We observe that the conductivity of FSPed pure copper decreases by some 9-10% at 250C, while the composite of Copper with Yttria particles retain most of the thermal conducting ability with a mild reduction of about 4-5%. Plausible explanation could be the hindrance of yttria particles for the recrystallization and grain growth. Any other possibilities?
There is no porosity in these materials as they are bulk. Yes, there are some dislocations as they are friction stir processed!... there is a drastic reduction in the grain sizes of FSPed samples when compared to that of pure copper sheet.
As far as I understand finer FSPed grains mean more grain boundaries and this is not good for the heat conductivity. On the contrary, recrystallized larger grains serve to enhancing it. The third phase particles like precipitates almost will not alter the conductivity unless their content is high enough but they will serve to stabilize the grain growth. Anyway you have to look at the structural changes occured to grain boundaries at 250C. Also there are TMAZ and HAZ zones which have different structures as compared to the nugget. What did happen in them at 250C?
It may happen that the FSPed grains within the HMAZ of HAZ retained some dislocation density and holding at 250 caused polygonization thus subdividing them into subgrains?
Another guess is intensive oxidation of copper by the grain boundary diffusion but I don't know what scheme is used for measuring the heat resitance with respect to FSP trace.
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