Speed of Movement Through A Solid Rod - Page 1
Forum section: electronics / general

author: German_EE
author_rank: Super Contributor
timestamp: September 13, 2019, 05:24:21 pm
content: I sometimes mentor a young neighbor, he's into electronics and well advanced for his age, unfortunately this also means that he sometimes asks awkward questions.The afternoon was spent showing that although radio waves travel at the speed of light an electrical signal doesn't, we proved this using two different lengths of coax and the scope. So, he says, how fast does something travel through solid material?Suppose you have a metal 1m rod, you push at one end and the other end moves in the same direction, is there a delay and, if so, what causes it?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2019, 06:11:15 pm
content: Mechanical movement is related to sound waves. So the speed to measure would be the speed of sound in the solid. This can actually be a possible experiment, e.g. with a a suitable sample and two piezo-transducers. The more indirect way is using just 1 transducer and look for resonance / refection.

author: Brutte
author_rank: Frequent Contributor
timestamp: September 13, 2019, 06:15:16 pm
content: Before any googling, I'd say that every dx has a mass and since cross section has a surface, there is a Young modulus involved (elasticity modulus). If at t=0 you apply force at free end then this has to be opposed by inertia of the dx section, stiffness when under tension that elongates dx, plus the force that pulls the next section. It is effectively a chain of dx masses tied together by (linear) springs.I think tension should be quite easily solved analytically because it seems a linear differential equation of a single variable (just a hint). The compression is more tricky because of the buckling (2D problem at least).

author: Dubbie
author_rank: Unknown
timestamp: September 13, 2019, 06:20:30 pm
content: I think this should answer your question thoroughly:https://booksite.elsevier.com/9780120864300/pdf/sample.pdfActually if you have a decent oscilloscope, with a few components such as piezo mics, this could be a fun thing to measure with him. Get a length of rebar and whack one end with a hammer and measure the delay between impulses at each end.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 13, 2019, 06:56:21 pm
content: The mechanical equivalents to inductance and capacitance, which give rise to propagation velocity and impedance, are elasticity and density.Just as we may approximate a transmission line as an inductor or capacitor (or some combination depending on what frequency range we're concerned with), a spring or mass is the low frequency approximation of a mechanical beam.Sound waves in a solid come in two directions (longitudinal or pressure, and transverse or shear), so there are more solutions than for EM waves (transverse only). There is a different modulus of elasticity for each mode, and so a different velocity and mechanical impedance.Beams also have many more degrees of freedom, beyond just the propagation modes; a rod, attached rigidly to "ground" at one end, has two directions of flex, one stretch, one twist, and all the standing wave modes for all of those axes. And all the couplings between those modes, which is why for example, when you twang such a rod (exciting the lowest standing wave mode in flex), the rod tip seems to go through orbits, eventually putting all the energy into a perpendicular flex direction, then back again and so forth; there is a small coupling between axes due to nonuniformity of the rod (I think?), which causes the oscillators to oscillate (the visual equivalent of a double-peaked IF coil).Or a coil spring and mass system, where the stretching converts to torsion and so on.So it's a bit of a rare convenience, or contrivance even, when we can approximate a mechanical system as few elements (i.e., springs and masses), so simulations or testing are best for analyzing the vibration response of mechanical systems; but such models are still useful as a representation for a particular mode, and are good learning tools.Tim

author: FreddieChopin
author_rank: Regular Contributor
timestamp: September 13, 2019, 06:58:59 pm
content: I also knew a skilled kid that wanted to learn common car repairs from me. Fortunately, just in time I learned that his single mother will often fill bogus child abuse charges against anyone helping him. BTFO'd and told him learning from me won't be possible after all because of safety reasons. Take care, know your rights and if possible record your every meeting. Being called pedophile will stick with you for years, even if you get your name cleared getting a job in your home town may be pretty much impossible.

author: Zero999
author_rank: Super Contributor
timestamp: September 13, 2019, 08:27:43 pm
content: Yes, there will be a delay between the end of the rod moving, after the opposite end was pushed.As mentioned above, it's related to the speed of sound. In very basic terms. When the rod is pushed, the end being pushed, will compress for a short period of time, like a spring and the other end won't start to move, until the compression has moved through the rod. An easy way to visualise this is a train being pulled by a locomotive. Anyone observing closely will notice that the wheels on the loco start moving before those on the last carriage, because the couplings stretch, when it starts accelerating, starting at the loco and finishing at the other end. The same will be true when it stops or the locomotive is pushing the train: the couplings will compress, starting at the loco's end.

