12 Age Hardening

Precipitation Hardening

orAge

Hardening

Consider an alloy where solid solubility decreases with decreasing temperature. The equilibrium structure is a q precipitate in a parent a phase

T

wt. % BA B

+

On the basis of dispersion hardening we would expect greater strength if we could create more interfacial surfaces. Therefore lets divide every particle and redistribute it – resulting in a Non-equilibrium structure

• We can attain this through a three-step heat treatment– Step 1: Solution treatment

• Heat into the single phase () region to erase the existing room-temperature structure and redissolve all B-atoms

• The temperature for this step is above the solvus but below the eutectic

• Hold to achieve chemical uniformity

T

wt. % BA B

+

X

– Step 2: Quench• Rapidly cool to suppress diffusion (no -phase will

form!)• Resultant is a supersaturated solid solution (’)

containing excessive amounts of dissolved B-atoms

T

wt. % BA B

+

– Step 3: Age• A controlled reheat within the 2-phase region

(temperature below the solvus) to activate diffusion• This produces a Coherent precipitate

T

wt. % BA B

+

Aging:

• Upon reheating the supersaturated B-atoms begin to cluster, but continue to maintain lattice positions within the parent phase structure– All planes and directions are continuous

throughout the cluster – hence, a “coherent” precipitate

Coherent Precipitate:

• Substantial distortion forms around a multi-atom cluster– This distortion acts to impede dislocation

movement numerous lattice spacings away– The coherent precipitate acts like a large barrier,

numerous times its actual size

Coherent Precipitate vs Solid Solution Strengthening

Overaging:• As the clusters continue to grow, the lattice is

further strained until it becomes energetically advantageous to break free and form a distinct second phase– phase particles form– Distinct - interfacial surfaces form– The particles now present a barrier or impedement of

their actual size.

• Coherency is lost– The material becomes weaker– This second phase formation is called “over aging”

Noncoherent and coherent precipitates

Overaging:Noncoherent and coherent precipitates

Example: 6061 Aluminum nails

http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6

Overaging:Noncoherent and coherent precipitates

Example: 6061 Aluminum nails

• Property variation during aging

Hardness

Aging time

Cluster formation and growth

Loss of coherency & onset of overaging

Strength vs Aging time

• Artificial aging– Requires elevated temperature to initiate aging

diffusion– Can age to peak strength conditions then drop

temperature, stop diffusion, and lock in the peak structure and properties

– Note: We must never reheat to diffusion temperatures because further diffusion will commence and over aging will occur with companion loss of strength

• Selecting an aging temperature– Lower temp – more ppt stronger– Lower temp – finer ppt stronger– Lower temp – longer aging time– Therefore we select the temp that produces

optimum properties in desired time

Hardness

Aging time

High T

Low T

Aging Temperature

• Natural aging– The diffusion required for aging can occur at

room temperature– Solution treat, quench (refrigerate in quenched

condition)– Ages when restored to room temperature

– EX: Aerospace rivets – head when soft but strengthen when in place!

• Requirements of age hardenable alloys– 1). Decreasing solubility with decreasing

temperature (must be able to cool from a single phase region through a solvus line to a two phase region)

– 2). Soft matrix with a strong, hard, possibly brittle precipitate

– 3). Precipitate must be coherent – 4). The alloy must be “quenchable”

• For age hardened materials, subsequent exposure to elevated temperature will result in loss of strength

• Consider the welding of an age hardened alloy