Stress-Induced Martensitic Transformation of Niti Shape Memory Alloy Thin Films at Elevated Temperatures

Shape memory alloys (SMAs) exhibit exceptional strength, along with unique mechanical and thermal recovery properties not found in conventional metals. These outstanding properties are preserved even at microscopic scales, offering advancements in micro-scale medical devices and electronic components that require both high strength and flexibility. In particular, shape memory alloy thin films can be directly deposited onto semiconductor chips and patterned into specific shapes, making them ideal for the development of various miniature devices.<br/>The unique properties of SMAs arise from the martensitic phase transformation process in response to heat and stress. While phase transformation characteristics persist even at small scales, it is known that these characteristics change as the size of the structure decreases. Additionally, since phase transformation is highly sensitive to the chemical composition and operating temperature, both factors must be considered to tailor the phase transformation behavior of small-scale SMAs. However, there are only limited studies on the phase transformation of SMAs under mechanical loading in micro/nano-scale.<br/>Therefore, we fabricated nickel-titanium (NiTi) SMA thin films and observed their temperature-dependent mechanical response. Using microelectromechanical systems (MEMS) processes, we produced 500 to 600 nm-thick freestanding thin film specimens along with Joule-heating tungsten microheaters. A custom-built micro-scale tensile testing equipment was installed inside a scanning electron microscope (SEM) to observe stress-induced martensitic transition of thin films at elevated temperatures. Interestingly, all thin films across the composition range from Ti-rich (47.4 at. % Ni) to Ni-rich (52.7 at. % Ni) exhibited martensite start temperatures (M_s) below 0 ^oC, approximately 50 ^oC lower than their bulk counterparts. Because of the low M_s, Ti-rich and equiatomic NiTi thin films show superelastic behavior around 40^oC, which is near the human body temperature. Moreover, Ti-rich thin films exhibit better mechanical stability than equiatomic thin films. The change in mechanical behavior with chemical composition could be attributed to different type of precipitates that can either expedite or suppress forward and reverse transformation processes.