Crystal structures and shape-memory behaviour of NiTi

Shape-memory alloys (SMAs) are a unique class of metal alloys that after a large deformation can, on heating, recover their original shape. In the many practical applications of SMAs, the most commonly used material is NiTi (nitinol). A full atomic-level understanding of the shape-memory effect in NiTi is still lacking, a problem particularly relevant to ongoing work on scaling down shape-memory devices for use in micro-electromechanical systems. Here we present a first-principles density functional study of the structural energetics of NiTi. Surprisingly, we find that the reported B19' structure of NiTi is unstable relative to a base-centred orthorhombic structure that cannot store shape memory at the atomic level. However, the reported structure is stabilized by a wide range of applied or residual internal stresses. We propose that the memory is stored primarily at the micro-structural level: this eliminates the need for two separate mechanisms in describing the two-way shape-memory effect.     

Osteoarthritic process modifies expression response to NiTi alloy presence

Nickel-titanium alloy (nitinol, NiTi) is a biomaterial with unique thermal shape memory, superelasticity and high damping properties. Therefore NiTi has been used in medical applications. In this in vitro study, the effect of NiTi alloy (with two surface modifications – helium and hydrogen) on gene expression profile of selected interleukins (IL-1β, IL-6 and IL-8) and matrix metalloproteinases (MMP-1 and MMP-2) in human physiological osteoblasts and human osteoarthritic osteoblasts was examined to respond to a question of the different behavior of bone tissue in the implantation of metallic materials in the presence of cells affected by the osteoarthritic process. The cells were cultivated in contact with NiTi and with or without LPS (bacterial lipolysaccharide). Changes in expression of target genes were calculated by 2^-ΔΔCt method. An increased gene expression of IL-1β in osteoarthritic osteoblasts, with even higher expression in cells collected directly from the metal surface was observed. In case of physiological osteoblasts, the change in expression was detected after LPS treatment in cells surrounding the disc. Higher expression levels of IL-8 were observed in osteoarthritic osteoblasts after NiTi treatment in contact with alloy, and in physiological osteoblasts without relation to location in combination of NiTi and LPS. IL-6 was slightly increased in physiological osteoblastes after application of LPS. MMP-1 expression level was obviously significantly higher in osteoarthritic osteoblasts with differences regarding the metal surface and location. MMP-2 expression was decreased in both cell lines after LPS treatment. In conclusion, results of present study show that the NiTi alloy and the treatment by LPS, especially repeated doses of LPS, change the gene expression of selected ILs and MMPs in human osteoblast cell cultures. Some of the changes were depicted solely to osteoarthritic osteoblasts.     

Mechanical response of nitrogen ion implanted NiTi shape memory alloy

In the paper a change of material (mechanical) parameters of NiTi shape memory alloy subjected to ion implantation treatment is investigated. The spherical indentation tests in micro- and nano-scale and tension test have been performed to study an evolution of local superelastic effect in different volumes of nonimplanted and nitrogen ion implanted NiTi alloy. The differential scanning calorimetry has been applied to measure the change of characteristic temperatures due to ion implantation treatment. The structure of implanted material has been investigated using electron microscopy technique. It has been found that the ion implantation process changes the properties not only in a thin surface layer but also in bulk material. In the layer the pseudoelastic effect is destroyed, and in the substrate is preserved, however its parameters are changed. The characteristic phase transformation temperatures in substrate are also modified.     

Rotating bending fatigue response of laser processed porous NiTi alloy

Porous implants are known to promote cell adhesion and have low elastic modulus, a combination that can significantly increase the life of an implant. However, porosity can significantly reduce the fatigue life of porous implants. Very little work has been reported on the fatigue behavior of bulk porous metals, specifically on porous nitinol (NiTi) alloy. In this article, we report high-cycle rotating bending fatigue response of porous NiTi alloys fabricated using Laser Engineered Net Shaping (LENS?). Samples were characterized in terms of monotonic mechanical properties and microstructural features. Rotating bending fatigue results showed that the presence of 10% porosity in NiTi alloys can decrease the actual fatigue failure stress, at 10⁶ cycles, up to 54% and single reversal failure stress by ~ 30%. From fractographic analysis, it is clear that the effect of surface porosity dominates the rotating bending fatigue failure of porous NiTi samples.     

High strain-rate compressive deformation behavior of the Al0.1CrFeCoNi high entropy alloy

High entropy alloy is a new class of structural metallic materials. No work, so far, has been carried-out to understand high strain-rate plastic deformation behavior and resulting microstructure. This work focuses on understanding the deformation behavior of an Al_0.1CrFeCoNi HEA at high strain-rate (HSR). HSR plastic deformation in compression mode was carried out using split-Hopkinson pressure bar. The pre- and post-deformation microstructures were studied using electron microscopes. A high strain-rate sensitivity of yield strength, significant work hardening, and profuse twinning are main characteristics observed during deformation of the alloy at HSR. Overall, the deformation behavior of the alloy was consistent with low stacking fault energy materials.     

