Twin Interface Research Articles

Metastable phase transformations and mechanical properties of an as-extruded TiAl-based alloy during two-step heat treatment

Metastable phase transformations of TiAl-based alloys provide a pathway for grain refinement and performance improvement. In this paper, the microstructure evolution of an as-extruded Ti-47Al-2Cr-0.2Mo alloy during quenching and tempering was investigated by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM), and the mechanical properties of the alloy were further evaluated by tensile tests. The results indicated that the analogous Σ3 coincidence site lattice (CSL) and γ/γ true twin interface appeared in both the Widmanstätten structure (γw) and feathery structure (γf), implying that sympathetic nucleation mechanism was responsible for the nucleation of the metastable phases. A 6 H intermediate phase with the long period stacking ordered structure (LPSO) was discovered after short-term water quenching, which provided an evidence supplement of the sympathetic nucleation mechanism. The transformation path of the metastable γw phase was proposed to follow the sequence of α/α2→6 H→γ. Apparent refinements of both grains and α2/γ lamellar colonies ((α2+γ)L) were exhibited after metastable phase transformation. The ultimate tensile strengths of the heat-treated alloy at whether room temperature (RT) or 800 °C were obviously improved, and the elongation at RT also kept at a considerable level.

Crack configuration influence on fracture behavior and stress shielding: insights from molecular dynamics simulations

The fracture behavior of the Ti–Al alloy is significantly affected by its nano-lamellar structure. However, further investigation is still required to fully comprehend how the initial crack configuration influences the lamellar Ti–Al’s deformation behavior. Although molecular dynamics simulations are a great way to study crack-tip interactions in interface-dominated microstructures, the design of the simulation can have an impact on the behavior that is predicted. To shed light on this matter and at the same time to understand the impact of the specific interface structure, a systematic study of crack-tip interface interactions in nano-lamellar two-phase Ti–Al was carried out. The type of interface and crack configuration were varied in these simulations to distinguish between the effects of the microstructure and the crack geometry. Results show that the semi-coherent pseudo twin ( γ/ PT) interface is the strongest barrier for crack propagation while the coherent true twin interface ( γ/ TT) is the weakest. After a thorough review of the contributing factors, it is evident that the orientation of the crack has a greater impact on its propagation than the aspect ratio of the crack. The stress shielding effectiveness of lamellar interfaces is strongly dependent on the crack configuration. However, regardless of the initial crack set-up, the coherent γ/ TT interface appears to be the most effective interface in terms of shielding.

Atomistic simulations of structure and motion of twin interfaces reveal the origin of twinning in NiTi shape memory alloys

Shape memory alloys (SMAs) are functional materials featuring unique shape recovery properties that make them suitable for key industrial applications. The essential process controlling this fascinating performance is microstructural twinning. Some twin systems are much more frequently observed than others, yet a fundamental mechanistic understanding of this evidence is missing, which hampers SMA design. Here, we consider the prototypical NiTi SMA to show that the emergence of twinning can be strongly affected by twin interface mobility. In this work, we adopt an integrated approach combining crystallographic theory, state-of-the-art atomistic modelling, topological model, and validation with high-resolution transmission-electron micrographs. Atomistic simulations confirm that the occurrence of twins is dictated by the driving force for twin boundary motion rather than interfacial energy. Moreover, our findings settle long-standing questions by elucidating the propagation mechanisms of twin interfaces, which the established martensite crystallography and kinetic theories could not address conclusively. The newly discovered understanding of the role of interface mobility in twin formation can be used to predict variant selection and guide the design of SMAs with improved functional performance.

