Producing a single crystal is expensive because of low mass productivity. Therefore, many metallic materials are being used in polycrystalline form, even though material properties are superior in a single crystal. Here we show that an extraordinarily large Cu-Al-Mn single crystal can be obtained by abnormal grain growth (AGG) induced by simple heat treatment with high mass productivity. In AGG, the sub-boundary energy introduced by cyclic heat treatment (CHT) is dominant in the driving pressure, and the grain boundary migration rate is accelerated by repeating the low-temperature CHT due to the increase of the sub-boundary energy. With such treatment, fabrication of single crystal bars 70 cm in length is achieved. This result ensures that the range of applications of shape memory alloys will spread beyond small-sized devices to large-scale components and may enable new applications of single crystals in other metallic and ceramics materials having similar microstructural features.

The objective of this paper is to examine the orientation dependence of plasticity and fracture in single-crystal superelastic Cu-Al-Mn shape memory alloy (SMA) bars. For this purpose, we prepared 14 single-crystal superelastic Cu-Al-Mn SMA bars of 15-mm diameter and 140-mm length. The crystal orientation was measured using electron backscatter diffraction. This work involved cyclic tension tests up to 10% strain and consecutive monotonic tension tests up to fracture. From these results, strong orientation dependence was observed in plasticity and fracture. Ductility was poor or moderate when the orientation of the specimen was close to the < 1 0 1> direction. On the other hand, a highly ductile response was observed when the orientation was close to the < 1 1 2>, <1 1 3>, or < 0 0 1> direction. In these specimens, the fracture strain ranged from 47% to 92%. The large rotation of crystal lattice and the propagation of slip band along the long distance in the longitudinal direction are the reasons for the highly ductile response. Such a highly ductile response is desirable in structural materials, especially in seismic applications for civil structures like buildings and bridges.

This study investigated the effects of the types and length of nonmetallic inclusions on fatigue properties in rotating bending fatigue testing of Ti-Ni alloy fine wire. It was fabricated to include titanium carbides Ti(C,O) and titanium oxides Ti4Ni2Ox as either single phases or a mixture of both phases as nonmetallic inclusions in Ti-Ni alloy. The fatigue strength of Ti-Ni alloy depended on the number of nonmetallic inclusions of a length of ≥2 μm. Compared with Ti(C,O), Ti4Ni2Ox is coarse. It also exhibited a trend of readily forming particles and void assemblies, which are a defect morphology that originates from nonmetallic inclusions and readily act as crack origins of fatigue fractures.

This paper examines the feasibility of mechanical splicing using a steel coupler to connect headed ends of superelastic Cu–Al–Mn alloy (Camalloy) bars and steel reinforcing bars to be used in concrete structures. Although threading of Camalloy is as easy as that of steel, mechanical splicing using threaded ends requires machining of Camalloy bars into dog-bone shape to avoid brittle fracture at the threaded ends. The machining process requires significant time and cost and wastes substantial amount of the material. This paper attempts to resolve this issue by applying mechanical splicing using steel couplers to connect headed ends of Camalloy and steel reinforcing bars. To study its feasibility, we prepare 3 specimens wherein both ends of each Camalloy bar (13 mm diameter and 300 mm length) are connected to steel reinforcing bars. The specimens are tested under monotonic, single-cycle, and full-cycle tension loading conditions. From these tests, we observed (1) excellent superelasticity with recoverable strain of around 6% and (2) large ductility with fracture strain of over 19%. It should be emphasized here that, in all the specimens, ductile fracture occurred at the locations apart from the headed ends. This is in sharp contrast with brittle fracture of headed superelastic Ni–Ti SMA bars, most of which took place around the headed ends. From the results of the microstructural analysis, we identified the following reasons for avoiding brittle fracture at the headed ends: (1) Precipitation hardening increases the strength around the boundary between the straight and headed (tapered) portions, where stress concentration takes place. (2) The strength of the straight portion does not increase significantly up to the ductile fracture if its grain orientation is close to 〈0 0 1〉.

Superelastic alloy (SEA) bars are widely used in structures subjected to moderate and strong earthquakes. Compared with conventional nickel-titanium (NiTi) SEAs, Cu-Al-Mn (CAM) SEAs has received increasing attention recently due to their cost-effectiveness and easier machinability. The authors’ previous research showed that despite their lower strength and limitations in the maximum length, the CAM SEAs have comparable superelastic strain recovery, a wider temperature range, and superior strain rate stability compared to NiTi SEAs. However, the previous research was limited to a few specimens and only conducted to a few hundred cycles without considering the full deterioration in the material properties. Besides, the existing research on CAM SEA was only limited to small sample sizes at room temperature, while the fatigue performance of large diameter CAM SEAs under low and high temperatures relevant for civil engineering structures has not been reported. To fill this knowledge gap, low-cycle fatigue performance of 20 mm diameter CAM SEAs was studied at room temperature 25℃, low temperature, -40℃, and high temperature, 50℃. Both single crystal and polycrystal CAM SEA were investigated to determine their feasibility as concrete reinforcement under repeated high strain loading cycles expected during an earthquake. Strain cycles up to 50,000 have been applied at a tensile strain amplitude of 5%. Variations in the superelastic properties were observed and analyzed, including the stress-strain curves, elastic modulus, transformation stresses, damping ratio and recovery strain. Stable hysteresis has been observed for cycles exceeding tens of thousands at all temperatures demonstrating the suitability of CAM SEAs for seismic applications in civil engineering structures.