Orientation Dependence of Plasticity and Fracture in Single-Crystal Superelastic Cu-Al-Mn SMA Bars

Since the advent of Ni-Ti shape memory alloys (SMAs) in 1963, tremendous progress has been made in the development and characterization of SMAs. Among many types of SMAs, Ni-Ti SMAs have been the most dominant in practical applications (Otsuka and Wayman 1998; Kainuma 2018). The difficulty in cold work of Ni-Ti SMAs, however, limits their use only to simple forms like wires and tubes. On the other hand, Cu-Al-Mn SMAs, discovered based on phase diagram study in the late 1990s, have excellent cold workability (Kainuma et al. 1996, 1998). The lower material cost and higher workability make Cu-Al-Mn SMAs strong candidates to pioneer new practical applications, such as seismic applications (Ozbulut et al. 2011; Chang and Araki 2016), which are the focus of this work. SMAs demonstrate a shape recovery property upon heating or unloading. The former is called shape memory effect and the later superelasticity. In seismic applications, superelasticity of SMAs is often exploited to minimize residual deformations in civil structures like buildings and bridges after earthquakes.

Due to the strong crystallographic anisotropy in transformation strain of Cu-Al-Mn SMAs—which is different from Ni-Ti SMAs—the mismatch of grain orientations at grain boundaries easily introduces dislocations in polycrystalline Cu-Al-Mn SMAs (Sutou et al. 2005, 2013). The accumulation of such dislocations degrades superelasticity and leads to intergranular fracture. One solution to this problem is texture control, where uniform grain orientation is obtained by cold work prior to heat treatment (Sutou et al. 2002). Another solution is single crystallization. It is well known that Cu-Al-Mn SMAs, as well as other Cu-based SMAs like Cu-Al-Ni (Otsuka et al. 1976) and Cu-Al-Be (Qiu and Zhu 2014), have excellent superelasticity in the form of single crystals. However, the cost of producing single-crystal Cu-based SMAs has been much higher than that of polycrystalline Ni-Ti SMAs. The Czochralski method and the Bridgman method are popular techniques for producing single crystals (Reed 2006); however, these methods have very slow crystal growth, which is a major disadvantage in commercialization of single-crystal SMAs. When SMAs are used in seismic applications, the use of large-diameter bars and/or thick plates is required. Commercialization of such large single-crystal SMAs has been considered extremely difficult, if not impossible.

Under such circumstances, a technique called cyclic heat treatment was developed, wherein only a repetition of heat treatments is necessary to accelerate the grain growth in Cu-Al-Mn SMA bars (Omori et al. 2013). This technique enables production of bamboo-like microstructures, wherein the size of grains is larger than the bar diameter. Although excellent superelasticity can be obtained by realizing the bamboo-like microstructure even in large-diameter bars, the risk of intergranular fracture remains if the mismatch of grain orientations is significant. To overcome this difficulty, the process of cyclic heat treatment was improved to further accelerate grain growth (Kusama et al. 2017). The improved process demonstrates that it is possible to produce large single-crystal superelastic bars of 15-mm diameter and 700-mm length within a reasonable time frame in the Cu-Al-Mn SMA. As such, improved cyclic heat treatment provides an avenue for commercializing large single-crystal superelastic Cu-Al-Mn SMAs in seismic application, which has been considered virtually impossible until now.

Of course, some issues remain to be addressed before commercialization of single-crystal Cu-Al-Mn SMAs. As mentioned above, strong anisotropy exists in mechanical properties of superelastic Cu-Al-Mn SMAs. Sutou et al. (2002, 2005, 2013) extensively studied the orientation dependence of superelasticity. It has been reported that the recoverable strain, due to transformation strain in the ${\mathrm{L}2}_1/6\mathrm{M}(18\mathrm{R})$ martensitic transformation, changes in the range of 2% and 10% in tension, depending on the loading direction (Sutou et al. 2002). The temperature dependence of critical stress for martensitic transformation (apparent yield stress) is inversely proportional to the transformation strain. Therefore, critical stress becomes high when the transformation strain is low at a fixed temperature. These mechanical properties related to superelasticity have been experimentally and theoretically investigated. Nevertheless, the orientation dependence of plastic and fracture response—or the response after reaching the maximum recoverable strain up to fracture—is still not well known. Because the ductility of materials plays a critical role in seismic applications, characterizing its orientation dependence is an essential task toward commercialization.