author: Red Squirrel
author_rank: Super Contributor
timestamp: September 13, 2019, 08:48:21 pm
content: Quote from: Kleinstein on September 13, 2019, 06:11:15 pmInteresting for some reason I always thought it was speed of light. It's actually kind of mind boggling to think that there even is a delay. Like if you push a long rigid rod you see the whole rod moves in unison, but in reality there is a wave of movement before the other end starts to move.

author: magic
author_rank: Super Contributor
timestamp: September 13, 2019, 09:23:41 pm
content: Quote from: Red Squirrel on September 13, 2019, 08:48:21 pmTry again with jelly instead of steel, that's certainly gonna be slower than light

author: coppercone2
author_rank: Super Contributor
timestamp: September 13, 2019, 09:27:25 pm
content: pull a tea/flower pot mat (whatever its called) across a table (the kind that is woven grid), its like atomsthe thing that prevents nice tables from being scratchedI think they even have ones that are a bunch of balls with weave between them to make a 'net'. they are pretty limp so you can see the space between the beads or weave intersections dilate before the whole object is moved. keep in mind you can put a tiny force on a steel rod that won't move the other end I think.. gravity would just be like glue. in a steady state condition with forces on it, it will be a slightly deformed objectbut its like 'over dampened'.. if you make a net with a buncha ball bearings glued together by springs. if he asks about 3 dimensions say its like graphenea box spring matress interior would work too,. if you can gut one.what could help you prove this would be dial / test indicators

author: StillTrying
author_rank: Super Contributor
timestamp: September 13, 2019, 09:35:57 pm
content: I've roughly measured that the speed of sound across a surface is 3 or 4 times faster than through air.I suppose you could glue a cheap piezo disk to one end of a metal bar and tap the other end with a small hammer using the metal to metal contact as the start time.Tapping cheap piezo disks in the ideal center position I've always found there's a delay of 2 or 3us before they start to output a voltage.https://www.romanblack.com/SonicRanging/Sonic_Ranging.htm

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 14, 2019, 01:53:35 am
content: From the Engineering Tool Box. There is an order of magnitude range of the speed of sound in "common" materials. Facts like this were used before digital to contrive delays and do other time shifting tasks.

author: German_EE
author_rank: Super Contributor
timestamp: September 14, 2019, 09:00:11 am
content: OK, thank you for all of the replies, it sounds logical that the propagation delay would be the speed of sound through a solid so, lacking two suitable transducers, I will have to think up some other way of demonstrating it.As for the other concern, I'm not too worried. For the first year or so he came with his mother and whilst we tinkered in the workshop she sat on the couch and read her gossip magazines. He now comes on his own and we're now friends with the entire family so trust seems to flow both ways.

author: woodchips
author_rank: Frequent Contributor
timestamp: September 14, 2019, 09:11:08 am
content: Don't forget that the very early, 1960's, electronic calculators used ultrasonic delay lines to store the number in. Quite a bit around to read if interested.

author: tggzzz
author_rank: Super Contributor
timestamp: September 14, 2019, 09:40:26 am
content: Quote from: woodchips on September 14, 2019, 09:11:08 ams/60/40/s/calculator/computer/https://www.computerhistory.org/storageengine/edsac-computer-employs-delay-line-storage/

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: September 14, 2019, 12:41:21 pm
content: After the young lad has grasped the idea of sound wave propagation and newtownian transfer of force, (I would begin with a Newton's cradle) you can then explain faster than sound propagation of high explosive detonation. Boys of all ages love that kind of thing.I think the prevailing theory is that light from the detonation wavefront causes micro detonation nucleation sites ahead of the main wave, thus ends up being considerable faster than local speed of sound.

author: GlennSprigg
author_rank: Super Contributor
timestamp: September 15, 2019, 11:41:22 am
content: Quote from: chickenHeadKnob on September 14, 2019, 12:41:21 pmI'm not sure if you are talking about the following, regarding "micro detonation" ?? (Investigated).On the latest new Mythbusters they noticed a light-flash, ahead of a bullet (ultra slow motion), penetratingsome ballistics gel, involving cavitation. They called the effect 'Sonoluminescence'. (Caused by sound).Other people have called it the "Diesel Effect", due to high pressures causing gasses to actually Detonate!!As in the following Youtube example....https://www.youtube.com/watch?time_continue=55&v=6yO6y48XiSIHowever, 'Sonoluminescence' seems to be a different thing, when a 'bubble' in liquid is collapsing after beingbombarded with sound waves, as shown in the following exaample....Interesting stuff, all the same !!

