Chapter 7: Dislocations and Strengthening Mechanisms

Alloying with impurity atoms that go into either substitutional or interstitial sites is called _______(another hyphenated word) strengthening.

solid-solution

____ hardening is the phenomenon whereby a ductile metal becomes harder and stronger as it is plastically deformed. Also called cold working.

strain

Polycrystalline materials are _____ than their single-crystal equivalents due to grain boundaries which are barriers to dislocation motion.

stronger

The combination of a slip plane and a slip direction is called a slip _____.

system

Recrystallization Temperature

temperature at which recrystallization just reaches completion in 1 hour

"Because macroscopic plastic deformation corresponds to the motion of large number of dislocations..."

the ability of a metal to plastically deform depends on the ability of dislocations to move

Stress-Strain Behavior vs. Temperature

σy and TS decrease with increasing test temperature %EL increases with increasing test temperature Why? Vacancies help dislocations move past obstacles.

Single Crystal Slip

(image)

Effects of Stress at Dislocations

-Repulsion -Attraction -Dislocation Annihilation: Perfect Crystal

Four Strategies for Strengthening:

1) Reduce grain size 2) Form solid solutions 3) Precipitation strengthening 4) Cold work (strain hardening)

Three Annealing Stages:

1. Recovery 2. Recrystallization 3. Grain Growth

Alloys are stronger than pure metals because: (3 reasons)

1.Impurity atoms that go into solid solution ordinarily impose lattice strains on the surrounding host atoms (tensile or compressive forces) 2.Lattice strain fields caused by impurity atoms interact with dislocations 3.Result in restricted dislocation movement(tensile fields oppose other tensile fields, etc.)

resolved shear stress

An applied tensile or compressive stress resolved into a shear component along a specific plane and direction within that plane.

3. Grain Growth

At longer times, average grain size increases: Small grains shrink (and ultimately disappear) Large grains continue to grow Grain Boundary area (and therefore energy) is reduced

(4) Cold Work (Strain Hardening)

Deformation at room temperature (for most metals). Common forming operations reduce the cross-sectional

Dislocations: Metals

Dislocation motion easiest: due to non-directional bonding and close-packed directions for slip

cold working

The plastic deformation of a metal at a temperature below that at which it recrystallizes.

recrystallization temperature

For a particular alloy, the minimum temperature at which complete recrystallization will occur within approximately one hour.

(1) Reduce Grain Size

Grain boundaries are barriers to slip. Barrier strength increases with increasing angle of misorientation. Smaller grain size: more barriers to slip

(3) Precipitation Strengthening

Hard precipitates are difficult to shear

solid-solution strengthening

Hardening and strengthening of metals that result from alloying in which a solid solution is formed. The presence of impurity atoms restricts dislocation mobility.

Cold Working vs. Hot Working

Hot working -> Deformation above Tr Cold working -> Deformation below Tr

(2) Form Solid Solutions

Impurity atoms distort the lattice & generate lattice strains. These strains can act as barriers to dislocation motion.

How grain sizes influence properties?

Metals having small grains- relatively strong and tough at low temperatures Metals having large grains- good creep resistance at relatively high temperatures

Dislocations: Covalent Ceramics

Motion difficult due to directional bonding

Dislocations: Ionic Ceramics

Motion difficult due to the need to avoid nearest neighbors of like sign

2. Recrystallization

New grains are formed that have low dislocation densities, are small in size, and consume and replace parent cold-worked grains.

1. Recovery

Reduction of dislocation density by annihilation

slip

Plastic deformation as the result of dislocation motion; also, the shear displacement of two adjacent planes of atoms.

lattice strain

Slight displacements of atoms relative to their normal lattice positions, normally imposed by crystalline defects such as dislocations, and interstitial and impurity atoms.

slip system

The combination of a crystallographic plane and, within that plane, a crystallographic direction along which slip (i.e., dislocation motion) occurs.

recrystallization

The formation of a new set of strain-free grains within a previously cold-worked material; normally an annealing heat treatment is necessary.

strain hardening

The increase in hardness and strength of a ductile metal as it is plastically deformed below its recrystallization temperature.

grain growth

The increase in average grain size of a polycrystalline material; for most materials, an elevated- temperature heat treatment is necessary.

recovery

The relief of some of the internal strain energy of a previously cold-worked metal, usually by heat treatment.

critical resolved shear stress

The shear stress, resolved within a slip plane and direction, that is required to initiate slip.

dislocation density

The total dislocation length per unit volume of material; alternately, the number of dislocations that intersect a unit area of a random surface section.

Anisotropy in stress(y)

can be induced by rolling a polycrystalline metal

Dislocations entangle with one another during ____ ____ making dislocation motion more difficult.

cold work

Even though a single grain may be favorably oriented with the applied stress for slip, it cannot ____ until the adjacent and less favorably oriented grains are capable of slip also.

deform

slip directions

directions of movement (highest linear densities)

A _____ (hyphenated word) material is harder and stronger than one that is coarse grained, because the former has a greater total grain boundary area to impede dislocation motion.

fine-grained

As cold work is increased Yield strength (σy) ________, Tensile strength (TS) ________, Ductility (%ELor %AR) ________.

increases increases decreases

"Even though a single grain may be favorably oriented with the applied stress for slip,"...

it cannot deform until the adjacent and less favorably oriented grains are capable of slip also; this requires a higher applied stress.

Dislocation Motion

movement of extra half-plane of atoms by breaking and reforming of interatomic bonds

slip plane

plane on which easiest slippage occurs (Highest planar densities, and large interplanar spacings)

_____ deformation corresponds to the motion of large numbers of dislocations.

plastic

If dislocations can't move, _____ _____ doesn't occur!

plastic deformation

"Virtually all strengthening techniques rely on this simple principle:"

restricting or hindering dislocation motion renders a material harder and stronger

The process by which plastic deformation is produced by dislocation motion is termed _____.

slip

A dislocation moves along a _____ ______ in a _____ _______ to the dislocation line

slip plane direction perpendicular