0. Introduction to Manufacturing
Metal Alloys (l.u. 2/15/10)

1. Introduction
The properties and behavior of metals (and alloys) depend on their:
Structure
Processing history
and
Composition
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2. Structure of Alloys
An alloy is the combination of two or more chemical elements, one being a metal.
Classification of alloys.
Ferrous: containing iron, second most abundant element (5% earth's crust).
Non-ferrous: no iron, usually more expensive than ferrous metals.
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3. (Black & Kohser, 2008, p. 125)
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4. Structure of Alloys (Cont.)
Solute- minor element (added to solvent)
Solvent- major element (host)
Solid solution- structure of the solvent is maintained (one alloying element is completely dissolved in another)
Phase- liquid, solid, gas – pure substance or solution (uniformity)
Some elements have limited solubility – results in mechanical mixture (clear boundaries)
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5. 1. Substitutional Solid Solutions
Substitutional Solid Solution (according to Hume-Rothery rules):
Must have similar crystal structures (e.g. FCC with FCC).
Difference between atomic radii less than 15% (same size atoms).
Brass (zinc + copper).
Copper Grains
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6. 2. Interstitial Solid Solutions
Interstitial Solid Solution - solvent atom has more than one valence electron (easier to control solute).
Atomic radius of solute atom is less than 59% of solvent (atom sizes differ greatly).
Example = Steel (iron + carbon)
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7. Intermetallic Compounds
Complex structures
Solute atoms present among solvent atoms = atomic bonding.
Strong, hard, and brittle
Ti3Al, Ni3Al, Fe3Al.
Aluminum Grains
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8. Two-phase Systems
Most alloys consist of two or more solid phases (alloy contains particles of single element OR grains are different).
Limited solubility (just as with sugar in water  Mechanical mixture).
Clear boundaries, mixture - each with its own properties.
Stronger and less ductile than solid solutions.
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9. Phase Diagrams
Pure metals have clearly defined melting or freezing points, and solidification takes place at a constant temperature.
Tool for understanding the relationship among temperature, composition, and phases present in a particular alloy system.
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10. Phase Diagrams (Cont.)
Alloys solidify over a range of temperatures, based on the composition of the mixture.
As the alloy cools the mixture begins to freeze, changing gradually to a solid (liquid/solid phases).
Temperature L
L+S S
Time
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11. Binary Phase Diagrams Complete Solid Solubility Two Phases
L+S
Temperature
Temperature S B
0% B
100% A
100% B
0% A
Composition
Solid Solution - Single Phase
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12. Nickel-Copper Diagram
(Black & Kohser, 2008, p. 75)
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13. Two-Phase Diagrams Limited solubility Two Phases
A + Liquid B
B + Liquid A B
Eutectic point
A + B
0% B
100% A
100% B
0% A
Solid Solution - Single Phase
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14. Two-Phase Lead-Tin Diagram
(Black & Kohser, 2008, p. 75)
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15. Two-Phase Lead-Tin Diagram
1) At the point of maximum solubility, 183 C, lead can hold up to 19.2% tin (and be in single phase solution)
2) Tin can hold up to 2.5% lead in single phase 183 degrees C).
3) 61.9% tin added to lead provides the lowest melting temperature (eutectic point)
(Black & Kohser, 2008, p. 75)
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16. Two-Phase Iron-Carbon Diagram
Most important phase diagram in manufacturing applications, since steels, cast irons, and cast steels are the most common engineering materials (versatile properties and relative low cost).
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17. Iron-Carbon Diagram (Cont.)
Solid Phases of the Iron-Carbon Diagram
Ferrite (-iron)
Austenite (-iron)
Cementite (iron-carbide)
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18. Ferrite (-iron) Soft, ductile, magnetic. BCC
Solid solution (0.022% carbon) almost pure iron.
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19. AUSTENITIC MANGANESE STEEL
Austenite
(-iron)
FCC - higher density than BCC, ductile at elevated temperatures (good formability)
Interstitial Solid
Solution (2.11% carbon)
Non-magnetic
AUSTENITIC MANGANESE STEEL (
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20. Cementite Iron carbide (Fe3C) 6.67% carbon
Hard & brittle Intermetallic Compound.
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22. Heat Treatment of Ferrous Alloys
The different microstructures of an alloy can be modified by controlled heating and cooling which will induce phase transformations and thus, changes in mechanical properties.
Heat treatment is one of the most common methods to improve or modify characteristics, since it can produce a variety of mechanical properties and improve service performance.
The effect of thermal treatment depends on the alloy, its composition, microstructure, degree of prior cold work, and rates of heating and cooling.
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23. Steel Microstructures
Perlite (eutectoid steel) - alternating layers of Ferrite and Cementite
fine or coarse perlite
 Spheroidite (spherical cementite) - tougher and harder than perlite
Bainite (very fine ferrite-cementite) - stronger and more ductile than perlite, same hardness
Martensite (tetragonal body-centered structure) - austenite cooled at high rate, hard and brittle (not practical)
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24. Heat Treatment Processes
Annealing: general term used to refer to the restoration of properties after cold work or heat treatment.
Normalizing: cooling cycle done in still air to avoid excessive softness in the annealing of steels.
Spheroidizing: improve properties of high-carbon steels.
Stress Relieving: reduce or eliminate residual stresses.
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25. Heat Treatment Processes (Cont.)
Tempering: reduce brittleness and residual stress, and increase ductility and toughness of previously hardened steels.
Hardening: heating and cooling rapidly (quenching)
Case Hardening: complete alteration of the microstructure and properties of just the surface of the material by heating within a particular atmosphere.
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