Not easy? material science
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I can answer this question consciously. I don't want to come, Zhihu's first novel. Don't flatter yourself, describe unprofessionally and ask for a pat ><
Many people have mentioned that this effect is not valid in the nanometer range, because the model used in nano-materials is different from that used in traditional materials. For material problems, most of them put forward reasonable models, which can explain the phenomenon of the problem, so different models can not be compared with each other. For traditional metal materials, it consists of small grains:
The above figure is a schematic diagram of three-dimensional and two-dimensional grains respectively. Fine grains mean that these grains are small and uniform in shape and size.
However, some defects will inevitably appear in the grains. These defects include tiny cracks and dislocations invisible to the naked eye. Because it is most relevant to this question, it is mentioned as two metaphors. Cracks are easy to understand, and the explanation of dislocations will tell you a story:
This is 193 in Britain. At Oxford University in London, Mr. frankl is studying his material deformation model. He said that the planes of atoms in matter move together. Everyone put their hands together (two rows of atomic planes), and when they rub together (all the bonds between atoms in the two rows of atomic planes are broken together), the atomic planes move with each other and the material is deformed. At this moment, my deskmate Taylor, a funny boy, stood up and said, Teacher, you talk a lot!
Then tell me what happened. Taylor: Teacher, you can't mop the carpet on the floor, but if I wear a bamboo pole in the middle of the carpet, make the carpet arch, move the arch slightly, and move the arch from side to side, it's equivalent to moving the carpet.
This is the famous carpet model, and this arch in the material is a dislocation, that is, the dislocation of the upper and lower atoms.
Where each lattice represents an atom.
Let's get down to business. When the material is stressed to a certain extent, the dislocations in the material will move, the dislocation source will start to produce dislocations, and the corresponding material will undergo plastic deformation. This phenomenon is called yielding, that is, the material is soft. When the force reaches a certain level, micro-cracks will appear in the material or the existing micro-cracks will expand, that is, the cracks will grow longer and larger, and the material will break if the cracks are too large.
The so-called plasticity refers to the ability of plastic deformation of materials, and the simple and straightforward understanding is the plastic deformation of materials before fracture. Therefore, if the microcracks in the material expand too fast, the fracture speed of the material will be faster and the plasticity will be worse. Strength refers to the ability of materials to resist deformation, and materials with high strength are not easy to deform (materials will bend a little when adding a lot of force). Generally speaking, in industry, the higher the strength, the worse the plasticity, but strength and plasticity are not one-to-one correspondence.
Next, I will talk about why fine lines can improve strength and plasticity, and finally get to the point. I'm exhausted.
Dislocations in grains will accumulate at grain boundaries, and at the same time, the accumulated dislocations will amplify the stress. The more they accumulate, the greater the magnification:
The larger the grain size, the more dislocations can be captured, which is easy to produce great stress on the grain boundary. The stress at the grain boundary urges the dislocation source in the next grain to begin to produce dislocation-induced deformation transfer (the force required by the dislocation source to start dislocation is greater than that required by dislocation movement). The smaller the grain, the less dislocations are captured, and the smaller the stress magnification, so more force is needed to start the dislocation source of the next grain (for example, the force required to start the dislocation of the next grain is 10, and the magnification of dislocation accumulation caused by large grains is 10, so the stress you need to add is 1, and the magnification of small grains is 5.
There are many reasons for the influence of fine grain on plasticity. After 1) grain refinement, the grain boundary area increases, the inclusions produced by segregation on the grain boundary decrease relatively, the interfacial bonding force increases and the plasticity improves. 2) Interfaces hinder the movement of microcracks. As mentioned above, the faster the microcrack grows, the easier it is for the material to break and the worse its plasticity. For cracks, the grain boundary is equivalent to a wall, and cracks are difficult to pass through. Now that there are more walls, cracks will naturally spread slowly and materials will not break easily. 3) Because the grains of the material are fine and uniform, the plastic deformation in the material is uniform (imagine that the grains in the material are large and small, and the plastic deformation caused by natural large grains is large), which reduces the formation of microcracks caused by large deformation concentration and urges the material to bear more overall plastic deformation before fracture (that is, it hinders the formation of microcracks).