Grain growth in Cu-Zn-Al-Mn shape-memory alloy

The kinetic grain growth has been determined in a Cu-Zn-Al-Mn alloy by the calculation of different grain-size parameters (perimeter, minimum and maximum diameter) and the ratio of the grain-boundary area per unit volume from measurements obtained at different temperatures and heat-treatment times. The growth order and activation energy have been evaluated.     

Biologically inspired crack delocalization in a high strain-rate environment

Biological materials possess unique and desirable energy-absorbing mechanisms and structural characteristics worthy of consideration by engineers. For example, high levels of energy dissipation at low strain rates via triggering of crack delocalization combined with interfacial hardening by platelet interlocking are observed in brittle materials such as nacre, the iridescent material in seashells. Such behaviours find no analogy in current engineering materials. The potential to mimic such toughening mechanisms on different length scales now exists, but the question concerning their suitability under dynamic loading conditions and whether these mechanisms retain their energy-absorbing potential is unclear. This paper investigates the kinematic behaviour of an ‘engineered’ nacre-like structure within a high strain-rate environment. A finite-element (FE) model was developed which incorporates the pertinent biological design features. A parametric study was carried out focusing on (i) the use of an overlapping discontinuous tile arrangement for crack delocalization and (ii) application of tile waviness (interfacial hardening) for improved post-damage behaviour. With respect to the material properties, the model allows the permutation and combination of a variety of different material datasets. The advantage of such a discontinuous material shows notable improvements in sustaining high strain-rate deformation relative to an equivalent continuous morphology. In the case of the continuous material, the shockwaves propagating through the material lead to localized failure while complex shockwave patterns are observed in the discontinuous flat tile arrangement, arising from platelet interlocking. The influence of the matrix properties on impact performance is investigated by varying the dominant material parameters. The results indicate a deceleration of the impactor velocity, thus delaying back face nodal displacement. A final series of FE models considered the identification of an optimized configuration as a function of tile waviness and matrix properties. In the combined model, the optimized configuration was capable of stopping the ballistic threat, thus indicating the potential for bioinspired toughened synthetic systems to defeat high strain-rate threats.     

Corrosion behavior of a laser-welded NiTi shape memory alloy

Corrosion behaviors of the laser-welded Ni–49.4 at.% Ti shape memory alloy and base metal in 0.9% NaCl solution were investigated by means of electrochemical techniques (the open circuit potential measurement, linear and potentiodynamic polarizations). The results showed that corrosion resistance of the laser-welded NiTi alloy is better than that of the base metal. Compared to the base metal, the laser-welded NiTi alloy exhibits higher open circuit potential, higher polarization resistance, a wider passive region and higher breakdown potential. The improvement of corrosion resistance of the laser-welded NiTi alloy is ascribed to a smoother, defect free surface and an absence of carbides.     

High-speed dislocations in high strain-rate deformations

In this paper, shear deformation at high strain rates is modelled within the framework of discrete dislocation plasticity. The question is addressed whether dislocation accelerations may be ignored at high strain rates. Furthermore, the usage of high-velocity stress and displacement fields are studied. The simulations take place in a computational cell representing Al and Cu that is sheared at a strain rate of 10⁶ s⁻¹. The computations show that the inertial effects may not be neglected. Furthermore, although the high-velocity stress and displacement fields yield significant differences locally with respect to their quasi-static counterparts, their effect on the overall stress–strain curve is negligible.     

Shock compaction of NiTi alloy powder

The shock recovery experiment for the equiatomic NiTi alloy powder was performed by the flyer impact technique. The powder samples with the initial density of 70% of full density were shock-treated in the dyer velocity range 0.65 to 1.7 km sec^?1. At the optimum flyer velocity of 1.3 km sec^?1, the powder sample is compacted up to 99.5% of the full density. With increasing flyer velocity, new pores are formed in the melted layer instead of the disappearance of initial interstices. Effects of the mechanical deformation and the annealing brought by the shock treatment evidently appear in the temperature dependence of electrical resistivity accompanied with the martensitic transformation. The shock state and the relaxation of the heterogeneous temperature by the shock treatment are estimated, which indicate the annealing condition caused by shock-loading and the formation of new pores.     

