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

Abstract

The present work addresses functional fatigue of binary NiTi and ternary NiTiCu (with 5, 7.5, 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.

Effect of Cu Content on Atomic Positions of Ti50Ni50−xCux Shape Memory Alloys Based on Density Functional Theory Calculations

“…TiNi-based shape memory alloys (SMAs) have been widely used in the aerospace and biomedical fields because of their superior shape memory effect (SME), high superelasticity, and favorable biocompatibility [1][2][3][4][5]. In order to fulfill the requirement of high response frequency in the automotive and aerospace industries, TiNiCu SMAs in which Cu substitutes for Ni have been widely studied due to their narrow martensitic transformation (MT) temperature hysteresis in comparison with TiNi binary alloys [6][7][8][9].…”

“…In order to design new SMAs possessing very small transformation temperature hysteresis, understanding the effect of alloying on martensite structures is of vital importance. Previously reported works have studied the effects of Cu content on transformation temperature [10], microstructure [7,8,11,12], stress-strain characteristic and fatigue life [13][14][15], as well as applications as actuators [5,16]. Earlier research has found that the Cu content in TiNiCu alloys has little influence on the MT temperature but obvious effect on the MT pathway [11,12,17].…”

Thermal cycling and isothermal deformation response of polycrystalline NiTi: Simulations vs. experiment

“…Other important features are the hysteretic width Dh H (feature F6), widths Dh A-M (feature F8) and Dh M-A for the austenite-to-martensite and martensite-to-austenite transformations, respectively, and the open loop strain per thermal cycle, DE cycle These quantities typically depend on R bias , composition, processing history and microstructural features such as single/polycrystalline orientation/texture, precipitate morphology, dislocation substructure and internal stress (e.g. [4]). …”

Mechanical Properties of NiTi-Based Foam with High Porosity for Implant Applications

“…Gil et al [14,15] showed that NiTiCu SMAs exhibit a much more narrow stress hysteresis, based on transformation stresses, as compared to binary NiTi SMAs. Fatigue testing of NiTiCu SMA actuator springs were conducted by Grossmann et al [16,17]. They reported that the functional fatigue resistance of NiTiCu SMAs is strongly improved by the addition of Cu.…”

“…Sehitoglu et al [18,19] and Biscarini et al [20] investigated the mechanical properties of NiTiCu single crystal SMAs under compression loading, and reported that the slip resistance increases when Ni-rich precipitates are present. To date, the mechanical behavior of bulk NiTiCu SMA is well established [7,[14][15][16][17][18][19][21][22][23][24]; however, few studies have focused on the effect of porosity on NiTiCu SMAs [25][26][27][28]. Goryczka et al [26,27] examined powder processing methods for producing homogenous NiTiCu SMAs.…”