No REX in HEA at high temperature?

[seep555]

I am working now on a high-entropy alloy which seems not to recrystallize at high temperature annealing (>0.5Tm).
It is hot forged and has grains of few tens of um with many nanocells inside them. At high T it seems that grains are growing but not recrystallizing and dislocations remain stable but change their arrangement into more equilibrium state.

My question is: do you have any ideas about this kind of phenomenon? Are there any materials which have similar properties? It seems very bizarre...

 

[MagBen]

tens of microns grains indicated you probably got dynamic recrystallization during hot working, especially when the finish forged temperature was high. Without strain, you won't get recrystallization regardless of high anneal temperature, but rather grain growth.

 

[seep555]

I see...
I also found this high density of disocations in Al alloy AA1235 after annealing... It was explained by nanoprecipitations blocking dislocations movement. My idea for now are nanofluctuations of chemical composition, and in case of HEAs this effect may be strengthened by lattice distortion and sluggish diffusion.

And are you sure that REX is not possible in this state? It seems that at lower T, around 600C (~0.4Tm) new grains are present in the structure... How can I explain that? In my opinion there is some change of phenomenon at around 0.5Tm...

 

[EdStainless]

I am with Ben on this, there are many alloys that because of pinning or simply the energy states require more than just heat for rex. Take a sample and cold work it, say 30-40%. Look to see if it is a deformed version of the original or if it rex during forming. If it is just deformed then try annealing at lower temps and see what happens. You will need new samples for each anneal trial, the stress relief that you get will take away from the driving force.

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P.E. Metallurgy, Plymouth Tube

 

[gearteeth]

Recrystallization is driven by the stored energy of deformation. As the other guys have said, a certain minimum amount of cold work is necessary for REX, this is known as critical deformation. The greater the amount of prior deformation, the lower the temperature for the start of recrystallization (more internal energy).

 

[seep555]

Thanks a lot!

And how would you explain that at low temperature (<0.5Tm) this mechanism seems to be different and there are small REX grains (or at least free of dislocations)?
And what about still very high density of dislocations after long heat treatment at T>0.5Tm? Isn't recovery about decreasing density of dislocations?