Metallic Grain Sizes

[NaukowiecGirl]

Hello, my chemistry topic is all about Metallic substances and their properties. I read that Annealing (which is heating just below melting point and allowing it to cool slowly) causes the metallic grains to expand in size. Why is that? Like, does heating a metal decrease grain size or something? What does heating do the the metallic structure? We're not being told why it happens, we're told it just does. Please help.

[studiot]

 

Hello, my chemistry topic is all about Metallic substances and their properties. I read that Annealing (which is heating just below melting point and allowing it to cool slowly) causes the metallic grains to expand in size. Why is that? Like, does heating a metal decrease grain size or something? What does heating do the the metallic structure? We're not being told why it happens, we're told it just does. Please help.

 

 

First some general advice.

 

This does not appear to me to be a homework question asking us to do your homework.

You would likely find more answers in the appropriate secience sections as many members do not bother with the homework section.

 

 

Now some facts.

 

A collection of small crystals has a larger surface area than a few large crystals of the same mass.

The larger the surface area the more energy the stuff has, due to increased surface energy.

 

Since matter seeks the lowest available energy state large crystals are energetically preferred.

This applies to all crystals, not just metallic ones.

So given the opportunity, small crystals will join together to form larger ones. (I will return to this)

 

However there is another mechanism at work.

When crystals form, they start to grow from what are called nucleation sites.

A nucleation site is a small zone in the cooling liquid that has dropped below melting temperature, or perhaps an impurity, or a concentration excess in a solution.

Either way a solid crystal starts to form and grow from that site.

The faster you cool the liquid, the more sites there are so greater numbers of smaller crystals form.

That is why you need to grow crystals slowly to achieve large ones.

 

Back to metal that has been solidified rapidly and therefore has small crystals.

The grains may be viewed as lots of crystals with different orientations 'stuck' together to form the metal.

As noted above, the smaller the crystals (grains) the larger their surface area so the seek to lower the energy by coalescing.

Gentle heat and slow cooling injects just enough activation energy to permit this.

 

Does this help?

[NaukowiecGirl]

Yay! Thank you so much! This does help a lot! I never knew anything like this. Thank you again. But may I ask, why does a substance want to achieve a lower surface energy? I understand that liquids like water will combine in order to reduce their mass:surface ratio, resulting in lower surface energy, but why would a substance want that? This may sound absolutely ridiculous, but has it got something to do with the activation energies of electrons in orbitals? (electrons want the lower energy shells)

[studiot]

No it is not to do with activation energy.

 

There are two main drivers in this universe.

 

1) The tendency of all things to move towards their lowest energy state.

 

This is why objects fall - they lower their potential energy

This is the condition for spontaneous chemical reactions and nuclear to occur

and so on

 

This is known as the minimum energy criterion.

 

2) The more difficult tendency of all things to move to spread out the available energy over the entire system

 

This is why sugar dissoves in water

This is why heat spreads out towards a uniform state

This is why osmosis, diffusion, electrophoresis and similar processes happen

This is why gases mix

 

This is known as the maximum entropy condition.

Entropy is a measure of the 'spread outness' of the energy.

 

Sometimes these two drivers work in the same direction, sometimes they work in opposition and a balance must be struck and sometimes only one is at work.

 

 

In many processes in order to commence the process energy must first be input to finally achieve lower energy.

This energy is called activation energy and is always lower than the final output or the process will not happen, even with the activation energy.

 

A simple example is a ball inside a bowl on a table.

 

The ball would move to a lower energy if it rolled off the table and fell to the floor, but it needs to have (activation) energy input to lift it over the sides of the bowl first.

The higher these sides, the greater the required activation energy.

 

Another example is the energy of the match we use to light a fire.

The energy we get out when the fire is burning is the result of the fuel moving to lower energy states, but this does not happen without the activation energy of the match.

So the stick or piece of paper can carry on for a very long time, perhaps indefinitely withour burning until that activation energy is supplied.

[NaukowiecGirl]

Ohh ok.... makes sense. So for processes, both chemical and physical, to achieve their lowest energy state, they must first receive a larger quantity of energy that will eventually reduce to zero once at its lowest state. And that is called activation energy. But I'm confused on entropy. Web sites have said that entropy measures the unavailable amounts of energy and that it measures the 'randomness' of the movement of particles. I'm confused about the meaning, what does it mean to measure unavailable energy? Thank you too for your help, I really appreciate it!!!

[studiot]

 

they must first receive a larger quantity of energy that will eventually reduce to zero once at its lowest state.

 

Not quite.

 

The energy state or total energy is larger but the activation energy is usually small compared to the total energies as in the diagram.

 

 

Look again at my description of the ball in the bowl.

 

The potential energy on the table is much larger than the activation energy needed to get the ball out of the bowl.

 

This situation is very important in chemistry since most reactions involve an activation energy.

Some types of catalysts work by reducing this activation energy.

 

I will leave discussion of entropy to another post since it is also important but a very large subject by itself and will involve several diagrams.

 

I am glad to see someone more interested in real science than some of the sillyness we see in other threads.

 

Keep it up.