Structural characteristics of irrational Type-II Twin interfaces

The Type-II Twin Boundary (TB) is a critical interface in functional materials whose irrational Miller-index identity has recently drawn significant research interest. This study establishes general structural characteristics of the Type-II twin interface, utilizing TBs in Shape Memory Alloys (SMAs) - TiPd, TiPt, and AuCd - as study targets. It is shown how the irrational identity of each TB is explained by the Terrace-Disconnection (T-D) structural topology. It is proposed that the terrace is the rational-plane nearest to the irrational TB in the reciprocal space, having integral Miller-indices of least magnitude. Crystallographic-registry on this terrace requires non-trivial coherence-strains. A novel kinematic-origin of the coherence-strain is proposed, coming directly from a transformation of the classical twinning deformation-gradient. This transformation revealed that the classical twinning-shear partitions into the coherence-strain and a new metric termed the “terrace-shear”. It is shown that the magnitude of shear relating the twin-structure to the matrix is the terrace-shear and not the twinning-shear, contrary to classical understanding. Furthermore, the Burgers vector of the twinning disconnection is shown to be related directly to the terrace-shear. The energy of each Type-II interface is determined from ab initio Density Functional Theory (DFT) calculations. It is shown that the energy-minimal atomic-structure on the terrace requires determination of a “lattice-offset” that is non-trivial and unknown apriori. In summary, this study expounds on T-D topological structure of Type-II twin interfaces, establishing methods to identify rational terraces, coherence strains, ab initio planar TB energies and revealing a unique partitioning of the twinning-shear exhibited by this class of interfaces.

FCC-Zr phase transformation-induced defect substructure in Zr2Si precipitates equilibrated in Si-modified Zircaloy-4

This work investigates the atomic scale substructure of interfaces in Zr2Si secondary phase precipitates formed by the stresses associated with the α-Zr (HCP) to γ-Zr (FCC) phase transformation and thermal effects in an experimental Si-modified Zircaloy-4 by means of transmission electron microscopy and CSL/DSCL (coincident site lattice/displacement shift complete lattice) models. Analysis of a high-angle twin interface reveals their characteristic principal primary Δg vectors, a typical feature of the singular interfaces, and the sessile twinning disconnections. The step height and length of the interfacial dislocation line, with the Burgers vector parallel to the habit plane, are analysed using the DSCL model. In the second case, a low-angle interface of mixed tilt-twist character is studied and facets of constant length along the densest-packed planes and ledges of inconstant height, a common feature of vicinal interfaces, are identified.

Effect of compositional minor adjustment on localized lamellar structure coarsening of TiAl alloys with different solidification modes

Possible lamellar coarsening in the as-cast microstructure of TiAl alloys significantly affects the subsequent processing of ingot metallurgy, and a systematic study of it is necessary. In this paper, two TiAl alloys with different solidification modes (TNM-45Al alloy and TNM-0.3Si-0.7 C alloy) were obtained by compositional minor adjustment basing on TNM alloy. The effect of different adjustment routes on the localized lamellar structure coarseing behaviour and the resulting differences in the uniformity of the nanohardness distribution were investigated by comparing with TNM alloy. It is found that increasing the Al content of the TNM alloy while keeping the solidification mode as β-solidification unchanged leads to severe localised lamellar coarsening in the S-segregation zone of TNM-45Al alloy, including continuous coarsening (CC) and discontinuous coarsening (DC). In contrast, the solidification mode of the TNM-0.3Si-0.7 C alloy with the addition of the strong α stabilizer elements Si and C is transformed into hypoperitectic solidification with severe peritectic segregation, but the lamellar structure exhibits surprising full-domain stability. The results show that the thermodynamic tendency caused by high Al content and the absence of net-like β-segregation leads to severe localised lamellar coarsening in TNM-45Al alloy. CC can be accomplished through the migration of interface defects. DC is formed by the nucleation and expansion of γ-phase Shockley incomplete dislocations, and the interfaces consist of the α2 /γ interface and the first discovered γ /γ twin interface. The lamellar coarsening of TNM-0.3Si-0.7 C alloy can be effectively suppressed by the solid solution of Si/C atoms and the near net-like β-segregation in the primary β-phase region. As a result, the distribution uniformity of the nanohardness of the lamellar structure in TNM-0.3Si-0.7 C alloy is excellent, with the variance decreasing from ±0.24 of TNM alloy to ±0.20, while it increases to ±0.37 for the TNM-45Al alloy.

