Solid solution strengthening

1. Definition

A phenomenon in which alloying elements are dissolved in the base metal to cause a certain degree of lattice distortion and thus increase the strength of the alloy.

2. Principle

The solute atoms dissolved in the solid solution cause lattice distortion, which increases the resistance of dislocation movement, makes slipping difficult, and increases the strength and hardness of the alloy solid solution. This phenomenon of strengthening the metal by dissolving a certain solute element to form a solid solution is called solid 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 have decreased.

3. Influencing factors

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

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

Interstitial solute atoms have a greater solid solution strengthening effect than replacement atoms, and because the lattice distortion of interstitial atoms in body-centered cubic crystals is asymmetric, their strengthening effect is greater than that of face-centered cubic crystals; but interstitial atoms The solid solubility is very limited, so the actual strengthening effect is also limited.

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

4. The degree of solid solution strengthening mainly depends on the following factors

The difference in size between matrix atoms and solute atoms. The greater the size difference, the greater the interference to the original crystal structure, and the more difficult it is for dislocation slip.

The amount of alloying elements. The more alloying elements added, the greater the strengthening effect. If too many atoms are too large or too small, the solubility will be exceeded. This involves another strengthening mechanism, the dispersed phase strengthening.

Interstitial solute atoms have a greater solid solution strengthening effect than replacement atoms.

The greater the difference in the number of valence electrons between the solute atoms and the base metal, the more significant the solid solution strengthening effect.

5. Effect

Yield strength, tensile strength and hardness are stronger than pure metals;

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

The conductivity is much lower than pure metal;

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

Work hardening

1. Definition

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

2. Introduction

A phenomenon in which the strength and hardness of metal materials increase when they are plastically deformed below the recrystallization temperature, while the plasticity and toughness decrease. Also known as cold work hardening. The reason is that when the metal is plastically deformed, the crystal grains slip and dislocations are entangled, which causes the crystal grains to elongate, break, and fiberize, and residual stresses are generated in the metal. The degree of work hardening is usually expressed by the ratio of the microhardness of the surface layer after processing to that before processing and the depth of the hardened layer.

3. Interpretation from the perspective of dislocation theory

(1) Intersection occurs between dislocations, and the resulting cuts hinder the movement of the dislocations;

(2) A reaction occurs between dislocations, and the formed fixed dislocation hinders the movement of the dislocation;

(3) The proliferation of dislocations occurs, and the increase in dislocation density further increases the resistance to dislocation movement.

4. Harm

Work hardening brings difficulties to the further processing of metal parts. For example, in the process of cold-rolling the steel plate, it will become harder and harder to roll, so it is necessary to arrange intermediate annealing during the processing process to eliminate its work hardening by heating. Another example is to make the surface of the workpiece brittle and hard in the cutting process, thereby accelerating tool wear and increasing cutting force.

5. Benefits

It can improve the strength, hardness and wear resistance of metals, especially for those pure metals and certain alloys that cannot be improved by heat treatment. For example, cold-drawn high-strength steel wire and cold-coiled spring, etc., use cold working deformation to improve its strength and elastic limit. Another example is the use of work hardening to improve the hardness and wear resistance of tanks, tractor tracks, crusher jaws and railway turnouts.

6. Role in mechanical engineering

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

After the parts are stressed, the local stress of certain parts often exceeds the yield limit of the material, causing plastic deformation. Due to work hardening, the continued development of plastic deformation is restricted, which can improve the safety of parts and components;

When a metal part or component is stamped, its plastic deformation is accompanied by strengthening, so that the deformation is transferred to the unworked hardened part around it. After such repeated alternating actions, cold stamping parts with uniform cross-sectional deformation can be obtained;

It can improve the cutting performance of low carbon steel and make the chips easy to separate. But work hardening also brings difficulties to the further processing of metal parts. For example, cold-drawn steel wire consumes a lot of energy for further drawing due to work hardening, and may even be broken. Therefore, it must be annealed to eliminate work hardening before drawing. Another example is that in order to make the surface of the workpiece brittle and hard during cutting, the cutting force is increased during re-cutting, and the tool wear is accelerated.

Fine grain strengthening

1. Definition

The method of improving the mechanical properties of metal materials by refining the crystal grains is called crystal refining strengthening. In the industry, the strength of the material is improved by refining the crystal grains.

2. Principle

Metals are usually polycrystals composed of many crystal grains. The size of the crystal grains can be expressed by the number of crystal grains per unit volume. The more the number, the finer the crystal grains. Experiments show that fine-grained metals at room temperature have higher strength, hardness, plasticity and toughness than coarse-grained metals. This is because the fine grains undergo plastic deformation under external force and can be dispersed in more grains, the plastic deformation is more uniform, and the stress concentration is less; in addition, the finer the grains, the larger the grain boundary area and the more tortuous grain boundaries. The more unfavorable the propagation of cracks. Therefore, the method of improving the strength of the material by refining the crystal grains is called grain refinement strengthening in the industry.

3. Effect

The smaller the grain size, the smaller the number of dislocations (n) in the dislocation cluster. According to τ=nτ0, the smaller the stress concentration, the higher the strength of the material;

The strengthening law of fine-grain strengthening is that the more grain boundaries, the finer the grains. According to the Hall-Peiqi relationship, the smaller the average value (d) of the grains, the higher the yield strength of the material.

4. The method of grain refinement

Increase the degree of subcooling;

Deterioration treatment;

Vibration and stirring;

For cold-deformed metals, the crystal grains can be refined by controlling the degree of deformation and annealing temperature.

Second phase reinforcement

1. Definition

Compared with single-phase alloys, multi-phase alloys have a second phase in addition to the matrix phase. When the second phase is uniformly distributed in the matrix phase with fine dispersed particles, it will have a significant strengthening effect. This strengthening effect is called the second phase strengthening.

2. Classification

For the movement of dislocations, the second phase contained in the alloy has the following two situations:

(1) Reinforcement of non-deformable particles (bypass mechanism).

(2) Reinforcement of deformable particles (cut-through mechanism).

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

3. Effect

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

to sum up

The most important factors affecting the strength are the composition, structure and surface state of the material itself; the second is the state of force, such as the speed of the force, the method of loading, simple stretching or repeated force, will show different strengths; In addition, the geometry and size of the sample and the test medium also have a great influence, sometimes even decisive. For example, the tensile strength of ultra-high-strength steel in a hydrogen atmosphere may drop exponentially.

There are only two ways to strengthen metal materials. One is to increase the interatomic bonding force of the alloy, increase its theoretical strength, and prepare a complete crystal without defects, such as whiskers. It is known that the strength of iron whiskers is close to the theoretical value. It can be considered that this is because there are no dislocations in the whiskers, or only a small amount of dislocations that cannot proliferate during the deformation process. Unfortunately, when the diameter of the whisker is larger, the strength drops sharply. Another strengthening approach 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 inhomogeneities (such as segregation), etc. These defects hinder the movement of dislocations and also Significantly improve the strength of the metal. Facts have proved that this is the most effective way to increase the strength of metals. For engineering materials, it is generally through comprehensive strengthening effects to achieve better comprehensive performance.