| Shape memory alloys (SMAs), as a kind of intelligent functional materials combining with perception and driving functions, are widely used in electronic communications, biomedicine, electromechanics systems, aerospace, robotics, civil construction, even in daily life and many other fields. Among the various SMAs, except Ni-Ti SMAs, Cu-based SMAs (e.g. Cu-Al-Ni, Cu-Zn-Al and Cu-Al-Mn alloys) are the most potential ones for large-scale industrial applications because of their low cost, high electrical and thermal conductivity, wide-range transformation temperature. Single crystalline Cu-based SMAs exhibit excellent shape memory properties which are almost equivalent to those of Ni-Ti SMAs. Nevertheless, the preparation of single crystal especially large size bulk is difficult. Ordinary polycrystalline Cu-based SMAs are also limited in many potential applications due to their low shape memory properties (such as superelasticity), low plasticity and poor fatigue life, and they are susceptible to intergranular cracking. Therefore, in this paper, to improve the poor properties of Cu-based SMAs, the extraordinary controls of solidification, forming and heat treatment were made to obtain special structures from the perspective of structure design.Through texture and grain boundary control by unidirectional solidification in this paper, the columnar-grained Cu71Al18Mn11 shape memory alloy was prepared, which possessed a strong<001>-oriented texture along the solidification direction and straight low-energy grain boundary. The alloy showed excellent superelastic strain improved from 3%(ordinary polycrystalline counterpart) to more than 10.1% and with a tiny residual strain of less than 0.3% after unloading. Reasons for the enhanced superelasticity as follows:(1) The martensitic transformation of all grains with strong<001>-oriented texture occurred at the same time under the tensile loading, which could avoid the significant stress concentration problem and transformation strain incompatibility at the grain boundaries due to the high elastic anisotropy in ordinary polycrystalline alloy. (2) High phase transformation strain could be obtained along<001> grain orientation. (3) Straight low-energy grain boundary and the absence of grain boundary triple junctions could significantly reduce the blockage of martensitic transformation at the grain boundaries. According to the effects of various structure factors on the superelasticity, the prior principles of structure design were proposed in the paper for high-performance Cu-based SMAs from most to least important:(1) obtaining grain orientation with high phase transformation strain; (2) increasing grain size or reducing GB area; (3) obtaining straight low-energy GBs, especially low-angle GBs; (4) trying to make GB direction parallel to external stress.When the angle between tensile direction and solidification direction (SD) ranged from 0° to 90° at the tensile tests, the superelastic strain of the columnar-grained Cu71Al18Mn11 alloy changed in a "V" shape and showed a large anisotropy. Meanwhile, the martensite transformation critical stress changed the opposite trend. The large anisotropy of the superelasticity was attributed to the combined effects of grain orientation and grain boundary, wherein the influence of the grain boundary had an obvious dependence on orientation. Utilized the advantage of the anisotropic characteristics, could make the columnar-grained Cu71Al18Mn11 alloy be applied into device design of specific energy absorption, shock and impact potentially, such as anisotropic shock isolators and dampers in high rise buildings and precision instruments. In addition, the columnar-grained Cu71Al18Mn11 alloy had good fatigue properties along SD. The alloy did not fracture after 1800 cycles at the fatigue strain of 10%. And the residual strain was less than 1% after 2400 cycles at the fatigue strain of 4%, which might make the alloy have potential in replacing Ni-Ti SMAs on the condition of repeated deformation.In order to improve the strength of the columnar-grained Cu71Al18Mn11 alloy, and to obtain the alloy with both high superelasticity and high strength, in this paper, the aging behavior of the columnar-grained Cu71Al18Mn11 shape memory alloy at low temperatures of 250℃-400℃ was investigated. The researches showed that the bainitic plates coherently precipitated within grains and at grain boundaries homogeneously, which improved the hardness and the strength of the alloy significantly, while its effects on superelasticity is not so obvious. The alloy with both excellent superelasticity (superelastic strain of 5%～9%) and high strength (the critical stress of 443 MPa-677 MPa) could be obtained after reasonable aging treatment. Combined with undirectional solidification and low temperature aging, the alloy with high superelasticity and high strength could be prepared, the functionally gradient materials with the properties changed in a wide-range could be made as well.Adopted two kinds of processes multipass high-temperature-rolling (HR) and hot-rolling+several times cold-rolling (HR+nCR), the influence of forming and heat treatment on the structure and properties of the columnar-grained Cu71Al18Mn11 shape memory alloy were studied in this paper. The research results showed that the alloy could deform to the rolling reduction of more than 80% at 800 ℃, and remained columnar-grained structure after the first pass HR. Then the recrystallization occurred when HR continued. After three passes HR and heat treatment at 800℃, the alloy had the superelastic strain of 5.9%. When the alloy annealed at 550℃ after the first pass HR reduction of 80%, the cold-rolling reduction of 50%～80% could always be reached. Both the alloys of annealing and cold-rolling after annealing at 550℃ were biphasic structure composed of a and β1, and still kept the columnar-grained structure. Compared with the ordinary polycrystalline one, the columnar-grained Cu71Al18Mn11 alloy tend to form the <011>-oriented texture along the SD after rolling deformation and recrystallization annealing, which were helpful to obtain high superelasticity. Then after grain growth heat treatment, the alloy still has strong<011>-oriented texture along the rolling orientation, and the superelastic strain of the alloy could reach 7%. |
