Microstructure, Shape Memory Effect and Functional Stability of Ti₆₇Ta₃₃ Thin Films

Abstract

Ti-Ta based alloys are an interesting class of high-temperature shape memory materials. When fabricated as thin films, they can be used as high-temperature micro-actuators with operation temperatures exceeding 100°C. In this study, microstructure, shape memory effect and thermal cycling stability of room-temperature sputter deposited Ti 67 Ta 33 thin films are investigated. A disordered a 00 martensite (orthorhombic) phase is formed in the as-deposited Ti 67 Ta 33 films. The films show a columnar morphology with the columns being oriented perpendicular to the substrate surface. They are approximately 200 nm in width. XRD texture analysis reveals a martensite fiber texture with {120} and {102} fiber axes. The XRD results are confirmed by TEM analysis, which also shows columnar grains with long axes perpendicular to the {120} and {102} planes of a 00 martensite. The shape memory effect is analyzed in the temperature range of -10 to 240°C using the cantilever deflection method, with special emphasis placed on cyclic stability. Ti 67 Ta 33 thin films undergo a forward martensitic transformation at M s % 165°C, with a stress relaxation of approximately 33 MPa during the transformation. The actuation response of the film actuators degrades significantly during thermal cycling. TEM analysis shows that this degradation is related to the formation of nanoscale v precipitates (5-13 nm) which form above the austenite finish temperature. These precipitates suppress the martensitic transformation, as they act as obstacles for the growth of martensite variants.

Compositional and Tribo‐Mechanical Characterization of Ti‐Ta Coatings Prepared by Confocal Dual Magnetron Co‐Sputtering

“…This memory effect was investigated by Motemani et al in Ti‐Ta coatings produced by magnetron sputtering. Montemani et al results indicated that by using magnetron co‐sputtering, it was possible to obtain the α″ martensitic phase in the as‐deposited room temperature films for Ta contents between 30 and 33 at%. They observed the martensitic transformation between 225–250 °C, making the material useful to produce high temperature actuators for micro‐electromechanical systems.…”

“…The martensitic phases also lead to other exotic properties, such as shape memory effects . This memory effect was investigated by Motemani et al in Ti‐Ta coatings produced by magnetron sputtering. Montemani et al results indicated that by using magnetron co‐sputtering, it was possible to obtain the α″ martensitic phase in the as‐deposited room temperature films for Ta contents between 30 and 33 at%.…”

Crystallographic Structure Analysis of a Ti–Ta Thin Film Materials Library Fabricated by Combinatorial Magnetron Sputtering

“…These films showed an actuation response at temperatures above 100 °C, which is promising for development of high-temperature microactuators for applications in microsystem technologies. However, those thin films exhibited functional fatigue upon thermal cycling due to precipitation of disperse, nanocrystalline ω particles . The ω phase grows coherently in the β parent structure upon quenching (athermal ω) or thermal aging (isothermal ω) .…”

“…Ti–Ta thin films are an attractive class of materials for the development of biomedical implants − and actuators. − As promising implant materials, these alloys show excellent biocompatibility, ,− a low Young’s modulus ,, and a relatively high yield strength. ,, Ti–Ta alloys are also categorized as high-temperature shape memory alloys (HT-SMA), as they exhibit a martensitic transformation (actuation response) from the cubic β austenite to the orthorhombic α″ martensite at temperatures above 100 °C. ,,− For a composition of Ti 70 Ta 30 , a martensite transformation start temperature ( M s ) of ≈180 °C , is obtained (on cooling); M s increases by 30 K for a decrease in Ta content of 1 at. % .…”

“…% . Transformation temperatures higher than 100 °C were also shown for thin films. ,, Therefore, Ti–Ta SMA can be utilized to develop high-temperature actuators, where the excellent cold workability of Ti–Ta is a benefit. , So far, most of the interest in Ti–Ta alloys was based on bulk materials beside a few reports about the thin film form. ,,,− Motemani et al. , have fabricated Ti 100– x Ta x ( x = 30 and 33) actuator thin films by magnetron sputtering at room temperature. These films showed an actuation response at temperatures above 100 °C, which is promising for development of high-temperature microactuators for applications in microsystem technologies.…”