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Tubular NiTi is a promising candidate refrigerant for eco-friendly elastocaloric cooling, but its severe functional degradation during cyclic phase transition (PT) is a key concern in the technology development. Here, plastic deformation of 6.7% is applied to NiTi tube by overstress training under 1900 MPa for five cycles to improve the cyclic PT stability without losing cooling efficiency. It is found that after 106compressive cycles under 1000 MPa, the overstress-trained NiTi tube exhibits small residual strain (0.5%), stable adiabatic temperature drop (ΔT = 11K) and improved coefficient of performance (COP = 55), showing high functional stability and good cooling performance. Transmission electron microscopy (TEM) observations show that the microstructure of the overstress-trained NiTi tube consists of 5–10 nm sized austenite (B2) and martensite (B19′) nanodomains with near-saturated dislocation density (ρ ≈ 5.153 × 1016 m−2). Such dislocation-enriched nanostructure effectively suppresses further formation and motion of dislocation during subsequent cyclic compression, thereby significantly enhancing the cyclic stability of the NiTi tube. The residual B19′ nanodomains and dense dislocations change the PT mode from nucleation-growth of the B19′ phase to direct growth of residual B19′ nanodomains, thereby reducing the dissipation of the PT process and increasing the COP of the NiTi tube. Our study provides an economic and effective method for to enhance the cyclic stability of bulk NiTi tubes for solid-state refrigeration.

Developing 3D graphics applications on consumer-grade mobile devices recently leads to a growing research interest. However, the limited computational and memory storage capacity of mobile devices makes it unsuitable to directly apply widely used rendering algorithms such as Precomputed Radiance Transfer algorithms and ray tracing algorithms, which require high computational cost and memory storage to provide visually appealing rendering results. In this paper, we present a framework for real-time rendering lighting effects for consumer-grade mobile devices. Our framework uses the shadow mapping algorithm, Concentric spherical representation, to generate soft shadows and seamless dual paraboloid map mipmap to store shadow maps. By utilizing the advantages of these two algorithms, we can real time render visually appealing soft shadows from multiple omnidirectional light sources for animated models on mobile devices. We also present a method to divide environment map lighting into several omnidirectional light sources, allowing us to use a bunch of shadow maps to approximate the shadows from environment map lighting. The rendering results show that our method can render visually appealing lighting effects, i.e. reflection effects and soft shadows, from the environment map lighting for animated models in real-time on consumer-grade mobile devices.

Elastocaloric cooling is a promising greenhouse-gas-free alternative to the conventional vapor-compression refrigeration which relies on high global-warming-potential refrigerants. However, the limited temperature span and cooling power in existing elastocaloric devices restrict the commercialization of this technology. Here, we build a compact compression-based regenerative elastocaloric device using a tubular NiTi regenerator of spiral cross-section. The large specific heat transfer area of the spiral structure enables fast heat transfer between NiTi and fluid, whereas a sufficient regenerative length facilitates a large temperature span along the fluid-flow direction. Our device achieves a temperature span of 50.6 K on the water side and a cooling power of 203 W, and it endures over 1 million compressive phase-transition cycles. These results surpass those of all existing elastocaloric devices, paving the way for the large-scale commercialization of this technology.

A grain size gradient NiTi shape memory alloy, which contains distributed grain sizes within one piece of NiTi sample, would combine superior performances, such as large hysteresis and high fatigue life, high ductility, and strength, due to its gradient structures. This study illustrates the fabrication process, microstructure, and mechanical behavior of both cold-rolled and grain size gradient NiTi. An clear gradient structure has been confirmed by the experiments of this paper. The temperature of the cold-rolled specimen decreases during loading and increases during unloading, which may be mainly caused by the the thermoelastic effect in the linear elastic region and also possible inverse elastocaloric effect (R-B2 phase transition) induced by Ti2Ni and Ni3Ti precipitates. The temperature of the gradient annealed specimen shows a gradient temperature profile, and the temperature decreases slightly followed by an increase during loading, and then decreases and increases during unloading. The main reason for this phenomenon may be the competition among the thermoelastic effect, elastocaloric effect (B2-R phase transition) and inverse elastocaloric effect (R-B2 phase transition) induced by Ti2Ni and Ni3Ti precipitates.