The objective of this paper is to study the orientation dependence of plasticity and fracture in single-crystal superelastic Cu-Al-Mn SMA bars. The key components of the present study are: (1) systematic investigation focusing on plasticity and fracture; and (2) use of large single-crystal SMA bars as test specimens. The study performed cyclic tension tests up to 10% strain and consecutive monotonic tension tests up to fracture. The test specimens were 14 single-crystal superelastic Cu-Al-Mn SMA bars of 15-mm diameter and 140-mm length, with a variety of grain orientations. The microstructure and the fracture surface of the SMA bars were analyzed to consider the possible reasons for the large difference observed in the plastic and fracture responses.

The black circles in Fig. 4(a) show the grain orientation of each bar along the longitudinal direction. As seen from the inverse pole figure, the grain orientations are distributed randomly around the orientations $<\begin{array}{ccc}1& 0& 1\end{array}>$, $<\begin{array}{ccc}1& 1& 2\end{array}>$, $<\begin{array}{ccc}1& 1& 3\end{array}>$, and $<\begin{array}{ccc}0& 0& 1\end{array}>$. Fig. 4(b) shows the numbering of the samples on the contour curves of the transformation strains [8, 9]. For later discussions, the numbering of the bar specimens was performed in increasing order of the estimated transformation strains. The specimens can be classified into four groups in accordance with the orientations. The first group, G1, is from No. 1 to No. 6, whose orientation is around $<\begin{array}{ccc}1& 0& 1\end{array}>$. The second, G2, is from No. 7 to No. 10, around $<\begin{array}{ccc}1& 1& 2\end{array}>$. The third, G3, is from No. 11 and No. 12, around $<\begin{array}{ccc}1& 1& 3\end{array}>$. And the fourth, G4, is No. 13 and No. 14, around $<\begin{array}{ccc}0& 0& 1\end{array}>$. Fig. 5 shows the stress–strain curves obtained from the tension tests. Table 1 shows the values of Young’s modulus $E$, the transformation stress ${\sigma }_t$, the fracture stress ${\sigma }_f$, the transformation strain ${\epsilon }_t$, and the fracture strain ${\epsilon }_f$ obtained from the stress–stress curves shown in Fig. 5. The definitions of these variables are schematically shown in Fig. 6. The measured values of the Vickers hardness $HV$, the martensite start temperature $M_s$, and the austenite finish temperature $A_f$ are also shown in Table 1. The orientation dependence of $M_s$ and $A_f$ is negligibly small and within the range of the measurement error. The variations in $HV$ are also minor, and no clear orientation dependence can be seen. On the other hand, strong orientation dependence exits in the other variables, which are discussed in more detail in the following sections.

We examined the orientation dependence of mechanical properties of single-crystal superelastic Cu-Al-Mn SMA bars, focusing on plasticity and fracture. The following conclusions can be drawn from the study. (1) It was confirmed that the orientation dependence in elastic and superelastic response can be predicted quantitatively by calculations based on the theories presented in the literature. (2) Strong orientation dependence was observed in plasticity and fracture. To the authors’ knowledge, such orientation dependence has not been reported in the literature. Ductility was poor or moderate when the orientation of the specimen was close to the $<\begin{array}{ccc}1& 0& 1\end{array}>$ direction. On the other hand, a highly ductile response, with fracture strain up to 92%, was observed when the orientation was close to the $<\begin{array}{ccc}1& 1& 2\end{array}>$, $<\begin{array}{ccc}1& 1& 3\end{array}>$, or $<\begin{array}{ccc}0& 0& 1\end{array}>$ direction. The large rotation of the crystal lattice and the propagation of the slip band along the long distance in the longitudinal direction are the main reasons for the highly ductile response. Such a highly ductile response is desirable in a structural material, especially for seismic applications.

Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.

This research was supported by Grant-in Aids for Challenging Research (Exploratory) #17K18825 and for the Promotion of Joint International Research #16KK0149 provided by the Japan Society for the Promotion of Science (JSPS). The authors thank Prof. Y. Sutou of Tohoku University and Prof. M. Nishida and Prof. em. K. Tsuzaki of Kyushu University for their helpful discussions. The authors are grateful to Mr. Y. Oizumi for his assistance in performing the tension tests.