Property and microstructure of Ni50.3Ti29.7Hf20 high-temperature shape memory alloys with different aging conditions

NiTiHf is a class of promising high-temperature shape memory alloys (SMAs) that find many applications. However, their complex martensitic microstructure and attendant thermomechanical properties are not well understood. In this work, we used solution-treated (precipitate-free) and aged (precipitate-bearing) Ni50.3Ti29.7Hf20 (at.%) SMAs as a model system. We observed that the presence of precipitates refines the martensite plates, reduces the number of martensite variants, and changes the orientation relationship between the martensite plates compared with the solution-treated counterpart. Furthermore, the aged samples exhibited higher transformation temperatures, narrower phase transformation temperature windows, improved thermal stability, and retained or even improved actuation strain. The improved thermomechanical properties observed in the aged samples are attributed in part to the reduction of the number of martensite variants and the change in martensite and twin interface characteristics, both of which are induced by the presence of precipitates. The findings of this study offer new information on the processing-property-microstructure relationship in NiTiHf-based SMAs. These insights can guide future materials design efforts, facilitating the development of advanced SMAs tailored for specific high-temperature applications.

Cyber-WISE: A Cyber-Physical Deep Wireless Indoor Positioning System and Digital Twin Approach.

In recent decades, there have been significant research efforts focusing on wireless indoor localization systems, with fingerprinting techniques based on received signal strength leading the way. The majority of the suggested approaches require challenging and laborious Wi-Fi site surveys to construct a radio map, which is then utilized to match radio signatures with particular locations. In this paper, a novel next-generation cyber-physical wireless indoor positioning system is presented that addresses the challenges of fingerprinting techniques associated with data collection. The proposed approach not only facilitates an interactive digital representation that fosters informed decision-making through a digital twin interface but also ensures adaptability to new scenarios, scalability, and suitability for large environments and evolving conditions during the process of constructing the radio map. Additionally, it reduces the labor cost and laborious data collection process while helping to increase the efficiency of fingerprint-based positioning methods through accurate ground-truth data collection. This is also convenient for working in remote environments to improve human safety in locations where human access is limited or hazardous and to address issues related to radio map obsolescence. The feasibility of the cyber-physical system design is successfully verified and evaluated with real-world experiments in which a ground robot is utilized to obtain a radio map autonomously in real-time in a challenging environment through an informed decision process. With the proposed setup, the results demonstrate the success of RSSI-based indoor positioning using deep learning models, including MLP, LSTM Model 1, and LSTM Model 2, achieving an average localization error of ≤2.16 m in individual areas. Specifically, LSTM Model 2 achieves an average localization error as low as 1.55 m and 1.97 m with 83.33% and 81.05% of the errors within 2 m for individual and combined areas, respectively. These outcomes demonstrate that the proposed cyber-physical wireless indoor positioning approach, which is based on the application of dynamic Wi-Fi RSS surveying through human feedback using autonomous mobile robots, effectively leverages the precision of deep learning models, resulting in localization performance comparable to the literature. Furthermore, they highlight its potential for suitability for deployment in real-world scenarios and practical applicability.

Lessons learned: Symbiotic autonomous robot ecosystem for nuclear environments

AbstractNuclear facilities have a regulatory requirement to measure radiation levels within Post Operational Clean Out (POCO) around nuclear facilities each year, resulting in a trend towards robotic deployments to gain an improved understanding during nuclear decommissioning phases. The UK Nuclear Decommissioning Authority supports the view that human‐in‐the‐loop (HITL) robotic deployments are a solution to improve procedures and reduce risks within radiation characterisation of nuclear sites. The authors present a novel implementation of a Cyber‐Physical System (CPS) deployed in an analogue nuclear environment, comprised of a multi‐robot (MR) team coordinated by a HITL operator through a digital twin interface. The development of the CPS created efficient partnerships across systems including robots, digital systems and human. This was presented as a multi‐staged mission within an inspection scenario for the heterogeneous Symbiotic Multi‐Robot Fleet (SMuRF). Symbiotic interactions were achieved across the SMuRF where robots utilised automated collaborative governance to work together, where a single robot would face challenges in full characterisation of radiation. Key contributions include the demonstration of symbiotic autonomy and query‐based learning of an autonomous mission supporting scalable autonomy and autonomy as a service. The coordination of the CPS was a success and displayed further challenges and improvements related to future MR fleets.

