precipitate size, precipitate spacing

Initially during aging precipitate spacing decreases because more precipitates are ____, what phenomenon explains why eventually this decrease levels out and spacing begins to increase?

nucleating, ostwald ripening

small precipitates (often), cause ____ misfit strains, since easy for dislocation transmission from matrix into precipitate → leads to ____ _____ as primary means of continued dislocation motion on slip plane

high, particle shearing

large precipitates (often), cause ____ misfit strains, have no particle shearing so continued dislocation motion occurs through ____ looping

low, orowan

There is a critical transition point in particle diameter! Increased resistance to shearing with ____ size, increasing size _____ looping because of ostwald ripening

increasing, decreases

Metal is strengthened by addition of solute atoms dissolved in solvent lattice, impedance of dislocation line motion by interaction with solute atoms on/above/below slip plane. What mechanism of strengthening is this?

solid solution

What three factors govern magnitude of solute hardening?

solubility, atomic size difference, solute strain field details

Does interaction and strengthening (in solid solution strengthening) occur between like or unlike stress-strain field characteristics?

like

what are the stress fields associated with common types of solute (dilation = ___, shear = ___) ? (1) substitutional (d/s) (2) interstitial symmetric (d/s) (3) interstitial asymmetric (d/s), (4) complex (d/s)

edge, screw, d, d, d+s, d+s

Breakdown of hall-petch relationship typically occurs at grain diameters below _____ but becomes very apparent on the nanometer scale because dislocation pile-ups ____ form in very small grains

one micron, cannot

in aging, peak aging time is achieved when there is a balance in what three factors of the microstructure?

precipitate size, density, and coherency

overaging occurs when particles grow too large and ____ coherency, ___ interfacial energy

lose, lowering

what ratio of T/Tm for metals is the temperature considered elevated or high homologous

.35

what is this strain behavior at constant stress called?

creep

transient strain accumulation at a declining creep rate, decreases with increasing time and plastic strain. dislocation cell structure develops to cause LEDS hardening, what stage of creep is this?

primary

Constant rate of creep strain accumulation, ____ steady state rate. what stage of creep is this?

minimum, secondary

When steady-state creep rate is achieved, dislocation structure hardening is balanced by dynamic recovery-softening (dislocation ___ ___ and diffusion ____)

cross slip, climb

Accelerating creep strain rate to rupture at time tR. Microstructure instability (precipitate coarsening, recrystallization) and microscopic cavity formation leads to fracture. what stage of creep is this?

tertiary

During creep, increasing stress or temperature causes a ____ in time of stage 1 and 2, steady state creep rate ____, and time and strain to failure fall

reduction, increases

the fact that these lines are parallel means what in terms of the microstructure and mechanism? if these lines are non-linear, then the microstructure changes with ____ and/or _____, the creep mechanism changes, or ____ creep mechanisms operate

they don’t change, temperature, stress, multiple

what is this phenomenon called when strain is held constant?

stress relaxation

During stress relaxation, the stressed material _____ deforms at stress below yield stress to fixed displacement. Stored elastic energy drives creep if temperature is sufficient to enable _____ ______ plasticity.

elastically, thermally activated

When T/Tm > 0.5 and the activation energy for creep is constant, then the single-general process controlling creep is ___ _______

self diffusion

Self-diffusion creep is good for high temperatures but breaks down at other stresses and materials because factors associated with dislocation _____ and _____ are not included but critical to the controlling mechanism.

structure, mobility

In dislocation slip (time-temperature dependent plastic deformation), strength decreases with ____ temperature but is time independent, so not technically creep? What range of T/Tm ratio and stress level is this typically active.

increasing, .35-.5, higher

In dislocation or power law creep (time-temperature dependent plastic deformation), it is typically active at T/Tm > ___ and ____ to ____ stresses. This describes the additional recovery mechanisms that become possible.

.6, moderate, high

A recovery mechanism possible described by dislocation creep, “solute drag”, describes how at higher T solute atoms are mobile, if the dislocation velocity is ___ and the solute diffusion rate is ___ then they can move together (typically at low creep rates).

Factors influencing dislocation velocity for drag:

solute atom diffusivity is higher then ___ drag, misfit parameter between dislocation and solute is higher then ___ drag, solute concentration higher then __ drag.

low, high, less, more, more

A recovery mechanism possible described by dislocation creep, “glide-climb”, time-dependent atom (vacancy) diffusion enables dislocation escape from blocking particles and plastic deformation, another mechanism occurs here instead! At T/Tm > 0.6 _____ diffusion occurs but at lower T ___ ____ diffusion occurs.

lattice, dislocation core

A recovery mechanism possible described by dislocation creep, “diffusional creep”, describes creep strain accumulation governed by diffusional mass transport and not dislocation movement. There are two means of diffusion, labeled as Coble and Nabarro-Herring, which correlates to lattice and grain boundary diffusion respectively? It is typically active at T/Tm > 0.6 and __ stress

Nabarro-Herring, Coble, low

to accomodate diffusional mass transport during high temperature deformation and avoid micro voids/cracks at grain boundaries, you have grain boundary ____

sliding

vacancies flow by diffusion from areas of high concentration (under _____) to areas of low concentration (under _____)

tension, compression

which creep occurs more rapidly, Coble (lattice) or Nabarro-Herring (grain boundary)?

Coble

How will we know if Nabarro-Herring or Coble Creep dominates? Remember grain-boundary diffusion is faster at ____ temperatures while lattice diffusion is faster at ____ temperatures.

lower, higher, faster creep rate dominates

On a deformation map, the boundaries represent what?

where each mechanism produces the same creep strain rate