Muddiest Points: Dislocations and Plastic Deformation of Metals

hello and welcome to this muddiest points video this screencast will go over dislocations and plastic deformation of metals some of the muddiest points received from students in regards to dislocations and plastic deformation where why do the ripples or dislocations occur in materials in the first place what exactly is a screw dislocation if a unit cell is most stiff about its closely packed direction why does it deform about it how is slip related to different crystal structures and how does it affect deformation and then ductility and plastic deformation was foggy so a little bit about plastic deformation plastic deformation is permanent deformation it means that the material won't ever bounce back or return to its original shape plastic deformation occurs as dislocations move along slip planes in metals plastic deformation happens all the time a couple real-world examples are if you had a soda can and crushed it in your hand you would never be able to get the soda can back to its original form so it would be permanently or plastically deformed another example is if you took a metal spoon and put it into cold ice cream and maybe when you pulled it out the spoon bends and eventually broke this would be an example of plastic deformation that led to failure car accidents are another example where you would see a lot of permanent deformation when the cars collide the bodies of the car actually absorb a lot of energy which leads to the permanent deformation and dents that you see during a car accident in dissipating this energy though they actually help keep the passenger safe so deformation does not have to be a bad thing dislocations form when materials solidify so there are already dislocations present in all types of materials the number of dislocations in a material can be controlled dislocations will form when stress is added to a material deformation occurs as dislocations move through a crystal lattice there are two types of dislocations the first being in edge dislocation an edge dislocation happens when an extra half plane of atoms is moved through a crystal lattice and comes out on the other side I want to share stress here and here is applied to a crystal lattice parallel to the slip plane and in a plane where slip is occurring this extra half plane of atoms shown here and here actually gets moved through the crystal lattice so you can see that it's here and here so you can see the edge of the plane here and here is where the dislocation is and this is where the edge dislocation gets its name as the stress continues here and here the plane is moved all the way through the lattice and emerges on the other side when this displaces the entire top half plane of atoms moving that part of the crystal lattice over a good model for understanding dislocation motion is the ripple in the rug model this model uses the idea of a rug placed on the floor which would represent two planes one on top of another so you'd have the blue plane here and then the green plane which you can't see but is underneath this one if you applied a force to the rug a ripple would form which you can see here which displaces the left edge of the rook the same thing would happen in a crystal lattice the left edge of the crystal was displaced by the shifted half plane as the force continues the ripple continues to move through the rug eventually the ripple emerges on the other side in the end the top plane is displaced by the same amount that the rebel displays the top plane in the second picture the same thing happens in a crystal lattice as the dislocation moves through it eventually emerges on the other side displacing the top half plane of atoms by one atomic spacing in the crystal lattice it takes a lot less energy to move a dislocation through the lattice than it would to shift an entire plane at once in the same way that it would be easier to shift a rug by moving ripples through than to slide the rug across the floor as a whole this is why materials to form the dislocation motion when stress is applied so if deformation occurs through dislocation motion how can it be stopped or slowed down dislocations moving or gliding on different planes can run into each other and get tangled as shown here this impedes dislocation motion so more stress would be required to activate motion of new dislocations to continue plastic deformation higher stress required for deformation means the material has higher strength because of this increasing the number of dislocations can actually strengthen the metal which is the idea behind cold working so we've already discussed an edge dislocation so now I'm going to discuss the next type of dislocation which is a screw dislocation with a screw dislocation a stress is applied here and here the front plane of atoms begins to shift so you can see that here the front plane of atoms is in blue and it is shifted over in reference to the back plane here so you can see the front plane here versus the back plane here the bonds attaching the top half plane to the bottom half plane break and then reform so you can see that the back here still has the original bonds here but the front has shifted over so the bonds have reformed here as stress continues here and here a dislocation glides across the lattice and eventually shifts the entire half plane over so you can see the dislocation line here which emanates from this point where the dislocation is moving across the crystal lattice here this type of dislocation is called a screw dislocation because the top is partially displaced here so if you were to actually travel along this dislocation it'd be like going down a spiral staircase or moving along the threads of the screw so if you went one spacing right one spacing down one spacing left and ones facing up you'd expect to be back where you started but instead it's actually off here just like if you had moved along the threads of a screw doing one full revolution around the screw should put you back in your original place but because of the threads you actually end up lower than you started the same thing happens with a screw dislocation the dislocation will continue to glide across the lattice until the entire top half plane is shifted over which you can see