Dispersion Strengthening and Eutectic Phase Diagrams Chapter 10 – 4th Edition Chapter 11 – 5th Edition

In the last chapter… We talked about alloys that form from materials that are completely soluble in each other We also learned not all materials are completely soluble In this chapter we’ll talk about what happens when there are multiple solid phases present

Dispersion Strengthening The existence of two or more phases can strengthen a material When that happens it’s called dispersion strengthening Matrix – present in large amounts Precipitate – present in smaller amounts

Guidelines Matrix should be soft to provide ductility Precipitate should be hard to provide strength – more precipitate results in higher strength Precipitate should be discontinuous Precipitate Matrix

Guidelines Smaller, more numerous precipitate particles give higher strength because they interfere with slip Round particles are better than sharp particles – Sharp particles can initiate a crack Good Not so Good

Guidelines Smaller, more numerous precipitate particles give higher strength because they interfere with slip Round particles are better than sharp particles – Sharp particles can initiate a crack Good Not good at all

Intermetallic Compounds Often precipitates are intermetallic compounds – metals that are bonded to each other Stoichiometric Compounds Non-stoichiometric Compounds have a range of compositions (often a blend of two or more stoichiometric compounds)

3-Phase Reactions In order to have a precipitate we must have 2 solid phases Matrix Precipitate There are a number of ways to create these phases – but we are going to look at the following 3-Phase reactions Eutectic L-> S1 + S2 (this chapter) Eutectoid S1 -> S2 + S3 (next chapter)

Let’s start with the Eutectic L → S1 + S2

Eutectoid S1 S2 S3 S2 + S3 S1 → S2 + S3

The Eutectic Phase Diagram Temperature Wt% Y X Y Liquid a b Eutectic Solidus Solidus Liquidus a + L b + L Solvus Solvus a + b

The Eutectic Phase Diagram Temperature Wt% Y X Y Liquid a b a + L b + L a + b In this portion of the diagram only a 2 phase reaction occurs – similar to what we observed in Chapter 9 The strengthening mechanism is solid solution strengthening

Lead – Tin Phase Diagram Temperature Wt% Y X Y Liquid a b a + L b + L a + b Pb Sn Lead – Tin Phase Diagram Amount of a Amount of b Sn

Cooling Curve Liquid Liquid + a Temperature a a + b Time

Solidification of a Lead-Tin Alloy Lead – Tin Phase Diagram Lead – Tin Cooling Curve Liquid Liquid Liquid + a Temperature a a + L Temperature a b + L b a + b a + b Amount of a Amount of b Wt% Sn Pb X Sn Y Time

How Does the Solid Form? We’ll talk in the next chapter about how to manipulate the particle growth to achieve the optimum distribution of the precipitate Liquid a L + a a + b This alloy is dispersion strengthened

Lead – Tin Phase Diagram Temperature Wt% Y X Y Liquid a b a + L b + L a + b Pb Sn Lead – Tin Phase Diagram Eutectic Amount of b Amount of a Sn

Cooling Curve for a Hypoeutectic System Liquid Liquid + a a + b Temperature L + a + b Time

Lets not look at how the solid forms just yet – it’s a little complicated Instead lets look at what happens if your over all composition is the eutectic composition

Temperature Wt% Y X Y Liquid a b a + L b + L a + b Pb Sn Lead – Tin Phase Diagram Eutectic Composition

Cooling Curve for a Eutectic System Plumber’s solder is a eutectic alloy of Pb and Sn. Why? Liquid a + b L + a + b Temperature Low, sharp melting point Time

How Does the Eutectic Solid Form? Liquid L + a + b Eutectic Solids are strong but generally have little ductility

Strength of Eutectic Alloys Each phase is solid solution strengthened Grain size affects the strength – well inoculated melts have smaller grain size Interlamellar spacing

Cobalt-Carbon Eutectic Scanning electron microscope image of cobalt-carbon eutectic. There is an irregular arrangement of graphite needles in a cobalt rich-phase matrix. http://www.npl.co.uk/server.php?show=conMediaFile.1613

Interlamellar Spacing The spacing is controlled by how long the grains are allowed to grow You can limit the spacing by reducing the solidification time – by removing heat faster Small interlamellar spacing results in high strength Interlamellar Spacing

Lead – Tin Phase Diagram Temperature Wt% Y X Y Liquid a b a + L b + L a + b Pb Sn Lead – Tin Phase Diagram Eutectic Amount of b Amount of a Let’s go back to the hypo eutectic composition Sn

How does solidification occur in a hypoeutectic system? L + a + b Liquid L + a Primary Phase is a (Proeutectic) Eutectic Microconstituent