Tensile behaviour of FRC under high strain-rate

This paper presents experimental results on two types of concrete reinforced with steel and polyvinyl-alcohol (PVA) fibres subjected to dynamic tensile loading. The tests were carried out by using a Modified Hopkinson Bar apparatus on fibre reinforced concrete notched-specimens under three different strain-rates (50, 100, and 200 s⁻¹). From the experiments it was found that there is a significant enhancement in tensile strength with increasing strain-rates. The dynamic tests on steel FRC with the smaller loading rate (50 s⁻¹) showed a strength similar to the one measured from static tests; however, for increasing loading rates, a remarkable decrease of post-peak strength and ductility occurs. In specimens with PVA fibres, an enhancement of the tensile strength was also observed and a significant reduction of fracture energy and ultimate deformation occurred. Some experimental aspects are also discussed as the specimen shape, its dimension, the loading rate as well as the different post-peak behaviour from static and dynamic tests.     

Multi-scale applications to high strain-rate dynamic fracture

Though the bulk of the Bodega Bay Multi-Scale Modeling workshop was devoted to understanding flow stress within the multi-scale paradigm, a dedicated session was devoted to Dynamic Failure and Fracture. Here, we review recent developments with emphasis on work presented at the workshop in the area of ductile dynamic fracture. The paper begins with a discussion of the relevant experimental observations, followed by an overview of the mechanisms of void nucleation and growth at high strain-rate, including dislocation processes (see the companion review by Bulatov in this issue). While the connection to the continuum is at its infancy, we present some directions that hold promise. Shear localization from the continuum perspective is presented in the companion review by Becker et al. This section finishes with a brief summary of issues that need to be resolved to apply the full apparatus of multi-scale modeling to dynamic fracture. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mechanical properties of additive laser-welded NiTi alloy

Laser welding would be a suitable joining technique for NiTi shape memory alloy if the mechanical properties of laser weld were improved. With this purpose, effects of additive on mechanical properties of laser-welded NiTi alloy have been experimentally studied. Welding specimens used in this study were 2 mm thick hot-rolled plates with a chemical composition of Ni50.9Ti49.1. (Ni50.9Ti49.1)-Ce2 (at.%) alloy foil or Ni47Ti44Nb9 plate was used as filler metal to add Ce or Nb element into NiTi laser weld metal. Both tensile strength and the toughness of additive-welding specimens were improved significantly compared with non-additive-welding specimen. The mechanical property improvement was attributed to the fine solidification NiTi grains and good grain-linking in weld center. The microstructure control mechanisms of these two additive welds were discussed.     

Thermomechanical topology optimization of shape-memory alloy structures using a transient bilevel adjoint method

We present a novel method for computational design of adaptive shape-memory alloy (SMA) structures via topology optimization. By optimally distributing a SMA within the prescribed design domain, the proposed algorithm seeks to tailor the two-way shape-memory effect (TWSME) and pseudoelasticity response of the SMA materials. Using a phenomenological material model, the thermomechanical response of the SMA structure is solved through inelastic finite element analysis, while assuming a transient but spatially uniform temperature distribution. The material distribution is parameterized via a SIMP formulation, with gradient-based optimization used to perform the optimization search. We derive a transient, bilevel adjoint formulation for analytically computing the design sensitivities. We demonstrate the proposed design framework using a series of two-dimensional thermomechanical benchmark problems. These examples include design for optimal displacement due to the TWSME, and design for maximum mechanical advantage while accounting for pseudoelasticity.     

Self-assembled nanosheets on NiTi alloy facilitate endothelial cell function and manipulate macrophage immune response

Stenting has been widely adopted for the treatment of cardiovascular diseases,but the complications such as in-stent restenosis and late stent thrombosis cannot be completely avoided,which are closely related to endothelial dysfunction and inflammatory response.In the present work,oxide nanosheets were grown on the surface of nearly equiatomic NiTi alloy by alkaline corrosion(AC),aiming at yielding favorable endothelial functionality and immune microenvironment.The results show nanosheets mainly composed of TiO2,Ni(OH)2,and K2TiO3 can be grown on the alloy in KOH solution of 2.5-15 M at room temperature.The AC-treated samples significantly promote endothelial cell(EC)functionality such as proliferation,migration,NO production,VEGF secretion,and angiogenesis.In addition,the sample grown in KOH of 15 M can switch macrophages to an anti-inflammatory M2 phenotype and up-regulate the gene expression of VEGF to facilitate EC functionality.These results demonstrate that the nanosheets can directly and indirectly up-regulate EC functionality,possibly leading to rapid re-endothelialization of the stents thus addressing the stent-related complications.     

NiTi合金微弧氧化的初步研究

在NiTi合金表面采用微弧氧化得到一层致密的氧化膜.采用X R D分析了薄膜的结构.结果表明在室温条件下在Na2SiO3电解液中通过微弧氧化可以在NiTi合金基体上得到一层氧化物膜层,具有硬度高、耐腐蚀性能强的特点,对于NiTi合金具有良好的保护作用.