Modeling the dynamic magneto-mechanical response of magnetic shape memory alloys based on Hamilton’s principle: The numerical algorithm

This paper is the second part of a variational modeling work on the dynamic magneto-mechanical response of magnetic shape memory alloys (MSMA). In the first part of this series work, the governing equations system for modeling the dynamic response of MSMA samples has been derived based on Hamilton’s principle, which contains the Maxwell’s equations, the magneto-mechanic equations, the evolution equations of internal variables and the twin interface motion criteria. In this paper, we aim to conduct comprehensive numerical simulations on the dynamic magneto-mechanical response of MSMA samples. To solve the governing system, the space–time finite element method is applied and a double-loop iterative numerical algorithm is proposed. By using this algorithm, the independent variables in the governing system and the motion of twin interfaces in the MSMA sample can be determined separately and iteratively. Besides that, a specific discretization of the sample is adopted to simplify the calculation of variant state distribution in the sample. To show the efficiency of the numerical algorithm, the magneto-mechanical response of a MSMA sample in stress-assisted dynamic field loading tests and field-assisted dynamic mechanical loading tests are studied. Based on the numerical results, some global response curves of the MSMA sample are plotted, which show good consistency with the experimental results. Furthermore, the current configuration of the sample and the distributions of some important physical fields in the sample can be simulated. The current work will be helpful for the design of novel actuators or energy harvesters with MSMA.

Intelligent emergency digital twin system for monitoring building fire evacuation

The provision of real-time and detailed evacuation information feedback is vitally significant for the formulation and adaptation of the onsite evacuation strategy. The conventional surveillance system relying on surveillance cameras is limited to processing video and fails to extract human behaviour or provide privacy protection. This work proposes an Intelligent Emergency Digital Twin system based on computer vision and deep learning. The system comprises (1) CCTV network, (2) YOLOv4 evacuee detector, (3) DeepSORT evacuee tracker, (4) Perspective transformer, and (5) Digital Twin interface. It enables the detection and tracking of evacuees, the calculation of their egress speed, and the protection of their privacy in a digital interface. The proposed system was evaluated in a staircase of an office building through two types of tests with positive results: ratio of successfully detecting is 100% in individual objects test and about 90% in multiple objects test. The evacuation data generated by the digital twin system would be useful for guiding evacuations out of fire scenarios. This proposed digital twin framework can lay the foundation for the implementation of smart human monitoring in fire scenarios for buildings.

The complexity of [formula omitted] twin interface structure and energetics in HCP materials

The quest for a deeper understanding of the mechanical properties of HCP alloys has been an ongoing effort. Due to the lack of a sufficient number of slip systems, twin nucleation and migration originate as a substantial plastic deformation mechanism in HCP metals. The paper uncovers the energetics and atomistic mechanism of the {112¯1}〈112¯6〉 which is one of the most prominent twin modes in HCP materials that has not been fully understood. In view of corrugated {112¯1}interfaces, the positioning of lattice/motif atoms needs to be clarified. First, we establish the initial twin boundary (TB) structure unambiguously via the lattice offsets resulting in volume invariancy and correct relative positioning of the twin variants. Secondly, the TB migration mechanism is determined from a crystallographic (zero net shuffle, Σs→=0) and energetic standpoints (minimum energy path). The Generalized Planar Fault Energy (GPFE) curves are calculated for Ti, Hf, Mg, and Zr. Such GPFE curves for this twin mode are calculated for the first time, and point to unusually high unstable energies (300-850 mJ/m2), and low migration barriers (<60 mJ/m2) for twinning accompanied with large shuffle magnitudes, |s→|, comparable to the Burger's vector (|s→|≅be>0.44Å).

Microstructure evolution induced by solidification and ferrite–austenite massive-like transformation in Fe–C alloys

The coarsening of austenite grains produced through a massive-like transformation from the ferrite to the austenite phase during/after solidification in Fe–0.45mass%C steel was examined through time-resolved X-ray diffraction measurements using time-resolved computed tomography. The δ phase solidification resulted in a single δ grain in the observation region (0.6 mm in diameter, 0.1 mm in height). The massive-like transformation produced multiple γ grains, and coarsening immediately followed the transformation. The coarsening in the observation region was completed even within 200 s. A correlation of crystallographic orientation between the γ grains was found in the γ grain structure. The massive-like transformation frequently produced fcc (111) twinned grains, and the twinned austenite grains tended to remain during coarsening because the twin interface energy is lower than the grain boundary energy. The massive-like transformation following the solidification contributed to the texture formation in the γ grain structure.

