@Introduction to Materials Science and Engineering kindly help here sir ,pi(di)^2 is the surface area of one whole grain, it's not the grain surface area boundary ,why are you then dividing by 2 to get the grain surface area boundary per grain?
Professor Rajesh Prasad, thank you very much for the explanation! Please could you explain why the grain boundary is not the volume of a sphere? And why is the grain boundary per grain only half of the area? Thank you in advance.
Grain boundary is the surface between crystals of two different orientations. So it does not have a volume but only an area. A given grain boundary is shared between two grains. Thus to determine the area per grain we divide it by 2. Suppose two cubical grains, each of size L , are sharing their common face as a grain boundary. Then the boundary are is L^2. But as two grains are sharing it the boundary area per grain is (L^2)/2.
Sir, wondering if this process continues through some of the cooling stage, or if it only occurs when the material is being held at a constant elevated temperature.
Dear Professor Prasad, How can the misorientation between two grains be compensated after grain growth? Does the smaller grain dissolve and contribute to the growth of the larger grain? Then, what is the difference between coarsening (Ostwald ripening) and grain growth?
Grain growth refers to the increase of average grain size of a single-phase material. Ostwald ripening is the increase in the average size of a second-phase particle in a two-phase microstructure (second phase: precipitate, 1st phase: matrix). The driving force for the two has a common source: a decrease in surface energy. But the nature of the surface is different in two cases. In the grain growth, it is the decrease in the grain boundary (alpha/alpha boundary) energy. In the Ostwald ripening, it is the decrease in phase boundary (alpha/beta boundary) energy.
@@rajeshprasad101 Thank you very much for your kind explanation! I also watched your coarsening lecture and it was clear that smaller particles dissolve and larger particles grow due to their free energy difference. Do we also need to consider free energy, depending on grain size, in grain growth? I can understand the driving force is a decrease in surface energy, but not sure why the orientation of larger grains become dominant at the late stage of grain growth. Although this sounds more natural than the opposite case (larger one becomes a part of smaller one) but I would like to know if there is any specific reason...
@@introductiontomaterialsscience I initially thought the final grain orientation is an intermediate orientation between two grains because they are sharing interface and possibly rotate to minimize misorientation. This might be beyond the topic of this lecture?
Grain growth occur when we maintain temperature above recrystallization temperature.. Grain boundaries also decrease at lower temperature and it is also driving force in this case.. So why grain growth can not occur at room temperature????
You are right. The driving force exists at all temperature. But for actual growth to happen atomic mobility is also required. This is negligible at low temperatures and starts becoming significant around recrystallisation temperature.
Sir, please can you tell me the difference between coarsening and grain growth. Are these two phenomenon same or there is some difference between these two ???
Grain growth relates to increase in average size of grains. One may call it grain coarsening also but grain growth is quite common. This should be distinguished from avearge increase in the size of precipitates in a matrix. This is called precipitate coarsening or Ostwald ripening.
sir,it is very clear that the finer grains contributes to larger strength.But when we talk about DUCTILITY AND TOUGHNESS ,then what is the relation of grain size to these two properties??...DO finer grains are more ductile in any cases??...its very confusing sir..plz help....Also,in many sources finer grains have high brittleness and less toughness ..why it so?
Ductility is a difficult property to explain. However, finer grains do lend to higher ductility and toughness. Please share with me cases where fine grains lead to brittleness.
Dear prof, coarse pearlite was more ductile as compared to fine pearlite and martensite.How can we say that fine grains lend to more ductility and toughness?
kindly help here sir ,pi(di)^2 is the surface area of one whole grain, it's not the grain surface area boundary ,why are you then dividing by 2 to get the grain surface area boundary per grain?
@@introductiontomaterialsscience Yes sir that is fine but what if may be there is a transition say from ferritic to austentic?(BCC to FCC).Obviously there is a volume change that is relatable.What if we have such instances.
@@rajorshikoyal4397 This will be an example of phase transformation and not grain growth. During grain growth the phase remains the same. Only the grain size increases.