This is the explanation of this problem. In fact, the strength and plasticity of materials are related to many problems. Here, we exclude other influences and only discuss the influence of grain size.
The logic may not be clear for the first time. This question is a bit professional, and it is estimated that no one pays attention and is sad. .
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Although no one is reading it, I'd better update it. Recently, I read a report about the failure of fine grain strengthening in nanometer range:
Recently, the research group of Mr. Wang Zhao of Frontier Institute recently published a paper on NanoLetters (impact factor 13.592), trying to give an answer: when the size of the material drops below 10 nm, atomic diffusion will increase rapidly due to the change of surface energy, and it is this micro-diffusion that makes the mechanical properties of the material change greatly, even breaking the hall that has been believed for many years. These results make it possible to explain the "smaller and weaker" phenomenon of nano-materials. In this paper, through Zener-Hollomon analysis, the relationship among micro-contact strength S, scale L, temperature T and strain rate R is deeply discussed, and a new model is given, which lays a certain physical foundation for subsequent experimental research. Interested in learning more can download and have a look.
Edited on 2015-10-15? Put away the comments? Thanks?
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Zhishimiao
In fact, as long as you continue to contribute professional answers, you will be found sooner or later. Look at the little pig god.
1 year ago
It is not easy to be indifferent (author)? Reply? Zhi Miao checks the dialogue.
Thank you for your encouragement. I can also share my knowledge and opinions.
1 year ago
Fan bowen
The answer is easy to understand. I studied materials science before. I want to ask another question, whether the existence of large-angle grain boundaries in the original structure can promote grain refinement in plastic deformation. Or is it that the finer the grain, the more large-angle grain boundaries there will be?
10 months ago
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20 160405 supplement, since it is the problem itself, put it at the beginning:
No wonder someone asks' why' instead of' isn't it'; Many people are also answering according to the idea of' yes'. After discussing with my brother the day before yesterday, he really found the source: fine grain not only improved the strength of polycrystalline, but also improved its plasticity and toughness. The finer the grains, the more grains per unit volume, and the same deformation amount can be dispersed into more grains during the deformation process, resulting in more uniform deformation, without causing excessive concentration of local stress and premature generation and development of cracks. ......
P572 Fundamentals of Material Science, Yu Yongning, 2006
In addition, someone mentioned the grain boundary slip of fine grains, which is a bit unclear. The condition of grain boundary slip (GBS) is diffusion. When the temperature does not reach a certain height, the contribution of GBS is very small. Its main use is superplasticity, but the premise of superplasticity is high temperature. This is not a topic with usual performance. The "strength" involved in superplasticity is the flow stress at high temperature, and the strength in the title is the tensile strength at room temperature.
1. Summarize its argument in one sentence: If you play the violin well, the symphony will be good.
2. Is this argument wrong? That's right. Playing the violin well has really improved the spread effect of the symphony to a certain extent. But that's not all. Not even the main part.
3. However, knowledge is divided into stages, and each stage is based on the previous stage. Made it stick said that different stages of learning have different stages of knowledge schema, that is, knowledge flow that can be used for oneself. Basic foundation, we can't talk about it all. If it is expanded, even 3000 pages of physical metallurgy can't go into any chapter in detail.
Next, I will talk about piano and orchestral instruments besides violin. Whether the whole symphony is good or bad in the end is judged by data.
Complement.
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The original text is as follows:
Don't be confused, strength and plasticity have always been mutually exclusive, which is the general trend. Without the support of special structure, the plasticity of fine grains is lower than that of coarse grains, and the plasticity is greatly reduced to nanometer level.
Rebecca, Chen Min, Zhou Feng, et al. High tensile plasticity of nanostructured metals [J]. Materials Science, 2002. Nature, 2002,419 (6910): 912-915.
To understand the relationship between strength and plasticity, it is necessary to deeply analyze these two indexes.
1, strength. Generally speaking, HP has a very good relationship.
The reason of strength is not only the grain boundary, but also the main source of some obstacles and dislocations. The root of dislocation density determines the strength of the material. The larger the grain boundary area per unit volume, the more dislocations per unit deformation and the higher the strength.
