An origin of functional fatigue of shape memory alloys

Abstract

Functional fatigue (FF) during thermal and mechanical cycling, which leads to the generation of macroscopic irrecoverable strain and the loss of dimensional stability, is a critical issue that limits the service life of shape memory alloys (SMAs). Although it has been demonstrated experimentally that such a phenomenon is related to microstructural changes, a fundamental understanding of the physical origin of FF is still lacking, especially from a crystallographic point of view. In this study, we show that in addition to the normal martensitic phase transformation pathway (PTP), there is a symmetry-dictated non-phase-transformation pathway (SDNPTP) during phase transformation cycling, whose activation could play a key role in leading to FF. By investigating crystal symmetry changes along both the PTPs and SDNPTPs, the characteristic types of defects (e.g., dislocations and grain boundaries) generated during transformation cycling can be predicted systematically, and agree well with those observed experimentally in NiTi. By analyzing key materials parameters that could suppress the SDNPTPs, strategies to develop high performance SMAs with much improved FF resistance through crystallographic design and transformation pathway engineering are suggested.

Tensile Deformation of Superelastic NiTi Wires in Wide Temperature and Microstructure Ranges

“…17 can be compared with the accumulated unrecovered strain. This proposition is somehow similar to the concept for the origin of functional fatigue of NiTi proposed recently by Gao et al [33] who claimed that cyclically deformed NiTi displays irreversible changes of atomic configurations (called a symmetry-dictated non-phase-transformation pathway). This proposition does not disregard the role of dislocation slip in cyclic instability, as articulated in Ref.…”

On the Fatigue Behavior of Nanocrystalline NiTi Shape Memory Alloys: A Review

“…functional degradation due to an increase in dislocation density [42,44]. From a stress perspective, functional fatigue transpires if the sum of the residual and applied stress is greater than the critical stress during martensitic transformation [45]. Furthermore, residual strain, change in transformation temperature, and hysteresis or lower dissipated energy all represents symptoms of functional fatigue [42].…”

Research on Shape Memory Alloys and Magnetorheological Fluids for Use in Pneumatic Actuators

“…( Figure 2). Based on theoretical assumptions contained in the mentioned works [3,[7][8][9][10][11][12][13][14][15][16][17], such calculations can be also used in chains containing SM elements. While heating the plate up to the austenitic temperature, the sinusoidal bending will be obtained:…”