Eutectic system

Summary

The script discusses the phase diagram of lead-tin solder alloy, traditionally used for soldering due to its low melting point. It explains the concept of solid solution strengthening in alloys, highlighting the melting points of lead (327°C) and tin (232°C). The lead-tin phase diagram reveals a unique eutectic point at 183°C, where a 62% tin alloy melts at a remarkably low temperature. The eutectic alloy solidifies at a single temperature, introducing new boundaries like the eutectic horizontal, solidus, and solvus in the phase diagram.

Takeaways

• 🔍 The script discusses the lead-tin solder alloy, traditionally common but now being phased out due to lead toxicity concerns.
• 🔨 Soldering requires a low melting point alloy for ease of melting, and the lead-tin alloy is an example of such a material.
• 💪 Alloys generally have better strength than pure elements, a concept known as solid solution strengthening.
• 🌡️ Lead has a melting point of 327 degrees Celsius, and tin has an even lower melting point of 232 degrees Celsius, making them suitable for alloying to create a low melting point solder.
• 📉 The addition of tin to lead is expected to lower the melting point of the alloy, which is beneficial for soldering applications.
• 📊 The lead-tin phase diagram is introduced, showing a non-linear relationship between composition and melting point.
• 📍 The phase diagram reveals a minimum melting point of 183 degrees Celsius for the lead-tin alloy at 62 weight percent tin.
• 🔬 The phase diagram includes unique features such as a non-continuous liquidus and two solidus lines, indicating the presence of two distinct solid phases, alpha and beta.
• 📐 The script explains the terms liquidus, solidus, and solvus, which are boundaries separating different phases in the phase diagram.
• 🧩 The eutectic point at 62 weight percent tin and 183 degrees Celsius is highlighted as a special composition where the alloy melts at a single, low temperature.
• 🌟 The eutectic alloy, eutectic composition, and eutectic temperature are defined, with the eutectic horizontal being a key feature in the phase diagram.

Q & A

Why is lead tin solder alloy being phased out in many contexts?

Lead tin solder alloy is being phased out due to concerns about the toxicity of lead.

What is the replacement for lead tin solder alloy?

Lead-free solder alloys are commonly used as a replacement.

Outlines

00:00

🔍 Study of Lead-Tin Solder Alloy and Phase Diagram Basics

This paragraph introduces the lead-tin solder alloy, traditionally used for soldering due to its low melting point, which is advantageous for the process. The text discusses the phasing out of lead due to toxicity concerns and the educational value of studying this alloy's phase diagram. It emphasizes the general rule that alloys have better strength than pure elements, a concept known as solid solution strengthening. The melting points of lead and tin are highlighted, and the expectation is set that combining these elements will result in an alloy with an even lower melting point, which is desirable for soldering applications. The paragraph ends with an introduction to the lead-tin phase diagram, explaining the axes and setting the stage for a deeper exploration of the alloy's properties.

05:04

📉 Understanding the Lead-Tin Phase Diagram and Eutectic Point

The second paragraph delves into the intricacies of the lead-tin phase diagram, correcting the initial linear approximation of the melting point and introducing the concepts of liquidus and solidus. It explains that the alloy's melting point is not a single temperature but occurs over a range, with the liquidus reaching a minimum at 183 degrees Celsius. The paragraph describes the phase diagram's unique features, including the non-continuous liquidus and solidus lines, and the presence of two distinct solid phases, alpha and beta. It also introduces the term 'eutectic', referring to the composition and temperature at which an alloy melts at the lowest possible temperature, and explains the term further.

Keywords

💡Lead tin solder alloy

A lead tin solder alloy is a mixture of lead and tin used for soldering, a process that joins metal parts together by melting the alloy and allowing it to flow into the joint. This alloy is significant in the video as it serves as a primary example for discussing phase diagrams and the properties of alloys. Traditionally, it was the most common solder alloy due to its favorable properties, but concerns over lead toxicity have led to its phasing out in many applications.

