Fig. 1. Abnormal grain growth, optical micrograph and reversible martensitic transformation of Cu-12.4Al-6.5Mn-3.2Fe alloy: (a) abnormal grain growth, the large-sized grain of about 56 mm was obtained only through annealing the cast alloy at 1173 K for 2 h; (b) optical micrograph of the single crystal consisted of dominant L21 parent; (c) reversible martensitic transformation of the single crystal measured by DSC test with a heating and cooling rate of 10 K min-1.

Fig. 2. Superelasticity and shape memory effect of Cu-12.4Al-6.5Mn-3.2Fe single crystal: (a) cyclic compressive stress-strain and corresponding strain-temperature curves with different deformation level; (b) reverse martensitic transformation start temperature (As) and functional characteristics depending on the deformation level.

Fig. 3. Evolution of macroscopic morphology and optical micrograph when deforming the Cu-12.4Al-6.5Mn-3.2Fe single crystal: (a) evolution of macroscopic morphology when Cu-12.4Al-6.5Mn-3.2Fe single crystal was repeatedly compressed to 7.5 %, 10 %, 12 %, 13 %, and then heated for shape recovery; (b-d) optical micrograph when Cu-12.4Al-6.5Mn-3.2Fe single crystal was deformed to 7.5 %, 10 % and 13 %, respectively. After each compression, the deformed sample was heated to 523 K for 5 min for shape recovery. The letters P and M respectively represent the L21-Cu2AlMn parent and 2H martensite.

Fig. 4. TMA curves under different pre-strains of 7.5 % (a), 8% (b), 10 % (c), 12 % (d) and 15 % (e) with a heating and cooling rate 0.1 K min-1. The black lines imply the dimension change of the deformed samples. The red lines represent the “jumping” distance of the recovered samples resulted from the instantaneous and complete shape recovery process when a critical temperature reaches as shown by the red arrows.

Fig. 5. Superelasticity (SE) and shape memory effect (SME) in the present Cu-Al-Mn-Fe single crystal comparison to the existing SMAs (Table 1).

Fig. 6. Microstructural evolution of Cu-12.4Al-6.5Mn-3.2Fe single crystal before and after the deformation, as well as after heating for shape recovery: (a) bright-field (BF) image and the SADP of L21 parent and (b) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image the alloy before deformation. (c-f) Microstructures of the alloy was deformed to the deformation of 7.5 % and unloading: (c) BF image and the SADP of 2H martensite, (d) dark-field (DF) image of 2H martensite and the SADP of L21 parent, (e) DF image of L21 parent, (f) magnifying BF image of the red dashed frame in (c), showing the distortions of the interfaces among the parent, martensite and nanoparticles. (g, h) Microstructures of the deformed alloy after heating to 523 K for 5 min for shape recovery: (g) BF image and the SADP of L21 parent, (h) HAADF-STEM image.

Fig. 7. Compressive stress-strain curves of Cu-12.4Al-6.5Mn-3.2Fe single crystal at room temperature with a constant pre-strain of 8% during fifty thermomechanical cycles. During each cycle, the sample was deformed to 8% and unloading, and followed by heating for shape recovery. A perfect shape memory effect of about 6% was obtained for each time.

Fig. 8. Microstructures of Cu-12.4Al-6.5Mn-3.2Fe single crystal deformed to 10 % and unloading: (a) TEM bright-field (BF) image of bcc β(FeAl) nanoparticles and 2H martensite; (b) TEM dark-field (DF) image of bcc β(FeAl); (c) DF image of 2H martensite; (d) BF image of 2H martensite and corresponding SAED pattern (inset).

Fig. 9. Schematic illustration of the pinning mechanism due to the presence of nanoparticles related to the superelasticity and shape memory effect resulted from the stabilization of the stress-induced martensite in the present Cu-Al-Mn-Fe single crystal. The black dots represent the parent, and the red dots represent the nanoparticles.