01. Strengthening solid solutions
Set ition
The phenomenon of solid solution of alloying elements in the matrix metal, causing a certain degree of lattice distortion, improves the strength of the alloy.
Principle
The solute atoms that are dissolved in the solid solution cause distortion of the network, which increases resistance to displacement movement and makes sliding difficult. As a result, the strength and hardness of the alloy solid solution increase. This phenomenon of metal strengthening through the formation of a solid solution by dissolving a specific solute element is known as solid solution strengthening.
When the solute atom concentration is ideal, the strength and hardness of the material can be improved, but its toughness and plasticity decrease.
Influencing factors
The higher the concentration of solute atoms, the more pronounced the strengthening effect will be, especially at low concentrations, where the effect is more significant.
The greater the difference in atomic size between the solute atom and the matrix metal, the stronger the strengthening effect.
Interstitial solute atoms have a stronger solid solution strengthening effect than replacement atoms. Furthermore, the lattice distortion caused by interstitial atoms in body-centered cubic crystals is asymmetric, resulting in a stronger strengthening effect than that in face-centered cubic crystals. However, 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 the solute atom and the matrix metal, the more pronounced the strengthening effect of the solid solution becomes. In other words, the yield strength of the solid solution increases as the concentration of valence electrons increases.
The degree of strengthening of the solid solution mainly depends on the following factors:
(1) The size difference between the matrix atoms and the solute atoms:
The greater the size difference, the more the original crystal structure is disturbed and the more difficult it becomes for dislocations to slide.
(2) The amount of alloying elements:
The greater the amount of alloying elements added, the stronger the strengthening effect.
If too many very large or very small atoms are added, the solubility will be exceeded. This results in another strengthening mechanism known as dispersion phase strengthening.
(3) The solid solution strengthening effect of interstitial solute atoms is greater than that of replacement atoms.
(4) The greater the difference in the number of valence electrons between the solute atom and the matrix metal, the more pronounced the strengthening effect of the solid solution becomes.
It is made
The yield strength, tensile strength and hardness are stronger compared to those of pure metal.
In most cases, the ductility is lower compared to that of the pure metal.
The electrical conductivity is significantly lower compared to that of the pure metal.
Solid solution reinforcement can improve resistance to creep or loss of strength at high temperatures.
02. Work hardening
D definition
With increasing cold deformation, the strength and hardness of metal materials increase, but the plasticity and toughness decrease.
Brief introduction
The phenomenon of increased strength and hardness of metallic materials, accompanied by a decrease in plasticity and toughness during plastic deformation below the recrystallization temperature, is known as cold work hardening.
The reason for this is that during plastic deformation of the metal, the grains slip and the dislocations become entangled, causing the grains to elongate, break and fibrose, resulting in residual stress within the metal.
The degree of work hardening is commonly expressed as the ratio of the microhardness of the surface layer after processing to that before processing, and the depth of the hardened layer.
From the perspective of displacement theory:
(1) The intersection of dislocations impedes their movement through the formation of shear dislocations;
(2) The reaction between displacements creates fixed displacements that further impede its movement;
(3) The proliferation of dislocations leads to an increase in the density of dislocations, further increasing the resistance to movement of dislocations.
Hurt
Work hardening makes further processing of metal parts challenging.
For example, during cold rolling, the steel sheet will become increasingly hard to the point that it can no longer be rolled. Therefore, it is necessary to include intermediate annealing in the processing process to eliminate heat hardening.
For example, in the cutting process, the surface of the workpiece becomes brittle and hard, causing accelerated tool wear, increased cutting force, and so on.
Benefits
Work hardening can improve the strength, hardness and wear resistance of metals, especially for pure metals and some alloys that cannot be strengthened through heat treatment.
Examples include cold-drawn high-strength steel wire and cold-wound springs, which use cold-working deformation to increase their strength and elastic limit.
For example, the track of tanks and tractors, the jaw plate of crushers, and the siding of railway tracks also use hardening to improve their hardness and wear resistance.
Role in mechanical engineering
The surface strength of metal materials, parts and components can be significantly improved through cold drawing, rolling and shot peening (as described in surface strengthening).