💡Phase diagram

A phase diagram is a graphical representation that describes the phase behavior of a system as a function of temperature and composition. In the context of the video, the lead-tin phase diagram is used to illustrate the melting points and phase transitions of the alloy at different tin concentrations. It is central to understanding the properties of materials and how they change with composition.

💡Toxicity of lead

The toxicity of lead refers to the harmful effects that lead can have on human health and the environment. In the script, it is mentioned as the reason why lead-based alloys like the lead tin solder are being phased out. This term is crucial as it relates to the environmental and health considerations in material selection and alloy development.

💡Solid solution strengthening

Solid solution strengthening is a process where the strength of a material is increased by adding a solute to a solvent, creating a solid solution. This concept is introduced in the video to explain why alloys generally have better strength than pure elements. It is exemplified by the lead tin solder alloy, where tin is added to lead to improve its mechanical properties.

💡Melting point

The melting point is the temperature at which a solid becomes a liquid. In the video, the melting points of lead (327 degrees Celsius) and tin (232 degrees Celsius) are discussed to illustrate how the addition of tin to lead can result in an alloy with an even lower melting point, which is desirable for soldering applications.

💡Liquidus

The liquidus is the line on a phase diagram that represents the temperature at which a mixture of liquid and solid begins to form. In the context of the lead-tin phase diagram, the liquidus is not a continuous line but has a minimum at 183 degrees Celsius, indicating the lowest temperature at which the alloy can be completely liquid.

💡Solidus

The solidus is the line on a phase diagram that represents the temperature at which the last solid phase melts to form a liquid. In the video, the solidus is described as breaking into two parts, each starting from the melting point of lead and tin, converging at the eutectic temperature of 183 degrees Celsius.

💡Eutectic alloy

A eutectic alloy is a specific composition within an alloy system that has a unique property of melting at a single, sharply defined temperature. The script mentions that at 62 weight percent tin, the lead-tin alloy has a eutectic composition that melts at a very low eutectic temperature of 183 degrees Celsius, making it an ideal material for soldering.

💡Eutectic temperature

The eutectic temperature is the specific temperature at which a eutectic alloy melts. In the video, it is highlighted that the eutectic temperature for the lead-tin alloy at 62 weight percent tin is 183 degrees Celsius, which is a critical point in the phase diagram and signifies the lowest melting point of the alloy.

💡Eutectic horizontal

The eutectic horizontal is a horizontal line on a phase diagram that represents the eutectic temperature at which the eutectic alloy melts. The video describes this as a significant feature in the lead-tin phase diagram, indicating the unique melting behavior of the eutectic alloy.

💡Solvus

The solvus is a boundary on a phase diagram that separates a single solid phase from a mixture of two solid phases. In the script, the solvus is introduced as a new boundary in the lead-tin system, where the alpha and beta solid phases coexist, and it delineates the regions where these phases are stable.

Highlights

The lead tin solder alloy is being phased out due to lead toxicity concerns, but remains a valuable system for studying phase diagrams.

Soldering requires a low melting alloy for easy melting, and alloys generally have better strength than pure elements due to solid solution strengthening.

Lead's low melting point of 327 degrees Celsius makes it a suitable candidate for soldering alloys.

Tin's even lower melting point of 232 degrees Celsius is used to further lower the alloy's melting point.

The lead-tin phase diagram is introduced as a more complex system than simple isomorphous diagrams.

The phase diagram reveals a non-linear relationship between alloy composition and melting point.

The lead-tin system exhibits a eutectic point at 183 degrees Celsius, which is a significant low melting temperature for the alloy.

The phase diagram shows a unique feature where the liquidus and solidus are not continuous lines but break into two parts, converging at the eutectic point.

Two distinct solid phases, alpha and beta, are present in the lead-tin alloy system.

The lever rule can be applied to determine the composition of two-phase regions in the alloy.

The phase diagram introduces new boundaries: liquidus, solidus, solvus, and the eutectic horizontal.

