The situation where components have a limited solubility. Once the limit of solubility is exceeded ,phase separation takes place into two solid 'A-rich' and 'B-rich' phases.
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Define 'solvus'
It describes the limit of solid solubility in an equilibrium diagram.
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What is the invariant point?
The point where the two liquidus lines meet.
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Define the solidus line
Represents the lowest temperature at which a liquid phase may exit for any alloy at equilibrium.
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What is the eutectic reaction?
It occurs at the invariant point , when the liquid phase turns into the alpha and the beta phase.
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State what is the Eutectic isotherm?
The temperature at which the eutectic reaction takes place.
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What are the general rules of an eutectic system?
1) Only 1 or at most 2 phases are in equilibrium within a region;2)Along the solidus line all phases are in equilibrium;3) Single phase regions are separated by a two phase region
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Give an equation for the density of a phase(using the solder system as an example)
ρ _α = 100/(c_sn(α)/ρ _Sn)+(c_pb(α)/ρ _pb)
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What does the eutectic solid look like for Sn-Pb alloy?
A lamellar structure is adopted where dark layers are Pb rich α and the bright layers are Sn rich β.
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How does the eutectic solid form?
Through diffusion from liquid layer where Pb diffuses towards the α layer and Sn towards the β layer.
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Why does the eutectic structure form in alternating layers?
The atomic diffusion of Pb and Sn need only occur over relatively small distances.
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Give a distinct feature of the cooling curve for a eutectic alloy?
The temperature remains constant as the eutectic forms so the cooling curve looks similar to that of a pure element.
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How is a binary eutectic phase diagram created?
Cooling curves for several compositions of the alloy are put together and suitable gradients are found. (the ime axis is removed and a composition axis is added)
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What happens when equilibrium is not maintained whilst cooling through the alpha + liquid region?
1)Grains of primary alpha have a non-uniform distribution of solute across the grains;2) The fraction of the eutectic alpha formed will be greater than for equilibrium.
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Define a 'constituent'
A visually distinct region which does not have to be chemically homogeneous.
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Define Hypoeutectic
When the composition of an alloy places it to the left of the eutectic.
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Define Hypereutectic
When the composition is on the right hand side of the eutectic.
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Other cards in this set
Card 2
Front
Define 'solvus'
Back
It describes the limit of solid solubility in an equilibrium diagram.
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