 

Edited March 30, 2016 by studiot

[NaukowiecGirl]

Ohh my goodness thank you so so much!! I finally understand it now; the gravitational potential energy is greater than the activation energy because it has more distance and thus energy to it compared to the small wall of the bowl which although requires some energy, it does not compare to the larger area/distance of GPE. Makes sense now, E2 is at a higher energy point but the majority of the table height belongs to the GPE, so GPE contains more energy overall. YAY I get it! I cannot thank you enough for your help! I really like your diagram too, it's a very good visual representation of the theory. Thank you for your encouragement too, I'll be sure to strive forward Thankyou.00 x 10^(50) !!!!

[studiot]

Fine, look out for the next post about entropy and surface energy.

 

BTW what are you studying?

 

You originally said this came up in Chemistry, so I have been trying to put things from a Chemistry point of view.

[NaukowiecGirl]

I'm currently studying Chemistry, Physics, Mathematics and other Arts. Grain sizes did show up in Chemistry, but I like to also see the Physics involved in Chemistry, like energy, etc. Seems like Physics and Chemistry are Intertwined with each other. It helps to understand a topic from a different view, cause then it makes more sense, like your explanation. Thank you again, I'll be sure to look out for entropy!!!

[studiot]

Entropy part 1

 

Entropy, like energy, is a derived quantity, not directly measurable. Today we have two quite different approaches entropy. It was one of the great triumphs of 19^th century science to show these different approaches to be concerned with the same physical property, albeit from different viewpoints. Confusion often arises if and when inappropriate parts of one view are mixed with the other.

 

Historically entropy was first introduced by the technologists and scientists interested in heat engines, particularly steam engines. These workers were concerned with plain down-to-earth matters that have real substance in out physical world that they could measure – We now call these observables.

 

The second approach was abstract and theoretical and founded in the Kinetic Theory.

Boltzman’s equation

Entropy, S = k log_e (W)

is one of the great fundamental equations of Physics that has been expanded beyond energetics and thermodynamics to other fields such as information theory.

 

This treatment is largely qualitative, for understanding, rather than quantitative for carrying out calculations. But it does involve some maths and underlying Physics. You should ask if I introduce anything you have not met before.

 

To start with something familiar we have to go back to the time of Hooke and Newton and consider stretching and cooling as in the two diagrams Fig1 and Fig2 which have given rise to two modern day experiments for A level students.

A great deal of Physics can be learned from both these two experiments that is not always put forward by the texts.

They did not have a fully formed concept of energy, let alone entropy, in those days but they gave us the beginnings of modern mechanics (Hooke’s Law) and Thermodynamics (Newton’s Law of Cooling).

 

 

In Hooke’s experiment we have two observables, the load or applied force and the extension.

We can plot a force/extension graph and use it to find the energy input (= work done) as the area under the graph.

Instead of getting hung up on the graph being a straight line two things to take from this

1) The use of observables to calculate a desired derived quantity.

2) The desired quantity (energy) is simply the area under the graph.

This was the world’s first use of such a diagram .

 

I will return to Fig2 later, but what do you think the observables are?

 

Fast forward through the next hundred years or so, to the time of James Watt, during which Joule and Count Rumford put energy on a firm basis. In particular they showed that heat is a form of energy, a property of matter, not a special fluid that we add matter or take away from it.

 

James Watt was concerned with steam engines and his directly observables were pressure, volume, temperature and mass.

He was the first person to emulate Fig1 by drawing a pressure-volume graph and realising that the area under this graph also represents energy or work, as in Fig3.

 

 

Clever mechanics soon created a mechanism to draw these PV diagrams, directly from the pressure gauges or indicators of the time.

Such diagrams became known as ‘indicator diagrams’.

 

Now this was all very well for pressure and volume, but what about temperature?

So let us start by looking at Fig2 more closely.

Newton’s cooling experiment (usually conducted with naphthalene today) is a temperature-time graph.

It has three portions.

Two decelerating curves where the temperature is falling according to Newton’s Law of Cooling.

But in between these is a flat, horizontal, section during which the temperature does not fall at all.

Yet heat is being lost all the time.

This is our first inkling of entropy, and our first connection to the kinetic theory.

 

Fast forward another hundred years whilst thermodynamics develops and the steam engineers want to link the other observable, temperature, to the energy input or outputs.

 

So they look for a simple property that can be paired with temperature to produce a graph where the energy is the area under the graph, like the indicator diagrams of Hooke and Watt.

Clausius came up with ‘entropy’ and it was given the symbol S.

 

Entropy is nothing more than the property on one axis of an indicator diagram that yields energy as the area under the graph.

 

They called this a T-S indicator diagram. Fig 4

 

 

Note that in order to create an indicator diagram using temperature as one variable they had to give up using only observables, but they set about producing tables of entropy for all conceivable situations and engineers of today use these to look up the entropy.

 

So there you have it

 

Energy = entropy times temperature

 

Or

 

Entropy = energy divided by temperature

So it is measured in energy per degree (Kelvin).

 

But in that century of development there were some twists.

Many types of energy were identified.

For entropy it is not any old energy but specifically heat energy.

Mechanical energy is not included.

So all the force times extension work under the Hooke graph in Fig1 is not included.

 

In Fig 2 I have marked two points, A and B.

Heat is being evolved but the temperature is not changing.

 

The entropy change = Area under the TS diagram in Fig4 and I have marked A and B here as well.

 

Today we have also identified many other pairs of variables that can be combined to form indicator diagrams.

Once such pairing is surface energy and surface area, which is what started this thread.

 

There are plenty of points to expand on here so I think that is enough for part 1.