Mechanical Softening Research Articles

Mechanical softening of thermoelectric semiconductor Mg2Si from nanotwinning

Nanotwinning exhibits strengthening effects in many metals, semiconductors, and ceramics. However, we show from ab-initio calculations that nanotwins significantly decrease the strength of thermoelectric semiconductor Mg2Si. The theoretical shear strength of nanotwinned Mg2Si is found to be 0.93 GPa, much lower than that (6.88 GPa) of flawless Mg2Si. Stretching the MgSi bond under deformation leads to the structural softening and failure of flawless Mg2Si. While in nanotwinned Mg2Si, the MgSi bond at the twin boundary (TB) is expanded to accommodate the structural misfit, weakening the TB rigidity and leading to the low ideal shear strength.

Phototriggered Depolymerization of Flexible Poly(phthalaldehyde) Substrates by Integrated Organic Light-Emitting Diodes.

We demonstrate phototriggered depolymerization of a low ceiling temperature ( Tc) polymer, poly(phthalaldehyde) (PPHA), via internal light emission from integrated organic light-emitting diodes (OLEDs) fabricated directly on flexible PPHA substrates with silver nanowire electrodes. The depolymerization of the PPHA substrates is triggered by absorption of the OLED emission by a sensitizer that activates a photoacid generator via energetically favorable electron transfer. We confirm with Fourier-transform infrared spectroscopy that the photon doses delivered by the integrated OLED are sufficient to depolymerize the PPHA substrates. We determine this critical dosage by measuring the operating lifetimes of the OLEDs whose failure is believed to be due to significant mechanical softening during the liquefaction of decomposed phthalaldehyde monomers.

(Invited) Distinctive Characteristics of Internal Fracture in Tough Double Network Hydrogels Revealed By Various Modes of Stretching

(Invited) Distinctive Characteristics of Internal Fracture in Tough Double Network Hydrogels Revealed By Various Modes of Stretching

Magnetic Non-Destructive Evaluation of Softening of Cold Rolled Low Carbon IF Steel

Electromagnetic techniques such as Magnetic Hysteresis Loop and Barkhausen Emission are applied to determine recovery and recrystallization in low carbon IF steel. Isothermal annealing at 450 and 620°C are carried to promote recovery and recrystallization. In present study, the magnetic non-destructive parameters such as coercivity and magnetic Barkhausen noise signal measurements, recovery and onset of recrystallization are monitored for annealing treatments. Mechanical softening is very small changes than magnetic softening at 450°C, while at 620°C, both the parameters are sensitive. Microstructure analysis examined through Optical microscopy at each annealing temperature in order to determine recovery and recrystallization. Keywords: IF steel, Recrystallization, MHL, MBE, Hardness, Optical microscopy

The Prediction of isothermal cyclic plasticity in 7175-T7351 aluminium alloy with particular emphasis on thermal ageing effects

Rapid thermal ageing has been observed in the literature for certain aerospace aluminium alloys under conditions which are foreseeable in their intended component applications. While several experimental programs have explored this phenomenon in the laboratory, efforts to incorporate these effects in unified constitutive models have, to date, been lacking. In the present work, a modified elastic-viscoplastic material model has been fitted to 7175-T7351 aluminium alloy data under fully-reversed uniaxial (strain-controlled) isothermal loading conditions at 160 °C and 200 °C. These temperatures were chosen in order to represent nominal and extreme (but still operationally relevant) conditions experienced by aero-engine gearbox components. The modified elastic-viscoplastic model is able to accurately predict the hysteresis loops of the strain-controlled fatigue data of each sample at 160 °C and 200 °C. Additionally, it was found that the isotropic hardening can be effectively de-coupled into material ageing and mechanical softening components.

Optimizing the electromechanical response in morphotropic BiFeO3

sMorphotropic phase boundaries (MPBs) in ferroelectrics are highly desired phase transition regions, where crystal structure becomes extremely susceptible to external stimuli leading to very large electromechanical responses with applications in high-performance piezoelectric actuators and sensors. Since the recent discovery of a strain-driven MPB in chemically simple oxides, especially in thin films of BiFeO3 (BFO), much of the research emphasis has been on the physical structural MPBs which spatially separate intimately mixed phases in corrugated nanoscale regions. Very large electromechanical responses and other coupled novel functionalities, i.e. magnetism and electronic conductivity have been observed at such physical structural MPBs. In this report, we investigate and identify a region in the thickness-dependent phase diagram of the morphotropic BFO thin films at which a superior piezoelectric performance is achieved. In this region, an electrically recoverable strain approximately 40% higher than the previously reported values for the morphotropic BFO is observed. This enhanced electromechanical coupling has been attributed to the degeneracy of the mixed-phase states and concurrent increased mechanical softening.

