Length scale dependent alloying and strain-rate sensitivity of Ti/Ni multilayers

In this paper, we prepared alloying Ti/Ni multilayers through changing modulation periods (λ) and subsequent annealing. Strain rate sensitivity of the multilayers was investigated by nanoindentation. It was found that the multilayer became full alloying with the highest value of hardness at λ=5.4 nm. Both the alloying degree and the corresponding hardness are remarkable dependent on λ. Only the multilayers accompanied by obvious stress-induced pop-in events exhibit negative strain rate sensitivity resulted from stress-induced martensitic transformation, while hardness of the other multilayers does not change with strain rate. In addition, the critical load for the first pop-in increases with strain rate, meaning that larger stress is needed to induce martensitic transformation at higher strain rate.