The application of Ti-Ni binary alloy at high temperatures is hindered by its low phase transition temperature. To enhance this transition temperature, researchers have explored the addition of metal Hf to Ti-Ni alloy. However, Ti-Ni-Hf high-temperature shape memory alloy exhibits brittleness and lacks favorable thermal deformation characteristics. Thus, a comprehensive investigation of its thermal deformation behavior is essential. During the hot working process, the material undergoes shape and microstructural changes, which are influenced by various processing factors. Consequently, optimizing processing parameters, including temperature, strain, and strain rate, is crucial for producing defect-free components with the desired microstructure. To optimize the hot working technology, single-pass compression tests on a Gleeble-3800 thermo-simulation machine were conducted and the hot deformation behavior and workability of the high-temperature shape memory alloy Ti30Ni50Hf20 were explored. These tests covered a temperature range of 700-900 oC and a strain rate range of 0.01-10 s-1. Flow stress-strain curves for Ti30Ni50Hf20 under different deformation conditions were generated and the evolution of the alloy's microstructure at varying deformation temperatures under a strain rate of 0.01 s-1, as well as at a deformation temperature of 900 oC with different deformation rates were examined. Utilizing a dynamic material model, a processing diagram was constructed and the impact of process parameters on the alloy's processing performance was analyzed. The results indicate that the recrystallization of Ti30Ni50Hf20 high-temperature shape memory alloy increases with the deformation temperature. This alloy exhibits negative temperature sensitivity and positive strain sensitivity, with flow stress increasing as the strain rate rises and decreasing with higher deformation temperatures. A constitutive equation for Ti30Ni50Hf20 high-temperature shape memory alloy during hot working is established, employing the Arrhenius hot deformation equation. The calculated strain activation energy was determined to be 527.447 kJ/mol. It revealed a consistent match between the theoretical and actual peak stress values. Through the assessment of the hot working diagram, the optimal processing parameters are identified as a deformation temperature in the range of 880-900 oC and a strain rate of 0.01-0.04 s-1.
Exploring novel magnetic Heusler alloys is of great significance for the development of a new generation of smart sensing materials. The A2BC type magnetic alloy, which comprises transition magnetic metal elements A and B and III-V main group element C (p-block element), has gained significant attention due to its various physical and chemical properties, including semimetallic magnetism, ferromagnetic shape memory effect, multicaloric effect, and superconductive effect. In this study, eight new A2BTi type magnetic functional alloys, including three Co-based alloys (Co2MnTi, Co2FeTi, and Co2NiTi), three Fe-based alloys (Fe2MnTi, Fe2CoTi, and Fe2NiTi), and two Ni-based alloys (Ni2FeTi and Ni2CoTi), were investigated for their phase stability against tetragonal distortion using first-principles calculation. The underlying mechanism for the stability of the L21 phase was discussed. The results show that valence electron concentration and magnetism are the key parameters in determining the structural stability of L21 phase in A2BTi type alloys. Co2NiTi, Fe2NiTi, and Ni2CoTi alloy samples, whose L21 structure is an unstable phase, were prepared, and their crystal structure, phase transformation, magnetic properties, electrical resistance, and mechanical properties were investigated experimentally. The results show that at 298 K, Co2NiTi is composed of an ordered face-centered cubic L12 structured matrix phase and a hexagonal Co3Ti-type second phase, Fe2NiTi is composed of a hexagonal Fe2Ti-type matrix phase and a tetragonal FeNi-type second phase, and Ni2CoTi has a single hexagonal Ni3Ti-type structure. The fact that no compound undergoes a first-order structural phase transition may be due to the weak stabilities of their L21 phases. Fe2NiTi and Ni2CoTi have strong magnetic properties and undergo a second-order Curie magnetic transition during cooling. Fe2NiTi has high compressive strength (1280 MPa), moderate compressive strain (5%), and large resistance (120 μΩ·cm), while Co2NiTi and Ni2CoTi have excellent compressive plasticity and small resistance. This phenomenon may be related to the different proportions of metallic and covalent bonding caused by the difference in valence electron concentration of the three alloys.
With the development of industry, people pay more and more attention to the hazards of vibration and noise in various fields. Besides adopting various vibration-reduction technologies, the demand for high-performance damping materials is also increasing to reduce vibration and noise. Among them, damping composites combine the advantages of different damping materials and superimpose multiple mechanisms to integrate their functions and structures, obtaining damping materials with excellent comprehensive performance. Herein, a novel damping composite was prepared using the sintering evaporation method and vacuum infiltration. This composite adopts the porous CuAlMn shape memory alloy as the skeleton, whose pores are filled with a composite composed of carbon nanotubes loaded with HfO2 particles and a viscoelastic polymer. Uniaxial compression test at room temperature and dynamic mechanical analysis were carried out on composite samples. The results show that when porosity of the skeleton is 80% and the mass fraction of carbon nanotubes is 1%, the compressive yield strength and elastic modulus of the composite are 27 MPa and 1040 MPa, respectively, and its density is only 2.11 g/cm3. Its loss factor is > 0.055 in the range of 0.1-200 Hz and 20-100oC, and its maximum value can reach 0.102. The elastic modulus, compressive yield strength, and loss factor of this composite increased by 1, 2, and 1.5 times, respectively, compared to those of the CuAlMn skeleton with same porosity. A three-phase model was utilized to analyze the damping mechanism of composite samples. The calculation results show that the primary damping mechanism of the proposed novel composite is interface damping.
