Yield Strength Of Metallic Materials And Its Influencing Factors

Yield strength refers to the material material begins to produce macro-plastic deformation when the stress. For the yield phenomenon is obvious material, yield strength on the yield point of stress - yield value; for the yield phenomenon is not obvious material, usually will be the stress - strain curve in order to provide for the occurrence of a certain amount of residual deformation as a standard, such as usually 0.2% residual deformation of the stress as the yield strength, the symbol for σ0.2 or σys.
Yield strength is usually used as an evaluation index of the mechanical properties of solid materials, and is the actual use limit of the material.
Intrinsic factors affecting yield strength
Intrinsic factors affecting yield strength are:
1. metal nature and lattice type - The yield strength of a pure metallic single crystal is determined by the resistance to dislocation movement. These resistances are differentiated between lattice resistance and resistance resulting from interactions between dislocations. The lattice forces are related to the dislocation widths and the Bergner vectors, both of which in turn are related to the crystal structure. The resistance generated by inter-dislocation interaction includes the resistance generated by the interaction between parallel dislocations and the resistance generated by the interaction between moving dislocations and forest dislocations. It is expressed by the formula: T=αGb/L, where α is the scaling factor, and because the density ρ is proportional to 1/L2, therefore, T=αGbρ1/2, which shows that an increase in density increases the yield strength.
2. Grain size and substructure - the effect of grain size is a reflection of the effect of grain boundaries, reducing grain size will increase the number of dislocation movement barriers and reduce the length of the dislocation plugging group within the grain, which will result in an increase in yield strength. Many metals and alloys of the yield strength and grain size of the relationship are consistent with Holpegger's formula σs = σj + kyd-1/2, in which σj is the dislocation in the base metal in the movement of the total resistance, also known as frictional resistance, which is determined by the crystal structure and the density of dislocations; ky is a measure of the grain boundaries on the strengthening of the contribution to the size of the pegging constant, or that the end of the slip band of the stress concentration coefficient; d for the average size of the grains. Subgranular boundaries act similarly to grain boundaries and also impede the movement of dislocations.
3. solute elements - pure metal into the solute atoms to form interstitial or replacement type solid solution alloy will significantly increase the yield strength, this is the solid solution strengthening. This is mainly due to the solute atoms and solvent atoms of different diameters, in the solute around the formation of the lattice distortion stress field, the stress field produces interaction, so that the dislocation movement is blocked, thereby increasing the yield strength.

4. second phase - engineering metallic materials whose microstructure is generally multiphase. The effect of the second phase on the yield strength is strongly related to whether the plasmas themselves can deform during the yield deformation of the metallic material. Accordingly, the second phase plasmas can be divided into two categories: non-deformable and deformable.
According to the dislocation theory, the dislocation line can only bypass the undeformable second-phase plasmas, and for this reason, the line tension of the bending dislocation must be overcome. The yield strength and rheological stress of metallic materials with non-deformable second-phase plasmas are determined by the spacing between the second-phase plasmas. In the case of deformable second-phase plasmas, the dislocations can be cut through and deformed together with the matrix, thereby also increasing the yield strength.
The strengthening effect of the second phase is also related to its size, shape, number and distribution, as well as the strength, plasticity and corresponding hardening properties of the second phase and the matrix, the crystallographic fit between the two phases and the interfacial energy. For the same volume ratio of the second phase, the elongated plasmas significantly influence the dislocation motion, so that the yield strength of a metallic material with such an organization is higher than that of a spherical one.
In summary, characterize the metal trace plastic deformation resistance of yield strength is a composition, organization is extremely sensitive to the mechanical properties of the index, affected by many intrinsic factors, change the alloy composition or heat treatment process can make the yield strength to produce significant changes.
Influence on the yield strength of extrinsic factors
1. temperature - the general temperature of the metal material yield strength decrease, however, the crystal structure of the metal material is different, the trend of change is not the same.
2. strain rate - when stretching, the loading rate increases, the strain rate increases, the strength of the metal material will increase. This is mainly because, any kind of metal has its own plastic deformation propagation speed, if the loading speed is greater than its own plastic propagation speed, will inevitably lead to an increase in the yield point. This is because if the loading rate is too fast, the rotation of the crystallographic plane in the direction of the external force is insufficient, and slip is impeded in the growth and expansion of the specimen, which manifests itself macroscopically in the form of an increase in the resistance to the initiating plastic deformation. This is, with the generation of deformation hardening, the spontaneous elimination of the hardening of the response can not be carried out, and deformation hardening will prevent the continued development of deformation, so to achieve the desired residual deformation, it is necessary to continue to increase the external force, which is also manifested as an increase in resistance to the initial plastic deformation.

3. Stress state - stress state on the yield strength of metal materials is also very important. The greater the component of shear stress, the more conducive to plastic deformation of the material, the lower the yield strength, so torsion than the yield strength of tensile low, tensile than the yield strength of bending low, the same stress state of the material yield strength is different, is not a change in the nature of the material, but the material in different conditions of the performance of the mechanical behavior of the different only. We usually say that the yield strength of the material generally refers to the yield strength in one-way stretching.
How to improve the yield strength
1. Alloying modification
Alloying modification is a common method to improve the yield strength of metals. Through the addition of elements in the metal, the formation of solid solution, precipitation hardening phase or interstitial solid solution, etc., to improve the microstructure of the metal, thereby improving the strength of the metal. For example, the addition of rare earth elements to aluminum alloys can significantly increase their yield strength.
2. Heat treatment
Heat treatment includes annealing, quenching and tempering methods. By controlling the temperature, time and cooling rate of heat treatment, the grain size of the metal is refined, the grain boundary is purified and the dislocation density is increased, so as to improve the yield strength of the metal. For example, quenching can significantly increase the yield strength and hardness of steel.
3. Cold work hardening
Cold work hardening refers to the increase of dislocation density through cold work deformation of metal and enhancement of strength and hardness of metal by hindering the movement of dislocations. Usually use compression, stretching, bending and other cold working methods. For example, copper can significantly increase its yield strength after tensile deformation.
4. Grain boundary engineering
Grain boundary engineering is a method to improve the yield strength of metals by utilizing the effect of grain boundaries on material properties. By controlling the interaction of metal grain boundaries and the hindering effect of dislocations, the yield strength of metals can be significantly improved. For example, the yield strength of copper can be significantly improved by adjusting the grain boundary angle and grain boundary morphology.
5. Surface treatment
Surface treatment is a method to improve the yield strength of metal through surface modification. For example, the use of chemical copper plating technology can make the steel surface to form a layer of uniform copper coating, so that the steel surface to form a new structure and organization, thereby improving its yield strength.
In summary, there are various methods to improve the yield strength of metallic materials, including alloying modification, heat treatment, cold work hardening, grain boundary engineering and surface treatment. In practical application, it is necessary to choose appropriate methods for improvement according to different material types and use environments.