precipitation hardening

Precipitation Hardening

Dr. H. K. KhairaProfessor in MSME

MANIT, Bhopal

Precipitation Hardening (or Age Hardening)

• Precipitation hardening is commonly used to process aluminum alloys and other nonferrous metals for commercial use. The examples are aluminum-copper, copper-beryllium, copper-tin, magnesium-aluminum, and some ferrous alloys

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Precipitation Hardening• the 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 is accomplished by appropriate heat treatments.• the process is called precipitation hardening because the small

particles of the new phase are termed "precipitates”.

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Precipitation Hardening• “age hardening" is also used to designate this procedure because

the strength develops with time, or as the alloy ages at designated temperatures below the “solvus” temperature.

• alloys that are hardened by precipitation treatments include Al-Cu, Cu-Be, Cu-Sn, and Mg-Al; and some ferrous alloys.

Solvus curve

Solvus curve

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Precipitation Hardening• Mechanism of Hardening:• During plastic deformation:– Zones or precipitates act as obstacles to dislocation

motion– Stress must be increased to “push” the dislocation

through the distribution of precipitates.– Consequently the alloy becomes harder and

stronger.

Precipitation Hardening in the First Aerospace Alluminum Alloy: The Wright Flyer Crankcase

• Aluminum has had an essential part in aerospace history from its very inception. An aluminum copper alloy (with a Cu composition of 8 wt%) was used in the engine that powered the historic first flight of the Wright brothers in 1903.

Modern AircraftMig–29

http://en.wikipedia.org/wiki/File:Mig-29_on_landing.jpg

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Age hardening - A special dispersion-strengthening heat treatment. By solution treatment, quenching, and aging, a coherent precipitate

forms that provides a substantial strengthening effect. Also known as precipitation hardening, it is a form of dispersion strengthening.

Age or Precipitation Hardening

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Coherent precipitate - A precipitate whose crystal structure and atomic arrangement have a continuous relationship with the matrix from which the precipitate is formed.

Coherent precipitate

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Precipitation Hardening• A composition that

can be precipitation hardened contains two phases at room temperature, but can be heated to a temperature that dissolves the second phase.

Al – Cu Equilibrium Diagram

http://www.substech.com/dokuwiki/lib/exe/fetch.php?cache=cache&media=age_hardening.png

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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

The aluminum-copper phase diagram and the microstructures that may develop curing cooling of an Al-4% Cu alloy.

Mechanism of Precipitation Hardening

• Formation of very small particles of a second, or precipitate, phase.

• During precipitation hardening, lattice strains are established at the precipitate-matrix interface.

• There is an increased resistance to dislocation motion by these lattice strains in the vicinity of the microscopically small precipitate particles.

http://www.substech.com/dokuwiki/lib/exe/fetch.php?cache=cache&media=age_hardening.png

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Mechanism of Hardening

Supersaturated α solid solution

Zones or precipitate phase (aging ) with lattice distortion

Equilibrium phase (Overaging) without distortion

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(a) A noncoherent precipitate has no relationship with the crystal structure of the surrounding matrix. (b) A coherent precipitate forms so that there is a definite relationship between the precipitate’s and the matrix’s crystal structure.

Lattice Strain

HardeningDue to Coherent Precipitate

Hardening toOver Ageing

Precipitation Hardening

• Small inclusions of secondary phases strengthen material

• Lattice distortions around these secondary phases impede dislocation motion

• The precipitates form when the solubility limit is exceeded

• Precipitation hardening is also called age hardening because it involves the hardening of the material over a prolonged time.

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Step 1: Solution Treatment Step 2: Ageing

Guinier-Preston (GP) zones - Tiny clusters of atoms that precipitate from the matrix in the early stages of the age-hardening process.

Microstructural Evolution in Age or Precipitation Hardening

Steps in Precipitation Hardening

• Precipitation hardening is accomplished by two steps1. Solution heat treatment

• During solution heat treatment all solute atoms are dissolved to form a single-phase solid solution Quenching or rapid cooling to room temperature to form a nonequilibrium supersaturated solid solution (to prevent diffusion and the accompanying formation of any second phase)

2. Ageing• The supersaturated solid solution is heated to an intermediate

temperature within the two-phase region. at this temperature diffusion rates become appreciable. The precipitates of the second phase form as finely dispersed particles.

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Precipitation Hardening• The Process:• Solution treatment, in

which the alloy is heated to a temperature above the solvus line into the alpha phase and held for a period sufficient to dissolve the beta phase.