The eutectic alloy, composed of 62 weight percent tin, melts at a single low temperature of 183 degrees Celsius.

The eutectic composition and eutectic temperature are key concepts in understanding the phase behavior of the lead-tin system.

The eutectic horizontal represents the unique melting behavior of the eutectic alloy in the phase diagram.

The lead-tin phase diagram provides a more nuanced understanding of alloy melting and solidification compared to simpler systems.

The phase diagram's complexity highlights the importance of understanding alloy composition and phase behavior for material properties.

The study of the lead-tin system offers insights into the development of new low melting point alloys for various applications.

Transcripts

As an example we take up the lead tin solder alloy.

So, this is used for soldiering and conventionally this was the most common solder alloy of course,

now because the concern of toxicity of lead it is being phased out in many contexts. But

still for the study of phase diagram it constitutes a nice system or a nice alloy to look at.

So, basically in soldering if you think of soldering, you need an alloy which can melt

easily; what you need is a low melting alloy. And you need alloy rather than a pure component

or a pure element because you also need a strength, and you will see during the development

of this course that alloys always have better strength than pure elements .

This is a general rule, this is called solid solution strengthening and we will have more

time to discuss that later in the course. So, lead in it makes a nice candidate for

this because lead itself has a melting point of 327 degrees Celsius, which is reasonably

low if you compare it with your previous example of copper and tin which were both having more

than 1000 degrees celsius as their melting point, lead has only a melting point of 327

degrees Celsius. And it still to strengthen it we have an alloying addition and we select

another alloying addition which is even lower melting point . So, the melting point of tin

is only 232 degrees celsius. So, hopefully since we are adding a lower

melting point alloy in a higher melting point system, probably we will get an alloy which

may have even lower melting point, which will be even better on the point of view of soldering

because you want to heat heat it to melt it and this heating will be easier or melting

will be easier if the melting point is low. So, let us look at what the lead tin phase

diagram looks like. So, here I am going to draw a lead tin phase

diagram for you, you are now familiar with this box for our binary alloy the x axis is

components. So, since its a lead tin alloy. So, the components are lead and tin, and we

decide to use weight percent tin as our x axis.

So, since its lead tin system 0 percent tin denotes led, 100 percent tin denotes pure

tin and we have the temperature axis. Now we do not have to have temperature axis running

in 2000 degrees Celsius, highest temperature the melting point is only 327 that is of pure

lead. So, we have that on the lead axis and we have 232 which is the melting point of

tin on the tin axis. And you have already seen that now we remember that if we want

let us say if we want the melting point of alloy of 62 weight percent you will soon see

why I am selecting something odd like 62`, but suppose I want a melting point of 62 weight

person tin alloy. Then we have already seen that our first approximation which was a linear

approximation was not very right my line is not very straight. So, excuse me for that

maybe I will try something, but at this time. So, if I draw a straight line and if I try

to predict a melting point I I get something close, but that is not the truth because now

you know better that for alloy the melting point is not happening at or melting is not

happening at one unique temperature, but over a range of temperature.

So, you see you saw in the copper nickel diagram, that there was a liquidus and solidus. So,

you can predict something like this that you here also you will have a liquidus and solidus

and this gives you an alloy which will melt at a lower temperature than the higher melting

component lead. And this is intuitively obvious because we are adding tin. So, we are we were

thinking that we are adding low melting component to a high melting component and that is bringing

down the melting point from 327 to somewhat lower temperature. However, in reality you

get even better result you get more than your expectation and the liquidus is not a continuous

line like this in the lead tin system if you see, the liquidus reaches a minimum at 183

degrees celsius . The liquidus is not a continuous line, it

comes in two parts one is starting from the melting point of lead and one is starting

from the melting point of tin and both converging at to a minimum point which is 183 degrees

celsius. So, this is very very interesting, this is more interesting than a continuous

liquidus like that and what does the solidus do? Solidus also makes an interesting term

and solidus actually breaks into two different solidus instead of a continuous solidus.

