Starting with a common microstructure, If I want to achieve a specific grain size, which heat treatment would give me lesser dislocation density than the other? 1. Grain growth at high temperature OR 2. Grain growth at lower temp for long time period?
This is quite clear: Temperature has more influence on grain growth than time. Temperature increase by 10-20°C often results in more than the 100-200 h additional time of the annealing. Of course, it is only for temperatures in the range of grain growth.
Thanks Vadim Verlotski. I agree with what you mentioned. But if we achieve a specific grain size by, what you say, 10-20 degree C OR 100-200 h annealing, do both these process result in equal dislocation density?
Dear Vibhor Atreya,
In case the activation energies for dislocation movement and grain boundary movement are equal and heterogeneous nucleation of the new grains prevails, it seems to me quite reasonably to assume full time/temperature compensation of the kinetics involved, provided that recrystallization indeed proceeds.
At higher temperature or higher the thermal activation, the growth of strain-free grains in the deformed matrix will be occurred. As a result, the dislocations density at the boundaries of the newly formed grains in the matrix is minimized, while at lowest temperature the growth of stress-free grains occurred within the matrix leading to higher dislocations density so both processes result in unequal dislocation density.
Best regards
It depends. Usually, high temperature reduced dislocation density. But dislocation density may be related to individual grain size, strain relax etc. I bet grain size distribution from high temperature and low temperature is different. Also there are more defects transformed into dislocations at high temperatures.
Pure grain growth does not have an order 1 influence on dislocation densities. The mechanisms of grain growth during annealing are motivated by a reduction of the interfacial energy of the microstructure. The real influence of the temperature on pure grain growth is in the mobility of the grain boundaries (kinetics) and the type of grain boundaries that are favoured (roughening/faceting phenomena).
However, two other thermally activated phenomena can occur during annealing. Recovery and recrystallization. The kinetics of these phenomena are extremely dependant on the initial microstructure (strain hardened, as-forged, etc...). Generally speaking however, higher temperatures will rid the microstructure of dislocations faster. There is no general rule however, seeing it is a case by case study of the initial state of the microstructure that most impacts the results.
I have attached a link (google books) to the "bible" of recrystallization and annealing phenomena. A great ressource for questions such as these.
https://books.google.fr/books?hl=en&lr=&id=52Gloa7HxGsC&oi=fnd&pg=PP1&dq=Rohrer+neural+network+Grain&ots=LgFfllQufA&sig=wmhMhyLcqelv5M0K-PZeDploLlE#v=onepage&q=Rohrer%20neural%20network%20Grain&f=false
I thank all for their inputs.
@Xu Wang: You mention that at higher temperatures, more defects transform into dislocations. Can you direct me to some literature about this phenomena which you may have come across? This may very prove helpful for me.
@Julien Fausty: Thank you for the resource. The reason I asked the question is because I observe increased dislocation with increased annealing temperatures in my samples, which, in my opinion doesn't confirm to the well estabilished fact of dislocation decrease with temperature. The initial microstructure are same and not deformed.
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