in this picture here and here in the end the top half plane of atoms is still shifted over just like it was with the edge dislocation because of that the final pictures look the same as they did with the edge dislocation this makes sense because both screw and edge dislocations are mechanisms for dislocation motion next I'll be discussing slip planes so in the schematics on the previous slides the bones were breaking along a line and then reforming in order to allow the dislocations to move this is easier along closely packed directions because the atoms are bonded more strongly which means they are closer together so it is easier for bones to break and then reform because of this metals to form along their closest packed directions on their closest packed slip planes so each of these closest pack directions on the specific slip plane would be a slip system so this schematic here is of an FCC crystal and this would have three slip systems so this one this one and this one on this slip plane deformation would occur along a single slip system so one direction on these parallel slip planes so you can see in this schematic here that the crystal is deforming along parallel slip planes along a single slip system so next I'll talk about the slip systems for the FCC and BCC crystals starting with the FCC crystal the subsystems in a material depend on that materials crystal structure so first I'm going to talk about slope systems in an FCC crystal the FCC or face centered cubic is the closest pack crystal with close packed planes and family one one one in this plane the atoms touch along face diagonals as you can see here which is this edge here this makes this one of the closed pack directions which would be in the one 100 family slip would occur along these directions in the one-one-one family of planes and each of these directions would be a slip system of the FCC crystal on the schematic here you can see the three slip systems of the 1 1 1 plane so I'm going to do a quick review of Miller indices if you wanted to find these directions on the 1 1 1 plane so to find this diagonal here I would draw my new axis here so we have X prime Y prime and Z prime and then for this diagonal you move one spacing and X 2 here so we went 1 you don't end up moving in Y so that's going to be a 0 and then you go down 1 and Z which is negative 1 and then you can connect your starting and ending points to see that you do get this face diagonal here so 1 0 negative 1 becomes 1 0 bar 1 which you can see is the same as this diagonal here so for the next diagonal here you can use the same axis here and for this one there is no movement along X so that's going to be a zero and then it goes one unit along Y so that's a 1 and then it goes down one and Z so that's a negative one and then again you connect your starting and your ending points and then this 0 1 negative 1 becomes a 0 1 bar 1 which you can see is this diagonal here so for this last diagonal here you have to move the axes because it does not go through this axis here so I moved my new axis to here so I have X prime Y Prime and Z prime and then for this diagonal it moved 1 in X so that's 1 negative 1 and Y so negative 1 and then did not move in Z so that's going to be a 0 and then you connect your starting and ending points to see that you did get this diagonal here and 1 negative 1 0 becomes 1 bar 1 0 which again is this diagonal here so you can see that with all three of these diagonals you end up getting a positive 1 a bar 1 or a negative 1 and a 0 so that's a quick review on how to find Miller indices to find the close-packed directions in the close-packed planes which would be the slip systems of your crystal so we've shown the three slip systems for the 1 1 1 plane here next we'll look at the slip systems of the 3 other planes in the 1 1 1 family here are the other 3 close packed planes and they're closed pack directions for an FCC crystal slip would occur along each of these directions or systems you can see that each of the close packed planes are in the family 1 1 1 and each of the close-packed directions are face diagonals and the family 1 1 o so total the FCC crystal has 4 slip planes these three here and then the one on the previous slide and each of these has 3 close-packed directions each of these directions in this plane would be a slip system which means that there are three slip systems per plane so 3 6 9 and then the three from the previous page means that there are 12 slip system is total for the FCC crystal all these slip systems mean that materials with an FCC crystal structure tend to be more ductile next I'm going to talk about slip systems in a body centered cubic or BCC crystal structure so unlike an FCC structure BCC crystals do not have a close-packed plane but they do still deform along close-packed directions the close-packed directions lie along body diagonals which you can see here the atoms are still touching even though the plane itself is not close-packed close-packed directions are group 1 1 1 which are body diagonals here and slip can occur along families of planes 1 1 0 which is this one here 1 1 2 and 1 2 3 so again a quick review of Miller indices to find this body diagonal I would set the new axis here have X prime Y Prime and Z prime to find this diagonal here I would go negative 1 and X 1 and Y and 1 and Z and connect the dots so we went negative 1 1 and 1 which becomes bar 1 1 and 1 and then you could do the same thing for the other body diagonal next we'll look at an example of each of the three families of planes on which slip can occur in a BCC crystal here are examples of the different slip planes for the BCC crystal unlike in an FCC crystal slip can occur in multiple planes so there are a lot of slope systems for BCC because there are no close packed planes though it is harder for dislocations to move so a greater stress is needed to cause deformation because of this BCC crystals tend to be less ductile you can see that although these planes look different each of them does have at least one body diagonal so the 1 1 o plane and that family of planes will have two body diagonals here the 1 1 2 plane has a body diagonal here and the 1 2 3 plane has a body diagonal here now I'm going to talk a bit more about a term that I've mentioned quite a