Now, you have two different solidus and they end at the same temperature of 183 degrees

Celsius. And then at 183 degree celsius you have a horizontal line going through this

minimum of the liquidus from one solidus end to another solidus end and finally, to complete

the phase diagram, you have two more lines two new lines which you have not seen in an

isomorphous diagram extending from this end to this end.

So, you can see this is a somewhat more complicated, but at the same time more interesting phase

diagram than the simple isomorphous diagram. So, since the earlier isomorphous expectation

is superimposed on this and is confusing the diagram let us look at the diagram in its

own. So, what we have seen is that you have liquidus

in two components. So, let us write down those boundary names

. So, you have liquidus we have not yet defined liquidus, but we can do that. So, liquidus

is any boundary which separates a single phase liquid with

a mixture of liquid and solid. So, this is a liquid liquid plus alpha phase boundary.

This is liquid plus liquid liquid and liquid plus beta phase boundary I have not yet mentioned

the phases. So, let me first label the phases in this diagram.

So, above the liquidus you have a liquid. These two end triangular regions are solid

phases and now you have two distinct solid phases. So, unlike isomorphous diagram where

a single phase was extending from one end to the other end, now you have two different

solid phases in the same alloy system. So, near the lead end you have alpha solid phase

and near the tin and you have the beta solid phase. These are the single phase field and

if you apply your lever rule you will get other two phase regions here. So, you can

see here one end alpha another end liquid. So, this is alpha plus liquid region here

one side liquid another side beta. So, this is liquid plus beta region and here one side

alpha one side beta. So, you have alpha plus beta region. So, all

the phase fields are labeled and now I am labeling the boundaries for you. So, this

is the liquidus boundary these boundaries are

the solidus. So, this is solidus and this is also solidus. So, solidus is a boundary

between single phase solid and solid plus liquid phase. So, any boundary.

So, we had seen liquidus and solidus in the isomorphous system the copper nickel system,

they are also the same definition is valid and when I say alpha not necessarily that

it has to be alpha then you can see here that this is a boundary between beta and liquid

plus beta and that is also solidus. So, alpha represents any solid. So, a solid with above

a boundary between a solid and the same solid plus liquid boundary will be called solid

as in any phase diagram. And finally, you have here these extra lines which was not

there extra in the sense of in comparison to the isomorphous system, they were not there

because you had a single solid phase, but now you have two solid phases.

So, there are boundaries which are delineating that these lines are called solvus . So, I

have solvus which is a boundary between a solid phase and then the mixture of two solids

alpha alpha plus beta. Of course, beta alpha plus beta is also another solvus there only

one boundary is not yet labeled and that is this horizontal line which is a very important

line in this diagram and this we call the eutectic horizontal eu tae tic eutectic horizontal.

So, you have liquidus, you have solidus, you have solvus and you have eutectic horizontal

the name eutectic comes from the meaning of eutectic is easy melting .

So, in this alloy system there is one composition and that is the 62 weight percent tin thats

why I had started at 62 remember. So, at 62 weight percent tin you have an alloy which

will melt at a very low temperature . So, if I take this alloy if I draw the composition

vertical at that alloy, you can see that this is liquid up to 183 degrees celsius and then

it will become solid. So, this alloy solidifies although its an alloy its solidifies as a

at a single temperature. So, this is specific alloy is called a eutectic alloy eutectic

alloy this particular vertical in eutectic alloy this composition. So, correspondingly

this composition at which the eutectic alloy exists this is called a eutectic composition

the temperature at which the eutectic alloy melts that will be called a eutectic temperature

. So, you have a eutectic alloy, eutectic composition

and eutectic temperature and the phase diagram has a horizontal line at the eutectic temperature

that is called the eutectic horizontal . So, in boundaries I have one more boundary, which

is called eutectic horizontal . So, you had met liquidus and solidus in the isomorphous

system also, the eutectic system gives you two more boundaries to new kinds of boundaries

the solvus boundary and the eutectic horizontal.