What is the effect of grain size on the mechanical properties of metals Why

After crystallization, the metal is polycrystalline composed of many grains, and the grain size can be measured in unit volumes.

The number of inner grains is described. The higher the number, the smaller the grains. For ease of measurement, it is often expressed in terms of the number of grains per unit section or the average diameter of the grains. The grain size of a metal has a great influence on many of the properties of the metal.

Grain size. The impact is, in essence.

Grain boundaries. Area size. The finer the grain, the larger the grain boundary area, and the greater the impact on performance.

Mechanical properties for metals at room temperature.

Generally, the finer the grain, the higher the strength and hardness, and the better the plasticity and toughness. This is because the finer the grain, the plastic deformation.

The smaller the concentration; And the finer the grains, the grain boundaries.

The more surfaces, the more tortuous the grain boundaries; The more opportunities there are for the grains to intersect with the middle of the grains, the less conducive to the propagation and development of cracks, the tighter they are to each other, and the better the strength and toughness. The table lists the grain size pairs.

Pure iron. influence on mechanical properties. As can be seen from the table, the fine grains play a great role in improving the mechanical properties of the metal at room temperature, so it is always desirable that the finer the grains of the steel material, the better.

However, in metal materials that work at high temperatures, it is not good for the grains to be too large or too small. Therefore, it is often desirable to get moderate.

Grain size. In some cases, it is desirable to have as much grain size as possible. For example, the manufacture of electric motors and transformers.

Silicon steel sheet. That's it, the coarser the grain, the less tape loss and the higher the efficiency. In short, grain size.

Right. Metallic properties.

The impact is manifold, to.

Case-by-case analysis.

Take the paragraph found on our teacher's courseware.

Hope it helps.

The effect of grain size is essentially the effect of grain boundary area size. The finer the grain, the larger the grain boundary area, and the greater the impact on performance. For the mechanical properties of metals at room temperature, generally the finer the grains, the higher the strength and hardness, and the better the plasticity and toughness.

This is because the finer the grains, the more the plastic deformation can be dispersed in more grains, so that the plastic deformation is more uniform and the internal stress concentration is smaller. Moreover, the finer the grain, the more crystal interfaces, and the more tortuous the grain boundaries; The more opportunities there are for the grains to intersect with the middle of the grains, the less conducive to the propagation and development of cracks, the tighter they are to each other, and the better the strength and toughness.

Grain sizeGrain size is a measure of the average size of the grains in a polycrystal. There are generally four ways to determine:

1) Determine the average size of the grains by microscopic observation;

2) Comparison with the specified standard grade chart under microscopic observation;

3) Count the number of grains in a certain area under the microscope to calculate the average size;

4) Observe the metal port and compare it with a specimen with a standard grain size.

The size of the grains after the metal crystallizes can be expressed in terms of the number of grains per unit volume. The higher the number of grains per unit volume, the finer the grains. Experiments have shown that the mechanical properties of fine-grained metals are higher than those of coarse-grained metals at room temperature.

This is mainly due to the fact that the finer the grains, the greater the number of grain boundaries, and the increase of resistance when the dislocation moves, which increases the plastic deformation resistance of the metal.

The physical properties of metal materials sometimes have a slow impact on the processing process. For example, the thermal conductivity of high-speed steel is poor, and a low speed should be used to heat up when forging, otherwise cracks are easy to occur; The thermal conductivity of the material has a significant impact on the temperature rise of the cutting tool. For another example, the melting points of tin-based bearing alloys, cast iron and cast steel are different, so the selected melting equipment and casting materials are very different.

Because by refining the grains, the mechanical properties of metal materials are improved: 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.

Usually metals are polycrystals composed of many grains, and the size of the grains can be expressed in terms of the number of grains per unit volume of penetration, and the greater the number, the finer the grains. Experiments show that fine-grained metals have higher strength, hardness, plasticity and toughness than coarse-grained ascender metals at room temperature.

This is because the plastic deformation of fine grains can be dispersed in more grains due to external force, and the plastic deformation is more uniform and the stress concentration is smaller. In addition, the finer the grain, the larger the grain boundary area, and the more tortuous the grain boundary, the less conducive to crack propagation.

The strength of the metal is related to both plasticity and grain size.

The first is strength, there is a Holpage formula, the strength of the material becomes stronger with the size of the grain, and the high strength will decrease greatly when the temperature rises, but this formula is not applicable to the time of nanocrystals, and the strength of nanocrystals is enhanced; However, the strength of single crystals does not follow the above rules, and the strength of single crystals is very strong and can still maintain strong strength at high temperatures.

Shaping means that the finer the grains, the better.

Methods of grain refinement, 1. Solidification control, including grain refiners (generally master alloys), grain refinement elements;

Shorten the solidification time, this should not be too much, too much will produce amorphous;

The solidification process applies external forces such as electromagnetic stirring to the physical fine grains.

2. Those that have been ingot can be forged and refined grains (this method can prepare nanocrystals under certain conditions, participate in the corresponding article of Academician Lu Ke); Cold working rolling can refine the grains in the rolling direction, and then crystallization can also refine the grains.