• Presents a novel computationally-efficient reconstruction of 3D RVE model of second-phase particle-strengthened AA7075-O sheet with real distribution of particles based on FIB-SEM techniques. • Three types of second-phase particles (i.e., irregularly-shaped iron-rich particle Al 3 Fe, elliptically-shaped particle MgZn 2 , and needle-like particle CuAl 2) are determined using SEM-based EDS and EBSD techniques. • Presents a systematic study of the overall elastic-plastic response by developing and applying a hybrid experimental analysis and computational modeling framework. • Good agreement is observed in the comparison of RVE results with that of in-situ SEM tensile tests, indicating that the real 3D microstructure-based RVE models can accurately predict the plastic deformation and interfacial failure evolution in AA7075-O sheet. This paper addresses the challenge of reconstructing randomly distributed second-phase particle-strengthened microstructure of AA7075-O aluminum sheet material for computational analysis. The particle characteristics in 3D space were obtained from focused ion beam and scanning electron microscopy (FIB-SEM) and SEM-based Electron Backscatter Diffraction/Energy Dispersive X-ray Spectrometry (EBSD/EDS) techniques. A theoretical framework for analysis of elastic-plastic deformation of such 3D microstructures is developed. Slip-induced shear band formation, void initiation, growth and linkage at large plastic strains during uniaxial tensile loading were investigated based on reconstructed 3D representative volume element (RVE) models with real-distribution of particles and the results compared with experimental observations. In-situ SEM interrupted tension tests along transverse direction (TD) and rolling direction (RD), employing microscopic-digital image correlation (μ-DIC) technique, were carried out to investigate slip bands, micro-voids formation and obtain microstructural strain maps. The resulting local strain maps were analyzed in relation to the experimentally observed plastic flow localization, failure modes and local stress maps from simulations of RVE models. The influences of particle size, shape, orientation, volume fraction as well as matrix-particle interface properties on local plastic deformation, global stress-strain/strain-hardening curves and interfacial failure mechanisms were studied based on 3D RVE models. When possible, the model results were compared with in-situ tensile test data. In general, good agreement was observed, indicating that the real 3D microstructure-based RVE models can accurately predict the plastic deformation and interfacial failure evolution in AA7075-O aluminum sheet. [ABSTRACT FROM AUTHOR]

The urgent need for sustainable cooling solutions has propelled the development of passive radiative cooling (RC) technology. However, the static nature of traditional RC (SRC) designs limits their adaptability to varying thermal demands. Dynamic RC (DRC) materials and systems have emerged as a cutting-edge solution, enabling cooling performance modulation in response to the changing environment. This review provides an overview of the latest advancements in DRC research and explores the innovative material and structural designs of various DRC systems, highlighting their immense potential for enhancing thermal management in building- and city-related applications. The incorporation of SRC and DRC into various systems is discussed, such as building envelopes, heating, ventilation, air-conditioning and cooling systems, and evaporative cooling. The application of SRC and DRC materials at the urban scale is explored, focusing on their widespread implementation on roofs, walls, and pavements to reduce surface and ambient temperatures and mitigate the urban heat island (UHI) effect. Looking ahead, the key challenges and opportunities for the next generation of DRC research are defined. Overall, this review serves as a roadmap for pushing the boundaries of sustainable cooling technology and reshaping the material and system development for the future built environment.

NiTi shape memory alloy (SMA) has been widely used in many fields for its superelastic and shape memory behaviors. The compressive fatigue life of NiTi is very high, but the tensile and bending fatigue life is relatively low. In this study, pre-strain laser shock peening (LSP) is introduced to solve these challenges. Different heterogeneous nanostructured (containing B2, B19′ phases, and precipitates including Ni3Ti and Ni4Ti3) gradient residual stress layers were created by controlling the pre-strain during LSP treatment. The highest bending fatigue life under the maximum tensile strain of 2.4% reached over 34,000 cycles, 33 times that of the untreated sample. The proposed pre-strain LSP method provides a new avenue to fabricate fatigue-resistant superelastic NiTi SMA.

