Explore the vast range of titles available.

Shape memory alloys (SMAs), such as Nitinol (i.e., NiTi), are of great importance in biomedical and engineering applications due to their unique superelasticity and shape memory properties. In recent years, additive manufacturing (AM) processes have been used to produce complex NiTi components, which provide the ability to tailor microstructure and thus the critical properties of the alloys, such as the superelastic behavior and transformation temperatures (TTs), by selection of processing parameters. In biomedical applications, superelasticity in implants play a critical role since it gives the implants bone-like behavior. In this study, a methodology of improving superelasticity in Ni-rich NiTi components without the need for any kind of post-process heat treatments will be revealed. It will be shown that superelasticity with 5.62% strain recovery and 98% recovery ratio can be observed in Ni-rich NiTi after the sample is processed with 250 W laser power, 1250 mm/s scanning speed, and 80 µm hatch spacing without, any post-process heat treatments. This superelasticity in as-fabricated Ni-rich SLM NiTi was not previously possible in the absence of post-process heat treatments. The findings of this study promise the fast, reliable and inexpensive fabrication of complex shaped superelastic NiTi components for many envisioned applications such as patient-specific biomedical implants.

Ultra-high strength of NiTiHfPd alloys have been promising for specific application areas of SMAs. Thus, the main objective of this study is to further understand the high strength behavior of the alloys through experimental and theoretical studies. Shape memory response of an ultra-high strength Ni 45.3 Ti 29.7 Hf 20 Pd 5 alloy was systematically investigated after aging at 550 °C for 5 h via constant-stress temperature cycling and constant-temperature stress cycling experiments. Shape memory behavior under a wide range of compressive stress levels from 300 to 1200 MPa was reported before and after stress cycling of 5000 times. The alloys showed a reversible strain of 1.3% against an ultra-high stress of 2 GPa. It is concluded that the combination of high strength and temperature capability mainly stems from high chemical complexity in addition to aging and will be quite advantageous in practical applications.

Porous NiTi scaffolds display unique bone-like properties including low stiffness and superelastic behavior which makes them promising for biomedical applications. The present article focuses on the techniques to enhance superelasticity of porous NiTi structures. Selective Laser Melting (SLM) method was employed to fabricate the dense and porous (32–58%) NiTi parts. The fabricated samples were subsequently heat-treated (solution annealing + aging at 350 °C for 15 min) and their thermo-mechanical properties were determined as functions of temperature and stress. Additionally, the mechanical behaviors of the samples were simulated and compared to the experimental results. It is shown that SLM NiTi with up to 58% porosity can display shape memory effect with full recovery under 100 MPa nominal stress. Dense SLM NiTi could show almost perfect superelasticity with strain recovery of 5.65 after 6% deformation at body temperatures. The strain recoveries were 3.5, 3.6, and 2.7% for samples with porosity levels of 32%, 45%, and 58%, respectively. Furthermore, it was shown that Young’s modulus (i.e., stiffness) of NiTi parts can be tuned by adjusting the porosity levels to match the properties of the bones.

Process parameters and post-processing heat treatment techniques have been developed to produce both shape memory and superelastic NiTi using Additive Manufacturing. By introducing engineered porosity, the stiffness of NiTi can be tuned to the level closely matching cortical bone. Using additively manufactured porous superelastic NiTi, we have proposed the use of patient-specific, stiffness-matched fixation hardware, for mandible skeletal reconstructive surgery. Currently, Ti-6Al-4V is the most commonly used material for skeletal fixation devices. Although this material offers more than sufficient strength for immobilization during the bone healing process, the high stiffness of Ti-6Al-4V implants can cause stress shielding. In this paper, we present a study of mandibular reconstruction that uses a dry cadaver mandible to validate our geometric and biomechanical design and fabrication (i.e., 3D printing) of NiTi skeletal fixation hardware. Based on the reference-dried mandible, we have developed a Finite Element model to evaluate the performance of the proposed fixation. Our results show a closer-to-normal stress distribution and an enhanced contact pressure at the bone graft interface than would be in the case with Ti-6Al-4V off-the-shelf fixation hardware. The porous fixation plates used in this study were fabricated by selective laser melting.

Abstarct Shape memory alloys (SMAs) have the ability to show large recoverable shape changes upon temperature, stress or magnetic field cycling. Their shape memory, material and magnetic properties (e.g. transformation temperatures, strain, saturation magnetization and strength) determine their prospects for applications from small-scale microelectromechanical systems to large scale aerospace and biomedical systems. It should be noted that properties of SMAs are highly temperature dependent. Generally, the conventional mechanical characterization methods (e.g, tension, compression, and torsion) are used on bulk samples of SMAs to determine those properties. In this article, it will be shown that indentation technique can be used as an alternative rapid method to determine some of the important shape memory properties of SMAs. Indentation response of a high-temperature NiTiHf alloy was determined as a function of temperature. A clear relationship between the work recoverable ratio and transformation temperatures, superelastic and plastic behavior was observed. This work shows that indentation response can be used to measure local superelasticity response, determine phase transformation temperatures and reveal the temperature intervals of the deformation mechanisms of shape memory alloys.