bit which is deformation this is a stress-strain curve and if you aren't familiar with one of these there is a money as points video analyzing stress-strain curves so the stress-strain curve has two different regions the elastic region here and the plastic region here and the elastic region the sample will recover its original shape so it will bounce back and it will be permanently deformed once the sample reaches the yield point here it enters the plastic region and begins to undergo plastic deformation and the plastic region the sample starts to become permanently deformed but when unloaded when the stress is taken away the sample will still recover its elastic strain this means that if you were to remove the stress the sample would recover some if you wanted to see how much a sample would recover after a certain point on the curve you could draw a line trace down from that point of the curve parallel to the elastic region so then this would be the plastic deformation for this point on the curve so here is the elastic recovery for a number of points on the curve including this point here which is failure so the plastic deformation at failure is this here and if you multiply this value by 100 you will get the percent elongation or the ductility of the sample one example showing the different stages of deformation is a tensile test for a tensile test stress is applied in these directions and the sample begins to stretch in the elastic region the sample stretches but returns to its original shape if stress is removed once the sample reaches this point here which is the yield point the symbol begins to permanently deform at this point plastic deformation begins initially there is uniform plastic deformation which means that there is a uniform reduction of area once the sample is under the maximum amount of stress that it can take at this point here you can find its tensile strength after this point here the sample begins to undergo necking which you can see here and here necking is a localized reduction of cross sectional area which means that the area of this part here is being reduced while the rest of the sample remains constant the necking continues to reduce the cross sectional area until it reaches this point here which is its breaking strength and where you get fracture and the sample fails stress-strain curves can also be used to compare the properties of different models some of the properties you can look at our yield strength tensile strength breaking strength percent elongation and toughness so first the yield strength I've mentioned yield strength before this is the point on the curve where you switch from the elastic to the plastic region which means you begin to get permanent deformation so for example that would be like here on this curve you can see that the yield strength differs for each of these materials the deform stainless steel sheet has the highest yield strength while magnesium has the lowest yield strength way down here a materials yield strength depends on a number of different factors the next feature you can read from a stress-strain curve is the tensile strength tensile stress is the maximum amount of stress a sample can withstand this can be seen here at the highest point on the curve once again that a forum stainless steel sheet has the highest tensile strength and magnesium has the lowest strength the breaking strength is the point at which the sample actually fractures in this case stainless steel actually has the highest breaking strength even though it didn't have as high of a tensile strength or a yield strength as to form stainless steel the next property you can find on a stress-strain curve is percent elongation the percent elongation is the amount that the sample plastically deformed before breaking this can be seen by the length of the curve so again to find the percent elongation you would go out to this point here where you found the breaking strength and draw a line parallel to the elastic modulus and then this value here multiplied by 100 would be the percent elongation so you can see that the annealed nickel alloy sheet had the largest percent elongation which makes it the more ductile metal normally strength and ductility are inverse properties the more ductile a metal usually the weaker it is in vice-versa a stronger metal is usually less ductile you can see that with the deform stainless steel here this material is very strong but it is less ductile it's percent elongation is very low the final property you can see from a stress-strain curve is toughness toughness is the area underneath the curve you can see I filled that in for each of the metals here and the annealed nickel alloys sheet has the largest area underneath the curve which means it has the highest toughness followed pretty closely by stainless steel the annealed nickel alloy sheet had a high toughness because it had both a high strength and a high percent elongation so those are some of the properties of materials that you can read from a stress-strain curve next I'll review what we've covered in the screencast so in summary both edge and screw dislocations occur when stress is applied to a metal the motion of these dislocations causes permanent plastic deformation dislocations move along close-packed directions on more closely packed slip planes and more barriers to impede motion of dislocations will strengthen metals so hopefully dislocations and plastic deformation of metals is a bit more clear now just review the muddiest points that we've answered today are why do the ripples or dislocations occur in metals in the first place what exactly is a screw dislocation if a unit cell is most stiff about its closely packed direction why does it deform about it how is slip related to the different crystal structures and how does it affect on and then ductility and plastic deformation was foggy so thank you for tuning in to this screencast on dislocation and plastic deformation of metals

Channel: MaterialsConcepts

Views: 69,239

Keywords: materialsconcepts, materials concepts, materials science, engineering, materials science engineering, metal, dislocation, plastic deformation, slip, mse, planes, miller indices, crystal, direction

Id: npWhCY5Bv2Y

Length: 23min 26sec (1406 seconds)

Published: Thu Oct 01 2015