Last time, I explained the four basic types of chemical bonds that are first taught in high school chemistry: metallic bonds, ionic bonds, covalent bonds, and molecular bonds.
As I mentioned at the end of the last installment, metals are very common in mechanical materials, so we will delve into the mechanisms and characteristics of metallic bonding.
Understanding the mechanisms and characteristics of metallic bonding naturally allows you to grasp the properties of metals logically, rather than relying on vague impressions.
Understanding the deformation mechanisms of metals and metallic bonds is crucial because it allows you to understand how differences in crystal lattices, which will be discussed later, affect actual materials (metals).
Metals, the mechanism of metallic bonding (Coulomb force)
First, let's recall the mechanism of metallic bonding that we explained last time.
Remember that one metal atom shares its extra electrons with other metal atoms.
If you'd like to learn more about the bonding mechanisms of atoms, please take a look here.
Now let's consider what forces actually act to bond metal atoms together.
First, please recall what an atom is.
As explained before, atoms are composed of neutrons, protons, and electrons.
In this system, protons carry a positive electric charge, while electrons carry a negative electric charge.
If you'd like to learn more about the composition of atoms, please take a look here.
The important point here is that there are objects with positive and negative electrical charges located a short distance apart.
As many people can easily imagine, positive and negative electricity come together.
To stick together meansThere is an attractive force at work between the positive and negative terminals.
The concept is the same as how the north and south poles of a magnet attract each other (magnetic force involves a magnetic field).
Just as the south poles and north poles of magnets repel each other, positive charges and negative charges attract each other.
like thisThe force generated by the positive and negative charges of electricity is called the Coulomb force.
Incidentally, since it was a theory established by Coulomb, it is called the Coulomb force.
Now that you have a grasp of the concept of Coulomb's force, let's consider its strength.
Some of you might be hesitant because we'll be using some simple mathematical formulas here, but they're not difficult, so please try your best to follow along.
Let's start by explaining the Coulomb force equation with a diagram.
The quantity that might be difficult to understand from the diagram is q [C: Coulomb], but for now, just think of it as representing the magnitude of electricity.
Coulomb's constant K is also important if we delve deeper into the subject, but for the purposes of this explanation, it's sufficient to understand it simply as a fixed constant.
This time, the three important images I want you to visualize using this equation $$ P=K\frac{+q1・-q2}{r^2}$$ are as follows.
1. If the amount of electric charge q increases, the Coulomb force P increases.
2. As the distance r between the charges decreases, the Coulomb force P increases significantly (charge q is constant).
3. As the distance r between the charges increases, the Coulomb force P decreases significantly (assuming the charges remain constant).
The particularly important images are 2 and 3, as shown in the following diagram.
This image is extremely important, so please keep it somewhere in your mind.
Incidentally, the repulsive forces between positive and negative charges are shown in the following diagram.
It's important to note here that the sign of the attractive force is negative, while the sign of the repulsive force is positive.
What I personally find interesting here is that Coulomb's force and gravity (attraction), which is very familiar to us in our daily lives, are very similar.
This was one of the things that impressed me when I learned it in high school.
The fact that Coulomb's force, one of the equations governing atoms so tiny they are difficult to see even with an electron microscope, and the equation of gravity, one of the equations governing the world we live in, are almost identical made me wonder if, from a broader perspective, my daily actions might be as insignificant as the behavior of atoms (I happen to enjoy assigning meaning to mathematical formulas).
Let's put that aside and return to the world of atoms.
Let's add the Coulomb force, which we've explained so far, to the diagram of metallic bonding.
This can be understood by considering the Coulomb force acting between positive and negative terminals.
Now, let's focus on the force acting on the electron $e^-$.The Coulomb forces acting on electrons from each atom (the forces are the same but opposite in direction) cancel each other out.
I think you can understand from the diagram, but the mechanism of metal atoms and metallic bonding isAtoms are held together by the Coulomb force, which is generated through electrons.
Of course, this mechanism occurs not only between neighboring atoms, but also between multiple surrounding atoms that share electrons.
It is the attractive force generated by this Coulomb force that holds many atoms together, allowing us to perceive metals as having a physical shape.
Based on this Coulomb force, when we consider the bonding of various metal atoms, we can see that atoms with a large number of shared electrons (large charge q) have a strong Coulomb force, and that the closer the atoms in a metallic bond are, the stronger the Coulomb force tends to be.