author: StillTrying
author_rank: Super Contributor
timestamp: September 15, 2019, 11:54:10 am
content: Quote from: German_EE on September 14, 2019, 09:00:11 amYou can often get a usable piezo disc from something that beeps such as an old smoke alarm.I've got some lying about I might try to measure how long a tap takes to travel through the length of a 30cm rule.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 15, 2019, 05:28:04 pm
content: Quote from: StillTrying on September 15, 2019, 11:54:10 amWith the easy way to mount piezo discs on a ruler you will be generating and receiving shear waves. Much slower than the compression waves. You can couple the disks to a ruler for compression waves but it will take some additional pieces.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 15, 2019, 07:25:09 pm
content: Quote from: German_EE on September 13, 2019, 05:24:21 pmIt depends on how hard and fast you "push".Normally, the force is propagated at the speed of sound in the solid, liquid, or gas. So, ifvsis the speed of sound in that, andLis the length or distance, the delaytist=vsL. Note thatvsis not a constant, but varies depending on pressure and temperature and composition; see e.g. the WikipediaSpeed of soundarticle.If, however, you hit it with something hard and fast, you will also get a supersonicshock waveracing ahead of the sonic/acoustic wave. (The definition of a shock wave is a change that propagates faster than the speed of sound in the material at hand.) Shock waves do not have any specific velocity, as their velocity and intensity will drop fairly rapidly; their range is relatively short as well, so the sonic/acoustic wave will eventually catch up to it (unless the piece of material is small). For say a meter long piece of steel, the impact at one end would have to be quite severe for the shock wave to reach the other end, so for anything largeish, I'd definitely go with the velocity of sound in said material for the answer here.The rest of this post is speculative, as I've observed it in simulations, but haven't yet encountered any actual papers confirming or rejecting this -- meaning, this can be actual physical phenomena, or just a peculiarity in molecular dynamic simulations, or just an error caused by me setting incorrect/unphysical initial conditions.When two hard solid surfaces impact at slower than sound speeds, a shockwave can be generated: consider something like a heavy hammer hitting a metal surface. This seems to occur because the initial contact occurs in a very short timespan, not as fast as the speed of sound, but with a lot of force (momentum) behind it, pushing the atoms much closer to each other than they get at ordinary acoustic wave phenomena. I believe this forceful fast "push" basically loads the surface atoms with sufficient kinetic energy exceeding normal acoustic phenomena, similar to what happens in supersonic impacts. (This also means that if my observation is correct and physical, the definition of a shock wave would not depend on thevelocityormomentumof the impact, but on thekinetic energydensity associated with it. It does make sense to me, though.)If my understanding based on simulations is correct (i.e., matches physical reality), it makes things like hammer forging really interesting. Not only does it affect the internal structure of the metal being forged due to physical deformation, but also at the atomic level. Shock waves are irreversible phenomena, and increase entropy; in solids, this means it causes scattered small (atom-scale) defects in the material. Metals are usually not monocrystalline, but formed from small grains of crystalline structure, separated by those grain boundaries. Given enough stress, these grains will slide. The larger the grains, the easier they will slide. So, the smaller the grains, the harder the metal. Adding atomic scale defects is kind of adding the smallest possible grains, that stressing the material will affect first; thus making the material stronger and harder. If my observation is correct, this explains why hammer-forging metals often gives the best tensile strength and hardness properties, compared to other methods.Heat treatment (annealing) fuses grains together, reducing hardness but gaining tensile strength. Very hard materials tend to shatter. Higher tensile strength means the material will have a little bit of give in it, so that it deforms rather than shatters: the structure has places where damage due to stress is localized, rather than the entire object shattering along large grain boundaries. I am not exactly sure of how or how much it affects scattered atom-scale defects in the material, as proper simulation of annealing is hard (it takes too long to simulate the practical rates at which annealing is done -- simulations basically doingquenchinginstead ofannealing, the difference being the rate of cooling), but I suspect they tend to remain intact in annealing compared to actual grains, because atomic-scale defects residing in a crystal lattice do not have any nearby defects to merge with, unlike grains who share boundaries.

