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This study investigates the microstructure and properties of functionally graded NiTi alloy bilayers. The NiTi layer is printed by laser powder bed fusion on a NiTiX (where X is Hf or Cu) substrate prepared by vacuum arc remelting. Specimens produced with different thicknesses of layers, but constant thickness ratio, are examined by optical and scanning electron microscopy prior to and postannealing process at 1000 °C for 16 h. Scanning electron microscopy– energy‐dispersive X‐ray spectroscopy and transmission electron microscopy studies reveal the presence of Ti2Ni and Ni4Ti3 precipitates in the as‐printed NiTi/NiTiCu samples and Ti2Ni type precipitates in as‐printed NiTi/NiTiHf. Digital image correlation quantifies residual strain in the as‐printed bilayer and enables strain relief to be monitored during heating. It has been shown that microcracks occurring along interfacial zones during the laser powder bed fusion are diminished after annealing heat treatment. The microcrack closure occurs by diffusion of third elements to the open microcracks, leading to precipitation and accumulation of third elements in the interfaces. Eventually, the as‐printed NiTi/NiTiCu sample displays two‐way shape memory effects with about 24.5% shape recovery. This work enhances understanding of controlling fabrication to yield tailored properties in additively manufactured functionally graded NiTi‐based materials. [ABSTRACT FROM AUTHOR]

This study reviews the recent works on the development and application of iron‐based shape memory alloy (Fe‐SMA), the so‐called memory‐steel, for steel structures. First, the studies on the material properties of Fe‐SMA in terms of shape memory effect and superelasticity are discussed. Next, the use of Fe‐SMA in prestressed strengthening of steel structures is explained, including the applications in strengthening of steel girders, connections, and fatigue crack repairs. Various strengthening solutions such as using mechanically anchored or adhesively‐bonded Fe‐SMA, as well as the studies on the behavior of the Fe‐SMA‐to‐steel bonded joints, are discussed. The use and application of Fe‐SMA for strengthening of a 113‐years steel bridge has been explained. In addition, studies on the innovative application of the Fe‐SMA as pipe couplers are presented. At the end, innovative ongoing research on the additive manufacturing of architected Fe‐SMA (4D‐printing) are discussed. [ABSTRACT FROM AUTHOR]

The pseudoelasticity behavior of memory‐steels has recently gained attention for applications in seismic‐resistant structures. The pseudoelasticity can be used to reduce residual deformations in building structures subjected to seismic loading. Fe‐based shape memory alloys (Fe‐SMA), which is also called as memory‐steel, have gained much attention because of their strong shape memory behavior and low material manufacturing cost, which justify their large‐scale application in construction. This research investigated the effects of heat treatment on microstructure and pseudoelasticity of a memory‐steel (Fe‐17Mn‐5Si‐10Cr‐4Ni‐1(V,C) %wt.). Various solution‐annealing treatments from 1000 to 1200 °C for 2 h and aging from 650, 750 and 850 °C were applied to investigate the microstructural evolution and pseudoelasticity behavior of the alloy. The results showed that increasing the solution annealing temperature caused grain growth and pseudoelasticity loss. On the other hand, the precipitation of (Cr,V)Cs after aging resulted in an improved pseudo‐elasticity. The large elastic strain field, which forms near the carbide precipitates, improves the pseudoelasticity. Furthermore, precipitates can provide preferential nucleation sites for the martensite phase and a high density of stacking faults in the austenite matrix. The study concludes that the best pseudoelasticity behavior was achieved after an aging at 750°C for 6 h. [ABSTRACT FROM AUTHOR]

Shape memory Ni-Ti-based thin films were deposited on glass substrates at room temperature by DC magnetron sputtering system from alloy targets. Two discs of Ti-rich targets (Ni41Ti54Cu5 and Ni47Ti53) were used to deposit two single-layers with chemical compositions of Ni45Ti50Cu5 and Ni50.8Ti49.2 and one bi-layer thin films with different chemical compositions similar to the single layers on glass substrates. The as-deposited Ni-Ti thin films were annealed to transform the amorphous to a crystalline material. Subsequently, the structures, phase transformations, and mechanical properties were characterized using grazing incidence X-ray diffraction (GIXRD), field emission electron microscope (FESEM), electrical resistance, and nanoindentation. The developed bi-layer thin film exhibited a two-step transformation B2 ↔ R ↔ B19' during cooling and heating with a narrow thermal hysteresis as compared to the single-layers. Moreover, super-elasticity effect and partial permanent strain appeared in the bi-layer because of composite structure. [ABSTRACT FROM AUTHOR]