Functional Fatigue and Tension–Compression Asymmetry in [001]-Oriented Co49Ni21Ga30 High-Temperature Shape Memory Alloy Single Crystals

Abstract

Conventional shape memory alloys cannot be employed for applications in the elevated temperature regime due to rapid functional degradation. Co-Ni-Ga has shown the potential to be used up to temperatures of about 400°C due to a fully reversible superelastic stress-strain response. However, available results only highlight the superelastic response for single cycle tests. So far, no data addressing cyclic loading and functional fatigue are available. In order to close this gap, the current study reports on the cyclic degradation behavior and tensioncompression asymmetry in [001]-oriented Co 49 Ni 21 Ga 30 single crystals at elevated temperatures. The cyclic stressstrain response of the material under displacement controlled superelastic loading conditions was found to be dictated by the number of active martensite variants and different resulting stabilization effects. Co-Ni-Ga shows a large superelastic temperature window of about 400°C under tension and compression, but a linear Clausius-Clapeyron relationship could only be observed up to a temperature of 200°C. In the present experiments, the samples were subjected to 1000 cycles at different temperatures. Degradation mechanisms were characterized by neutron diffraction and transmission electron microscopy. The results in this study confirm the potential of these alloys for damping applications at elevated temperatures.

Cyclic Degradation of Co49Ni21Ga30 High-Temperature Shape Memory Alloy: On the Roles of Dislocation Activity and Chemical Order

“…Interestingly, the stress hysteresis starts to decrease again above a temperature of about 400°C. Whereas the evolution of the stress hysteresis was divided into two stages in the previous study [30] the current data reveal a third stage above 375°C hinting at a significant change in the underlying microstructure evolution. As is well known, r crit values for forward and reverse transformations correlate with martensite start (M s ) and austenite start (A s ) values, respectively.…”

“…Neutron diffraction experiments and TEM analysis were performed on both, samples tested in the present study (cycling at 400°C) and in previous work (cycling in the temperature range from 100 to 300°C [30]). The neutron diffraction experiments were conducted with the single crystal Laue diffractometer SXD [33] at the ISIS neutron source, Rutherford Appleton Laboratory, Oxfordshire.…”

“…Co-Ni-Ga HT-SMAs are promising candidates for high-temperature damping and actuation applications since these exhibit superelasticity and shape memory behavior in a temperature range up to 400°C [29,30]. Processing of polycrystalline Co-Ni-Ga materials can be significantly improved by the precipitation of the disordered secondary c-phase (A1), which makes the alloy distinctly better workable than its counterpart, Co-Ni-Al [18].…”

“…Due to a lack of data in the open literature reporting on fatigue mechanisms at higher temperatures, the current authors recently addressed the tension-compression asymmetry and discussed the degradation mechanisms dictating functional instability up to 300°C using transmission electron microscopy (TEM) and neutron diffraction analysis [30]. The results unequivocally revealed, that the degradation of the functional performance up to 300°C, under both tensile and compressive loads, is mainly related to an enhanced dislocation formation, whereas a contribution of diffusion-controlled mechanisms, i.e., changes in the degree of chemical order, was found to be of less importance [30]. Focusing on the changes in the degree of chemical order, a recently published study reported on the effect of aging in the stressinduced martensitic (SIM) phase in Co-Ni-Ga single crystals under compression [29].…”

Finite element implementation and simulation of the functional fatigue in shape memory alloys

“…The related effect of functional fatigue which occurs during pseudoelastic loading is modeled by subdividing the diffusionless solid/solid phase transformation into a reversible and an irreversible process which is an experimentally motivated ansatz for the complex material behavior, see e.g. [1]. Thus, we take into account a reversible and an irreversible volume fraction for the austenitic and several martensitic phases.…”