4 Strengthening Processes That Make Metals Stronger

Strengthening Processes That Make Metals Stronger

1. Solid solution strengthening

1.1 Definition

The solid solution of alloy elements in the base metal results in a certain degree of lattice distortion, which improves the strength of the alloy.

1.2 Principle

The solute atoms dissolved in the solid solution cause lattice distortion, which increases the resistance of dislocation movement and makes it difficult to slip, thus increasing the strength and hardness of the alloy solid solution.

The phenomenon of strengthening metal by dissolving some solute elements to form solid solution is called solid solution strengthening.

When the concentration of solute atoms is appropriate, the strength and hardness of the material can be improved, but the toughness and plasticity are decreased.

1.3 Influencing factors

The higher the atomic fraction of solute atoms, the stronger the strengthening effect, especially when the atomic fraction is very low.

The larger the size difference between solute atoms and base metal atoms, the greater the strengthening effect.

The results show that interstitial solute atoms have better solid solution strengthening effect than replacement atoms, and the lattice distortion of interstitial atoms in BCC crystal is asymmetric, so the strengthening effect of interstitial atoms is greater than that of FCC crystal;

However, the solid solubility of interstitial atoms is very limited, so the actual strengthening effect is also limited.

The larger the difference of valence electron number between solute atom and base metal, the more obvious the solution strengthening effect is, that is, the yield strength of solid solution increases with the increase of valence electron concentration.

1.4 The degree of solution strengthening

It mainly depends on the following factors:

1) The size difference between matrix atoms and solute atoms.

The larger the size difference is, the greater the interference to the original crystal structure will be, and the more difficult the dislocation slip will be.

2) The amount of alloying elements.

The more alloying elements added, the greater the strengthening effect.

If too many too large or too small atoms are added, the solubility will be exceeded.

This involves another strengthening mechanism, dispersed phase strengthening.

3) The results show that interstitial solute atoms have more solid solution strengthening effect than replacement atoms.

4) The larger the difference of valence electron number between solute atom and base metal, the more significant the solid solution strengthening effect.

1.5 Effect

The yield strength, tensile strength and hardness are better than pure metal;

In most cases, the ductility is lower than that of pure metal;

The conductivity is much lower than that of pure metal;

Creep resistance, or strength loss at high temperature, can be improved by solution strengthening.

2. Work hardening

2.1 Definition

With the increase of cold deformation degree, the strength and hardness of metal materials increase, but the plasticity and toughness decrease.

2.2 Introduction

Work hardening refers to the phenomenon that the strength and hardness of metal materials increase while the plasticity and toughness decrease when they are deformed below the recrystallization temperature, which is also called cold work hardening.

The cause is that when the metal is plastically deformed, the grains slip, entanglement of dislocations occurs, making the grains elongated, broken and fibrillated, and residual stresses are generated inside the metal, etc.

The degree of work hardening is usually expressed by the ratio of the microhardness of the surface layer and the depth of the hardened layer.

2.3 Explanation from dislocation theory

1) The dislocation intersects with each other, and the cutting steps hinder the dislocation movement;

2) The reaction between dislocations results in the formation of fixed dislocations which hinder the movement of dislocations;

3) The increase of dislocation density further increases the resistance of dislocation movement.

2.4 Hazards

Work hardening brings difficulties to the further processing of metal parts.

For example, in the process of cold rolling, the harder the steel plate will be rolled, so it is necessary to arrange intermediate annealing in the process of processing to eliminate the work hardening by heating.

Another example is to make the surface of the workpiece brittle and hard in the cutting process, so as to accelerate the tool wear and increase the cutting force.

2.5 Benefits

It can improve the strength, hardness and wear resistance of metals, especially for those pure metals and some alloys that can not be improved by heat treatment.

For example, cold drawn high strength steel wire and cold rolled spring are used to improve their strength and elastic limit.

Another example is that the hardness and wear resistance of the tracks of tanks and tractors, jaw plates of crushers and turnouts of railways are improved by work hardening.

