Search Results (5)

Shape memory alloys (SMAs) are types of materials that can restore their original shape upon severe or quasi-plastic deformation, being exposed to specific external stimuli, including heating, electric current, magnetic field, etc. They are a category of functional materials that provides superelasticity as a significant material property. The roots of this unintentional discovery were in the 20th century, and later it attracted the attention of various industries, including aerospace, medical, mechanical, manufacturing industries, etc. Later developments mainly focused on improving the properties of these materials. One of the ways in which this is achieved is the application of intensive plastic strains on SMAs through severe plastic deformation (SPD) methods, leading to extreme grain refinement. Superelasticity is a key characteristic of SMAs and is known as the capacity of a polycrystalline material to display extremely high elongations before failure, in a typically isotropic way, with an approximate strain rate of 0.5. Utilization of SPD techniques can also affect and lead to superior superelasticity responses in SMAs. Several SPD methodologies have been introduced over the decades, to produce ultrafine-grained and even nanostructured materials, including constrained groove pressing, equal-channel angular pressing, high-speed high-pressure torsion, accumulative roll bonding, etc. This paper aims to present a clear view of the mechanical properties and microstructure evolution of shape memory alloys after processing by some SPD methods, and to show that SPD methods can be a great option for developing SMAs and expanding their industrial and technological applications. Full article

To reduce the effects of earthquakes on the ancient Small Wild Goose Pagoda, a shape memory alloy-suspension pendulum damping system (SMA-SPDS) is developed by combining superelastic SMAs with damping pendulum theory. A MATLAB/Simulink simulation model of the SMA-SPDS is established and tested on a 1:10 scale model of the Pagoda. After verifying and comparing the simulation data with experimental results, a shock absorption analysis is performed on the prototype Pagoda. The optimum engineering design for the prototype structure of the Small Wild Goose Pagoda using SMA-SPDS for shock absorption protection in the future is put forward. The results show that the performance of the SMA-SPDS system is stable, and it can improve the integrity of the original structure of the Pagoda for better performance during earthquakes. In addition, with an increment in seismic intensity, the SMA-SPDS shows an apparent controlling effect. The Simulink simulation results of the model structure of the Small Wild Goose Pagoda are in good agreement with the test results. The Simulink simulation method can simulate the seismic response of the model structure of the Small Wild Goose Pagoda well, with and without SMA-SPDS, to obtain a more real damping effect of setting SMA-SPDS on the prototype structure; the engineering optimization of the location, quantity, and system performance parameters of SMA-SPDS in the prototype structure of the Small Wild Goose Pagoda has a remarkable effect, which can make the damping effect of SMA-SPDS reach more than 43% floor. Full article

Using the methods of electron microscopy and X-ray analysis in combination with measurements of the electrical resistance and magnetic susceptibility, the authors have obtained data on the peculiar features of pre-martensitic states and martensitic transformations, as well as subsequent decomposition, in the alloys with shape memory effect of Cu–14wt%Al–3wt%Ni and Cu–13.5wt%Al–3.5wt%Ni. For the first time, we established the microstructure, phase composition, mechanical properties, and microhardness of the alloys obtained in the nanocrystalline state as a result of severe plastic deformation under high pressure torsion and subsequent annealing. A crystallographic model of the martensite nucleation and the rearrangements β₁→β₁′ and β₁→γ₁′ are proposed based on the analysis of the observed tweed contrast and diffuse scattering in the austenite and the internal defects in the substructure of the martensite. Full article

It is generally accepted that severe plastic deformation (SPD) has the ability to produce ultrafinegrained (UFG) and nanocrystalline materials in bulk. Recent developments in high pressure torsion (HPT) processes have led to the production of bimetallic composites using copper, aluminum or magnesium alloys. This article outlines a new approach to fabricate multilayered Ni-Ti nanocomposites by a patented SPD technique, namely, high speed high pressure torsion (HSHPT). The multilayered composite discs consist of Ni-Ti alloys of different composition: a shape memory alloy (SMA) Ti-rich, whose Mf > RT, and an SMA Ni-rich, whose Af < RT. The composites were designed to have 2 to 32 layers of both alloys. The layers were arranged in different sequences to improve the shape recovery on both heating and cooling of nickel-titanium alloys. The manufacturing process of Ni-Ti multilayers is explained in this work. The evolution of the microstructure was traced using optical, scanning electron and transmission electron microscopes. The effectiveness of the bonding of the multilayered composites was investigated. The shape memory characteristics and the martensitic transition of the nickel-titanium nanocomposites were studied by differential scanning calorimetry (DSC). This method opens up new possibilities for designing various layered metal-matrix composites achieving the best combination of shape memory, deformability and tensile strength. Full article

Microstructural evolution of NiTi shape memory alloy (SMA) with a nominal composition of Ni_50.9Ti_49.1 (at %) is investigated on the basis of heat treatment and severe plastic deformation (SPD). As for as-rolled NiTi SMA samples subjected to aging, plenty of R phases appear in the austenite matrix. In terms of as-rolled NiTi SMA samples undergoing solution treatment and aging, Ni₄Ti₃ precipitates arise in the austenite matrix. In the case of as-rolled NiTi SMA samples subjected to SPD and aging, martensitic twins are observed in the matrix of NiTi SMA. With respect to as-rolled NiTi SMA samples subjected to solution treatment, SPD, and aging, neither R phases nor Ni₄Ti₃ precipitates are observed in the matrix of NiTi SMA. The dislocation networks play an important role in the formation of the R phase. SPD leads to amorphization of NiTi SMA, and in the case of annealing, amorphous NiTi SMA samples are subjected to crystallization. This contributes to suppressing the occurrence of R phase and Ni₄Ti₃ precipitate in NiTi SMA. Full article