2. Plasticity. Usually related to work hardening. The hardening ability can be maintained, that is, the deformation increases and the strengthening increases, so that the strength of the local thinning area due to deformation will increase more because of large deformation, thus making up for the problem of low bearing capacity caused by thinning, making the load of each section of the sample close, and the deformation continues, thus better maintaining plasticity. Hardening ability is inversely proportional to grain size. For a detailed description and model, see.
Mechanical properties of materials by Marc Andremeyers, and work hardening by Krishan Kumachawla.
For dislocation, because there are too many people, the carriage becomes a stuffy tanker and can't be squeezed in. Therefore, unless there are other mechanisms to allow local areas to harden at the beginning of slimming, necking at the beginning of slimming will inevitably lead to fracture within a small amount of deformation. Low elongation is inevitable.
3. To sum up, in general, strength and plasticity are doomed to be chosen, which has been proved by years of practice and theory. Many topics derived from the H-P relationship 60 years ago can often be linked with science and nature because some people are constantly trying to find an "unusual" condition, so as to realize the "simultaneous" improvement of both. However, please note that both of them can be realized at the same time, but they are published in Mat's advanced and above nature sub-journals, and most of them can be summarized not only by the word "fine crystal", but also by special microstructures. So simply speaking, fine lines can improve strength and plasticity at the same time, which is only the undergraduate level. In addition, we can now give examples of Shuanggao, most of which came to the laboratory under bad conditions. How many examples of actual production can cross this "extraordinary" requires field workers to think hard.
In addition, please think about whether the mutual exclusion relationship between the two has really been eliminated under "unusual" circumstances (I have been thinking about this issue for some time). Whether this question should be a question that should ask "yes" first and then "why".
What is the reference frame of 1 and height? This standard must be clear, otherwise people who fish in troubled waters will confuse the audience and write it on paper, which will make people confused but unable to understand. The strict conclusion of double heights needs a strict contrast premise. For example, compared with coarse grain, other parameters of fine grain and coarse grain must be on the same starting line, and there is only one parameter, grain boundary ratio. If so, at least the data I have made concludes that the elongation of coarse grains is high. All the papers and monographs I read also support the conclusion of mutual exclusion. Anyone who has read a paper with double height and only one variable, please increase the granularity and we will discuss it.
2, fine crystal double height = open a small stove. If other variables are added at the same time of fine crystallization, it can be double high. It is understandable to mistakenly think that the height is high and not aware of the fraud.
3, multi-channel double height. There will be many results of double height mentioned below. Interested friends can consider whether this double height is caused by fine grain or other reasons. It is only necessary to note that the microstructure affects the properties, and the grain boundary ratio is only a small point in a series of microstructures including grain size, grain size distribution, grain boundary properties, second phase volume fraction, morphology and properties.
Supplement to friends who look at this issue from an academic point of view:
Is there any way to achieve double height?
Yes, although it is not a fully mature system.
1, it is necessary to understand that the factors affecting strength and plasticity seem to be mutually exclusive, but in fact there are other relations.
Strengthening needs dislocation fixation; Plasticity requires dislocations to move easily.
However, this easy exercise is superficial. What the actual ductility requires, of course, is not the free movement of a single dislocation, but the "easy movement and simultaneous movement" of dislocation movement in the whole sample. If this linkage is not easy to understand, imagine hundreds of people waiting in line for the bus. If everyone can queue up consciously and move in an orderly and efficient way, everyone will get on the bus soon. On the contrary, if some people in front crowd to the door together and the door is blocked, no one can get on the bus smoothly, and the people behind have no chance to move forward, let alone get on the bus. Here, the people behind can't do the "easy action" required by the overall plasticity because they don't have the opportunity to exercise, and the plasticity they carry hangs up before they go on stage, which is a waste. The simplest understanding is that a mouse excrement has ruined a pot of porridge. Therefore, to ensure the overall plasticity, we must not allow local "crowding" and no mouse excrement. What shall we do? Divide it equally. We must try our best to distribute the elongation of the material evenly to every place, every grain and every finer unit. It is certain that the Qianli levee was destroyed by the ant nest, and the significance of plasticity lies in the germination of the ant nest to the collapse of the levee. What we have to do is to slow down the expansion and collapse of the cave and let the wounded die slowly, although they will definitely die.