This work presents mechanical properties of the NiTi polycrystalline superelastic shape memory alloys (SMA) of 5 different grain sizes under high-speed impacts. The amorphous, nanocrystalline (40, 80, 120 nm) and coarse grain (20 μm) sheets are manufactured with cold rolling and suitable heat treatments. A Hopkinson tensile bar is used to perform tests up to 45 m/s. High-speed camera system and digital image correlation method are used to get the strain field and particle velocity field at a sampling frequency of 2×106frames/s with a resolution of 924×768 pixels. Nominal stress-strain curves are obtained for all the sheets with a strain rate of about 1000 s−1and they have a similar evolution to the quasi-static case but with much higher stress levels. The rate sensitivity is increased with the grain size and the stress level can reach up to a 70% growth for a coarse grain sheet but be totally insensitive for the amorphous sheet in the strain rate from 10−4to 103s−1. A single transformation front can be found under high-speed impact (45 m/s) at the early loading stage. The speed of the transformation front is calculated from strain time histories and the highest front speed of 811 m/s is observed which is never observed before. It also reveals that the front speed depends also on the grain size. With the same loading speed, the bigger the grain size is, the slower the transformation front speed is.

Many established, but also potential future applications of NiTi-based shape memory alloys (SMA) in biomedical devices and solid-state refrigeration require long fatigue life with 107–109duty cycles1,2. However, improving the fatigue resistance of NiTi often compromises other mechanical and functional properties3,4. Existing efforts to improve the fatigue resistance of SMA include composition control for coherent phase boundaries5–7and microstructure control such as precipitation8,9and grain-size reduction3,4. Here, we extend the strategy to the nanoscale and improve fatigue resistance of NiTi via a hybrid heterogenous nanostructure. We produced a superelastic NiTi nanocomposite with crystalline and amorphous phases via severe plastic deformation and low-temperature annealing. The as-produced nanocomposite possesses a recoverable strain of 4.3% and a yield strength of 2.3 GPa. In cyclic compression experiments, the nanostructured NiTi micropillars endure over 108reversible-phase-transition cycles under a stress of 1.8 GPa. We attribute the enhanced properties to the mutual strengthening of nanosized amorphous and crystalline phases where the amorphous phase suppresses dislocation slip in the crystalline phase while the crystalline phase hinders shear band propagation in the amorphous phase. The synergy of the properties of crystalline and amorphous phases at the nanoscale could be an effective method to improve fatigue resistance and strength of SMA.

A pre-strain laser shock peening method is proposed to fabricate fatigue-resistant nanocrystalline NiTi with graded nanolayers and compressive residual stress layers. Grain size gradient surface layer of about 100 μm thick is fabricated on a 1.5-mm-thick bulk nanocrystalline NiTi plate. It is found that the nanostructure shows a gradient distribution from middle region of the plate to its laser-treated surface, and the grain size at the treated surface is about 5 nm. B2, B19′ phase and Ni4Ti3, Ni3Ti precipitates are found at the treated surface. The nanohardness at the laser-treated surface reaches 10 GPa. Residual stress profile on the laser-treated plate cross-section is measured by a focused ion beam-digital image correlation technique. The measured maximum residual compressive stress is about 1.2 GPa at the laser-treated top surface, while there is residual tensile stress of about 200 MPa in the middle region. Four-point bending displacement-controlled experiments show that the fatigue life of the NiTi sample increases about seven times after laser treatment. The work demonstrates that pre-strain laser shock peening without surface coating is an effective method to fabricate fatigue-resistant nanocrystalline NiTi with gradient grain size and compressive residual stress layers.

We report recent progress in tailoring the thermal expansion (TE) of nanocrystalline (NC) NiTi by microstructure hierarchical design and control without composition change. Fabrication and characterization methods are outlined and preliminary results of both experiment and mechanism-based modeling are presented to understand and get insight into the unusual TE phenomena. The important roles of the intrinsic thermal expansion anisotropy of B19′ lattice and the suppression of phase transition by the extrinsic fabricated microstructure (cold rolling and annealing, grain size, defects, textures and volume fractions of nanoscaled B2 and B19′ lattices) in the overall macroscopic TE behaviors of the superelastic NC NiTi polycrystal SMAs are emphasized.

Various types of bacterial form adhesive aggregates on surfaces known as biofilms, which are of interest to researchers in a great variety of fields. In this paper, a continuum model based on diffusion-reaction is proposed and extended finite element method (XFEM) coupled to level set is employed to simulate the biofilm growth on ager substrate. Numerical results show the quantitative relationship between colony morphology and nutrient deletion. Comparing with experiment observation, we approve that diffusion limited regime is more appropriate than growth limited regime to describe Bacillus subtilisbiofilm growth on MSggager substrate. Our model can be extended to investigate detailed biological process in biofilm growth such as cell subdivision and osmotic pressure induced transportation.