author: StillTrying
author_rank: Super Contributor
timestamp: September 15, 2019, 07:47:52 pm
content: Quote from: CatalinaWOW on September 15, 2019, 05:28:04 pmThere's only one disc in my version.I usually tap the very center of the back of the disc, which usefully also gives a metal to metal electrical connection to measure the output response time.I'd have hard flat surface, piezo disc, end of rule in contact with center of disc, and the tap the other end of the rule with the small metal bar, so nearly all compression wave.Getting the front of the disk in solid contact with hard a flat surface (a small slate tile) is usually the hard bit because of the wires soldered on that side.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 15, 2019, 07:58:53 pm
content: Quote from: Nominal Animal on September 15, 2019, 07:25:09 pmHmm, atomic scale defects can't last very long at forging temperature, though -- diffusion is fast on the order of microns (hence the transformation of micron-scale structures in steel, on heating and cooling, and at different rates). On the atomic scale, I'd suppose it's imperceptible, essentially instantaneous?QuoteI'll admit to not knowing the particulars of atomic motion in solids very well, though.If impacts can cause surface shocks, that would be interesting for a cold process like shot peening. I don't know of any special properties, though. It's most often used just for cleaning?Tim

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 15, 2019, 11:32:45 pm
content: Quote from: T3sl4co1l on September 15, 2019, 07:58:53 pmIt depends on how the defects interact with the grain boundaries.I guess I should have run a few simulations before airing my speculations, and look at grain merging at forging temperatures; and compare the situation when there are a small number of atom-scale defects in each grain!Like I said, it is speculation, nothing verified; I just thought it might be interesting to others as well.Problem is, such simulations take a long time, because the simulation timesteps (femtoseconds) are so far from the event timescales here. Monte Carlo simulations (finding the minimum energy configuration ignoring time scales completely) won't necessarily help either, as the grain boundary and atomic defect may effectively "repulse" each other, depending on atom species and structures involved, requiring higher-energy events for merging to occur; that should be investigated first.(This is also one of the nasty cases to simulate using traditional potential models, because both surface and bulk effects are involved. For many metals, potential models can be fitted well to either surface or to bulk properties, but poorly to both. The sizes (number of atoms, or electrons rather) needed is also such that ab initio/quantum mechanical computing models are definitely too slow to do this.)Note that higher temperatures cause grain boundaries to merge, increasing the grain size. However, hammer forging is said to "align atoms", but in a different way to merging boundaries. As far as I understand, exactly what happens in hammer forging at the atomic scale is as of 2019, unclear; explanations exist, but they are more like hand-waving than actual results from peer-reviewed research. We do know hammer forging affects the material properties, and know the macro/human-scale effects of it pretty well in detail; it's just that at the atomic scale, we do not know exactly what and how that happens. I guess nobody has seen it interesting enough to devote the massive computation needed to find out.Quote from: T3sl4co1l on September 15, 2019, 07:58:53 pmThey vary (depending on atom species, whether it is the same as the lattice, and on the type of lattice), but generally, atoms are more mobile even in room-temperature solids than one would think. So, I don't think you are off the mark here; there is just stuff happening at the atomic scale that is unclear (at least to me), in interesting and real-world useful phenomena.For hammer-forged materials, we must also remember that the lattices are not pure, but contain different species of atoms. Various steels in particular are a bitch to model, because iron-chromium interaction is particularly complex, and the other ones like zinc, tin, carbon are not much easier either.Even at room-temperature, these "impurities" (or dopants or whatever) do wander around even in steel. (I wonder how many physicists have weirded themselves out because of that, after taking psychedelics; "solids" are not exactly static, after all. I haven't tried any myself, so I don't know.) It is difficult to say whether the single-atom impurities are just structural, or involve specific species of atom; or whether the shockwave breaks multi-atom impurities; or what. The grain boundaries tend to be both structural and little bit surface-like, too.For example, in ferrochrome (my specialty; 13-18% chrome, rest iron), the chromium atoms wander so that the density near the surface is highest, and lowest in the bulk material, with the transition zone several hundred nanometers, about micrometer scale. The chromium atoms aren't scattered, either, but form clusters (larger near the surface, smaller closer to bulk), and almost stalactite-like distributions of said clusters. This was verified only about a decade ago, by cutting and imaging a specially prepared sample at very low temperatures. Even at room temperatures, cutting a new surface tends to redistribute the chromium atoms (speed depending on temperature, obviously) fast enough for us humans to not really find any significant difference between "new" and "old" surfaces, except in presence of very corrosive materials perhaps.This distribution is at the core