Study On Phase Transformation Behavior And Mechanical Properties Of Ni-Ti Based Alloy Films

Ni-Ti based alloy films have been widely applied in microelectromechanical systems (MEMS) due to their unique shape memory effect, pseudoelasticity and other properties. With miniaturization development of the devices, feature sizes (such as thickness, grain size, etc) of thin films continuously decrease, their phase transformation behaviors and mechanical properties exhibit obvious size dependence, and some abnormal physical and mechanical properties have been found. Working reliability and life of the devices can not be predicted precisely. On the other hand, the phase transformation behaviors and mechanical properties of Ni-Ti based alloy films are closely related with their components, chemical compositions, phase compositions and heat-treatment processes. Therefore, to investigate effects of feature sizes, components, etc on phase transformation behaviors and mechanical properties has significant scientific values for design and preparation of the alloy films with fast response, high phase transformation temperatures and excellent mechanical properties.Based on these considerations above, we have utilized magnetron sputtering method to prepare a series of ternary Ni-Ti-Al thin films and Ti/Ni multilayers by changing deposition and annealing temperatures. The chemical compositions, phase compositions and microstructures of the films were characterized by energy dispersive X-ray spectrum, X-ray diffraction, scanning electron microscope and transmission electron microscope. The phase transformation behaviors were characterized by four-probe resistance testing method and mechanical properties (including pseudoelasticity, hardness, reduced modulus and strain rate sensitivity) of the films were tested by nanoindentation. Effects of feature sizes (including grain size and modulation period), indentation depth, precipitates and strain rate on phase transformation behaviors and mechanical properties of the Ni-Ti based alloy films were investigated, and the corresponding mechanisms were discussed. The main conclusions are as follows:(1) Nanoscale L21 phase-Ni43Ti38gAl19 alloy films with high pseudoelasticity were prepared and it is found that the pseudoelasticity and hardness of the films exhibit obvious size dependence. The pseudoelasticity gradually increases with the decrease of grain size and indentation depth, and reaches the maximum value of 92.7% at the smallest grain size of 12 nm and indentation depth of 30 nm. The reason for evolution of pseudoelasticity vs. grain size is that the elastic strain and critical stress to induce martensitic transformation increase with the decrement of grain size. With decreasing the grain size from 28 to 12 nm, the hardness of the films increases initially and then decreases, reaching the maximum value of 13.8 GPa at grain size of 16 nm. When the grain size is less than or equal to 16 nm, the hardness vs. grain size exhibits an inverse Hall-Petch effect.(2) Growth of Ni-rich precipitates can not only increase martensitic transformation temperatures and hardness of Ni-Ti-Al alloy films, but also decrease the phase transformation hysteresis. Martensitic transformation start temperature of the Ni49.7Ti45.3Al5 films deposited at various temperatures exceeds room temperature. With the growth of Ni-rich precipitates, martensitic transformation temperatures increase, the transformation hysteresis reduces by a half, and two-stage B2â†'Râ†'B19’ transformation path changes into B2â†'B19’ transformation. Martensitic transformation temperature of Ni44Ti32Al24 Heusler alloy films increases from -37.5 to -33.7℃ with growth of Ni-rich precipitates, and the films show near-zero transformation hysteresis. Hardness of the Ni49.7Ti45.3Al5å'ŒNi44Ti32Al24 films increases by 12.8% and 22% with the increasing Ni-rich precipitates, respectively.(3) Combining Ti/Ni multilayer technique, in-situ heating deposition and annealing process, we have prepared Ni-Ti alloy films with super-high hardness. The results have confirmed our initial design about strengthening performance by importing layer interfaces. Both alloying process and hardness of the annealed Ti/Ni multilayers are dependent on modulation period. With decreasing the modulation period to 5.4 nm, the multilayer has fulfilled complete alloying and the layer interfaces have disappeared. The hardness of the as-deposited and annealed Ti/Ni multilayer first increases and then decreases with decreasing the modulation period, and the maximum hardness is found at the modulation period of 5.4 nm. With increase of indentation depth, an anomalous hardness softening is observed in the annealed multilayers with modulation period of 13.5 and 27 nm, which is simultaneously accompanied by pop-in events and sudden drop of indentation load in load-depth curves. The pop-in events are caused by stress-induced martensitic transformation and the critical load to induce martensitic transformation increases with the decreasing modulation period.(4) Effects of strain rate on mechanical properties and phase transformation behaviors have been investigated by nanoindentation. It is found that hardness of the annealed multilayers with modulation period of 13.5 and 27 nm shows negative strain rate sensitivity and the hardness in the other multilayers does not change with strain rate. Stress-induced martensitic transformation is the dominating mechanism for present negative strain rate sensitivity and the critical load to induce martensitic transformation increases gradually with the increasing strain rate. It is due to more transformation latent heat, derived from exothermic austenite to martensite transition, cannot dissipate quickly under a higher strain rate, which leads to more stabilization of the austenitic phase. So, it needs larger driving force of phase transformation.