12 age hardening
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Age hardeningTRANSCRIPT
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Precipitation Hardening
orAge
Hardening
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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
+
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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
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• 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
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– 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
+
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– 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
+
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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
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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
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Coherent Precipitate vs Solid Solution Strengthening
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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”
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Noncoherent and coherent precipitates
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Overaging:Noncoherent and coherent precipitates
Example: 6061 Aluminum nails
http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6
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Overaging:Noncoherent and coherent precipitates
Example: 6061 Aluminum nails
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• Property variation during aging
Hardness
Aging time
Cluster formation and growth
Loss of coherency & onset of overaging
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Strength vs Aging time
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• 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
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• 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
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Aging Temperature
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• 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!
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• 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”
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• For age hardened materials, subsequent exposure to elevated temperature will result in loss of strength
• Consider the welding of an age hardened alloy