Structural and Mechanical Effects of Calcium on the Lipid Bilayer

Calcium is a ubiquitous divalent cation involved in fundamental biochemical and biophysical processes. Its integral roles in cell signaling, protein-mediated fusion, and action potential propagation have been well studied with a variety of techniques. Calcium is also commonly maintained at strong asymmetric concentration gradients, which often span several orders of magnitude, across mammalian cell membranes in proper physiological conditions. Yet, there has been a comparative lack of investigation of the effect of divalent cations on lipid bilayer structure and mechanical properties. Of the few studies done, experimental methods are limited to undefined resolutions and indirect spectroscopic or fluorescent techniques. This is in part due to the inherent difficulty in preparing well aligned samples with ions for high resolution experiments; common multilayer preparation techniques are subject to miscibility limits and sample segregation. Computationally, molecular dynamics simulations have predicted a calcium “condensing” effect that is associated with bilayer thickening and increased hydrocarbon chain order. However, these methods are limited by still-developing approaches to modeling divalent cation structure and interactions. Here, we report lamellar x-ray diffraction measurements of lipid bilayer thickness in partially charged multilayers containing calcium. Additionally, we report an apparent calcium-induced mechanical softening effect in giant unilamellar vesicles (GUVs) as measured by micropipette aspiration. This effect is marked by a measurable reduction in the elastic stretching coefficient of GUVs in calcium-rich environments. Both the thickening and mechanical softening effects may be important for proteins and molecules that couple directly to the lipid bilayer in physiological calcium concentrations. Future investigations involving PG-containing membranes and bacterial protoplasts may help lead to a better understanding of how invading bacteria thrive in calcium-rich environments, or if calcium-rich membrane domains have some non-trivial influence on the mechanisms of antibiotics and/or host defense antimicrobial peptides.

Tuning the piezoelectric and mechanical properties of the AlN system via alloying with YN and BN

Recent advances in microelectromechanical systems often require multifunctional materials, which are designed so as to optimize more than one property. Using density functional theory calculations for alloyed nitride systems, we illustrate how co-alloying a piezoelectric material (AlN) with different nitrides helps tune both its piezoelectric and mechanical properties simultaneously. Wurtzite AlN-YN alloys display increased piezoelectric response with YN concentration, accompanied by mechanical softening along the crystallographic c direction. Both effects increase the electromechanical coupling coefficients relevant for transducers and actuators. Resonator applications, however, require superior stiffness, thus leading to the need to decouple the increased piezoelectric response from the softened lattice. We show that co-alloying of AlN with YN and BN results in improved elastic properties while retaining some of the piezoelectric enhancements from YN alloying. This finding may lead to new avenues for tuning the design properties of piezoelectrics through composition-property maps.

Multiscale damage evolution in polymer bonded sugar under dynamic loading

Ultra-high-speed and high spatial resolution digital image correlation based experiments are performed to understand the sub grain level local failure mechanisms in polymer bonded explosives under dynamic loading. Polymer bonded sugar that mimics the mechanical properties of polymer bonded explosives is used in this study. The experiments are conducted in a split Hopkinson pressure bar setup at an intermediate strain rate. From the digital image correlation analyses, high strain localization was observed predominantly in the regions filled by the polymer binder surrounding the sugar crystals. The damage evolution was quantified by calculating the Poisson's ratio and volumetric strain as a function of loading. It was observed that the damage incubation is started at a very small global strain and causes mechanical softening in the material that leads to shear band formation. Also, the possible hotspot mechanism in the material was seen to be frictional heating of crystals and de-bonding of the polymer binder.

The softening effect of heat-treated strengthened Al–Zn–Mg alloy in welding process

Weld joint softening occurs during the welding process of heat-treatable aluminum alloys and strongly influences the mechanical properties. In this work, the softening of heat-treated Al–Zn–Mg alloy was studied in the multipass welding process. By Gleeble-3500 thermal–mechanical simulator, the heat treatment and tensile test with welding thermal cycles were carried out to simulate the microstructure evolution and mechanical softening during multipass welding. After that, the softening mechanism of the HAZ was analyzed by microstructure analysis. The results indicate that the heat-treated Al–Zn–Mg alloy exhibited obvious softening after several thermal cycles with peak temperature higher than 200[Formula: see text]C, and this phenomenon is worse with increasing peak temperature. Based on the microstructure analysis, it was found that the reinforcement phase changes according to the applied thermal cycles, which strongly affects the strength of Al–Zn–Mg alloys.

Linking macroscopic rejuvenation to nano-elastic fluctuations in a metallic glass

Metallic glasses are structurally heterogeneous below a certain length scale. Here we demonstrate how elastic heterogeneities change in response to a macroscopic cyclic, but elastic, loading protocol. Fluctuations in local elastic properties are spatially resolved across the surface of a bulk sample by evaluating nano-scale contact resonances of an atomic force microscope. The findings indicate a significant increase in nano-elastic fluctuations due to loading. The distribution of these fluctuations broadens symmetrically and almost three fold, revealing how the atomically disordered structure is driven further out of equilibrium at the nano-scale. Macroscopically, the stress-driven elastic heterogeneities lead to a rejuvenation and therefore a new structural state of the bulk metallic glass, characterized by a higher energy release during heating and mechanical softening. These results show how macroscopic energy storage in metallic glasses may be linked to the development of nano-scale elastic fluctuations.

