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In the present work, a single solution treatment as well as solution/aging treatments were employed to regulate the precipitation behavior, martensitic transformation, mechanical and functional properties of EBF3-fabricated NiTi shape memory alloys. Under the optimized post-process heat treatment conditions, i.e., solution treatment at 1000 oC for 10 h followed by subsequent aging treatment at 500 oC for 4 h, the previously existing elongated Ti 4 Ni 2 O x oxides were partially dissolved and interrupted into granular form and relatively uniformly distributed in the NiTi matrix. Meanwhile, the preexisting nanoscale Ni 4 Ti 3 precipitates gradually transformed from clustered or segregated granular to a lenticular shape with the increase of the aging time. The martensite transformation routes also underwent an accompanying alteration, which changed from a one-step (B2-A ↔ B19'-M) to two-step (B2-A → R → B19'-M) transformation. Moreover, Ni 4 Ti 3 precipitated with a coherent relationship of {123} <111> B2-A //{11–20} <0001> Ni4Ti3 which increase the material strength by hindering dislocation movement and improve plasticity by providing a transition domain for accommodating deformation at the interface. The superelastic stability and narrow response hysteresis promoted by the presence of the R-phase are affected by residual (100) compound twins martensite in the NiTi matrix and the permanent plastic deformation activated in (011) [001] slip system of the B2-A phase. This deteriorates the superelastic recovery ratio from 83.6 to 57.4%, as observed after 10 loading-unloading cycles. The post-heat treatment regime presented in the current investigation can be used to modulate the material precipitation, phase transformation, as well as the mechanical and functional properties of EBF3-fabricated NiTi alloys. These results allow to extrapolate the potential for carefully selected post-heat treatment scheduled for additively manufacturing NiTi shape memory alloys. [Display omitted] • Post-heat treatment can regulate precipitation, martensitic transformation and properties of EBF3-fabricated NiTi SMAs. • Nano-scale Ni 4 Ti 3 alters the transformation routes and improves the strength-plasticity trade-off. • Superelastic stability and narrow response hysteresis are promoted by the presence of R-phase. • Residual (100) compound twins and dislocation activated in (011)[001] system of B2-A deteriorates the superelasticity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Interlayer remelting strategies can regulate the microstructure evolution, martensitic transformation, and superelasticity of EBF3-fabricated NiTi SMAs. • A one-step reversible transformation (B2 ↔ B19′) occurred upon heating/cooling. • The critical stress (σMs) has a strong dependence on the martensitic transformation behavior. • The broadening and stabilization of martensite is the key factor for the deterioration of the superelastic response. In this work, different interlayer remelting strategies are applied to regulate the microstructure evolution, martensitic transformation, and superelastic features of NiTi shape memory alloys prepared using the EBF3 additive manufacturing technique. The NiTi deposits prepared under different remelting beam currents are all composed of the B2 austenite, residual B19′ martensite, and submicron-scale Ti 4 Ni 2 O x precipitates, and exhibit a one-step phase transformation (B2 ↔ B19′). Meanwhile, the crystallographic orientation, grain boundaries, and residual strain of these alloys present a distinct variation with the application of different remelting beam currents. During mechanical testing, the critical stress (σMs) of the EBF3-fabricated NiTi alloys was seen to possess a significant dependence on the martensitic transformation behavior, namely with the amount of B19′ martensite and the corresponding M s. However, the broadening and stabilization of lamellar martensite created by dislocation pile-ups and plastic deformation in the cyclic loading–unloading procedure is the key reason for the deterioration of the superelastic response of the NiTi deposits. This work confirms that the mechanical and functional performances of NiTi alloys produced via EBF3-technique can be modified upon the application of proper interlayer remelting strategies, which can be extrapolated to the directed energy deposition of shape memory alloys or other metal components. [ABSTRACT FROM AUTHOR]

In this work, NiTi shape memory alloys parts were additively manufactured using electron beam freeform fabrication (EBF3) under different processing parameters, including the beam current, travel speed, and wire feeding speed. The forming quality, phase composition, microstructure change, crystallography, martensitic transformation, shape memory and superelastic responses were systematically investigated. All deposits mainly consisted of B2-austenite at room temperature, and a handful of B19′-martensite and submicron-sized Ti 4 Ni 2 O x precipitates were also detected. The martensitic transformation of NiTi alloys prepared by EBF3-technique possessed an individual reversible path between B2 and B19′ upon heating/cooling. The optimized deposit possessed the best comprehensive properties, where the values of the relative density, shape memory recovery and superelastic recovery ratios were 99.6%, 98.95%, and 55.78%, respectively. Furthermore, the dependence of the martensitic transformation behavior on the thermomechanical condition and the relationship between plastic deformation and phase transformation during superelastic deformation are discussed in detail. Our work details that the EBF3 provides a suitable way for the complex fabrication of large-scaled parts based on shape memory alloys. • NiTi shape memory alloys parts were additively manufactured using electron beam freeform fabrication under different processing parameters. • The fabricated NiTi alloys presented a single reversible martensitic transformation (B2 ↔ B19') upon heating/cooling. • The optimized deposit had shape memory recovery and superelastic recovery ratios of 98.95%, and 55.78%, respectively. • The dependence of the martensitic transformation behavior on the thermomechanical condition is discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel iron-based shape memory alloy was successfully deposited by wire and arc additive manufacturing. • As-deposited material was predominantly composed by FCC γ phase with a columnar dendritic morphology. • High mechanical performance was observed for yield strength, fracture stress, elongation, cyclic repetibility and hardness. • Uniaxial loading induced the FCC γ → HCP ε phase transformation and resulted in a ductile fracture. Shape memory alloys (SMA) are a class of smart materials with inherent shape memory and superelastic characteristics. Unlike other SMAs, iron-based SMAs (Fe-SMA) offer cost-effectiveness, weldability, and robust mechanical strength for the construction industry. Thus, applying these promising materials to advanced manufacturing processes is of considerable industrial and academic relevance. This study aims to present a pioneer application of a Fe–Mn–Si–Cr–Ni–V-C SMA to arc-based directed energy deposition additive manufacturing, namely wire and arc additive manufacturing (WAAM), examining the microstructure evolution and mechanical/functional response. The WAAM-fabricated Fe-SMAs presented negligible porosity and high deposition efficiency. Microstructure characterization encompassing electron microscopy and high energy synchrotron X-ray diffraction revealed that the as-deposited material is primarily composed by γ FCC phase with modest amounts of VC, ε and σ phases. Tensile and cyclic testing highlighted the Fe-SMA's excellent mechanical and functional response. Tensile testing revealed a yield strength and fracture stress of 472 and 821 MPa, respectively, with a fracture strain of 26%. After uniaxial tensile loading to fracture, the γ → ε phase transformation was clearly evidenced with post-mortem synchrotron X-ray diffraction analysis. The cyclic stability during 100 load/unloading cycles was also evaluated, showcasing the potential applicability of the fabricated material for structural applications. [ABSTRACT FROM AUTHOR]