Dear Vinod,

I cannot associate the serrations with any metallurgical phenomena and I do believe this may be some testing artifact. The high strain rates would not allow any diffusion-controlled machanism to operate and cause the serrations.

Are you using the Gleeble system?

Kind regards, Fulvio

Thanks Dr. Fulvio for your observations. Yes, I am using Gleeble system. These tests have been conducted using Hydrawedge. But, I am of the opinon that it a metallurgical phenomenon and not related to machine. We have observed similar serrations in some other materials also.

sorry sir, i never do this.

Dear Dr. Vinod,

In my opinion it is related to Hydra-wedge system. We are also working on this problem. We have observed even the peaks with higher magnitude in some Al alloys, they are systematic always at same coordinate point. And magnitude increases with temperature. Could you try to deform the sample with turn off heating during deformation.

Thanks Surya for your observation. We have done all the deformation with heat ON using Hydrawedge. But, if it is related to Hydrawedge, the serrations should appear at all temperature, strain rate and more so on regular basis and not at a gap. We can confirm it with Heat mute off.  

@Dr. Fulvio, I am almost sure that it is a metallurgical phenomenon. But, can you check with DSI whether it is possible with Hydrawedge. I want to be sure from machine point of view.

It may have to do with the formation of shear bands. Also the strain rate sensitivities are a function of temperature. The strain rate sensitivity may be positive or negative depending on the temperature. Since there is practically no carbon, therefore,there is more likelihood of formation of shear bands and a transition between positive and negative strain rate sensitivities leading to ositive and negative serrations