Hi hossein

As you know, yield stress is the threshold for dislocations to start slipping and is independent of dendrite cell size, but UTS is related to parameters that disturb dislocations mobility. The more barrier in microstructure the more increase in UTS. Some examples for barriers: grain boundaries, dendrite arms, ppts, inclusions and so on.

Know, do you know why elongation decreases as dendrite cell size increases?

Good Luck

Dear Hossein

In addition to what  Nima Tabatabaei said, the yield strength value usually can be  determined within the linear- elastic region of the stress versus the strain plot of the Al alloys.  In contrast, the UTS value  normally can be determined within the nonlinear elastic-plastic region of the stress versus the strain plot of the alloys.   Consequently, the UTS is dependent on  dislocations mobility in the elastic-plastic region of the stress versus strain plot of the alloys.   Since the dislocations mobility is affected by the size of the dendrite arm size., therefore, the UTS value would be effected as well.   In other words, the yield strength value is independent of the dislocation mobility in the elastic region of the stress versus strain plot of the alloys.

Hello everybody

 I saw the same effect in Hadfield steel. One possible explanation: smaller dendrites  increase the amount of microporosity and the percentage volume of second phases.