Invalid scheme: simple 1, solute; ; 2, precipitation; 3. Particle size. These factors often hinder dislocation. It's like beating the earth of the levee with a rammer, which strengthens the strength of the levee. However, if a nest is formed, even if it is random, this reinforcement is unable to repair the nest. Moreover, the stronger the matrix, once a loophole appears, it will basically accelerate the expansion of the loophole, and the collapse is a very fast process. Therefore, strengthening the matrix can not delay the corrosion resistance of the matrix after damage, which are two different things.
2, if you want to double your height, plasticity is the difficulty. If you want plasticity, you need to share the deformation equally. In order to distribute the deformation evenly, there needs to be a mechanism: when there are signs of local ant nests, go up and block the budding holes. This hole is blocked, but generally speaking, the damage (material elongation) is bound to increase gradually, so if these injuries are dispersed, each part will be distributed to other places in a smaller dose to prevent all the injuries from concentrating here and causing rapid collapse and wasting the resistance of other areas. The smaller the dose, the more points are shared, and the more evenly shared. Everyone bears the damage that he should bear and exerts his plasticity as a whole. This is good plasticity. The core index to realize this damage sharing is strain hardening, that is, work hardening. If you hit him, he will become stronger, and the injury will always bully the weak and fear the hard, thus forcibly transferring the injury to other areas. At the same time, all areas were injured once, and everyone got a round of seven-injury boxing, and their bodies were strengthened. Of course, the intensity of the coming injury becomes worse.
Hurt and struggle, do it again.
Pure fine lines must be strain-damaged and work-hardened, because fine lines limit the attack of injury, just like in a cage, you can't even hurt yourself and your fist can't be opened. The body will not be strengthened unless it is hit. If it is not strengthened, there will be insufficient resistance. It can only wait for the damage to collapse, and its plasticity is naturally not good. So it is wrong for me to say that fine crystals are high and low, which is the level of civil science. The teacher said this only because you are still in the undergraduate stage; In the doctoral stage, no teacher will tell you this. If the teacher says this, it only means that the teacher doesn't read books or documents.
If you can reach the double height, you must achieve self-injury in the cage in order to prepare for systematic injury. What can we do? We can't beat ourselves, and the space is narrow. It's also very simple. Just hit yourself with a massager, electric batons, soldering irons, pacemakers, piston hammers, and anything that is not limited by space.
Specifically, it is to implant enough work-hardening tissues or structures in the matrix during deformation, and constantly beat yourself when the damage strikes violently, trying to be on an equal footing with the market. What is effective now, at least as reported, seems to be effective: science, hard intermetallic compounds, such as B2 (natural, Pohang), bimodal grains (large responsible for plasticity, small responsible for strength, natural, Johns Hopkins), all of which are theoretically feasible, at least what is made by thumb is feasible, although most of the actual products are not feasible or greatly discounted. It is also important to know the size effect.
In addition, it needs to be clearly distinguished, and there is a clear difference between industrial production and laboratory research. At least according to my data experience, extraordinary performance is always the second-class pursuit in production, and qualified is the first class.
Edited on 20 16- 10-02? 3 1 comment? Thanks?
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Stick to your heart? mechanical engineering
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Metals are polycrystals composed of many grains. The size of grains can be expressed by the number of grains per unit volume. The more numbers, the finer the grains. Fine grained metals have higher strength, hardness, plasticity and toughness than coarse grained metals at room temperature. This is because the plastic deformation of fine grains can be dispersed in more grains when subjected to external force, and the plastic deformation is uniform and the stress concentration is small; In addition, the finer the grain size, the larger the grain boundary area and the more tortuous the grain boundary, which is not conducive to crack propagation. Therefore, the method of improving the strength of materials by refining grains will become fine grain strengthening in industry.
At room temperature, the strength and hardness of grain boundaries are greater than those in grains, and the grain boundaries increase after grain refinement, which is bound to improve the strength and hardness of materials. The plastic deformation process is accompanied by dislocation movement. Grain boundaries will hinder dislocation movement. The finer the grain, the more grain boundaries, and the stronger the hindrance to dislocation movement. Many dislocations gather at the grain boundary, forming dislocation network and dislocation wall, which brings more difficulties to the further deformation of materials. The plasticity of the material will also be improved accordingly.
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