2.6 Role in mechanical engineering

After cold drawing, rolling and shot peening (see surface strengthening), the surface strength of metal materials, parts and components can be significantly improved;

After the part is stressed, the local stress in some parts often exceeds the yield limit of the material, causing plastic deformation.

Because the work hardening limits the continuous development of plastic deformation, the safety of parts and components can be improved;

When metal parts or components are pressed, the plastic deformation is accompanied by strengthening, which makes the deformation transfer to the surrounding parts.

The cold stamping parts with uniform cross-section deformation can be obtained by repeated alternating action;

It can improve the cutting performance of low carbon steel and make the chip easy to separate.

However, work hardening also brings difficulties to the further processing of metal parts.

For the cold drawn steel wire, because of work hardening, further drawing energy consumption is large, or even broken, so it must be annealed in the middle to eliminate work hardening before drawing.

For example, in order to make the surface of the workpiece brittle and hard in the cutting process, the cutting force is increased and the tool wear is accelerated.

3. Fine-grain strengthening

3.1 Definition

The method of improving the mechanical properties of metal materials by refining grains is called fine-grain strengthening.

In industry, the strength of materials is improved by refining grains.

3.2 Principle

Usually, metals are polycrystals composed of many grains.

The size of grains can be expressed by the number of grains per unit volume.

The more the number, the finer the grains.

The results show that the fine-grain metal has higher strength, hardness, plasticity and toughness than the coarse grain metal at room temperature.

This is because the plastic deformation of the fine grains can be dispersed in more grains, the plastic deformation is more uniform, and the stress concentration is smaller;

In addition, the finer the grains, the larger the area of the grain boundary, the more tortuous the grain boundary, and the more unfavorable to the crack propagation.

Therefore, the method of improving material strength by refining grain is called fine grain strengthening in industry.

3.3 Effect

The smaller the grain size is, the smaller the number of dislocations (n) in the dislocation cluster is, the smaller the stress concentration is, and the higher the strength of the material is;

According to the Hall-Page relation, the smaller the average value of grain (d), the higher the yield strength of the material.

3.4 Methods of grain refinement

Increase the degree of supercooling;

Metamorphic treatment;

Vibration and stirring;

For cold deformed metals, the grain size can be refined by controlling the deformation degree and annealing temperature.

4. Second phase strengthening

4.1 Definition

In addition to the matrix phase, there is a second phase in the multiphase alloy.

When the second phase is uniformly distributed in the matrix with fine particles, it will have a significant strengthening effect.

This strengthening effect is called second phase strengthening.

4.2 Classification

For the movement of dislocations, there are two kinds of second phases in the alloy:

1) The strengthening effect of non-deformable particles (bypass mechanism).

2) The strengthening effect of deformable particles (shear mechanism).

Both dispersion strengthening and precipitation strengthening are special cases of second phase strengthening.

4.3 Effect

The main reason for the strengthening of the second phase is the interaction between them and dislocations, which hinders the movement of dislocations and improves the deformation resistance of the alloy.

5. Conclusion

The most important factors affecting the strength are the composition, microstructure and surface state of the material itself;

Secondly, the stress state, such as the speed of loading, loading mode, simple tension or repeated stress, will show different strength;

In addition, the geometry and size of the specimen and the test medium also have great influence, sometimes even decisive.

For example, the tensile strength of ultra-high strength steel in hydrogen atmosphere may decrease exponentially.

There are two ways to strengthen metal materials.

One is to improve the bonding force between atoms of the alloy, improve its theoretical strength, and make perfect crystals without defects, such as whiskers.

The strength of iron whiskers is close to the theoretical value, which can be attributed to the fact that there are no dislocations in the whiskers or only a few dislocations that can not proliferate during deformation.

Unfortunately, when the diameter of whisker is larger, the strength will drop sharply.

Another way of strengthening is to introduce a large number of crystal defects into the crystal, such as dislocations, point defects, heterogeneous atoms, grain boundaries, highly dispersed particles or inhomogeneity (such as segregation).

These defects hinder the movement of dislocations and obviously improve the strength of the metal.

As it turns out, this is the most effective way to improve the strength of metal.

For engineering materials, it is generally through the comprehensive strengthening effect to achieve better comprehensive performance.

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