[MUSIC] Dear students in today's lecture we are going to talk about dynamic recovery and dynamic recrystallization. So in previous cases, we have defined what is cold working, right? So cold working is the deformation below recrystallization temperature. And along this line of logic, we can also define hot working. So, hot working is just the deformation above this recrystallization temperature. And then the intermediate temperature levels you can also define this warm working, okay? So, the hot working processes by intuition is fairly complex than cold working. Because in hot working, you have recrystallization and recovery, and you also have deformation. And in cold working, you only have deformation. And that is the reason why your hot working is usually much much more complex than cold working. And you usually have a competition between your work hardening and recrystallisation recovery softening. And the hot working can be divided into two types. One is dynamic recovery, dynamic recrystallization, which occur under mechanical load at high temperatures. And the other type is so called static recovery and the static recrystallization. And this occur after the withdrawal of external load at high temperatures. So, you can easily understand this so called static recovery and recrystallization. Are essentially the same as what we have discussed previously. The only difference is that the heat generated is from the residual heat during this hot working process. And the properties of the final product is as we have discussed, is determined by the competition between hardening and softening mechanisms during hard working. Especially in the dynamic recovery and dynamic recrystallization processes. And in particular, the deformation working will lead to the strain hardening or work hardening of your material. However, your dynamic recovery and recrystallization will lead to the softening of your mechanical properties. So, we will start our discussion from dynamic recovery stress-strain response first dynamic recovery curve. So consider we have a deformation at constant temperature and constant strain rate. So it's just for your information. Usually, when temperature is at constant, when you increase the strain rate, your stress for plastic deformation will go up. And if strain rate keeps at constant, if your temperature goes up, your stress required for plastic defamation will usually reduce. So that's the usual case in general. So anyhow, during dynamic recovery where your deformation temperature is high enough for recovery. And when you do a tensile test like this, when you go beyond your elastic regime. Initially you usually will have a work hardening regime, which is associated with the accumulation of dislocations, increase in dislocation density, and the entanglement of dislocations. However, what happens is that the recovery process which is associated with the annihilation of point defects, and the rearrangement of dislocations. They will usually or they will gradually catch up with this hardening process, and leading to softening. And eventually, you will have competing two processes balanced each other and then you will have a relatively flat and cost of flow stress during plastic deformation, okay? So, this is this scenario of your dynamic recovery and for dynamic recrystallization, the situation is fairly more complex. So, this will the shape of the stress strain response upon dynamic recrystallization really depends on the screen rate of the test. In the case of high strain rate test, where you are pulling the sample or compressing the sample at high velocities. The shape of the stress-strain curve will be something like this, okay? So when you load the sample beyond a certain stress point, then the stress and the associated stored energy into the system provides sufficient driving force for recrystallization to take place. And then eventually, you will have a strain hardening at the beginning because dislocation accumulation increase in dislocation densities. And eventually it will be balanced, balanced by your recrystallization at this maximum stress value. So that afterwards, you will have a gradually decreasing or softening process, which is dominated by your recrystallization softening kinetics, okay? And eventually, because you are at a high strain rate, because you are at a high strain rate, the baseline of your strain hardening deformation process will dominate. So you end up with a fairly constant flow stress for plastic deformation. So the next two slides are just a descriptions of what I have said. And the situation will be different if you have a low strain rate test. So at a low strain rate test, the regime one will be the same, right? You still need this critical stress in order to have sufficient driving force for dynamic recrystallization to take place. However, in the low strain rate regime, you will have a similar kinetics of your hardening process, and your softening process. So the similar kinetics will end up and will result in this periodic modulation of your stress-strain responses. So at low strain rates you will usually have this periodic modulation and zigzag shaped curve are your stress-strain response. So in today's lecture, we have talked about the dynamic recovery and dynamic recrystallization. Thank you very much. [MUSIC]