We report recent advances in the experimental and theoretical study of grain size (GS) effects on the thermal and mechanical properties of nanostructured NiTi polycrystalline shape memory alloy (SMA). It is shown that when GS< 60 nm, the superelastic stress-strain hysteresis loop area (H) of the polycrystal decreases rapidly with GSand tends to vanish as GSapproaches 10 nanometers. At the same time, the temperature dependence of the transition stress also decreases with GSand eventually approaches zero, leading to a wide superelastic temperature window and breakdown of the Clausius-Claperyon relationship. Rate dependence of the stress-strain responses is significantly reduced and the cyclic stability of the material is improved by the nanocrystallization. It is proposed that the emergence of such significant changes in the behavior of the material with GSreduction originate from the large increase in the area-to-volume ratios of the nanometer-thick interfaces (grain boundary and Austenite-Martensite (A-M) interface) in the polycrystal. In particular, with GSreduction, interfacial energy terms will gradually become dominant over the bulk energy of the crystallite, eventually bring fundamental changes in the phase transition responses of the material. Modelling strategy leading to the establishment of quantitative relationships among GS, grain boundary, A-M interfaces and the macroscopic responses of the material are outlined.

Abstract

Recent experiments [1]revealed many new phenomena of the macroscopic domain patterns in the stress-induced phase transformation of a superelastic polycrystalline NiTi tube during tensile loading. The new phenomena include deformation instability with the formation of a helical domain, domain topology transition from helix to cylinder, domain-front branching and loading-path dependence of domain patterns. In this paper, we model the polycrystal as an elastic continuum with nonconvex strain energy [2]and adopt the non-local strain gradient energy to account for the energy of the diffusive domain front. We simulate the equilibrium domain patterns and their evolution in the tubes under tensile loading by a non-local Finite Element Method (FEM). It is revealed that the observed loading-path dependence and topology transition of domain patterns are due to the thermodynamic metastability of the tube system. The computation also shows that the tube-wall thickness has a significant effect on the domain patterns: with fixed material properties and interfacial energy density, a large tube-wall thickness leads to a long and slim helical domain and a severe branching of the cylindrical-domain front.

Martensite domain formation, evolution and annihilation are widely observed in stress-induced phase transformation of superelastic NiTi polycrystalline shape memory alloys. By the calculation of the thermodynamic driving force and the incorporation of friction kinetics of the interface, the domain morphology and its evolution were successfully simulated by the interface-tracking technique. The computational results agree well with the experimental observation of tensile strips. Based on theoretical and computational results, we discussed the effects of critical driving force and the existence of metastability on the transition between different domain patterns.

The Poisson’s ratio of two-dimensional hexagonal crystals has been widely studied due to its fundamental and fantastic nature. However, the issue involved in the regulation strategy and in the bounds of Poisson’s ratio of two-dimensional hexagonal crystals has not been addressed. In this work, we predict that the Poisson’s ratio of two-dimensional hexagonal crystals can be controlled by modifying the structural interaction therein, where the lower bound and upper bound are −1∕3and +1, respectively. Furthermore, molecular simulations verify these predictions. Finally, the underlying mechanism is revealed as the interplay between two deformation modes (i.e., bond stretching and angle changing). This work provides an universal regulation strategy to tune the Poisson’s ratio of two-dimensional hexagonal crystals, and determines fundamental limits on the Poisson’s ratio of two-dimensional hexagonal crystals.

Bacterial flagellar filament can undergo a stress-induced polymorphic phase transition in both vitro and vivo environments. The filament has 12 different helical forms (phases) characterized by different pitch lengths and helix radii. When subjected to the frictional force of flowing fluid, the filament changes between a left-handed normal phase and a right-handed semi-coiled phase via phase nucleation and growth. This paper develops non-local finite element method (FEM) to simulate the phase transition under a displacement-controlled loading condition (controlled helix-twist). The FEM formulation is based on the Ginzburg-Landau theory using a one-dimensional non-convex and non-local continuum model. To describe the processes of the phase nucleation and growth, viscosity-type kinetics is also used. The non-local FEM simulation captures the main features of the phase transition: two-phase coexistence with an interface of finite thickness, phase nucleation and phase growth with interface propagation. The non-local FEM model provides a tool to study the effects of the interfacial energy/thickness and loading conditions on the phase transition.