Poor machinability with conventional machining processes is a major shortcoming that limits the manufacture of NiTi components. To better understand the effects of phase state on the machining performance of NiTi alloys, cutting temperature, tool-wear behavior, cutting force components, tool-chip contact length, chip thickness, and machined surface quality data were generated from a NiTi alloy using precooled cryogenic, dry, minimum quantity lubrication (MQL), and preheated machining conditions. Findings reveal that machining NiTi in the martensite phase, which was achieved through precooled cryogenic machining, profoundly improved the machining performance by reducing cutting force components, notch wear, and surface roughness. Machining in the austenite state, achieved through preheating, did not provide any benefit over dry and MQL machining, and these processes were, in general, inferior to cryogenic machining in terms of machining performance, particularly at higher cutting speeds.

Laser powder bed fusion has been widely investigated for shape memory alloys, primarily NiTi alloys, with the goal of tailoring microstructures and producing complex geometries. However, processing high temperature shape memory alloys (HTSMAs) remains unknown. In our previous study, we showed that it is possible to manufacture NiTiHf HTSMA, as one of the most viable alloys in the aerospace industry, using SLM and investigated the effect of parameters on defect formation. The current study elucidates the effect of process parameters (PPs) on the functionality of this alloy. Shape memory properties and the microstructure of additively manufactured Ni-rich NiTiHf alloys were characterized across a wide range of PPs (laser power, scanning speed, and hatch spacing) and correlated with energy density. The optimum laser parameters for defect-free and functional samples were found to be in the range of approximately 60–100 J/mm3. Below an energy density of 60 J/mm3, porosity formation due to lack-of-fusion is the limiting factor. Samples fabricated with energy densities of 60–100 J/mm3 showed comparable thermomechanical behavior in comparison with the starting as-cast material, and samples fabricated with higher energy densities (>100 J/mm3) showed very high transformation temperatures but poor thermomechanical behavior. Poor properties for samples with higher energies were mainly attributed to the excessive Ni loss and resultant change in the chemical composition of the matrix, as well as the formation of cracks and porosities. Although energy density was found to be an important factor, the outcome of this study suggests that each of the PPs should be selected carefully. A maximum actuation strain of 1.67% at 400 MPa was obtained for the sample with power, scan speed, and hatch space of 100 W, 400 mm/s, and 140 µm, respectively, while 1.5% actuation strain was obtained for the starting as-cast ingot. These results can serve as a guideline for future studies on optimizing PPs for fabricating functional HTSMAs.

The shape memory behavior of [111]-oriented Ni_（51）Ti_（49） （at.%） single crystals was investigated after stressassisted aging at 500 °C for 1.5 h under a compressive stress of-150 MPa.It was found that a single family of Ni_4Ti_3 precipitates with two crystallographically equivalent variants was formed after aging under compressive stress.Stressassisted aging resulted in tensile two-way shape memory effect strain of 1.56% under-5 MPa.Thermal cycling under-600 MPa resulted in a transformation strain of-2.15%,while the subsequent thermal cycling under-5 MPa resulted in a tensile two-way shape memory effect strain of 2.2%.

The shape-memory properties of Ni 45.3 Ti 34.7 Hf 15 Pd 5 and Ni 45.3 Ti 29.7 Hf 20 Pd 5 polycrystalline alloys were determined through superelasticity and shape-memory tests in compression. It has been revealed that the Ni 45.3 Ti 34.7 Hf 15 Pd 5 has a maximum transformation strain of 3.8 % and work output of up to 30 J cm −3 , while the Ni 45.3 Ti 29.7 Hf 20 Pd 5 has a maximum transformation strain of 2.6 % and work output of up to 20 J cm −3 at 700 MPa. Two-way shape-memory strains of 0.6 and 0.85 % were obtained in Ni 45.3 Ti 34.7 Hf 15 Pd 5 and Ni 45.3 Ti 29.7 Hf 20 Pd 5 alloys, respectively. The Ni 45.3 Ti 34.7 Hf 15 Pd 5 showed superelasticity at 90 °C with recoverable strain of 3.1 %, while high hardening of Ni 45.3 Ti 29.7 Hf 20 Pd 5 limited its superelastic behavior. Microstructure of the Ni 45.3 Ti 34.7 Hf 15 Pd 5 alloy was revealed by transmission electron microscopy, and effects of composition on the lattice parameters of the transforming phases and martensite morphology were discussed.

This study presents the shape memory behavior of Ni-rich NiTi shape memory alloy fabricated by Laser Powder Bed Fusion Additive Manufacturing (L-PBF-AM) before and after post-processing heat treatment. The microstructural features and thermo-mechanical responses were systematically investigated to understand the effects of processing on the behavior of the specimens. It was shown that the L-PBF-AM process improves the functionality of NiTi components by illustrating perfect superelastic behavior at higher-temperature windows compared to the casted ingot. In addition, it was revealed that shape memory responses were tailored by altering hatch distance, which significantly controls the texture formation along the building direction. After post-processing treatments, transformation temperatures were increased, hysteresis was decreased, and the strength of the samples was significantly improved. The aged L-PBF-AM sample with a smaller hatch distance (80 µm) and intense [001] texture illustrated perfect superelastic behavior with a recoverable strain of 7% and a superelastic temperature span of about 100 °C.