Effect of cyclic deformation on the pseudoelasticity characteristics of Ti-Ni alloys

Stability and fatigue are terms often referred to when discussing the durability of the shape memory effect. This paper considers the meaning of these terms and how they relate to the macroscopic memory response and the microstructural accommodation response of NiTi alloys. The results of previous workers are analysed and compared to establish how the three parameters that apparently control the macroscopic memory effect -temperature, stress and strain, are controlled themselves by the alloy's microstructure. It will be shown that repeated phase changes will alter the microstructure and hysteresis of the transformation and in turn this will lead to changes in transformation temperatures, transformation stresses and transformation strains. Both superelastic and memory effects are considered simultaneously so as to emphasise the similarities between the respective stress/strain and strain/temperature degradation effects. Although the mechanisms of both are often regarded as physically separate it is shown that in fact the macroscopic changes observed during cycling are a result of similar microstructural changes. It is also shown how as a result of the inherently large transformation strains internal cracking may occur and that even apparently stable microstructures can fail catastrophically via conventional crack growth mechanisms.

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“…the gradient gradually becomes steeper). These effects may be seen in the hysteresis profiles shown in Figure 2 [17].…”

Section: Superelastic Cyclingmentioning

confidence: 81%

A review of shape memory stability in NiTi alloys

Morgan

Friend

2001

J. Phys. IV France

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“…Work output, being the product of the transformation strain and the applied stress, also decreases with increasing transformation temperature (i.e., higher Pd content) (Figure 14 30 . At high stress levels, where the transformation strains are relatively equivalent (such as 197 and 295 MPa), an increase in work output was still observed.…”

Section: Discussionmentioning

confidence: 99%

“…At stresses below this maximum, the transformation strain increased with stress, because the higher stresses were able to cause more of the martensite twin variants to reorient toward an orientation that favored the applied stress during cooling, therefore making more strain available to be recovered during the subsequent heating cycle during the transition to austenite. Eventually, at some level of stress, this reorientation effect is maximized, but further increases in stress become high enough to prevent full recovery of all the reoriented martensite, [30][31][32] and so the transformation strain level peaks and begins to decrease. Work output, being the mathematical product of the transformation strain and the applied stress, increased with applied stress for all of the stress levels reached (Figure 10), but would have eventually reached a maximum if even higher stress levels were attained, as our previous work on Ni 19.5-Ti 50.5 Pd 30 in compression has shown.…”

Section: B Stress-free Transformation Temperaturesmentioning

confidence: 99%

“…5 Pd x (x = 15,20,25,30, and 46 at. pct) were vacuum induction melted in graphite crucibles under a protective argon atmosphere using high-purity elemental constituents (99.98 wt pct Ni, 99.995 wt pct Pd, and 99.95 wt pct Ti) and cast into a 25.4-mmdiameter by 102-mm-long copper mold.…”

Section: A Materials Processing and Characterizationmentioning

confidence: 99%

“…The most notable exceptions are more recent studies by Cai et al [12] on Ni 19.4 Ti 50.6 Pd 30 , Noebe et al [13] on Ni 19.5 Ti 50. 5 Pd 30 , and Kumar et al [14] on Ni 10 Ti 50 Pd 40 . These studies were predominantly limited to higher Pd containing alloys.…”

Section: Introductionmentioning

confidence: 99%

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Characterization of Ternary NiTiPd High-Temperature Shape-Memory Alloys under Load-Biased Thermal Cycling

Bigelow

Padula

Garg

et al. 2010

Metall Mater Trans A

121

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While NiTiPd alloys have been extensively studied for proposed use in high-temperature shapememory applications, little is known about the shape-memory response of these materials under stress. Consequently, the isobaric thermal cyclic responses of five (Ni,Pd) 49.5 Ti 50.5 alloys with constant stoichiometry and Pd contents ranging from 15 to 46 at. pct were investigated. From these tests, transformation temperatures, transformation strain (which is proportional to work output), and unrecovered strain per cycle (a measure of dimensional instability) were determined as a function of stress for each alloy. It was found that increasing the Pd content over this range resulted in a linear increase in transformation temperature, as expected. At a given stress level, work output decreased while the amount of unrecovered strain produced during each load-biased thermal cycle increased with increasing Pd content, during the initial thermal cycles. However, continued thermal cycling at constant stress resulted in a saturation of the work output and nearly eliminated further unrecovered strain under certain conditions, resulting in stable behavior amenable to many actuator applications.

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Implementation and testing of passive control devices based on shape memory alloys

Dolce

Cardone

Marnetto³

2000

Earthq Engng Struct Dyn

444

244

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SUMMARYTwo families of passive seismic control devices exploiting the peculiar properties of shape memory alloy (SMA) kernel components have been implemented and tested within the MANSIDE project (Memory Alloys for New Seismic Isolation and Energy Dissipation Devices). They are special braces for framed structures and isolation devices for buildings and bridges. Their most important feature is their extreme versatility, i.e. the possibility to obtain a wide range of cyclic behaviour * from supplemental and fully re-centring to highly dissipating * by simply varying the number and/or the characteristics of the SMA components. Other remarkable properties are their extraordinary fatigue resistance under large strain cycles and their great durability and reliability in the long run. In this paper, the working mechanisms of the SMA based devices are outlined and the experimental tests carried out to verify the above-mentioned properties are extensively described.

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Effect of cyclic deformation on the pseudoelasticity characteristics of Ti-Ni alloys

Cited by 622 publications

References 4 publications

A review of shape memory stability in NiTi alloys

A review of shape memory stability in NiTi alloys

Characterization of Ternary NiTiPd High-Temperature Shape-Memory Alloys under Load-Biased Thermal Cycling

Implementation and testing of passive control devices based on shape memory alloys

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