1.17 Microstructure Development in an Off-Eutectic Alloy

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Из курса от партнера Georgia Institute of Technology

Material Processing

171 оценки

Georgia Institute of Technology

Material Processing

171 оценки

Have you ever wondered why ceramics are hard and brittle while metals tend to be ductile? Why some materials conduct heat or electricity while others are insulators? Why adding just a small amount of carbon to iron results in an alloy that is so much stronger than the base metal? In this course, you will learn how a material’s properties are determined by the microstructure of the material, which is in turn determined by composition and the processing that the material has undergone. This is the second of three Coursera courses that mirror the Introduction to Materials Science class that is taken by most engineering undergrads at Georgia Tech. The aim of the course is to help students better understand the engineering materials that are used in the world around them. This first section covers the fundamentals of materials science including atomic structure and bonding, crystal structure, atomic and microscopic defects, and noncrystalline materials such as glasses, rubbers, and polymers.

Из урока

Phase Diagrams and Phase Equilibria

This course picks up with an overview of basic thermodynamics and kinetics as they pertain to the processing of crystalline materials. The first module deals with phase diagrams - charts that tell us how a material will behave given a certain set of variables such as temperature, pressure, and composition. You will learn how to interpret common and complex phase diagrams and how to extract useful information from them.

1.11 Deviations from Ideal Behavior10:41

1.12 Eutectic Phase Diagram10:24

1.13 Determination of Phase Boundaries7:54

1.14 Eutectic Microstructure Development8:05

1.15 Equilibrium Cooling of an Off-Eutectic Alloy7:18

1.16 Equilibrium Cooling of an Off-Eutectic Alloy - Calculations6:08

1.17 Microstructure Development in an Off-Eutectic Alloy6:03

Познакомьтесь с преподавателями

Thomas H. Sanders, Jr.
Regents' Professor
School of Materials Science and Engineering

Welcome back.

In this lecture we are actually going to look at some

real microstructures of off-eutectic alloys.

The first alloy we're going to consider is the aluminum copper alloy, again.

In this particular case we have two phases that form behind the advancing liquid.

So we have three phases in equilibrium which is what we should have

at the eutectic temperature.

When the material comes all the way down in temperature for

an alloy which is off you tactic.

What we see in our microstructure

is a composite structure that is made up of the black areas.

Our solid solution alpha.

So that's aluminum with a certain amount of copper dissolved in it.

The background is made up of

the alpha phase plus the aluminum two copper, intermetallic compound.

Now, we're looking at this in a particular technique in

the SCM where the elements, like copper,

which are heavier than aluminum, they wind up in dark field being bright.

And the aluminum, which has a lower atomic number in comparison, is dark.

So, we see that microstructure of the Eutectic in the background

between the primary alpha phase that forms.

So this is exactly what we calculated in the previous lesson.

Now we're just going to look at the microstructure at a slightly higher

magnification.

So now what we've got is the dark regions are the alpha solid solution.

And in between those primary alpha phases we have

the material that is the liquid of

the composition that transformed into alpha and theta.

Now, if you go back to the picture that helped us come

up with the understanding of the way these structures develop,

one of the things that's very important for us to understand is that in order for

the lamellar structure that we've been describing to develop,

what we need to have is a cooperative diffusion of process.

Where A moves from in front of the beta phase to the alpha phase and

component B moves in the other direction.

And what happens then is at that solid/liquid interface,

what we're able to do is to produce the lamellar eutectic.

Now, the key that

must be maintained is that in order to have this cooperative diffusion.

We essentially have to have a plainer interface or a relatively flat interface

where the A and B can move in opposite directions.

So we don't want one of the phases, either the alpha phase or

the beta phase to advance.

Because as a result of one phase going faster

than the other phase that's forming.

This very pivotal co diffusion process can't occur.

Now as it turns out Not all of the microstructures

have this regular lamellar eutectic.

When we look at a material like aluminum silicon,

now recall aluminum is a simple metallic material.

Silicon, on the other hand, is not.

Silicon can behave like a covalently bonded material.

And when we look at the phase diagram ,what we see is the high melting

temperature, silicon.

The low melting temperature of aluminum.

So this is a Eutectic.

And as it turns out this is a very popular eutectic especially in engines and things

of that nature where what you need is a good hard face that's going to maintain

where in the particular automobile engine that you're working with.

So if we look at a blown up section of this phase diagram, on the aluminum-rich

side we actually see that there is some solubility for silicon and aluminum.

But looking at the larger diagram, over the entire composition range,

it's clear that there is essentially no solubility for aluminum and silicon.

Now when we look at our eutectic microstructure, it's going to appear

different than the eutectic microstructure that formed in the aluminum copper case.

What we have in the background is the white region.

And that white region represents the aluminum that formed as primary aluminum,

with a little bit of silicon dissolved in it.

If we look between those primary alpha grains,

what we're going to see is alternating dark and light.

The dark is representing the silicon, and the light is representing the aluminum.

So we have this structure analogous to the structure we have with aluminum-copper.

But if we look at higher magnification and we look at the details

in the aluminum silicon eutectic you find that we don't have a regular structure.

We have a silicon and

aluminum that have formed but they hadn't formed at the same rate.

And they form a structure that's not regular.

So some of the microstructures that happened with respect to systems that

are eutectic, they may solidify as the regular eutectic or

alternatives as the eutectics that's described in this particular micrograph.

Thank you.

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