Solid solution strengthening

1.Definition 

A phenomenon in which the solid solution of alloying elements in the matrix metal causes lattice distortion to a certain extent so that increasing the strength of the alloy.

2. Principle

Solute atoms in the solid solution cause lattice distortion, which increases the resistance of dislocation motion and makes slip difficult to carry out, thus increasing the strength and hardness of the alloy solid solution. This strengthening of a metal by dissolving into a certain solute element to form a solid solution is called solution strengthening. When the concentration of solute atoms is appropriate, the strength and hardness of the material can be increased, but its toughness and plasticity can be decreased.

3. Factors Affecting 

The higher the atomic fraction of the solute atom, the greater the reinforcement, especially when the atomic fraction is very low. The larger the size difference between the solute atom and the base metal atom, the stronger the strengthening effect will be. The interstitial solute atoms have a greater solid solution strengthening effect than the replacement atoms, and the lattice distortion of interstitial atoms in the volumetric centered cubic crystals is asymmetric, so the strengthening effect is greater than that in the planar centered cubic crystals. But the solid solubility of interstitial atoms is very limited, so the actual strengthening effect is also limited. The greater the difference in the number of valence electrons between solute atoms and base metals, the more obvious the strengthening effect of solid solution is, that is, the yield strength of solid solution increases with the increase of the concentration of valence electrons

4. Degree of solid solution strengthening

Mainly depends on the following factors:

（1）Size is different between matrix atoms and solute atoms. The larger the size difference, the greater the disturbance to the original crystal structure 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 you add too many atoms too large or too small, you will exceed the solubility. This involves another reinforcement mechanism, decentralized phase reinforcement.

 (3) Interstitial solute atoms have greater solid solution strengthening effect than displacement atoms.

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

5. Effect

The yield strength, tensile strength and hardness are stronger than those of pure metals.

In most cases, ductility is lower than that of pure metals; It conducts electricity much less than pure metal; Creep resistance, or strength loss at high temperatures, can be improved by solid solution strengthening.

Hardening

1.Definition

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

2.Introduction

The phenomenon that the strength and hardness of a metal material increase while the plasticity and toughness decrease during plastic deformation below the re-crystallization temperature, known as cold hardening. The reason is that during the plastic deformation of the metal, the grains slip and dislocation intertwine, which makes the grains elongate, break and fibrosis, and residual stresses are generated inside the metal. The degree of work hardening is usually expressed by the ratio of the microhardness of the surface layer after and before working and the depth of the hardened layer.

3. From the perspective of dislocation theory

(1) Cross-cutting occurs between dislocations, resulting in cutting order hindering dislocation motion;

 (2) The dislocation reacts with each other and the fixed dislocation formed hinders the dislocation movement;

(3) Dislocation proliferation occurs, and dislocation density increases to further increase dislocation motion resistance.

4. Harm

Work hardening brings difficulties to the further processing of metal parts. If in the process of cold rolling steel plate will be more and more hard rolling so that rolling does not move, so in the processing process to arrange intermediate annealing, through heating to eliminate its work hardening. Such as in the cutting process to make the surface of the workpiece brittle and hard, so as to accelerate tool wear, increase the cutting force.

5. Advantage

It improves the strength, hardness and wear resistance of metals, especially for pure metals and certain alloys that cannot be strengthened by heat treatment. For example, cold-drawn high-strength steel wire and cold-rolled spring are used to improve their strength and elastic limit. Also be like the track of tank, tractor, jaw plate of crusher and turnout of railroad also use work to harden to improve its hardness and wear resistance.

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 parts are stressed, some parts of the local stress often exceed the yield limit of the material, causing plastic deformation, due to the work hardening limit of plastic deformation continues to develop, can improve the safety of parts and components; When metal parts or components are pressed, their plastic deformation is accompanied by reinforcement, which transfers the deformation to the surrounding unworked hardened parts. The cold stamping parts with uniform cross section deformation can be obtained by such repeated alternating action. It can improve the cutting performance of low carbon steel and make the chip easy to separate. But the work hardening also brings difficulties to the further processing of metal parts. Such as cold drawn steel wire, due to work hardening make further drawing energy consumption, or even be broken, so must be through intermediate annealing, eliminate work hardening and then drawing. For example, in order to make the surface of the workpiece brittle and hard, and then cutting to increase the cutting force, accelerate the tool wear.

Fine-grain strengthening

1. Definition

The method of improving the mechanical properties of metal materials by refining grains is called fine grain strengthening. In industry, the method of refining grains is used to improve the strength of materials.

2. Principle 

Generally, a metal is polycrystalline with many grains. The size of the grains can be expressed as the number of grains per unit volume. The more the number, the finer the grain. Experiments show that fine-grained metals have higher strength, hardness, plasticity and toughness than coarse-grained metals at room temperature. This is because the plastic deformation of fine grains under external force can be dispersed in more grains, and the plastic deformation is more uniform and the stress concentration is smaller. In addition, the finer the grains are, the larger the grain boundary area is, and the more tortuous the grain boundary is, the more unfavorable the crack propagation is. Therefore, the method of improving the material strength by refining grain is called fine grain strengthening.

3. Effect

The smaller the grain size, the smaller the number of dislocation (n) in the dislocation cluster, the smaller the stress concentration and the higher the strength of the material. According to the Hall - ligand relation, the smaller the average (D) of the grains is, the higher the yield strength of the material will be.

4. Method of grain refinement

Increase supercooling degree; Metamorphic treatment; Vibration and stirring; Grains can be refined by controlling deformation degree and annealing temperature for cold deformed metals.

Second phase reinforcement

1. Definition

In comparison with single-phase alloys, there are second phases in addition to matrix phase. When the second phase is evenly distributed in the matrix phase with fine dispersed particles, a significant strengthening effect will occur. This reinforcement is called secondary reinforcement.

2. Classification

For dislocation motion, the second phase of the alloy contains the following two conditions :(1) the strengthening effect of non-deformable particles (bypass mechanism). (2) Strengthening effect of deformable particles (cutting mechanism). Both dispersion strengthening and precipitation strengthening belong to the special cases of second phase strengthening.

3. Effect

The second reason is the interaction between them and the dislocation, which hinders the dislocation movement and improves the deformation resistance of the alloy.

Conclusion

The most important factors affecting the strength are the composition, structure and surface state of the material itself. Secondly, the state of stress, such as the speed of afterforce, loading mode, simple stretching or repeated stress, will show different strength; In addition, the geometry and size of the specimen and the test medium also have a great influence, sometimes even decisive, such as the tensile strength of ultra-high strength steel in hydrogen atmosphere may decrease exponentially. There are only two ways to strengthen the metal materials. One is to improve the atomic binding force of the alloy, increase its theoretical strength, and make a complete crystal without defects, such as whiskers. Given that the whisker strength of iron is close to the theoretical value, it can be assumed that this is because there are no dislocations in the whisker, or only a small number of dislocations that cannot proliferate during deformation. Unfortunately, when the whisker diameter is large, the strength drops sharply. Another strengthening approach is to introduce a large number of crystal defects, such as dislocation, point defects, heterogeneous atoms, grain boundaries, highly dispersed particles or inhomogeneity (such as skewness), etc. into the crystal. These defects hinder dislocation movement and will significantly improve the metal strength. This has proved to be the most effective way to increase the strength of the metal. For engineering materials, it is generally through the comprehensive strengthening effect to achieve better comprehensive performance.