Diffusional Transformations

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En provenance du cours de University of California, Davis

Materials Science: 10 Things Every Engineer Should Know

724 notes

University of California, Davis

Materials Science: 10 Things Every Engineer Should Know

724 notes

We explore “10 things” that range from the menu of materials available to engineers in their profession to the many mechanical and electrical properties of materials important to their use in various engineering fields. We also discuss the principles behind the manufacturing of those materials. By the end of the course, you will be able to: * Recognize the important aspects of the materials used in modern engineering applications, * Explain the underlying principle of materials science: “structure leads to properties,” * Identify the role of thermally activated processes in many of these important “things” – as illustrated by the Arrhenius relationship. * Relate each of these topics to issues that have arisen (or potentially could arise) in your life and work. If you would like to explore the topic in more depth you may purchase Dr. Shackelford's Textbook: J.F. Shackelford, Introduction to Materials Science for Engineers, Eighth Edition, Pearson Prentice-Hall, Upper Saddle River, NJ, 2015

À partir de la leçon

Making Things Fast and Slow / A Brief History of Semiconductors

Welcome to week 5! In lesson nine we’ll deal with how to make things fast and slow. We’ll examine the lead-tin phase diagram and look at its practical applications as an example of making something slowly. Then we’ll evaluate the TTT diagram for eutectoid steel, and compare diffusional to diffusionless transformations with the TTT diagram, monitoring how we make things rapidly. Lesson ten is a brief history of semiconductors. Here, we discuss the role of semiconductor materials in the modern electronics industry. Our friend Arrhenius is back again, and this time we’re applying the Arrhenius Relationship to both intrinsic and extrinsic semiconductors. We’ll also look at combined intrinsic and extrinsic behavior.

Introduction to Phase Diagrams10:19

The Lead-Tin Phase Diagram1:47

The Competition Between Instability and Diffusion4:13

The TTT Diagram for Eutectoid Steel5:42

Diffusional Transformations3:12

Diffusionless Transformations4:21

Rencontrer les enseignants

James Shackelford
Distinguished Professor Emeritus
Department of Chemical Engineering and Materials Science

Next, let's develop the full story of microstructural

development in this eutectoid steel

over a wide temperature range where diffusion is the dominant factor.

However, now we're getting into more subtlety as we start to talk about heat

treatment and varying these rates.

Let's say that we now, back to the black ink.

Let's say that we now cool down to about 500 degrees Centigrade, very rapidly.

What would happen there upon cooling,

we're going to get a somewhat different story.

It will be basically the same overall result, but because

we're now holding at 500 degrees Centigrade,

what we see is we end up now with a fine perlite at

the end because that diffusion coefficient is starting to play a role now.

It's taking longer for the atoms to re-ring

themselves from this single uniform homogeneous

phase of the 0.77 percent carbon in solid solution.

To this very carbon-depleted alpha iron ferrite and

then the carbon-rich cementite compound.

So, in this case

even though the cross points here are somewhat different here.

We're getting a more rapid onset of the product formation,

50% point occurring earlier in the final completion of the formation

of the complete microstructure is fine perlite, is created more rapidly.

The important point is that the structure becomes finer as we go down to

lower temperatures.

And then as we go further still, let's say we go down to 300 degrees Centigrade.

Round out the diffusional transformation story.

Now we'll do a, theoretically, instantaneous crunch down to 300 degrees

Centigrade, [COUGH] and then hold, This for an extended period of time.

We'll see now that diffusion is really dominating the story.

And so it takes us much longer to get the incipient formation

of another phase, another phases.

Longer still for the 50% point, and then longer still for

the completion of the formation.

But now what we see, of course, is that we really don't have

time to form that very nice lamellar

alternating layers of perlite, but the somewhat

busier microstructure of bainite,

and finer yet dispersion of alpha and Fe3C.

The important thing is we go to increasingly fine grained structures.

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