In other words, it becomes easier to understand what effects differences in lattice shape have, such as a smaller crystal lattice and stronger bonding forces when atoms are closer together (details will be explained next time).
このDifferences in bonding strength are largely related to the strength and hardness of metal atoms and metallic bonds (substances).
Looking more closely at the actual bonding mechanisms of atoms involves complex concepts such as the influence of electron shell orbitals, electron spin, and quantum mechanics considering the relationship between neutrons and protons. However, for a general understanding of the bonding mechanism, imagining the Coulomb force should be sufficient to avoid major errors.
Next, we will explain dislocations, which are an important form of deformation in metallic bonds.
Metal deformation, dislocations
There are three main types of deformation forms for metal atoms and metallic bonds.
The type of deformation changes depending on the amount of deformation, but let's start by considering the small amount of deformation.
Deformation within the elastic range (small deformation)
First, let's visualize a metallic bond formed in a 2x4 grid (Coulomb force is also included).
Let's apply an external force Q horizontally to the first row of this 2x4 metallic bond.
As a result, the metallic bond deforms under the external force Q as shown in the following diagram.
As shown in the diagram, the Coulomb force, which is a binding force, resists the external force Q by deforming diagonally.
Focusing on a single atom, the relationship of forces is as shown in the following diagram.
If the external force is small enough to maintain this balance, removing the external force will return the metallic bond to its original 2x4 configuration.
このThe property of metals to return to their original shape is extremely important and is called elasticity.
Furthermore, the range of external forces and deformations that allows the material to return to its original metallic bond and shape is called the elastic region, and it is a very important concept.
Machines that utilize this elasticity, such as screws and springs, are indispensable to humankind, and I think you can somewhat understand their importance (human civilization could not exist without this property).
Incidentally, in the context of materials mechanics, when considering metal atoms and metallic bonds as a larger material, the force generated by a horizontal external force is called shear force.
If you're interested in material mechanics, including stress and shear force, please take a look here (chemistry, mechanical materials, and physics/material mechanics are interconnected).
Deformation due to dislocation, plastic deformation (large deformation)
Next, let's try applying a larger external force Q2 to the atoms in the first row.
As the external force Q2 increases, it surpasses the Coulomb force, the force that binds atoms together, causing the bonds between the atoms to break.
I think this situation is easy to visualize.
Here, a very interesting phenomenon occurs involving metals and metallic bonding.
Here, I'd like you to recall the characteristics of metallic bonding: in metallic bonding, electrons are shared throughout the entire material, electrons move freely, and electrons readily recombine.
Due to its excellent recombinability, if a large external force Q2 is applied and the metallic bond deforms and breaks, it will recombine with the nearest atom that moved to the new location with even greater force.
The reason why they can recombine with a stronger force can also be basically explained by Coulomb's force.
Before deformation, the total number of free electrons (-q) in a metallic bond supported four bonds. However, after deformation and recombination, the total number of free electrons (-q) remains unchanged, but the number of bonds decreases to three.
In other words, the number of electrons per atom increases, and -q becomes larger, so the Coulomb force, which is the bonding force, increases (in reality, it's more complex, involving not only electrons but also changes in interatomic distance, but the image is important).
If the external force Q2 is removed here, it will have rebonded with the neighboring atom and cannot return to its original shape.
This deformation mechanism is called dislocation, and the deformed state is called plastic deformation (extremely important).
Metal atoms and metallic bonds are prone to dislocations, which gives them important properties such as elongation and toughness.
For example, while ductile iron deforms when bent or stretched due to dislocations, plastics like ABS resin and glass (although they can't be directly compared due to their different bonding mechanisms) do not easily generate dislocations, so they break easily when bent or stretched.
The ductility and toughness of this metal are extremely useful to humankind and are used in processes such as forging, which involves shaping metal by tempering, sheet metal pressing, rolling, and rolling (without these properties, human civilization could not exist).
A simple forging process would look like the following diagram.
We process it in this way to transform it into a form useful to humankind.
Next, an even stronger external force Q3 is applied.
If the external force Q3 is weaker than the bond force after recombination, the metallic bond will deform slightly but will still be able to withstand the force.
