Metals II: Processing

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Metals IIProcessing Characterization

• Lecture 6
• February 5, 2009

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Background

• Last class we discussed different materials,
  their structure, properties and applications
• Structure and properties of metals are
  controlled by different processing techniques
• The ductility and malleability of metals allow
  for unique processing
• Blacksmiths utilized these techniques for
  centuries
• Many (but not all) metals can be processed to
  improved mechanical properties
• Modern processing relies on the use of
  deformation and heat, but with more control than
  the smiths had
• The resulting microstructure gives insight into
  the properties

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Metals Processing Summary
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Metal Processing Methods

• Cast products
• 1. Melt metal
• 2. Cast to final shape in a mold (investment
  casting or lost wax)
• Powder metallurgy (PM)
• 1. Form metal into a fine powder
• 2. Compact powder and sinter or hot press at
  high temperature (gt0.5 of the melting
  temperature, TM)
• Other specialized forming methods
• Chemical vapor deposition (CVD)
• Superplastic deformation at high temperature
• Electroforming
• Melt spinning

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Other Processing Methods

• Mechanical processing
• Cold working
• Hot working
• Thermal processing
• Annealing
• Recovery, recrystalization and growth
• Heat treatments
• Both of these are used to control properties of
  the final product

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Cold Working - Strain Hardening

• Material properties can be altered by processing
  a material so that its grain or phase (crystal)
  structure changes
• Cold Working
• Plastic deformation of metal below the softening
  or recrystallization point, but commonly at or
  about room temperature
• Reduces the amount of plastic deformation that a
  material can undergo in subsequent processing and
  requires more power for further working
• Increase in sy and hardness - method for
  strengthening many metals
• Can make the material brittle
• Extent of cold working reduction in area or
  reduction in thickness
• CW 100(Ao A)/Ao or CW 100 (do d)/do
• Achieved by forging, rolling, extruding, and
  drawing

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Metal Forming Methods

• Forging Pressing of a solid sample in a die to
  produce a desired shape. This can be done by a
  single continuous compression or by multiple
  successive shocks
• Rolling Pressing of a sample through sets of
  parallel rollers, which may also impart a
  particular shape to the metal
• Extrusion Pushing a solid sample through an
  orifice die by means of a piston or screw pushing
  a ram
• Drawing Similar to extrusion, but the sample is
  pulled through an orifice by means of a tensile
  force applied at the down-stream end of the
  process

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Metal Working Methods

• Change in cross-sectional area
• CW (A0 A)/A0 x 100

Rolling
Forging
A
Ao
A
Ao
Extrusion
Drawing
A
A
Ao
Ao
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How Cold Working Works

• Control of the dislocations in the material
• The yield strength is completely controlled by
  the onset of dislocation movement.
• No dislocation motion, no plastic deformation
• Therefore impeding dislocation motion increases
  the yield strength of metals
• When metals plastically deform two things occur
• Existing dislocations in the material begin to
  move
• New dislocations are rapidly created and move.
• Higher the density of dislocations, the more they
  become entangled with each other making it harder
  for all of them to move.

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Dislocations and Their Motion

• Two dislocations will get in each others way and
  will prevent the motion of both dislocations
• Near a dislocation the lattice is strained
  additional dislocations would increase the local
  strain
• Imagine putting another
  ½ plane of atoms in
  here
• Two alike dislocations
  would repel each other
• Dislocations occur in many
  planes with many
  orientations

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Effect on sy and Ductility
Trade off in properties
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Hot Working

• Mechanical deformation above recrystallization
  temperature
• Metal remains soft and relatively ductile during
  processing
• Reduced chance of cracking
• Does not introduce permanent dislocations
• Hot working is important to fabrication processes
  because it allows increased plastic deformation
• The continuous recrystallization that takes place
  as the piece is deformed prevents the increase in
  strength and reduction in ductility produced by
  cold working
• Achieved through forging, rolling, extruding and
  drawing

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Annealing

• Heating a cold-worked metal above a
  recrystallization temperature (0.30.5 TM)
  eliminates most of the defects (dislocations,
  etc)
• In 1 hr, substantial amount of recrystallization
  occurs
• Heating process referred to as annealing
• Annealing consists of heating to a high enough
  temperature followed by cooling at a suitable
  rate
• During annealing, metals undergo recovery and
  recrystallization
• Highly-strained grains are replaced by new
  strain-free grains
• Amount of recrystallization is dependent on both
  time and temperature
• Annealing leads to
• Reduction in yield strength and hardness and
  increase in ductility as the dislocations are
  removed
• Increase ductility, softness
• Development of desired microstructure and
  properties
• Cold-working and annealing are often cycled to
  assist in production

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Recovery

• When the cold-worked metal is annealed
• Recovery occurs first (at ?0.1TM)
• Thermal energy allows some dislocation motion
• Dislocation density goes down slightly due to
  annihilation and rearrangement
• Hardness and ductility are almost unchanged
• Major changes come from recrystallization that
  occurs at higher temperatures (at ?0.30.5TM)

