On the effect of alloy composition on martensite start temperatures and latent heats in Ni–Ti-based shape memory alloys

Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys

Effect of temperature and texture on the reorientation of martensite variants in NiTi shape memory alloys

The present work addresses functional fatigue of binary NiTi and ternary NiTiCu (with 5, 75, and 10 at pct Cu) shape memory (SM) spring actuators We study how the alloy composition and processing affect the actuator stability during thermomechanical cycling Spring lengths and temperatures were monitored and it was found that functional fatigue results in an accumulation of irreversible strain (in austenite and martensite) and in increasing martensite start temperatures We present phenomenological equations that quantify both phenomena We show that cyclic actuator stability can be improved by using precycling, subjecting the material to cold work, and adding copper Adding copper is more attractive than cold work, because it improves cyclic stability without sacrificing the exploitable actuator stroke Copper reduces the width of the thermal hysteresis and improves geometrical and thermal actuator stability, because it results in a better crystallographic compatibility between the parent and the product phase There is a good correlation between the width of the thermal hysteresis and the intensity of irrecoverable deformation associated with thermomechanical cycling We interpret this finding on the basis of a scenario in which dislocations are created during the phase transformations that remain in the microstructure during subsequent cycling These dislocations facilitate the formation of martensite (increasing martensite start (M S ) temperatures) and account for the accumulation of irreversible strain in martensite and austenite

Elementary Transformation and Deformation Processes and the Cyclic Stability of NiTi and NiTiCu Shape Memory Spring Actuators

One goal of the Consortium for the Advancement of Shape Memory Alloy Research and Technology is to compile the collective design experiences of our member organizations into a single medium that researchers and engineers may use to make efficient and effective decisions when developing shape memory alloy (SMA) components and systems. Recent work toward this goal is presented through the framework of six fundamental design aspects we have identified, which include evaluation, alloy selection, processing and fabrication, testing and properties, modeling, and system integration considerations including control system design. Each aspect is documented in the light of enabling the design engineer to access the tools and information needed to successfully design and develop SMA systems. Application of these aspects is illustrated through case studies resulting from our own SMA designs. It is shown that there is not an obvious single, linear route a designer can adopt to navigate the path from concept to product. Each application brings unique challenges that demand a particular emphasis and priority for each engineering aspect involved in the development of a system actuated by SMAs.

Shape memory alloy actuator design: CASMART collaborative best practices and case studies

Among the multifarious engineering applications of NiTi shape memory alloys (SMAs), their use in actuator applications stands out. In actuator applications, where the one-way effect (1WE) of NiTi SMAs is exploited, SM components are often applied as helical coil springs. Ingots are generally used as starting materials for the production of springs. But before SM actuator springs can be manufactured, the processing of appropriate wires from NiTi ingots poses a challenge because cold and hot working of NiTi SMAs strongly affect microstructure, and it is well known that the functional properties of NiTi SMAs are strongly dependent on their microstructure. The objective of the present paper is therefore to produce binary Ni50Ti50 and ternary Ni40Ti50Cu10 SMA actuator springs, starting from ingots produced by vacuum induction melting. From these ingots springs are produced using swaging, rolling, wire drawing and a shape-constraining procedure in combination with appropriate heat treatments. The evolution of microstructure during processing is characterized and the mechanical properties of the wires prior to spring-making are documented. The mechanical and functional characteristics of the wires are investigated in the stress-strain-temperature space. Finally, functional fatigue testing of actuator springs is briefly described and preliminary results for NiTi and NiTiCu actuator springs are reported.



Untersuchungen zur Herstellung und zu den Eigenschaften von NiTi- und NiTiCu-Aktorfedern



NiTi-Formgedachtnislegierungen (FGL) zeichnen sich durch eine hohe Attraktivitat fur verschiedene Aktorik-Anwendungen aus. Dabei werden FGL haufig in Form von zylindrischen Federn verwendet, wobei der Einwegeffekt genutzt wird. Die Herstellung von solchen Aktor-Federn ist jedoch keinesfalls trivial. Sowohl die Herstellung von geeignetem Drahtmaterial als auch die Formgebungsbehandlung stellen in gewisser Weise eine Herausforderung dar. Die funktionellen Eigenschaften von NiTi-FGL hangen sehr stark von mikrostrukturellen Randbedingungen ab, und erforderliche thermomechanische Behandlungen sind mit weit reichenden mikrostrukturellen Veranderungen verbunden. Das Ziel dieser Arbeit war, eine Prozesskette zur Herstellung von Ni50Ti50 und Ni40Ti50Cu10-Aktorfedern ausgehend vom Gussmaterial zu erarbeiten und metallkundlich zu charakterisieren. Aus den Gussblocken wurden durch Rundkneten, Walzen, Drahtziehen und speziellen Formgebungsbehandlungen Aktorfedern hergestellt. Dabei wurden die mikrostrukturellen und funktionellen Anderungen der beiden FGL charakterisiert. Zum Schluss wurden die mechanischen und funktionellen Eigenschaften der verschiedenen Draht-Materialien und der fertigen Federn untersucht. Dabei wurde ein besonderes Augenmerk auf die funktionelle Ermudung der Aktorfedern gelegt.

C H Grossmann

Papers

Processing and property assessment of NiTi and NiTiCu shape memory actuator springs