of how these materials resist oxidization, i.e. how rust-free/corrosion resistant materials actually happen at the atomic scale.Quote from: T3sl4co1l on September 15, 2019, 07:58:53 pmIndividual grains don't have much momentum or kinetic energy, so I'm not sure if they generate any shock waves; even if they do, it probably won't travel very far. The impacts do cause surface atoms from both to speed off.However, if we get the grains or particles at much higher speeds, you do get some very interesting effects.For example, inexplosion weldingthe surface atoms get such high energies, they become plasma (stripped of all electrons); this plasma is caught between the two surfaces, and scours the surfaces absolutely clean, to a depth of several atom layers. (A friend simulated this as part of their masters thesis a decade ago.)Explosion welding is mostly used in cladding materials with other materials, essentially similar to shot peening, except that the shot is tiny, tiny particles of the cladding material, the velocities are high, and it is done in vacuum. (You can also do it at the macroscopic scale, with actual slabs of material, but then the energies are such that the target objects tend to be damaged more than is appropriate.. using smaller "shot" yields less damage.)Anyway, even in explosion welding, there is no significant shock wave. If there is one, it only reaches a few dozen atom layers or less. This could be because the individual impactors in explosion welding are quite small, often tiny flakes or powder; but they do have quite high velocities. (For the macroscopic slab experiments, the overall damage is so high it is difficult to say what the shock wave contributed, if anything.)The kinetic energy at the surface results in some of those atoms turning to plasma (being stripped of all their electrons), and this plasma has a scouring/cleaning effect on the surface.If you reduce the shot media to individual atoms, ions, or clusters of atoms, you getimplantation. At high enough velocities, the velocity controls how deep the implanted ion or cluster gets before "detonating"; the path to the surface tends to contain very little damage, very localized to the path the ion or cluster traveled through. In simulations, a shock wave can be observed, but it has little effect compared to the other "damage". Ion implantation is of course a very widely used technique in semiconductor industry, and to produce various special materials.At very slow velocities, with individual ions or clusters, you get (physical) deposition, often used to create variousthin filmsin industry. (Chemical deposition and epitaxy are more common than physical deposition, however.)In summary, my speculation/suspicion is that the shock waves are relatively insignificant in other phenomena, but significant in metal forging, where hard objects impact with relatively slow speeds. Again, I could be wrong, but if I am right, this explains some interesting and useful effects.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2019, 01:19:32 am
content: Quote from: Nominal Animal on September 15, 2019, 11:32:45 pmIndeed.QuoteYeah, such is the nature of "tightly coupled" systems. Not much you can do but simulate a block of it, assuming the volume is large enough, and the boundary conditions appropriate, to obtain a meaningful result. Hugely intensive.And, chemistry in general: it's hard to appreciate that atoms flick around on the fs and sub-nm scale, when your chemical reaction takes ten fricking hours. But they do indeed, and figuring out the mechanics of those reactions, on such small scales of time and space, is a very recent thing; and very cool!QuoteYeah; it comes to mind that atomic defects in graphite are annealed at a mere ~250C (Wigner effect), while the overall crystal (i.e., including grain boundaries) is annealed at something ridiculous like 3000C. Obviously, the energy associated with the Wigner effect is exceptional, but plenty of other examples are out there. Sodium in silica glasses, for example (sodium ion impurity causes time-variable threshold voltage in MOSFETs at room temperature). And carburization of steel doesn't require all that much heat (hm, I suppose the main reason higher time*temps are used, is more just to get a thicker case hardening?).QuoteOoh, speaking of zinc and tin, and a few other metals under certain conditions -- "tin cry" is apparently a shock wave. That's associated with sudden slippage of particular planes with relatively high energy, and uh, I forget if twinning something or other is involved as well. Now I'm wondering, is there any defect "debris" associated with the dissipation of that energy?QuoteOhhh, that's cool as hell!Now, would that manifest as, say, you partially dissolve a sample in acid, and it's always got a higher Cr concentration than when you dissolve the whole thing? Does that only apply to crystals (diffusion blocked by grain boundaries) so you have to dissolve a very small amount (~um) or use large-grained material, to get a Cr-rich solution? Would that be hidden because you'd expect to dissolve more Cr anyway (high reduction potential)? (Or, is that "very corrosive" enough not to show the effect, and we're talking more like atmospheric corrosion processes?)Seems kind of funny, compared to chromium in solution: it tends to form "hard" ligands, i.e. that have fairly high binding energy and equilibriate very slowly with the solution. Obviously, the chemical environment of Cr surrounded by Fe atoms is more... slippery.Still so much crazy stuff we don't know about d orbitals, let alone f's... or extended p (hybrid p/d?) orbitals for that matter ("expanded octet" and beyond).And there's still boron...Tim

