Strain heterogeneity at the microstructural level plays a vital role in the deformation and fracture behaviour of dual or multi-phase materials. In the present work, the strain heterogeneity, localization and partitioning arising at the sub-micron scale during elevated temperature (170 °C) tensile deformation of an Mg-5Al-3Ca alloy was investigated using quasi in-situ μ-DIC experiments. The results reveal that the strain is mainly carried by the α-Mg phase, while the intermetallic Laves phase plays a critical role in that strain concentrations build up at the α-Mg matrix and Laves phase interfaces, hence, reducing the overall deformability of the alloy. In quasi in-situ and bulk material analysis at elevated temperature, cracks were observed to nucleate in the Laves phase, at i) the intersection points of slip lines in the α-Mg matrix with the Laves phase and ii) the twin intersections with α-Mg/Laves phase interfaces and iii) twin transmissions across α-Mg/Laves phase interfaces. Euler number analysis has shown that the (inter-)connectivity of the Laves phase decreases with deformation. Finally, cracks grow preferentially along the Laves phases until the material fractures.
The effect of Si on Fe-rich intermetallics formation and mechanical properties of heat-treated squeeze cast Al-5.0Cu-0.6Mn-0.7Fe alloy was investigated. Our results show that increasing Si content promotes the formation of Al15(FeMn)3(SiCu)2 ($α$-Fe), and varying the morphology of T (Al20Cu3Mn2) where the size decreases and the amount increases. The major reason is that Si promotes heterogeneous nucleation of the intermetallics leading to finer precipitates. Si addition significantly enhances ultimate tensile strength and yield strength of the alloys. The strengthening effect is mainly owing t…
The effects of tensile stresses on isothermal bainitic transformation were studied in the case of a 35MV7 steel. The modification of transformation kinetics and the presence of transformation plasticity is shown in a first step. Furthermore the effect of stress on the morphological modifications of the ferrite laths is illustrated. The role of the stress on these changes is analysed.
The influence of Fe content on the three-dimensional (3D) morphology of Fe-rich intermetallic phases (Fe phases), Al2Cu, and pores and mechanical properties of cast Al-5.0Cu-0.6Mn alloys with 0.5 and 1.0 wt. % Fe are characterized using synchrotron X-ray tomography and a tensile test. The results show that both Fe phases and Al2Cu exhibit a complex 3D network structure, and the pores are irregular with complex interconnected and near-globular shape. As the Fe content increases from 0.5 % to 1.0 %, the volume fraction and equivalent diameter of Fe phases decrease, whereas both their interconnec…
Martensitic transformation induced by cooling NiTi wire under various tensile stresses: martensite variant microstructure, textures, recoverable strains and plastic strains
The deformation mechanisms of an extruded Mg-5Y-0.08Ca (wt. %) alloy were analyzed by means of micropillar compression tests on single crystals along different orientations -- selected to activate specific deformation modes -- as well as slip trace analysis, transmission electron microscopy and transmission Kikuchi diffraction. The polycrystalline alloy presented a remarkable ductility in tension (~32%) and negligible differences in the yield strength between tension and compression. It was found that the presence of Y and Ca in solid solution led to a huge increase in the CRSS for <a> basal s…
The time evolution of the pore size distributions and mechanical properties of amorphous solids at constant pressure is studied using molecular dynamics simulations. The porous glasses were initially prepared at constant volume conditions via a rapid thermal quench from the liquid state to the glassy region and allowing for simultaneous phase separation and material solidification. We found that at constant pressure and low temperature, the porous network becomes more compact and the glassy systems relocate to progressively lower levels of the potential energy. Although the elastic modulus and…
We present a molecular dynamics study of force patterns, tensile strength and crack formation in a cohesive granular model where the particles are subjected to swelling or shrinkage gradients. Non-uniform particle size change generates self-equilibrated forces that lead to crack initiation as soon as strongest tensile contacts begin to fail. We find that the coarse-grained stresses are correctly predicted by an elastic model that incorporates particle size change as metric evolution. The tensile strength is found to be well below the theoretical strength as a result of inhomogeneous force tran…