Of course, removing the external force Q3 will return it to its plastically deformed shape. However, it cannot return to the very first shape, 2 rows and 4 columns.
This amount of deformation is called permanent deformation, or in the terminology of materials mechanics, permanent strain.
Rupture
Next, when an external force Q4 exceeding the rebonding force is applied to the atom in the first row, the dislocations of atoms 2, 3, and 4 repeatedly break and rebond, eventually leading to complete fracture.
This series of movements constitutes the mechanism of deformation of metal atoms and metallic bonds.
A graph showing the relationship between the stress σ (sigma) and deformation, or more precisely, the strain ε (epsilon), generated by an external force Q applied to the metallic bonds of this metal is called a stress-strain diagram, and it looks like the following graph (a fundamental concept in mechanics of materials).
I've explained the details of this stress-strain diagram on this blog, so please take a look if you're interested.
When the elastic region, plastic region (dislocation generation), and fracture point, which were explained in this section, are superimposed on this graph, the result is as shown in the following figure.
Naturally, the relationship between external force and deformation varies depending on the atom (material), and even with the same atom (material), it changes depending on the crystal and crystal lattice.
Paradoxically, if we understand atoms (materials), crystals, and crystal lattices, we can predict how much deformation will occur under a given external force, which makes it useful for humankind.
If we don't understand this, we can't decide on the materials, so simply drawing the shape in the design isn't very useful.
Therefore, understanding crystals and crystal lattices is extremely important.
The above explains the deformation mechanism of metal atoms and metallic bonds.
Summary
Let's summarize things as usual.
First, let's look at the mechanism of bonding forces.
1. In metallic bonding, the positive charge of protons and the negative charge of electrons create an attractive force that binds them together.
2. Positive and negative charges attract each other due to the Coulomb force.
3. If the amount of electric charge q increases, the Coulomb force P increases.
4. As the distance r between two charges decreases, the Coulomb force P increases significantly, and conversely, as the distance r increases, the Coulomb force P decreases significantly (assuming the charges remain constant).
Next is a summary of the deformation mechanisms of metal atoms and metallic bonds.
1. Metal atoms and metallic bonds, even if their bond state is broken by an external force, will recombine with nearby atoms.
2. The deformation that occurs when an atom recombines with a nearby atom is called a dislocation, and this deformation is called plastic deformation.
3. Deformation caused by external forces that does not result in dislocations returns to its original shape when the external force is removed; this is called elastic deformation.
4. The relationship between the deformation of metal atoms and metallic bonds and external forces can be represented macroscopically using a stress-strain diagram from the principles of materials mechanics.
This content is important not only for mechanical designers but for everyone involved with machinery, so I hope you will try to understand it as much as possible.
In particularThe image of the Coulomb force that determines the bonding force between metal atoms is important, and it is crucial to understand that the Coulomb force is inversely proportional to the distance $r^2$.
moreoverThe concepts of elasticity in the deformation state of metals and plasticity in which dislocations occur are also important (the concept of elasticity is also important).
What's interesting here is how high school physics (Coulomb force) and high school chemistry (atomic bonding) connect to mechanical materials and material mechanics used in university and actual design work.
Simply memorizing individual facts or concepts won't be useful in practice. Therefore, it's crucial to integrate knowledge and other academic subjects in your mind (for example, while Japanese language and mathematics may seem opposite, they are completely intertwined in terms of logical thinking).
Focusing on microscopic phenomena allows us to understand them by studying chemistry and mechanical materials, while a slightly more macroscopic perspective leads to physics and mechanics of materials.
Further narrowing the scope to the micro level leads to quantum mechanics, and narrowing the scope to the macro level connects to mechanical dynamics, from actual machine engines to everyday consumer goods like motorcycles and cars, to cutting-edge rockets and large-scale machinery like next-generation clean energy generators (of course, IT is also related in a broad sense).
In other words, subjects like mathematics, physics, and chemistry taught in compulsory education and high school are definitely connected to cutting-edge science, so try your best to understand them (this blog will help you understand the connections between these fields).
For that reason, this blog repeatedly emphasizes that imagery is more important than rote memorization.
Next time, we will finally explain metal crystals and their structures.
To those who found this article helpful in understanding design:
While there is basically no textbook covering this content, and it is my own original work, I will introduce the textbook that I have been using since I was a student.
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