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http://www.mse.ncsu.edu/zhu/ �
Objectives/outcomes: You will learn the following: •Classification of steels
•Roles played by alloy elements
•Hardenability test
•Al alloys
•Solution treatment, quenching, aging
•Aging and natural aging
•Mechanism of hardening: GP zone, metastable coherent phase and stable
incoherent phase
MSE200 Lecture 15 (CH. 9.4, 9.5.1)
Engineering Alloys Instructor: Yuntian Zhu
http://www.mse.ncsu.edu/zhu/ �
Low Alloy Steels
• Limitations of plain carbon steels:
Limited strength
Not deep hardenable
Low corrosion resistance (rust easily)
Rapid quenching leads to crack and distortion
Poor impact resistance at low temperature (Titanic)
• Alloy steels: add alloying elements like manganese,
nickel, chromium, molybdenum and tungsten.
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Classification of Alloy Steels
• First two digits: Principle alloying element.
• Last two digits: % of carbon. All are wt%
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Distribution of Alloying Elements
• Distribution depends upon compound and carbide forming
tendency of each element.
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Effects of Alloying Element on Eutectoid Temperature
• Mn and Ni lower eutectoid temperature.
• They act as austenite stabilizing
element.
• Tungsten, molybdenum
and titanium raise
eutectic temperature.
• They are called ferrite
stabilizing elements.
Stainless steels are
mostly austenite steels
They are softer
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Hardenability
• Hardenability: how easy the steel can be hardened by quenching.
• Hardenability depends on
Composition
Austenite grain size
Structure before
quenching
• Jominy hardenability test:
Cylindrical bar (1 inch dia and 4
inch length with 1/16 in flange
at one end is austenitized and one
end is quenched.
Rockwell C hardness is measured
up to 2.5 inch from quenched end.
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Cooling rate varies with position
•The hardenability test measures the
capability of a steel to be quenched
to form martensite
•Martensitic transformation is the
primary tool to obtain high strength
and high hardness in steel
Only for steel
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http://www.mse.ncsu.edu/zhu/ �
Aluminum Alloys
• Precipitation Strengthening (age hardening):
Creates fine dispersion of precipitated particles in
the metal to hinder dislocation movement.
• Basic steps :
Solution treatment: Alloy sample heated to and
held at a temperature to dissolve second-phase
particles to form solid solution
Quenching: Sample then quenched to room
temperature in water
freeze the solid solution
Aging: Solutionized and quenched sample is
then aged to form finely dispersed particles.
Form large # of fine particles, dispersed
Do not have
martensitic
transformation
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Decomposition Products Created by Aging
• Super saturated solid solution is in unstable condition.
• Alloy tends to seek a lower energy state by
decomposing into metastable or equilibrium phase.
•Aging: Heating the supersaturated solid solution at T> RT and hold to
form second phase particles
•Natural aging: leave it at the RT to form second phase particles
•The key is to form fine, uniformly dispersed particles
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Effects of Aging on Strength
• Aging curve: Plot of strength or hardness versus aging time.
• As aging time increases
alloy becomes stronger
harder and less ductile
– # of particles increases
• Overaging decreases
strength and hardness.
– Particles become larger
– # of particles decrease
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Example - Al 4% Cu Alloy
• Al -4% Cu is solutionized at about 5150C
• Alloy is rapidly cooled in water.
• Alloy is artificially aged in 130 – 1900C
• Structures formed :
GP1 Zone: At lower aging temperature, copper atom is segregated in supersaturated solid solution.
GP2 ( ´´ phase): Tetragonal structure, 10-100 nm diameter (coherent)
´ Phase: Nucleates heterogeneously on dislocation (incoherent)
Phase: Equilibrium phase, incoherent (CuAl2) (incoherent)
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Correlation of Structure and Hardness
• GP1 and GP2 Zones increases hardness by stopping dislocation movement.
• At 1300C when ’ forms, hardness is maximum.
• After ’ forms, GP2
zones are dissolved
and ’ gets coarsened
reducing hardness
•
GP1 Zone
GP2 Zone ( )
´
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An example of high strength and high ductility via
nanostructure and age-hardening
NS Al7075 after tensile testing
NS Al7075 before tensile testing
Zhao, et al, Advanced Mater. 18, 2280–2283 (2006).
NS: Nanostructured
NS+P: NS + particles
http://www.mse.ncsu.edu/zhu/ �
HW
Reading assignment for lecture after the Test 3:
10.1, 10.2, 10.4.1, 10.4.4