The behavior of PbTiO$_3$ under uniaxial strains and stresses is investigated from first-principles calculations within density functional theory. We show that irrespectively of the uniaxial mechanical constraint applied, the system keeps a purely ferroelectric ground-state, with the polarization aligned either along the constraint direction ($FE_z$ phase) or along one of the pseudo-cubic axis perpendicular to it ($FE_x$ phase). This contrasts with the cases of isotropic or biaxial mechanical constraints for which novel phases combining ferroelectic and antiferrodistortive motions have been pr…
In order to analyse the effect of strain on the magnetic properties of narrow-band manganites, the temperature and field dependent susceptibilities of about 8.5 nm thick epitaxial Pr0.7Ca0.3MnO3 films, respectively grown on (001) and (110) SrTiO3 substrates, have been compared. For ultrathin samples grown on (001) SrTiO3, a bulk-like cluster-glass magnetic behaviour is found, indicative of the possible coexistence of antiferromagnetic and ferromagnetic phases. On the contrary, ultrathin films grown on (110) substrates show a robust ferromagnetism, with a strong spontaneous magnetization of abo…
We demonstrate the crossover from indirect- to direct band gap in tensile-strained germanium by temperature-dependent photoluminescence. The samples are strained microbridges that enhance a biaxial strain of 0.16% up to 3.6% uniaxial tensile strain. Cooling the bridges to 20 K increases the uniaxial strain up to a maximum of 5.4%. Temperature-dependent photoluminescence reveals the crossover to a fundamental direct band gap to occur between 4.0% and 4.5%. Our data are in good agreement with new theoretical computations that predict a strong bowing of the band parameters with strain.
In this work the global and local mechanical properties of the magnetocaloric intermetallic LaFe11.2Si1.8 alloy are investigated by a combination of different testing and characterization techniques in order to shed light on the partly contradictory data in recent literature. Macroscale compression tests were performed to illuminate the global fracture behavior and evaluate it statistically. LaFe11.2Si1.8 demonstrates a brittle behavior with fracture strains below 0.6 % and widely distributed fracture stresses of 180-620 MPa leading to a Weibull modulus of m = 2 to 6. The local mechanical prop…
Finite-temperature properties of graphene monolayers under tensile stress have been studied by path-integral molecular dynamics (PIMD) simulations. This method allows one to consider the quantization of vibrational modes in these crystalline membranes and to analyze the influence of anharmonic effects in the membrane properties. Quantum nuclear effects turn out to be appreciable in structural and thermodynamic properties of graphene at low temperature, and they can even be noticeable at room temperature. Such quantum effects become more relevant as the applied stress is increased, mainly for p…
We analyze stress transmission in wet granular media in the pendular state by means of three-dimensional molecular dynamics simulations. We show that the tensile action of capillary bonds induces a self-stressed particle network organized in two percolating "phases" of positive and negative particle pressures. Various statistical descriptors of the microstructure and bond force network are used to characterize this partition. Two basic properties emerge: 1) The highest particle pressure is located in the bulk of each phase; 2) The lowest pressure level occurs at the interface between the two p…
In this paper, the dilatational response of porous solids with pressure-insensitive matrix displaying strength differential (SD) effects is investigated. To this end, micromechanical finite-element analyses of three-dimensional unit cells are carried out. The matrix behavior is governed by the isotropic form of Cazacu et al. (2006) criterion that accounts for SD effects through a parameter k. Simulation results are presented for axisymmetric tensile loadings corresponding to fixed values of the stress triaxiality for the two possible values of the Lode parameter, LP. For moderate and high stre…
The deformation behaviour of the intermetallic Al$_{2}$Cu-phase was investigated using atomistic simulations and micropillar compression, where slip on the unexpected {211} and {022} slip planes was revealed. Additionally, all possible slip systems for the intermetallic phases were further evaluated and a preference for the activation of slip systems based on their effective interplanar distances as well as the effective Burgers vector is proposed. The effective interplanar distance corresponds to the manually determined interplanar distance, whereas the effective Burgers vector takes a potent…