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 16, 2019, 05:03:02 am
content: Quote from: T3sl4co1l on September 16, 2019, 01:19:32 amYup. In Wigner defects, apparently the interstitial atoms tend to be paired with vacancies, explaining the large energy output when they combine, and the relatively low temperature needed to combine them. (Energy-wise, there is a potential barrier between each pair, with the barrier energy corresponding to about 520 K temperature. There is nothing specific that causes the energy barrier, it just emerges out of the structure; similar to how I speculate that tiny defects caused by shock waves are not as easily merged to grain boundaries compared to grains merging in ferrous metals.)The defects release quite a bit of energy (per defect) when they combine. Reminds me of how the atomic structure of Price Rupert's drops has residual stresses, essentially balanced internal pressure in the glass itself. A pressure wave from the right direction, and it shatters to tiny pieces.Quote from: T3sl4co1l on September 16, 2019, 01:19:32 amI haven't seen any (mention or simulation) where lattice plane slippage resulted in characteristic additional defects. That does not say much, though.I do know that if the energy density (local energy released) is high enough, you get all sorts of weird stuff happen, that is very hard to simulate realistically.(When I started learning about MD simulations, I once stabilized a molten droplet in solid Morse copper using acoustic waves for tens of thousands of time steps by mistake... I basically slammed two blocks of solid Morse copper unphysically hard together. And Morse copper is a very simple model too, not very "realistic" at all.)Quote from: T3sl4co1l on September 16, 2019, 01:19:32 amProblem is, chromium and iron have very different solubilities in acids (probably would have to use hydrochloric acid), so it'd be hard to dissolve a fixed "depth" or "layer" of the sample. (Consider e.g. Damascus steel, where different types of steel are mixed, and the end result etched in acid; the steels have different solubilities, so one type is etched more, possibly blackens?, and creates the contrast between the two.)However, if you shear off a thin slice of the surface, and analyze that, then yes: such a surface slice has much higher concentration of chromium than in the bulk material.In fact, if you do this in very low temperatures, and shear off a corner of the material, you can image the surface with scanning electron microscope, and get an actual image showing the chromium structure as it changes further away from the surface. This was, IIRC, what was done in the article I mentioned. (Darn it, can't find it on the web, and am too lazy to spin up my backup HDD with it. Don't even remember which one has it..)(I'm much more interested in creating better simulators than in running simulations myself, btw.)Quote from: T3sl4co1l on September 16, 2019, 01:19:32 amMy chemistry-fu is very weak, and I don't know how the presence of acid or other solvent would affect the Cr migration to the surface..Quote from: T3sl4co1l on September 16, 2019, 01:19:32 amFe and Cr atoms interact by *both*panddorbitals (which is the reason EAM models need two bands to model their interaction), which means there are actually magnetic effects involved.. darn hard to model in a physically relevant manner. There are features, like the formation energy of vacancies depending on the Cr concentration, that make the Cr migration toward surfaces "slippery"; it's not like the Cr atoms in there just wander randomly around all the time. It is darned interesting, and not least in that FeCr alloys are a good candidate for replacing tungsten (W) surfaces in fusion reactors. (Standard steel becomes very brittle in months, but tungsten is difficult and expensive to make and shape.)Quote from: T3sl4co1l on September 16, 2019, 01:19:32 amHeck, find a single potential model that can describe water (H2O) at the atomic level (not just at the molecular level) as liquid, solid, and gas correctly (including freezing, melting, sublimating, and deposition/desublimating), and you can likely make a career out of just that. (You almost certainly need a complicated H-O potential model to do that, with a relatively looong range compared to typical models. I.e., horrible. Many have tried.)

author: Psi
author_rank: Super Contributor
timestamp: September 16, 2019, 05:57:04 am
content: If you placed a little ant on a huge metal/concrete 4 lane motorway overpass bridge, guess what, the bridge will bend a little under the weight of the ant.You would be hard pressed to actually measure how much it bends, but it does bend, it has to.