Age hardening (metals)

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How does solid solution strengthening work?
Interaction with misfitinng solute atom with dislocation strain field inhibits dislocation motion. Small amounts of alloying elements distort the crystal lattice.
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What is age hardening?
The dispersion of fine ppt particles which act as obstacles to dislocation motion.(they either have to cut,climb or bow to get past particles)
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What are the two methods/types of Substitution?
1)Large atoms which create compressive stress;2) Small atoms which create a tensile stress.
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Explain solute hardening and how it can be optimised
Stress fields intersct wit dislocations:impedes mobility and increases alloy strength. Smaller atoms with high solubility would provide the most solid strengthening (as may strain fields would be created).
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Give the criteria for successful strengthening?
1)High room temperature solid solubility.2)Atomic misfit to create local compressive or tensile strains
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How can hardening be done through the addition of precipitates?
Pptation of alloying elements from supersaturated solid solution.The fine particles provide potent strength as they resist the motion of dislocations.
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What are the steps of adding precipitates into a metal?
1)Solution heat treatment;2)Rapid quenching;3) Ageing.
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Explain solution heat treatment (step 1)
Heating the alloy so that it is in a single phase region to dissolve all of the alloying elements into the matrix
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Explain Rapid quenching (step 2)
The matrix is cooled quickly to trap alloying elements in the matrix as a supersaturated solid solution.
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Explain Ageing (step 3)
Natural ageing: Alloying elements are precipitated at room temperature. Artificial ageing: The elements are ppt at a slightly elevated temperatures.
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Define 'nucleation' in terms of precipitation hardening
The formation of clusters of a precipitate phase
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What are the two things that the nucleus could turn into?
Coherent particle: the structure is similar to the matrix and there are some connections to the matrix; Incohrent particle: Has a different structure + an interface/boundary between the two.
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What is the driving force of the nucleation? (precipitation hardening)
The stored energy due to super saturation.
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What is the driving force opposed by?
Interfacial energy and Strain energy.
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Describe interfacial energy
It is required to create a new interface between the ppt nucleus and the matrix.
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Describe strain energy
It is required as the ppt nucleus will usually have a misfit with the matrix.
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Give the equation for the energy change when the precipitate forms.
ΔG_cluster =4/3 πr^3ΔG_v +ΔG_s+4πr^2γ ] where ΔG_v: driving force; ΔG_s:strain energy; γ:Interfacial energy.
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What does the equation assume? (delta_G_cluster)
That the nucleus is spherical with radius, r.
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Explain the relationship between the ppt size and the interfacial energy
The greater the size of the ppt the greater the interfacial energy. (as the surface surrounding the ppt gets larger also)
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What is the nucleation rate and what is it sensitive to/dependent on?
The number of new ppt forming per second. It is sensitive to the energy barrier.
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Give an equation for the energy barrier
G^* = 16/3 π ( γ^3/(ΔG_v +ΔG_s)^2
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How can a phase nucleate quickly?
If they have a reduced energy barrier to nucleation they will nucleate quickly even if they are not in the most stable phase.
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What kind of particle is likely to form first during age hardening?
Coherent particles.
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What is a metastable phase?
They have a low energy barrier to nucleation than the eqm ppt.and they nucleate more rapidly.
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How can metastable phases form?
The driving force for metastable precipitates are less than that of equilibrium precipitates. Thus after they form there is some driving force left for the formation of the eqm ppt (thus eqm is reach after a long time)
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What are the 3 different interface types?
1)Coherent; perfect matching of atoms across planes (lowest γ<0.2Jm^-2) 2)Semi-coherent:most atoms match but interfacial dislocations required to accommodate misfit (γ=0.2-0.5) 3)Incoherent: No matching of atoms across interface(γ=0.5-1)
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Give the typical precipitate sequence (and why it occurs)
Coherent-->Semi-coherent--> Inchorent (sequence progresses with increased interfacial energy and a decrease in the overall energy of the system).
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Why does precipitation hardening strengthen a material?
Ppt are barriers to dislocation movement.
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How are precipitates barriers to dislocations
Dislocations get past ppt by: cutting= small coherent ppt,bowing(going around ppt)=Incoherent ppt, cross slip and climbing (happens at high temperatures)
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How do small precipitates strengthen a material?
1)Coherency strain hardening: when strain field around ppt interacts with dislocation inhibiting motion;2)Chemical hardening: Where dislocations cut through ppt creating a new interface requiring extra stress.
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How do large precipitates strengthen a material?
1)Particle bowing:The stress required is determined by particle spacing;2)Climbing: Requires diffusion and is slow at low temperatures;3)Cross slipping:Only stops scre type dislocations. It is when they are moved into a new crystal plane.
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How do large precipitates strengthen a material?
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Other cards in this set

Card 2

Front

What is age hardening?

Back

The dispersion of fine ppt particles which act as obstacles to dislocation motion.(they either have to cut,climb or bow to get past particles)

Card 3

Front

What are the two methods/types of Substitution?

Back

Preview of the front of card 3

Card 4

Front

Explain solute hardening and how it can be optimised

Back

Preview of the front of card 4

Card 5

Front

Give the criteria for successful strengthening?

Back

Preview of the front of card 5
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