High entropy alloys are finding significant scientific interest due to their exotic microstructures and exceptional properties resulting thereof. These alloys have excellent corrosion resistance and may find broad range of applications from bio-implants, aerospace components and nuclear industry. A critical performance metric that determines the application worthiness of the alloys is the resilience of stressed structural members in a corrosive environment. This study reports the results from a novel experimental setup to quantify the corrosion rate under uniaxial tensile stress in a single ph…
We develop and solve a continuum theory for the piezoelectric response of nanotubes under applied uniaxial and torsional stresses. We find that the piezoelectric response is controlled by the chiral angle, the aspect ratio, and two dimensionless parameters specifying the ratio of the strengths of the electrostatic and elastic energies. The model is solved in two limiting cases and the solutions are discussed. These systems are found to have several unexpected physical effects not seen in conventional bulk systems, including a strong stretch-twist coupling and the development of a significant b…
On the basis of a previous theoretical approach to the plastic flow of highly refined materials, a physical explanation for diffusion bonding is essayed, which yields closed--form equations relating the bonding progress with time, temperature, applied pressure and the constants characterizing the material. Excellent agreement with experiment is attained, with no adjustable parameter. In the novel scheme, diffusion bonding is caused by the interpenetration of the two sufaces at the grain level. The process is driven by the strong tensile stress field induced in the plane of the interface by the…
Thermal properties of graphene display peculiar characteristics associated to the two-dimensional nature of this crystalline membrane. These properties can be changed and tuned in the presence of applied stresses, both tensile and compressive. Here we study graphene monolayers under tensile stress by using path-integral molecular dynamics (PIMD) simulations, which allows one to take into account quantization of vibrational modes and analyze the effect of anharmonicity on physical observables. The influence of the elastic energy due to strain in the crystalline membrane is studied for increasin…
The demand of fast and power efficient spintronics devices with flexibility requires additional energy for magnetization manipulation. Stress/and strain have shown their potentials for tuning magnetic properties to the desired level. Here, we report a systematic study for the effect of both tensile and compressive stresses on the magnetic anisotropy (MA). Further the effect of stress on the domain structure and magnetization relaxation mechanism in a perpendicularly magnetized Co/Pt film has been studied. It is observed that a minimal in-plane tensile strain has increased the coercivity of the…
Shock-wave profiles of sapphire (single-crystal Al2O3) with seven crystallographic orientations were measured with time-resolved VISAR interferometry at shock stresses in the range 16 to 86 GPa. Shock propagation was normal to the surface of each cut. The angle between the c-axis of the hexagonal crystal structure and the direction of shock propagation varied from 0 for c-cut up to 90 degrees for m-cut in the basal plane. Based on published shock-induced transparencies, shock-induced optical transparency correlates with the smoothness of the shock-wave profile. The ultimate goal was to find th…
Enhanced mechanical properties and microstructural stability of ultrafine-grained biodegradable Zn-Li-Mn-Mg-Cu alloys produced by rapid solidification and high-pressure torsion
Based on the atomistic studies presented in Part I we develop analytical yield criteria for single crystals that capture the effect of shear stresses other than the Schmid stress (non-glide stresses) on the shear stress needed for dislocation glide (Peierls stress). These yield criteria characterize a non-associated plastic flow that originates owing to the complex response of 1/2<111> screw dislocations to an applied stress tensor. Employing these criteria we identify the operative slip systems for tensile/compressive loading along various axes within the standard stereographic triangle and d…
Three-dimensional (3D) simulations of ferroelectric hysteresis and butterfly loops have been carried out based on solving the time dependent Ginsburg-Landau equations by using a finite volume method. The influence of externally mechanical loadings with a tensile strain and a compressive strain on the hysteresis and butterfly loops has been studied numerically. The 3D ferroelectric domain formation and its evolution have also been presented in the paper. The simulation results successfully reveal the macroscopically nonlinear response to the applied stresses and electric field.