Pb-Sn HypoEutectic Composition This image is 60-40 lead-tin solder showing the dark dendrites of primary lead surrounded by Pb-Sn eutectic. Scale bar is 667 micrometers Used with permission of Ruth I. Schultz Kramer Scientist, Dept. of Materials Science and Engineering, Michigan Technological University

Higher magnification of solder showing varying structure of the Pb within the two phase Pb-Sn eutectic, which surrounds the primary lead dendrites. Scale bar is 100 micrometers long. Used with permission of Ruth I. Schultz Kramer Scientist, Dept. of Materials Science and Engineering, Michigan Technological University http://www.mse.mtu.edu/slides/slide_2.html

Lead – Tin Phase Diagram Temperature Wt% Y X Y Liquid a b a + L b + L a + b Pb Sn Lead – Tin Phase Diagram Eutectic As the alloy cools from the eutectic temperature, there is a rearrangement WITHIN each microconstituent of the amount of  and β Amount of Eutectic Microconstituent Amount of a existing as the proeutectic or primary microconstituent Sn

Lead – Tin Phase Diagram Temperature Wt% Y X Y Liquid a b a + L b + L a + b Pb Sn Lead – Tin Phase Diagram As the alloy cools from the eutectic temperature, there is a rearrangement WITHIN each microconstituent of the amount of  and β Eutectic The amount of β existing in the eutectic microconstituent changes with temperature The amount of  existing in the eutectic microconstituent changes with temperature Sn

Lead – Tin Phase Diagram Temperature Wt% Y X Y Liquid a b a + L b + L a + b Pb Sn Lead – Tin Phase Diagram The PRIMARY microconstituent has no beta at the eutectic temperature, but particles grow within in it as the temperature decreases Eutectic Amount of β in the primary phase Amount of  in the primary phase Sn

50-50 alloy of Pb and Bi - HypoEutectic Note: This is lead and bismuth – but the idea is the same Eutectic Microconstituent Primary – Lead rich phase Used with permission of Ruth I. Schultz Kramer Scientist, Dept. of Materials Science and Engineering, Michigan Technological University http://www.mse.mtu.edu/slides/slide_2.html

Temperature Wt% X Y Liquid a b a + L b + L a + b Pb Sn Lead – Tin Phase Diagram Hypereutectic Composition

What happens during the solidification of a hypereutectic system? L + a + b Liquid L + b Primary Phase is b Eutectic Microconstituent

Which is Best? It depends on your design requirements Eutectic Hypoeutectic Hypothetical Alloy Dispersion Strengthening Hypereutectic Solid Solution Strengthening Strength Composition

Let’s do some calculations Assume you have an alloy that is 40 wt% Sn 60 wt% Pb What is the composition at room temperature of the  (Pb rich) phase? β (Sn rich) phase? What fraction of the alloy is ? β ? What fraction is Eutectic microconstituent? Proeutectic microconstituent?

This version of the phase diagram will be easier to use At room temperature the Pb rich phase is approximately 2% Pb At room temperature the Pb rich phase is approximately 2% Sn At room temperature the Sn rich phase is approximately 100% Sn

Fraction of each phase At room temperature the Pb rich phase is approximately 2% Pb  Fraction Sn rich phase (β) Fraction Pb rich phase ()

Fraction of each microconstituent At room temperature the Pb rich phase is approximately 2% Pb  Fraction Proeutectic microconstituent Fraction Eutectic microconstituent

Other Phase Diagrams Containing 3-Phase Reactions All we’ve looked at are phase diagrams with a eutectic Remember, a eutectic is a point where L-> a + b There are lots of other possible 3 phase reactions, and lots of much more complicated phase diagrams

3-Phase Reactions Eutectic L-> S1 + S2 Eutectoid S1 -> S2 + S3 Peritectic S1 + L1 -> S2 Peritectoid S1 + S2 -> S3 Monotectic L1 -> S1 + L2 We will be primarily concerned with Eutectic and Eutectoid Reactions

Consider the following hypothetical phase diagram taken from Askeland (pg 270)

L1 Monotectic d Eutectic L2 Peritectic g Temperature b Eutectoid a g + d L1 + g L2 L1 + L2 L2 + b L2 + g g b Temperature g + b Eutectoid a + g a Peritectoid a + b a + m m m + b X Y

Three Component Phase Diagrams So far we have only looked at 2 component alloys Three component phase diagrams can be very complicated – they require a three dimensional phase diagram A 2-D slice can tell us a lot though

Z 80 % 60 % 40 % 20 % Wt% X 80% 60% 40% 20% 20% 40% 60% 80% Wt % Y Wt% Z Y X

Ternary Phase Diagrams Used to plot the liquidus An isothermal plot shows the phases present

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