In the past decade, a unique composite system consisting of Nb nanowire and NiTi shape memory alloy matrix has attracted considerable attention. One of the works published in Science proposed that the NiTi-Nb composite has superior properties, including high strength (1.65 GPa), low Young's modulus (25.8 GPa), and quasi-linear superelasticity (6.4%). In particular, given the quasi-linear superelasticity of this composite, (1) continuous stress-induced martensitic transformation occurred even at the beginning of tensile loading, which indicated that the external stress required to start the transformation was reduced to almost zero; (2) the transformation (stress-strain) curve is a “hardening type” rather than a “plateau type,” with apparent Young's modulus of 25.8 GPa, and (3) the amount of quasi-linear superelasticity deformation is 6.4%, which is higher than that of conventional binary NiTi alloy. This work focused on the quasi-linear superelasticity property. Thus, an in situ NiTi-Nb composite was prepared by vacuum induction melting, hot forging, and wire drawing. Microscopic analysis showed that Nb nanowires were distributed in parallel inside the nanocrystalline NiTi matrix along the wire axial direction. Quasi-linear superelasticity was obtained after 9% pre-deformation, with a yield stress of 1.7 GPa, apparent Young's modulus of 34 GPa, and quasi-linear superelasticity deformation of ~5.5%, which is similar to the result proposed in Science. In situ synchrotron XRD measurements were conducted to analyze the effect of pre-deformation on the coupling effect between NiTi and Nb nanowire. The origin and deformation mechanism of the quasi-linear superelasticity were systematically studied.Results revealed that coupling tensile stress in NiTi, which was generated by pre-deformation, increased gradually with the increase of the pre-deformation strain, thereby providing a driving force for stress-induced martensitic transformation. The external stress required to start the transformation could be reduced to almost zero in some local areas as a result of the coupling tensile stress. The initial velocity of transformation increased with the increase of the coupling tensile stress in NiTi. Therefore, a continuous transformation with relatively high velocity was obtained even at the beginning of tensile loading after a proper pre-deformation. Furthermore, the gradient distribution of coupling tensile stress inside B2-NiTi led to the “hardening-type” transformation (stress-strain) curve.
The NiTi alloy is a key material in aerospace and biomedical fields owing to its excellent superelasticity and high shape memory effect. Laser directed energy deposition (LDED), as an advanced additive manufacturing technology, made the preparation of NiTi alloys with high shape memory effect possible. In this study, the NiTi alloy was fabricated via LDED using Ni and Ti powder feedstock. The microstructure, phase content, and phase transformation of the alloy were analyzed by XRD, phase fitting, SEM, EDS, and DSC. Next, the shape memory effect was tested using compressed cylindrical samples. When the laser energy density was low, several Ni4Ti3 phases were produced in the NiTi alloy. The Ni4Ti3 phase disappeared with an increase in the laser energy density. When the laser energy density was 20.0 J/mm2, the NiTi alloy showed a high compressive breaking strength of 2878 MPa and a compression failure strain of 34.9%, and the sample also showed a shape recovery rate of 88.2% after 20 cyc of compression.
The postprocessing/machining of NiTi shape memory alloys (SMAs) is extremely challenging and difficult due to their low thermal conductivity and the high reactivity of ready-made NiTi parts. As a typical metal additive manufacturing technology, selective laser melting (SLM) offers significant advantages and can directly fabricate complex metallic parts, effectively address the problems of cold workability and machinability for NiTi parts. By establishing the relationship between processing parameters, microstructure, functional properties, and revealing the underlying mechanisms for altered phase transformation behavior and functional properties of SLM NiTi SMAs, it can serve as a theoretical foundation for expanding the applications of SLM NiTi SMAs. As a result, this paper comprehensively evaluates the formability, phase transformation behavior, microstructure, mechanical properties, and thermomechanical properties of SLM NiTi SMAs. Additionally, the design of SLM porous NiTi SMAs, as well as their biocompatibility, are discussed. Eventually, the future development trend and critical problems in studying SLM NiTi SMAs are investigated.
Cu-Al-Ni alloys are not yet widely used due to issues related to their coarse grains, unacceptable plasticity, and poor thermal stability. Here, the physical and mechanical properties, as well as the corrosion behavior, of Cu-Al-Ni alloys doped with Y element (Cu-13Al-4Ni-xY (x = 0.2, 0.5, mass fraction, %)) were studied. XRD, OM, SEM, TEM, electronic universal testing machine, and electrochemical workstation were used to characterize the microstructures and measure the properties of the Cu-13Al-4Ni-xY alloys. The results showed that at room temperature, the microstructure of Cu-13Al-4Ni-xY alloys was mainly an 18R martensite matrix. The (Cu, Al, Ni)4Y second phase was characterized to have a hexagonal structure. The mechanical properties of the Cu-13Al-4Ni-xY alloys improved as the Y element content increased. For example, when the Y content was increased from 0% to 0.5%, the compressive fracture strain increased from 10.5% to 19.3% and the fracture strength increased from 580 to 1185 MPa. Additionally, the fracture type of the alloy changed from intergranular to transgranular with the addition of Y. Finally, the results from electrochemical experiments showed that the corrosion behavior of the alloys decreased slightly with the addition of Y.