The effect of tensile stress applied during cooling of binary glasses on the potential energy states and mechanical properties is investigated using molecular dynamics simulations. We study the three-dimensional binary mixture that was first annealed near the glass transition temperature and then rapidly cooled under tension into the glass phase. It is found that at larger values of the applied stress, the liquid glass former freezes under higher strain and its potential energy is enhanced. For a fixed cooling rate, the maximum tensile stress that can be applied during cooling is reduced upon …
Adding sufficient tensile strain to Ge can turn the material to a direct bandgap group IV semiconductor emitting in the mid-infrared wavelength range. However, highly strained-Ge cannot be directly grown on Si due to its large lattice mismatch. In this work, we have developed a process based on Ge micro-bridge strain redistribution intentionally landed to the Si substrate. Traction arms can be then partially etched to keep only localized strained-Ge micro-blocks. Large tunable uniaxial stresses up to 4.2% strain were demonstrated bonded on Si. Our approach allows to envision integrated straine…
Niobium Titanium Nitride as a High Tensile Stress Material for Nanomechanics
Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften（瓦尔特·迈斯纳研究所，巴伐利亚科学院）；Physics Department, TUM School of Natural Sciences, Technical University of Munich（慕尼黑工业大学自然科学学院物理系）；Munich Center for Quantum Science and Technology (MCQST)（慕尼黑量子科学与技术中心）；Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology（查尔姆斯理工大学微技术与纳米科学系）；School of Computation, Information and Technology, Technical University of Munich（慕尼黑工业大学计算、信息与技术学院）；TUM Center for Quantum Engineering (ZQE)（慕尼黑工业大学量子工程中心）
Deformation heterogeneities within microstructures of polycrystalline shape memory alloys (SMAs) during superelastic stressing are studied using both experiments and simulations. In situ X-ray diffraction, specifically the far-field high energy diffraction microscopy (ff-HEDM) technique was used to non-destructively measure the grain-averaged statistics of position, crystal orientation, elastic strain tensor and volume for hundreds of austenite grains in a superelastically loaded nickel-titanium (NiTi) SMA. This experimental data were also used to create a synthetic microstructure within a fin…
First-principles calculations of the epitaxial-strain phase diagram of perovskite SrCoO$_{3}$ are presented. Through combination of the large spin-phonon coupling with polarization-strain coupling and coupling of the band gap to the polar distortion, both tensile and compressive epitaxial strain are seen to drive the bulk ferromagnetic-metallic (FM-M) phase to antiferromagnetic-insulating-ferroelectric (AFM-I-FE) phases, the latter having unusually low elastic energy. At these coupled magnetic-ferroelectric metal-insulator phase boundaries, cross responses to applied electric and magnetic fiel…
Turing patterns on non-fluctuating surfaces under mechanical stresses
National Institute of Technology (KOSEN), Ibaraki College（茨城工业高等专门学校）；Argopilot Coorporation（Argopilot公司）；Institute for Liberal Arts, Institute of Science Tokyo（东京科学大学通识教育学院）；Department of Mathematics, Faculty of Science, Hokkaido University（北海道大学理学部数学系）；Institute of Fluid Science (IFS), Tohoku University（东北大学流体科学研究所）；ELyTMaX, CNRS-Universite de Lyon-Tohoku University（CNRS-里昂大学-东北大学 ELyTMaX）
Using the notion of eigenstrain produced by the defects formed in a material exposed to high energy neutron irradiation, we develop a method for computing macroscopic elastic stress and strain arising in components of a fusion power plant during operation. In a microstructurally isotropic material, the primary cause of macroscopic elastic stress and strain fields is the spatial variation of neutron exposure. We show that under traction-free boundary conditions, the volume-average elastic stress always vanishes, signifying the formation of a spatially heterogeneous stress state, combining compr…
A non-local model for the description of twinning in polycrystalline materials in the context of infinitesimal strains: application to a magnesium alloy
Machine-learned multi-orbital tight-binding (MMTB) Hamiltonian models have been developed to describe the electronic characteristics of intermetallic compounds $\rm Mg_2Si, Mg_2Ge, Mg_2Sn$, and $\rm Mg_2Pb$ subject to strain. The MMTB models incorporate spin-orbital mediated interactions and they are calibrated to the electronic band structures calculated via density functional theory (DFT) by a massively parallelized multi-dimensional Monte-Carlo search algorithm. The results show that a machine-learned five-band tight-binding model reproduces the key aspects of the valence band structures in…
Using Monte Carlo (MC) simulations, we study the skyrmion stability/instability as a response to uniaxial mechanical stresses. Skyrmions emerge in chiral magnetic materials as a stable spin configuration under external magnetic field $\vec{B}$ with the competition of ferromagnetic interaction and Dzyaloshinskii-Moriya interaction (DMI) at low temperature $T$. Skyrmion configurations are also known to be stable (unstable) under a compressive stress applied parallel (perpendicular) to $\vec{B}$. To understand the origin of such experimentally confirmed stability/instability, we use the Finsler g…
Complex morphologies and microstructures that emerge during materials growth and solidification are often determined by both equilibrium and kinetic properties of the interface and their crystalline anisotropies. However limited knowledge is available for the alloying and particularly the compositionally generated elastic effects on these interface characteristics. Here we systematically investigate such compositional effects on the interfacial properties of an alloy model system based on the phase-field-crystal analysis, including the solid-liquid interfacial free energy, kinetic coefficient,…