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Recrystallization

• Recrystallization
• New grains nucleate and grow at the expense the
  highly-strained grains until the whole of the
  metal consists of strain-free grains
• Nucleation usually occurs in the most deformed
  portion of the grain boundary or slip plane
• Driving force is the strain energy of the
  deformed grains
• Dislocation density returns to original value
  (before cold working)
• Hardness and ductility return to original value
• Recrystallization also used to control grain size
• High temperatures and long crystallization times
  can lead to grain growth of the strain-free
  grains (driven by reduction in grain boundary
  area) tend to produce a large grain size
• Grain growth due to surface tension - big grains
  eat little grains
• High initial deformation tends to produce small
  recrystallized grains

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Cold Working and Annealing
Cold-worked material has greatly increased
dislocation density
Annealing leads to recovery
Starting material with low dislocation density
Fully recrystallized metal with new (smaller)
strain-free grains
Further annealing leads to nucleation and growth
of new grains
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Recrystallization Growth
Callister, Mat Sci Eng an Intro, 5th ed.
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Strengthening Metals

• Metals can be strengthened by several
• basic mechanisms
• Work hardening
• Refinement of grain size
• Precipitation hardening
• Strengthening by a dispersion of fine second
  phase particles
• Solid solution strengthening
• Substitutional atoms
• Interstitial atoms

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Effect of Grain Size on Strength

• Grain boundaries stop dislocation motion due to
  discontinuity in crystals
• High energy/force is needed to move dislocations
  into a neighboring grain
• In larger grains, more dislocations pileup
  pushes them to new grains
• Strengthening is quantified by the following
  relationship
• sy sy,o k/dm
• where sy,o and sy ? original and new yield
  stress, d ? grain diameter, k ? constant, m½
• The increases in strength at very small grain
  sizes can be enormous
• Nanostructured metals with grain sizes from 20
  to 200 nm have very high strengths
• The strengthening is limited for very small
  grains, grain slipping will occur

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How To Control Grain Size

• Cold worked material
• high dislocation density
• lot of stored energy
• very strong
• not very ductile

• Recrystallized material
• low dislocation density
• no stored energy
• weaker
• ductile

process

• Energy is lowered by nucleating new grains from
  inside the old ones.
• New grains grow, eating the cold worked grains
  along the way
• Result is a completely new grain structure
• Amount of cold work
• More cold work more stored energy easier
  nucleation more nucleation sites smaller
  grain size
• Time and temperature of annealing
• Lower temperature slower diffusion smaller
  grain size
• Shorter annealing time less diffusion smaller
  grain size

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Effect of Annealing Temperature
Callister, Mat Sci Eng an Intro, 5th ed.
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Precipitation Hardening

• Precipitation hardening process in which small
  particles of a new phase (crystal) precipitate in
  a matrix to harden/strengthen materials
• Particles impede the movement of the dislocations
• Age Hardening is another name some metals
  form precipitates at room temperature over time
• Precipitation hardening occurs in 2 stages

Large t needed to move dislocation through or
around precipite
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Precipitation Hardening Steps

• Step I Solution heat treatment
• Heat composition, Co (A B atoms), to
  temperature, To, until all B atoms dissolves into
  A (i.e. 2nd phase removed) and one phase is
  achieved
• Alloy is quenched to T1 (diffusion-less process)
• Single phase is metastable
• Diffusion rates are often too slow at T1
• 2nd phase would take long periods of time to
  precipitate out
• Step II Precipitation heat treatment
• Supersaturated single phase solid (B conc is
  above solubility limit)
• Heat to T2 where kinetics allow for controlled
  diffusion of B to form 2nd phase
• Forms finely divided 2nd phase
• Finally, alloy is cooled to stop precipitation

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Characterization of Metals

• Mechanical testing will give various mechanical
  properties (s-e diagrams)
• Optically examination of the microstructure
• Gives general information on material behavior
• Determines or confirms structure-property
  relationships
• Optical imaging techniques are used with
  appropriate surface preparation
• Grinding or polishing, followed by etching
• Chemical activity of a surface depends on crystal
  orientation
• For polycrystals, etching characteristics vary
  between grains
• Affects the reflection of light and therefore
  appearance
• Grains boundaries are accentuated (more
  chemically active)

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Other Imaging Techniques

• TEM transmission of an electron beam through a
  thin sample
• Scattering or diffraction of the beam occurs and
  is imaged
• Often used to study dislocations
• SEM reflection of an electron beam off the
  surface of a sample
• Reflection (or back scattering) of the beam is
  imaged
• Does not require polishing/etching, but a
  conductive surface is necessary (coating added to
  non-conducting materials)
• Used to study microstructure, fracture behavior
• SPM sharp tipped probe near the surface, scans
  in 2D (includes AFM and STM techniques)
• AFM tip contacts surface and deflects as
  surface height changes
• STM tunneling current travels from tip to
  surface, changes in current are constructed into
  an image
• Both give 3D images (a topographical map) at the
  atomic scale
• Used to study a variety of surface features

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Summary

• Dislocations and grain size are controllable
  variable
• Can be manipulated to achieve desirable
  properties
• Trade-offs occur between ductility and strength
• Combinations of cold working and heat treatment
  are used
• Cold-working creates highly strained grains with
  many dislocations
• Annealing relieves strain and allows grains to
  recrystalize and grow
• Solid solutions and precipitates also lead to
  stronger materials
• Mechanical testing and imaging are combined to
  understand structure-property relationships