When parts are subjected to stress, local stresses in some areas can often exceed the yield strength of the material, leading to plastic deformation. However, hardening restricts the continued development of plastic deformation, thereby improving the safety of parts and components.
When a metal part or component is stamped, plastic deformation is accompanied by reinforcement, resulting in the transfer of the deformation to the surrounding hardened, unworked part.
Through repeated alternating actions, cold-stamped parts with uniform cross-sectional deformation can be obtained and the cutting performance of low-carbon steel can be improved, making chip separation easier.
However, work hardening also makes further processing of metal parts challenging. For example, cold drawn steel wire becomes difficult to stretch further due to work hardening, requiring a significant amount of energy and may even break. As a result, it must be annealed to eliminate hardening before being drawn again.
Similarly, in the cutting process, making the workpiece surface brittle and hard through hardening increases the cutting force and accelerates tool wear during recutting.
03. Strengthening fine grains
D definition
The method of improving the mechanical properties of metallic materials through grain refinement is known as fine-grain strengthening.
In industry, grain refining is used to improve the resistance of materials.
Principle
Metals are usually made up of many grains and are called polycrystals. Grain size can be expressed in terms of the number of grains per unit volume, with a larger number indicating finer grains.
Experiments show that fine-grained metals have greater strength, hardness, plasticity and toughness compared to coarse-grained metals at room temperature. This is because plastic deformation caused by external forces in fine grains can be dispersed into more grains, leading to more uniform plastic deformation and reduced stress concentration.
Furthermore, the finer the grain, the larger the grain boundary area will be and the more tortuous the grain boundary will become, making it difficult for cracks to propagate.
Therefore, the method of increasing material strength through grain refinement is known as fine grain strengthening in industry.
It is made
The finer the grain, the fewer dislocations (n) present in the dislocation cluster, resulting in lower stress concentration and greater material strength.
The fine grain strengthening law states that the more grain boundaries that are present, the finer the grains.
According to the Hall-Petch relationship, the smaller the average grain size (d), the higher the yield strength of the material.
The grain refinement method:
Methods for refining cold-deformed metal grains include:
- Increasing hypothermia
- Modification handling
- Vibration and shaking
The grain size can be controlled by adjusting the degree of deformation and the annealing temperature.
04. Strengthening the second phase
D definition
Compared to single-phase alloys, multiphase alloys contain a second phase in addition to the matrix phase.
When the second phase is uniformly dispersed as fine particles within the matrix phase, a significant strengthening effect, referred to as second phase strengthening, results.
Classification
The second phase contained in the alloy has the following two effects on the movement of dislocations:
(1) Non-deformable particle strengthening effect (bypass mechanism).
(2) Deformable particle strengthening effect (cutting mechanism).
Both dispersion strengthening and precipitation strengthening are special cases of second-phase strengthening.
It is made
The strengthening of the second phase is mainly due to the interaction between the second phase and the dislocations, which prevents the movement of the dislocations and increases the alloy's resistance to deformation.
Conclusion
The strength of metallic materials is mainly affected by their composition, microstructure and surface state.
The second factor is the stress state, such as the applied force rate and loading mode, which can result in different strengths, for example, the tensile strength of ultra-high strength steel can decrease when tested in hydrogen atmosphere .
The geometric shape and size of the sample and test medium also have a significant impact and can sometimes be decisive.
There are only two ways to strengthen metal materials:
- Improve the interatomic bonding strength of the alloy and prepare complete crystals without defects such as whiskers. Iron whiskers have strength close to the theoretical value due to the absence or small number of dislocations that do not multiply during deformation. However, its strength decreases rapidly as its diameter increases.
- Introduction of a large number of crystalline defects into the crystal, such as dislocations, point defects, heterogeneous atoms, grain boundaries, highly dispersed particles or heterogeneities (such as segregation). These defects impede the movement of dislocations and significantly increase the metal's strength. This is the most effective way to increase metal strength.
In engineering materials, strength is generally improved through a comprehensive reinforcing effect to obtain better overall properties.
