Phase diagrams (calculations and eqm vs non-eqm)

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What information can phase diagrams give?
1)The phases that are present; 2) The compositions of these phases; 3) The % and fraction of the phases
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How can phase concentrations be determined?
1)Draw a tie line across the two-phase region at the temperature of the alloy;2)Note the intersections of the tie line and the phase boundaries;3)Perpendiculars are dropped from the intersections to the composition axis which the composition is read.
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What are the steps in finding phase fractions?
Lever rule: 1) Locate the overall alloy composition on the tie line 2)Take the length of the tie line from the overall alloy composition to the phase boundary for the other phase and divide by the total tie line length.
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Give the equation for mass fraction (composition in wt%)
W_L =S/R+S = C_alpha-C_0/C_alpha-CL ; W_alpha +W_L =1
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State the equation for volume fraction
V_alpha = v_alpha/v_alpha+v_beta ] v:volume V:volume fraction
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how are volume fraction and mass fraction related?
v_alpha =W_alpha/p_alpha ] p:density
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Define equlibrium cooling
When cooling occurs slowly the phase equilibrium is continuously maintained
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What happens during non-equilibrium cooling?
Cooling rates are rapid thus equilibrium is not maintained. Diffusion rates in liquid phase are significantly faster than in solid phase.
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How does non-equilibrium cooling affact phase fractions?
There is a greater proportion of liquid present for non-equilibrium cooling. The solidus line is shifted to higher concentrations.
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How does the solidification temperature compare for non-equilibrium and equilibrium cooling?
Complete solidification occurs at a lower temperature.
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What are the consequences of non-equilibrium cooling?
There is: non-uniform distribution of the two elements in the grains; a concentration gradient across the grains,
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Describe the concentration gradient
The centre of the grains are rich in the high T_m element and the con. of the low T_m element increases with position from the grain centre to the grain boundary.
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What happens if the alloy created from non-equilibrium cooling is reheated?
Grain boundary regions would melt first resulting in a sudden loss of mechanical integrity.
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What is coring and when does it happen?
It happens when a heated alloy cools in non-equilibrium conditions. It is what produces the concentration gradient across the grains.(the conc. of the low T_m element.
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Other cards in this set

Card 2

Front

How can phase concentrations be determined?

Back

1)Draw a tie line across the two-phase region at the temperature of the alloy;2)Note the intersections of the tie line and the phase boundaries;3)Perpendiculars are dropped from the intersections to the composition axis which the composition is read.

Card 3

Front

What are the steps in finding phase fractions?

Back

Preview of the front of card 3

Card 4

Front

Give the equation for mass fraction (composition in wt%)

Back

Preview of the front of card 4

Card 5

Front

State the equation for volume fraction

Back

Preview of the front of card 5
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