Abstract
The present work provides a brief overview on structural and functional fatigue in shape memory alloys (SMAs). Both degenerative processes are of utmost technological importance because they limit service lives of shape memory components. While our fundamental understanding of these two phenomena has improved during the last two decades, there are still fields which require scientific attention. NiTi SMAs are prone to the formation of small cracks, which nucleate and grow in the early stages of structural fatigue. It is important to find out how these micro-cracks evolve into engineering macro-cracks, which can be accounted for by conventional crack growth laws. The present work provides examples for the complexity of short crack growth in pseudoelastic SMAs. The importance of functional fatigue has also been highlighted. Functional fatigue is related to the degeneration of specific functional characteristics, such as actuator stroke, recoverable strain, plateau stresses, hysteresis width, or transformation temperatures. It is caused by the accumulation of transformation-induced defects in the microstructure. The functional stability of SMAs can be improved by (1) making phase transformations processes smoother and (2) by improving the material’s resistance to irreversible processes like dislocation plasticity. Areas in need of further research are discussed.
Highlights
Shape memory technology has evolved into mature materials engineering field [1]
Two types of shape memory effects (SMEs), a thermal memory and mechanical memory, are exploited for advanced applications in aerospace, automotive, construction and environmental engineering, and in the field of medical technology, e.g., [1,2,3,4,5,6]. Both types of SMEs rely on the martensitic transformation, a solid-state transformation where a high-temperature phase austenite transforms into a low-temperature phase martensite on cooling/mechanical loading [7, 8]
For the field of shape memory technology, it is important that the formation of martensite is strongly governed by the chemical composition and the microstructure of an alloy [13,14,15,16,17,18]
Summary
Shape memory technology has evolved into mature materials engineering field [1]. Two types of shape memory effects (SMEs), a thermal memory (one/two-way effect, 1/2 WE) and mechanical memory (pseudoelasticity, PE), are exploited for advanced applications in aerospace, automotive, construction and environmental engineering, and in the field of medical technology, e.g., [1,2,3,4,5,6]. Keywords Shape memory alloys Á Fatigue Á Microstructure Á Short crack growth Á High entropy alloys Research on structural fatigue in SMAs has often been motivated by the requirement for medical implants to withstand a high number of loading/unloading cycles [27, 53, 54].
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