Thomas Lehnert

Characterization of shape-memory alloy thin films made up from sputter-deposited Ni/Ti multilayers

Abstract A novel fabrication process for Ni–Ti shape-memory alloy thin films is presented. This process is based on the appropriate annealing of sputter-deposited Ni/Ti multilayers. X-ray diffraction shows that interdiffusion of the two constituents results either in the amorphization of the multilayer structure after annealing at 330°C or in the recrystallization as a Ni–Ti intermetallic compound after annealing at temperatures above 400°C. A single 30 min annealing step in the temperature range from 400 to 800°C is sufficient to obtain Ni–Ti films showing martensitic phase transformations and the shape-memory effect. The influence of increasing annealing temperature on the transformation behavior is investigated by differential scanning calorimetry. The evolution of the transformation temperatures is found to be qualitatively similar to conventional sputter-deposited Ni–Ti films. The corresponding microstructure is studied by transmission electron microscopy. A very fine-grained structure is observed even after annealing at 800°C. The film composition can be varied by adjusting the thickness ratio of the individual Ti and Ni layers. Transformation curves of films with nominal compositions of 49.5 and 54.0 at.% Ti are compared. It is demonstrated that Ni–Ti films made up from multilayers may possess an intrinsic “two-way” shape-memory effect, which is a very interesting feature in view of the development of thin film micro-actuators.

A new fabrication process for Ni–Ti shape memory thin films

Abstract A new fabrication method for Ni–Ti thin films showing martensitic phase transformation is presented. A multilayer of up to 100 alternating pure Ni and pure Ti layers is deposited in an automated DC magnetron sputter system optimized for homogeneous deposition on large substrates. The single layer thickness is in the range of 10–20 nm. The multilayer is subjected to different heat treatments in order to allow for interdiffusion of Ni and Ti and subsequent recrystallization as a Ni–Ti intermetallic compound. The martensitic and austenite phase in the resulting films could be identified by means of X-ray diffraction. Differential scanning calorimetry (DSC) measurements monitor the transformation behavior of films with different composition and indicate the existence of an R-phase. The microstructure of the annealed films is investigated by transmission electron microscopy (TEM). The composition and transformation temperatures can be adjusted by varying the thickness ratio of the initial Ni and Ti layers. These films show a well pronounced shape memory effect for certain annealing procedures.

Transformation properties and microstructure of sputter-deposited Ni-Ti shape memory alloy thin films

The influence of annealing parameters on the martensitic phase transformation in sputter-deposited Ti rich Ni-Ti films is systematically studied by differential scanning calorimetry and by transmission electron microscopy. The annealing temperature range extends from the crystallization temperature of the films up to 900°C. For increasing temperature, multiple phase transformations, transformations via an R-phase or direct martensite/austenite transformations are observed. A similar behavior is found for increasing annealing time. Related changes of the film microstructure, such as the strongly varying distribution of round Ti2Ni precipitates in the grains, are analyzed. Transformation temperatures could be shifted over a wide range by adjusting the film composition from 48 to 54 at.% Ti. The corresponding transformation curves, grain structure as well as nature and amount of precipitates were investigated. No subsequent annealing process is required for films deposited on substrates heated above about 500°C. In this case, the as-deposited films have a very fine-grained and homogeneous microstructure.