1. Solid solution strengthening

definition

       The alloying element is dissolved in the base metal to cause a certain degree of lattice distortion, thereby improving the strength of the alloy.

principle

       The solute atoms dissolved in the solid solution cause lattice distortion, which increases the resistance of dislocation movement and makes it difficult for slip to proceed, thereby increasing 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, while its toughness and plasticity have decreased.

Influencing factors

       The higher the atomic fraction of solute atoms, the greater the strengthening effect, especially when the atomic fraction is very low. The greater the difference in atomic size between the solute atoms and the matrix metal, the greater the strengthening effect.

       Interstitial solute atoms have a larger solid solution strengthening effect than replacement atoms, and because the lattice distortion of interstitial atoms in body-centered cubic crystals is asymmetric, its 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 between 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.

Degree of solid solution strengthening

       Mainly depends on the following factors:

       (1) The size difference between matrix atoms and solute atoms. The larger the size difference, the more disturbed the original crystal structure, and the more difficult it is for dislocations to slip.

       (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, solubility will be exceeded. This involves another strengthening mechanism, dispersed phase strengthening.

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

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

Effect

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

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

       Conductivity is much lower than pure metals;

       Creep resistance, or loss of strength at elevated temperatures, can be improved by solution strengthening.

2. Work hardening

definition

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

Introduction

       When a metal material is plastically deformed below the recrystallization temperature, the strength and hardness increase, while the ductility and toughness decrease. Also known as cold hardening. The reason is that when the metal is plastically deformed, the grains slip and dislocations entangle, which makes the grains elongated, broken and fibrillated, and residual stress is generated inside 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.

Explained from the perspective of dislocation theory

       (1) Intersections occur between dislocations, and the resulting cut steps hinder the movement of dislocations;

       (2) The reaction occurs between dislocations, and the formed fixed dislocations hinder the movement of dislocations;

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

harm

       Work hardening makes further processing of metal parts difficult. 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 to eliminate its work hardening by heating. Another example is to make the surface of the workpiece brittle and hard during cutting, thereby accelerating tool wear and increasing cutting force.

benefit

       It can improve the strength, hardness and wear resistance of metals, especially for those pure metals and certain alloys that cannot be increased by heat treatment. Such as cold-drawn high-strength steel wire and cold-coiled spring, etc., use cold working deformation to improve their strength and elastic limit. Another example is the crawler of tanks, tractors, jaws of crushers and turnouts of railways, etc., which also use work hardening to improve their hardness and wear resistance.

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 in some parts often exceeds the yield limit of the material, causing plastic deformation. Due to the work hardening, the continued development of plastic deformation is limited, 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 surrounding unwork hardened part. After such repeated alternating action, cold stamping parts with uniform cross-sectional deformation can be obtained;

       It can improve the cutting performance of mild steel and make chips easy to separate. However, work hardening also brings difficulties to further processing of metal parts. Such as cold drawn steel wire, due to work hardening, further drawing consumes a lot of energy, and even it is broken, so it must be annealed in the middle to eliminate work hardening before drawing. Another example is that in order to make the surface of the workpiece brittle and hard, the cutting force is increased during cutting, and the tool wear is accelerated.

3. Fine grain strengthening

definition

       The method of improving the mechanical properties of metal materials by refining grains is called grain refinement strengthening, and industrially, the strength of materials is improved by refining grains.

principle

       Usually metals are polycrystals composed of many grains, and the size of grains can be expressed by the number of grains per unit volume. The more the number, the finer the 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 plastic deformation of the fine grains under external force can be dispersed in more grains, the plastic deformation is more uniform, and the stress concentration is small; in addition, the finer the grains, the larger the grain boundary area, and the more tortuous The more unfavorable for crack propagation. Therefore, in the industry, the method of improving the strength of the material by refining the grains is called fine-grain strengthening.

Effect

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

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

Methods of grain refinement

       increase supercooling;

       Deterioration treatment;

       Vibration and stirring;

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

4. Second phase strengthening

definition

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

Classification

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

       (1) Strengthening of non-deformable particles (bypassing mechanism).

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

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

Effect

       The main reason for the strengthening of the second phases is their interaction with dislocations, which hinders the movement of dislocations and improves the deformation resistance of the alloy.

Summarize

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

       There are only two ways to strengthen metal materials. One is to improve the interatomic bonding force of the alloy, improve its theoretical strength, and obtain a complete crystal without defects, such as whiskers. The strength of the iron whiskers is known to be close to the theoretical value, which is believed to be because there are no dislocations in the whiskers, or only a small amount of dislocations that cannot propagate during deformation. Unfortunately, when the diameter of the whiskers is larger, the strength drops 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 inhomogeneities (such as segregation), etc. These defects hinder dislocation movement and also Significantly improves metal strength. This has proven to be the most effective way to increase the strength of metals. For engineering materials, generally through the comprehensive strengthening effect to achieve better comprehensive performance.