MatSci Ch 7 Concepts

plastic deformation- how it occurs

• crystalline materials: occurs primarily through dislocation motion and slip

• movement of dislocations allows atomic planes to move one row at a time rather than forcing an entire plane to shift simultaneously

  • reduces the stress required for deformation.

theoretical strength vs. actual strength

perfect crystal:

• requires simultaneous breaking and reforming of enormous numbers of atomic bonds during deformation

• require extremely high stresses

real materials:

• have dislocations, which allow deformation to occur incrementally

• only atoms near the dislocation move at a given moment

• reduces the required stress by orders of magnitude

edge dislocation

• created when an extra half-plane of atoms is inserted into a crystal lattice

• end of the extra plane creates a region of lattice distortion.

• with applied stress, dislocation can move through the crystal and leave permanent slip behind

• cumulative effect produces macroscopic plastic deformation

• strongly influence strength and ductility

screw dislocation

• shear forces produce a spiral or helical distortion in the crystal lattice

• displacement occurs parallel to the dislocation line

• can move on multiple planes more easily than edge dislocations

• mobility contributes significantly to plastic deformation in many metals

slip

• process by which plastic deformation occurs through dislocation motion

  • one region of the crystal shifts relative to another

• macroscopic: slip creates permanent deformation

• microscopic: sequential movement of atoms along preferred crystallographic planes and directions

  • occurs most easily on those with highest atomic packing density

    • allow dislocations to move with less resistance

    • easiest pathway for atomic motion

• dominant plastic deformation mechanism in metals

slip system

• consists of a slip plane and a slip direction

• plastic deformation occurs only on available slip systems

• greater the number of slip systems, the easier it is for a crystal to accommodate deformation

• EXAMPLE: FCC metals more ductile

  • possess many closely packed slip systems

  • plastic deformation can occur relatively easily

  • sustain large plastic strains before fracture

• EXAMPLE: HCP metals less ductile

  • few independent slip systems at room temperature

  • plastic deformation becomes more difficult

  • tend to fracture after smaller amounts of deformation

resolved shear stress

• component of applied external stress that acts parallel to a specific slip plane and in a specific slip direction within a crystal lattice

• slip begins when the resolved shear stress reaches a critical value

• yielding depends not only on applied stress but also on crystal orientation

critical resolved shear stress (CRSS)

• minimum resolved shear stress required to initiate dislocation motion

• yielding when applied stress produces a resolved shear stress that equals or exceeds the CRSS

Schmid's Law

• plastic deformation begins when resolved shear stress on a slip system reaches CRSS

• relates applied tensile stress to resolved shear stress

• crystal orientation strongly influences when yielding begins

  • Two specimens made from the same material can yield at different stresses if their crystal orientations differ

• anisotropy in single crystals and helps predict yielding behavior

  • anisotropy: material's physical properties change depending on the direction you measure them

Burgers vector

• describes the magnitude and direction of crystal distortion associated with a dislocation

• lattice displacement produced by the defect

• determines many dislocation properties, including energy and mobility

dislocation interaction

• dislocations produce strain fields in the surrounding crystal

• strain fields interact

• (orientation-dependent) dislocations may attract, repel, annihilate, or become pinned

• interactions are responsible for many strengthening phenomena because they make additional dislocation motion more difficult

strain hardening

• occurs when a metal becomes stronger during plastic deformation

  • dislocation density increases dramatically

  • further deformation becomes increasingly difficult; loses its ability to deform plastically

• increased number of dislocations leads to more interactions and more barriers to motion

  • greater stress is required to continue deformation; resistance created by existing dislocations increases the required stress

  • yield strength, tensile strength, and hardness all increase during cold work

strengthening: fundamental principle

• all strengthening methods operate by reducing dislocation mobility

  • dislocations cannot move easily = plastic deformation becomes more difficult

• yielding corresponds to dislocation movement

  • restricting this movement raises strength

grain-size strengthening

• grain boundaries act as barriers to dislocation motion

  • dislocation cannot easily continue because the neighboring grain has a different orientation

• result: increased strength and hardness

• Hall-Petch equation

solid-solution strengthening

• solute atoms are added to a crystal

  • create local lattice distortions because their size differs from the host atoms

  • distortions produce stress fields that interact with dislocations

  • makes dislocation motion more difficult and raises strength

precipitation hardening

• introduces small particles throughout the material = obstacles to dislocation movement

• either cut through the particles or bend around them, both of which require additional energy

  • higher stresses are needed for deformation

annealing

• heat treatment used to reduce the effects of cold work

• stored strain energy decreases and the material becomes softer and more ductile

• occurs through three stages: recovery, recrystallization, and grain growth

  • recovery: dislocations rearrange/annihilate each other (reduce internal stresses)

  • recrystallization: new strain-free grains nucleate and grow in material. dislocation density decreases; strength/hardness decrease; ductility increases

  • grain growth: larger grains consume smaller grains. reducing grain-boudary area, lowering grain-boundary energy.

strengthening & annealing relation

• strengthening: increase resistance to dislocation motion

• annealing: restores dislocation mobility and removes stored strain energy

• both processes have opposite effects on strength and ductility

dislocations importance

Without dislocations, metals would be extremely strong but virtually impossible to shape

• enable manufacturability and limit strength

• strength controlled by dislocation mobility

  • move easily: soft/ductile

  • hindered by grain boudaries/solute atoms/precipitates: stronger

hardness

• measures resistance to localized plastic deformation, usually through indentation

• indentation requires dislocation motion = hardness is closely related to strength