Sir this is one of the best material science class ever. I was about to clap my hands when you explained the TTT diagram and how quench and age gives fine ppt.
@@letslearnmechanicalengineering if the precipitate is coherent, then the phase boundary is uniform enough (to the main lattice) to facilitate slipping of planes
great video sir, really helpful. I think GP Zones and the consequent precipitates and coherency should have been discussed, but otherwise great to understand the basic concept and i understand if it was intentionally left out during course design
Precipitation is phase transformation only. If it's true is the nucleation of precipitate happening homogeneous (as observed from diagram drawn in this video)
Sir, during MARAGING process in low C alloy(Co,Ni) steel, after quenching the steel it goes through age hardning process. Sir, my question is, in MARAGING process this age hardening process is carried out at what temperature, just above the martensite start temperature or just below the martensite start temperature? Also, if possible, please briefly explain the entire process of MARAGING. Sir, different sources online are providing conflicting answers.
Sir, why natural aged parts/material will get lower hardness after completion of their NA(96Hrs), there also having more time to form ppts with fast nucleation
After some time all precipitates have already formed. Thus no new precipitates can form. But the existing fine distribution of precipitates start to become coarse with time. This increases the interparticle distance, thus reducing the resistance to the dislocation motion and reduction of hardness.
That was so nice presentation. thank you sir I would love to ask if you can hold the same presentation but now using ternary phase diagram and talk about precipitation hardening if possible at any time from now
As we precipitate will increase the hardness value, can we apply this method to explain the hardness enhanced by addition of reinforcement in matrix composite material.
Yes. Dispersion hardening (say thoria-dispersed Ni) and hardening due to reinforcement in metal matrix composites can be explained similarly. Of course, in these cases there will be no time-dependence.
The two terms are synonymous. Age hardening refers to the fact that hardness increases with time(age). Precipitation hardening refers to the fact that the hardness increases due to the formation of second phase precipitates.
The way it is done in industry that peak hardness is attained by artificial aging at a higher temperature. Once this hardness is obtained the product is used at room temperature. At room temperature, although averaging will eventually take place, but the rate is rather low. So the component will maintain its hardness for the expected product life.
Precipitation at grain boundaries require less driving force and thus can happen even at higher temperature in comparison to precipitation inside the grains. Thus in slow cooling precipitates can form, but as the transformation temperature is high, most precipitates will form at gb. This is not good for hardening. This is the reason why quenching is required. Quenching, followed by ageing allows precipitation to happen at much lower temperature. Due to higher driving force available at low temperatures, precipitate form inside the grains as well, leading to more effective hardening.
You have said in your previous video that no change in the microstructure of Al alloy was found. But in this video, there is a microstructural change (change in the distribution of precipitates) which proves that " Property is a function of microstructure". Is that not correct?
It all depends on the scale of observation of the microstructure. The size of precipitated causing increase in hardness is very small and cannot be observed at the optical microscopy level. So for the initial investigators, like Wilm, no change in the microstructure was observed although there was change in the hardness. This was the surprise element. But later, with help of more powerful electron microscope fine precipitated were indeed observed. So, as you state, property is indeed a function of microstructure.
Sir, dislocation bowing is a mechanism of over ageing or ageing???because initially you said that dislocation loop leads to the difficulty in the dislocation motion ,leading to an increase in hardness ,but by using stress formula you proved hardness decreases??
When a dislocation negotiates a precipitate distribution it can bow between two nearby precipitates. The stress required for bowing is inversely proportional to the distance between the precipitates During aging, as more precipitates form, this distance between the precipitates decreases so the stress required for bowing increases. During overaging, the no new precipitates are forming. The existing fine distribution of precipitates changes to a coarse distribution with the same volume but less number of precipitates. This increases the distance between the precipitates thus reducing the stress.
@@syamukrishnan @34:04 it's mentioned loops increases hardness in coarse ppt, but when u refer formula it seems contradicting, but both statements are correct in their place.This is how I think it to be: 1. Loops increase hardness, so overaged alloys must have hardness higher than pure Al 2. But when u refer to formula, u are basically comparing stress required for fine ppt and coarse ppt
No, it is not really a nucleation problem, as the product phase is already present as fine particles. Only the average size of the particle increases. Thus in overaging the entire driving force comes from the surface energy. In heterogeneous nucleation, surface energy is a part of the driving force. The other part is the volume energy.
hi sir. i want to ask. if i want to do age hardening and hardness test afterwards, should i wait the material to cool down first after age hardening or should i immediately do the hardness test of the hot specimen?
