Anomalous Behavior During Nano‐Compression Superelastic Tests on Cu‐Al‐Ni Shape Memory Alloy Micro Pillars

Abstract

Shape memory alloys are one of the most important families of functional materials due to superelasticity and shape memory properties. In particular Cu‐Al‐Ni alloys exhibit these properties at nanoscale, becoming potentially useful to design new smart MEMS. In this work, an anomalous behavior observed during nano‐compression superelastic tests on Cu‐Al‐Ni shape memory alloy micro pillars is reported. The study is approached by nano‐compression tests on micro and nano pillars milled by focused ion beam. The anomalous behavior on the superelastic effect is manifested by a sudden stabilization of the stress‐induced martensite, which does not undergo the reverse transformation. A transition, from superelastic to pseudoelastic‐twinning behavior, takes place during the nano‐compression tests, and is evaluated through the load‐depth curves and quantified by the mechanical damping coefficient η, which undergoes a sharp change from ultra‐high damping, η > 0.1, down to η ≈ 0.01. The stabilization of the martensite occurs under two different experimental conditions, along cycling and by applying a very high stress during superelastic tests. In both cases, a further recovery of the initial superelastic behavior is registered. The mechanisms responsible for the observed stabilization and recovery are discussed, together with the implications and further requirements for technological applications in MEMS.

Dissecting functional degradation in NiTi shape memory alloys containing amorphous regions via atomistic simulations

“…Our simulations on the pillars with thinner shell thicknesses (0.5 and 1.0 nm) under relatively large maximum stresses (1.11 and 1.27 GPa) reveal the already detailed sudden stabilization of the martensite phase. As also discussed above, this finding is qualitatively similar to the results of the experiments on CuAlNi SMA pillars with diameters of 550 and 1700 nm [15]. However, even considering the smallest simulated shell thickness ( : 0.5 nm), the / ratio in the present calculations is 0.02 and therefore about a factor of two larger than the / ratio of the experimental submicron-or micron-sized pillars (0.009 -0.012) [15].…”

“…As also discussed above, this finding is qualitatively similar to the results of the experiments on CuAlNi SMA pillars with diameters of 550 and 1700 nm [15]. However, even considering the smallest simulated shell thickness ( : 0.5 nm), the / ratio in the present calculations is 0.02 and therefore about a factor of two larger than the / ratio of the experimental submicron-or micron-sized pillars (0.009 -0.012) [15]. Thus, in order to explain the similarity between the experimental and simulation results despite the difference in the / ratio, the difference in the number of cycles needs to be considered.…”

“…9(c-e)] observed here can be utilized to explain the previous experimental results by Gómez-Cortés et al [15] on CuAlNi SMA pillars. Specifically, this work [15] reported a loss of superelasticity quantified by a sudden increase of residual strain after a certain number of cycles (specifically after the 104 th cycle) although the initial (first) cycle started with an almost complete recovery of the original shape. This anomalous sudden increase in residual strain was more pronounced when the number of cycles or the maximum load increased.…”

“…At these highest temperatures, neither a phase transformation nor nucleation of dislocations is observed, yet residual strain still occurs and can only be interpreted by the plastic deformation of the amorphous shell region. We performed additional simulations using single crystal pillars with different diameters (15,20,25, and 30 nm) and fixed thickness of the amorphous shell (0.5 nm) to investigate the size effect (i.e., greater residual strain in smaller pillars) observed in previous experiments [9,10]. The results shown in Fig.…”

“…Unfortunately, the previous studies [9][10][11][12]15] revealed that NiTi SMAs at the small scale suffer from a significant deterioration of superelasticity, even much stronger than their bulk counterparts. SMAs prepared at the small scale generally exhibit an incomplete recovery of the initial shape after mechanical loading and unloading and show a residual strain in their stress-strain response.…”