• Quenching to room temperature to create a supersaturated solid solution

• Precipitation Treatment; alloy is heated to a temperature below Ts to cause precipitation of fine particles of beta phase.

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The aluminum-rich end of the aluminum-copper phase diagram showing the three steps in the age-hardening heat treatment and the microstructures that are produced.


• By quenching and then reheating an Al-Cu (4.5 wt%) alloy, a fine dispersion of precipitates form within the Grains.

• These precipitates are effective in hindering dislocation motion and consequently, increasing alloy hardness and strength

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Precipitation Hardening• aging can also occur at room temperature for some alloys

(natural aging).• Data represented as hardness or tensile strength vs aging time

(log scale) for T-constant.• Yield Strength increases as zones or precipitates form• Strength reaches a peak value and then decreases (overageing)

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Ageing

Effect of Ageing Temperature on Strength

Effect of Ageing Temperature on Ductility

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Compare the composition of the a solid solution in the Al-4% Cu alloy at room temperature when the alloy cools under equilibrium conditions with that when the alloy is quenched.

Composition of Al-4% Cu Alloy Phases


Figure 1 - The aluminum-copper phase diagram and the microstructures that may develop during cooling of an Al-4% Cu alloy.

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SOLUTION

In Figure - 1, a tie line can be drawn at room temperature. The composition of the α determined from the tie line is about 0.02% Cu. However, the composition of the α after quenching is still 4% Cu. Since α contains more than the equilibrium copper content, the α is supersaturated with copper.

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Effects of Aging Temperature and Time

The effect of aging temperature and time on the yield strength of an Al-4% Cu alloy.


Overaging in Precipitation Hardening:

• With increasing time, the strength or hardness increases, reaches a maximum, and finally diminishes.

• This reduction in strength and hardness that occurs after long time periods is known as overaging.

• Diagram shows strength as a function of the logarithm of aging time at constant temperature during the precipitation heat treatment.

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The operator of a furnace left for his hour lunch break without removing the Al-4% Cu alloy from the furnace used for the aging treatment. Compare the effect on the yield strength of the extra hour of aging for the aging temperatures of 190oC and 260oC.

Effect of Aging Heat Treatment Time on the Strength of Aluminum Alloys


Fig – 2 The effect of aging temperature and time on the yield strength of an Al-4% Cu alloy.

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SOLUTIONFrom Fig – 2

At 190oC, the peak strength of 400 MPa (60,000 psi) occurs at 2 h (Figure 11.13). After 3 h, the strength is essentially the same.

At 260oC, the peak strength of 340 MPa (50,000 psi) occurs at 0.06 h. However, after 1 h, the strength decreases to 250 MPa (40,000 psi).

Thus, the higher aging temperature gives lower peak strength and makes the strength more sensitive to aging time.

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The magnesium-aluminum phase diagram is shown in Figure. Suppose a Mg-8% Al alloy is responsive to an age-hardening heat treatment. Design a heat treatment for the alloy.

Design of an Age-Hardening Treatment


Fig – 3 Portion of the aluminum-magnesium phase diagram.

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SOLUTION

Fig – 3

Step 1: Solution-treat at a temperature between the solvus and the eutectic to avoid hot shortness. Thus, heat between 340oC and 451oC.

Step 2: Quench to room temperature fast enough to prevent the precipitate phase β from forming.

Step 3: Age at a temperature below the solvus, that is, below 340oC, to form a fine dispersion of β phase.

Requisite Features on Phase Diagrams for Precipitation Hardening:

An appreciable maximum solubility of one component in the other, of the order of several percent.

The alloy system must display decreasing solid solubility with decreasing temperature.

The matrix should be relatively soft and ductile, and the precipitate should be hard and brittle.

The alloy must be quenchable. A coherent precipitate must form.

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Use of Age-Hardenable Alloys at High Temperatures

Typical Precipitation Hardened Alloys

• Al 2014 Forged Aircraft Fittings, Al Structures 2024 High strength forgings, Rivets 7075 Aircraft Structures, Olympic Bikes Cu Beryllium Bronze: Surgical Instruments, Non sparking tools, Gears Mg AM 100A Sand Castings AZ80A Extruded products Ni Rene' 41 High Temperature Inconel 700 up to 1800F Fe A-286 High Strength Stainless 17-10P

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Figure 11.14 Microstructural changes that occur in age-hardened alloys during fusion welding: (a) microstructure in the weld at the peak temperature, and (b) microstructure in the weld after slowly cooling to room temperature.

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