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

http://www.mse.ncsu.edu/zhu/ �

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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http://www.mse.ncsu.edu/zhu/ �

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

http://www.mse.ncsu.edu/zhu/ �

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

http://www.mse.ncsu.edu/zhu/ �

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