Artificial intelligence for the metaverse: A survey

Along with the massive growth of the Internet from the 1990s until now, various innovative technologies have been created to bring users breathtaking experiences with more virtual interactions in cyberspace. Many virtual environments have been developed with immersive experience and digital transformation, but most are incoherent instead of being integrated into a platform. In this context, metaverse has been introduced as a shared virtual world that is fueled by many emerging technologies. Among such technologies, artificial intelligence (AI) has shown the great importance of enhancing immersive experience and enabling human-like intelligence of virtual agents. In this survey, we make a beneficial effort to explore the role of AI, including machine learning algorithms and deep learning architectures, in the foundation and development of the metaverse. As the main contributions, we convey a comprehensive investigation of AI-based methods concerning several technical aspects (e.g., natural language processing, machine vision, blockchain, networking, digital twin, and neural interface) that have potentials to build virtual worlds in the metaverse. Furthermore, several primary AI-aided applications, including healthcare, manufacturing, smart cities, and gaming, are studied to be promisingly deployed in the virtual worlds. Finally, we conclude the key contribution and open some future research directions of AI for the metaverse. Serving as a foundational survey, this work will help researchers, including experts and non-experts in related fields, in applying, developing, and optimizing AI techniques to polish the appearance of virtual worlds and improve the quality of applications built in the metaverse.

On mechanical twinning in tetragonal lattice

ABSTRACT The theory of mechanical twinning is revisited for the case of face-centred tetragonal lattices. The motivation is an imprecision in the determination of twinning shear vector magnitude, which occurs repeatedly in the literature. The magnitude of this vector describing the mutual shear of two adjacent crystallographic planes in the process of twin formation is a function of the tetragonality of the lattice c/a. Therefore, we introduce the c/a-dependent factor f which has to be applied to the magnitude of shearing vector instead of the commonly use factor 1/6, which is correct only for perfect cubic lattices. The theory is verified by ab initio calculations of the generalised planar fault energy curves for three tetragonal materials: the nonmodulated martensite phase of Ni2FeGa magnetic shape memory alloy, γ-TiAl intermetallic and pure In. Moreover, the calculations show that the additional modification of shear vector is caused by structural optimisation due to short-range interactions in the vicinity of twin interface, especially for lattices with large deviation of c/a from 1. Such modification cannot be simply predicted from the lattice geometry.

Effects of γ/γ lamellar interfaces on translamellar crack propagation in TiAl alloys

Lamellar interfaces and their structures play a significant role in the fracture behavior of fully lamellar (FL) TiAl alloys. However, the effects of different lamellar interfaces on crack tip behavior are not clear. To this end, the interaction between three types of γ/γ lamellar interfaces, which are termed by true-twin, rotational-order, and pseudo-twin interfaces, and a translamellar crack in FL-TiAl alloy have been investigated by molecular dynamics simulation. By analyzing the atomic structure evolution of crack propagation and the effects of interfaces on the crack growth rate, we found that the introduction of γ/γ lamellar interfaces enhanced the crack resistance of a crack model without interfaces. Furthermore, the resistances of the γ/γ lamellar interfaces against translamellar crack propagation are in the order of rotational-order> pseudo-twin> true-twin. Specifically, crack growth is almost arrested at the rotational-order interface; the crack propagation nearly stops after crossing through the pseudo-twin interface; while the crack penetrates the true-twin interface and continues to propagate in adjacent lamellae. In addition, as crack resistance of the lamellar interface improves, the dislocation density in the corresponding model also increases. Among the three kinds of γ/γ lamellar interfaces, the true-twin interface effectively screens the stress field at the crack tip, whereas the pseudo-twin and rotational-order interfaces do not. • Existence of lamellar interfaces enhance the crack resistance of interface-free model. • Rotational order interface exhibits the most desirable crack resistance. • True twin interface has the worst resistance to crack propagation. • Interface migration has a positive effect on improving crack resistance of interface. • True twin interface has the effective screening on stress field at the crack tip.

