Applications and processing of metals

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Chapter 11

• Applications and processing of metals
• ( Precipitation Hardening )

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Types of metal alloys

• Groups of metal alloys
• Ferrous alloy (iron is the prime constituent).
• Nonferrous alloys.
• Steels Iron-carbon alloys that may contain
  appreciable concentrations of other alloying
  elements. Carbon content is normally less than
  1.0 wt.
• Cast irons Ferrous alloys with carbon contents
  above 2.14 wt (usually 3.0-4.5 wt C).

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Taxonomy of Metals
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Classification for various ferrous alloys
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Steels
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Nonferrous alloys
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Thermal processing of metals
Annealing Heat to Tanneal, then cool slowly.
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Precipitation hardening of metals(Heat treatment)

• Strength and hardness of some metal alloys may be
  enhanced by the formation of extremely small
  uniformly dispersed particles of a second phase
  within the original phase matrix.
• This strengthening is accomplished by phase
  transformations induced by heat treatment.
• Age hardness is also used to designate the
  process since strength develops with time, or as
  the alloy ages.

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Precipitation hardening (Cont.)

• Requisite features of phase diagrams of alloy
  systems for precipitation hardening
• Appreciable maximum solubility of one component
  in the other (on the order of several percent).
• Solubility limit that rapidly decreases in
  concentration of the major component with
  temperature reduction.
• Composition of precipitation-hardenable alloy
  must be less than the maximum solubility.

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Precipitation hardening
Particles impede dislocations. Ex Al-Cu
system Procedure --Pt A solution heat
treat (get a solid solution) --Pt B
quench to room temp. --Pt C reheat to
nucleate small q crystals within a
crystals. Other precipitation systems
Cu-Be Cu-Sn Mg-Al
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Precipitation hardening (Cont.)

• Types of precipitation hardening process
• Solution heat treatment
• Precipitation heat treatment.

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Temperature vs. time plot showing both solution
and precipitation heat treatment for
precipitation hardening.
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Precipitation hardening (Cont.)

• Solution heat treatment
• Quenching (rapid cooling) prevent diffusion and
  the formation of a new phase. Resulting phase
  will be supersaturated in B atoms (alloy
  relatively soft and weak). ? Phase is retained
  at room temperature for relatively long periods
  (long diffusion rate at room temperature).
• Precipitation heat treatment
• The characteristics of the ? phase and the
  strength and hardness of the alloy depends on the
  precipitation temperature T2 and the aging time
  at this temperature.

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Precipitation effect on TS, EL
2014 Al Alloy
TS peaks with precipitation time.
Increasing T accelerates process.
EL reaches minimum with precipitation time.
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Effect of aging on dislocation motion
Peak-aged --avg. particle size 64b
--closer spaced particles efficiently stop
dislocations.
Over-aged --avg. particle size 361b
--more widely spaced particles not as
effective.
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SUMMARY
Steels increase TS, Hardness (and cost) by
adding --C (low alloy steels) --Cr, V,
Ni, Mo, W (high alloy steels) --ductility
usually decreases w/additions. Non-ferrous
--Cu, Al, Ti, Mg, Refractory, and noble
metals. Hardenability --increases with
alloy content. Precipitation hardening
--effective means to increase strength in
Al, Cu, and Mg alloys.