Anomalous softening behavior in Ti/Ni multilayers with ultra-high hardness

We utilize alloying of Ti/Ni multilayers to prepare Ti−Ni alloy films and investigate effects of modulation period (λ) on the alloying and mechanical properties. It is found that the annealed multilayers with the λ of 2.7–54 nm exhibit ultra-high hardness, far exceeding co-sputtering Ti−Ni alloy films, and the maximum hardness is found at λ=5.4 nm. Attractively, another anomalous hardness softening occurs at λ of 13.5 and 27 nm with increase of indentation depth besides the softening at λ=5.4 nm, which is simultaneously accompanied by the pop-in events including displacement jumps and sudden drop of load. The pop-in events are attributed to stress-induced martensitic phase transformation and the critical load for martensitic transformation increases with the decrease of λ.