Twinning damping interface design and synergistic internal friction behavior in FeMnCrCo high-entropy alloys

Vibration and noise reduction has always been important problem to be solved in the fields of rail transit, aerospace, marine engineering, and so on. In this paper, a novel Fe65-XMn20Cr15CoX (X = 5, 10, 15, 20) dual-phase high-entropy damping alloy with excellent properties were prepared by vacuum melting. The dependence of the alloy damping behavior on the twin interface, magnetic properties and phase interface was emphatically analysed. The effects of Co content on its microstructure, damping behavior and magnetic properties were investigated. The results show that with the increase of Co content, ε martensite gradually transforms to γ austenite, and the number of twins increases gradually. The peak damping (Q−1) increased from 0.0442 to 0.0595, an increase of 34.6%. The coupled effects of magneto-mechanical hysteresis, twin interface motion, and phase interface motion provide the alloy's excellent damping properties. The alloy has a higher damping internal friction peak of around 200 °C, which is due to the phase transition from ε to γ at the temperature. By adjusting the Co content, the phase content and twinning and other microstructures of the alloy can be regulated, and multiple damping mechanisms can be coupled to achieve higher damping performance.

Contextualisation of information in digital twin processes

Digital twins are required to process a large amount of data during operation, in order to achieve specific tasks, over the lifetime of the physical twin that they relate to. One important feature of processing data is the identification of trust in both the underlying data and processed information that arises from the data. Trust, as it is defined here, will typically be built from several contributory sources. While there are both quantitative and qualitative sources of trust, this paper focuses on the qualitative aspects of trust via the transparency of the algorithmic process that is available in the crystal-box modelling. The crystal-box idea is also extended to include the concept of a ‘crystal-box workflow’. The key idea is that in order to assist the user of the digital twin to interpret the results they are presented with, via the digital twin interface, the information needs to be contextualised. This work shows an example of how this can be done for a vibration testing (specifically modal testing) example on a scaled three-storey structure. The information is contextualised for the user via ‘profiles’, which collate and augment the processed information together. In particular, synthetic results are generated in order to augment a limited set of physically recorded data, and these synthetic results are then used to assist the user in contextualising the physically recorded data. Implementation results are shown using an open-source digital twin code called DTOP-Cristallo.

New explanation for the existence of B19′ phase in NiTi alloy from the perspective of twinning martensite

The ground state structure of NiTi martensite in experiments and existing theoretical calculations is the focus of ongoing disagreements. Based on Waitz et al.’s (2004, 2007) experiments, we use the first principle calculations to investigate the effect of (001) twin interface and thickness on the stability of NiTi martensite. Moreover, we determine the interface of the (001) martensitic twins observed in the experiment as the interface of twinned supercell-1 (TSC-1), in which the lattice parameters of individual unit cells are close to the experimental measurements of B19′. Furthermore, the energy per atom for TSC-1 is always between that of B19′ and B33 and does not vary with the twin thickness. Considering the structural similarity of TSC-1 and B33, an energy barrier between TSC-1 and B33 via the minimum enthalpy pathways was demonstrated. The influence of the (001) twin interface could proves the rationality of the existence of B19′ in experiments.

Utilizing twin interfaces to reduce lattice thermal conductivity of superlattice

Twin interfaces are easily formed in superlattices due to their lower interfacial energy. However, there are relatively few studies on their effect on the thermal conductivity of superlattices, and the conclusions are unclear. In particular, the degree of influence of the presence of twin interfaces on the thermal conductivity is inconsistent. Therefore, the thermal conductivities of silicon/germanium superlattices with twin interfaces were studied by non-equilibrium molecular dynamics simulations. It was found that the twin interface destroys coherent phonon transport, causes phonon localization, and leads a decrease in the thermal conductivity. The degree of influence of the twin interface on the thermal conductivity is strongly dependent on the period length, the system length, and temperature. Furthermore, phonon density of states, phonon participation rate, and spectral heat flow calculations were employed to deduce the phonon transport mechanisms in superlattices with twin interfaces.