Abstract

This chapter focuses on the crystallization of TiNi shape memory alloy thin films. These materials are commonly sputter-deposited in an amorphous form and require high-temperature thermal treatments to create their crystalline (actuating) form. The microstructures that emerge during crystallization depend on the nucleation and growth kinetics. This chapter briefly surveys crystallization theory and methods to determine these kinetic parameters such as calorimetry, X-ray diffraction and microscopy. Novel microscopy methods have also been developed to provide a robust description that can give rise to the prediction of microstructures. In addition to presenting these tools, this chapter will also survey various factors that influence crystallization and microstructural development, which include annealing temperature, composition, substrate materials and film thickness.

Introduction

The crystallization of nickel-titanium thin films is of important scientific concern since the as-deposited form of these films is commonly amorphous and requires a high-temperature annealing step to create their crystalline form. It is well-known that the resulting microstructures dictate the phase transformation behavior of these unique materials [1]. As such, an understanding of the structure–property link can give rise to the development of more reliable devices based on them [1, 2]. Unlike the melting of materials, crystallization does not occur at a unique temperature, but hinges on both temperature and time. For TiNi thin films, the general rule of thumb for crystallization is an annealing step near 500 °C for one hour [3, 4]. However, there is a wide range of processing conditions in the literature from very high temperatures to no annealing at all. Such an assortment in processing conditions produces different films.