3D polycrystalline discrete element method (3PDEM) for simulation of crack initiation and propagation in granular rock

A three-dimensional Voronoi tessellation model based on the distinct element method (DEM) is proposed to model the representative part of the microstructures of granular brittle rocks. Regularization is employed to decrease the frequency of polyhedrons with large edge ratio and contributes to a higher efficiency for element meshing. Sensitivity analyses are performed for a series of micro contact parameters in accordance with the macro responses observed in laboratory experiments (e.g. the uniaxial compression test, the Brazilian disc test and the triaxial compression test). Verifications by simulating the spalling test and plate impact test indicate that the 3D polycrystalline discrete element method (3PDEM) can be employed for efficiently simulating nonlinear mechanical behaviors, large deformation, strain softening and rock dynamics.

Impact of hydration on the micromechanical properties of the polymer composite structure of wood investigated with atomistic simulations

A model of the secondary layer of wood cell wall consisting of crystalline cellulose, hemicellulose, and lignin is constructed and investigated with molecular dynamics simulations in the full range of hydration: from dry to saturated state. The model is considered a composite with the cellulose fibrils embedded in hemicellulose and lignin, forming a soft amorphous matrix. Its complex structure leads to nonlinear and anisotropic swelling and mechanical weakening. The water diffusivity through the pores is affected by an interplay between stiff cellulose fibers and weakening amorphous polymers. The formation and breaking of hydrogen bonds within the polymers and at the interfaces is found to be the underlying mechanism of adsorption-induced mechanical softening. The model is tested for adsorption isotherm, mechanical moduli, hydrogen bonds, and water diffusivity that all undergo a substantial change as the hydration increases. The determined physical and mechanical properties, changing with hydration, agree qualitatively with experimental measurements.

Assessing the recovery and recrystallization kinetics of cold rolled microalloyed steel through coercive field measurements

Annealing after cold rolling brings about the activation of recovery and recrystallization in microalloyed steels. The importance of recovery has been most often associated with its effects on the recrystallization kinetics. However, recovery has gained particular importance as an alternative heat treatment under the name “back annealing” or “recovery annealing”. In the present work, various annealing treatments were applied to a Nb-microalloyed steel in the range of temperatures and times where recrystallization is not complete. As a consequence, a large set of tensile strength-ductility pairs was obtained, even for conditions in which recrystallization was avoided. Through non-destructive magnetic coercive field measurements, recovery and partial recrystallization were monitored for each annealing treatment. Magnetic softening is significantly greater than mechanical softening. The variation in recovery in terms of temperature and time is highly affected by the presence of Nb in solution in the hot band (before cold rolling). At low recovery annealing temperatures, 350–450°C, Nb solute drag on dislocations is the main mechanism that controls recovery, whereas at 550°C, Nb strain induced precipitation leads to a recovery plateau in terms of coercive field.

In-situ atomic force microscopy observation revealing gel-like plasticity on a metallic glass surface

It has been decade-long and enduring efforts to decipher the structural mechanism of plasticity in metallic glasses; however, it still remains a challenge to directly reveal the structural change, if any, that precedes; and dominant plastics flow in them. Here, by using the dynamic atomic force microscope as an “imaging” as well as a “forcing” tool, we unfold a real-time sequence of structural evolution occurring on the surface of an Au-Si thin film metallic glass. In sharp contrast to the common notion that plasticity comes along with mechanical softening in bulk metallic glasses, our experimental results directly reveal three types of nano-sized surface regions, which undergo plasticity but exhibit different characters of structural evolution following the local plasticity events, including stochastic structural rearrangement, unusual local relaxation and rejuvenation. As such, yielding on the metallic-glass surface manifests as a dynamic equilibrium between local relaxation and rejuvenation as opposed to shear instability in bulk metallic-glasses. Our finding demonstrates that plasticity on the metallic glass surface of Au-Si metallic glass bears much resemblance to that of the colloidal gels, of which nonlinear rheology rather than shear instability governs the constitutive behavior of plasticity.

Elastic and mechanical softening in boron-doped diamond

Alternative approaches to evaluating the hardness and elastic properties of materials exhibiting physical properties comparable to pure diamond have recently become necessary. The classic linear relationship between shear modulus (G) and Vickers hardness (HV), along with more recent non-linear formulations based on Pugh’s modulus extending into the superhard region (HV > 40 GPa) have guided synthesis and identification of novel superabrasives. These schemes rely on accurately quantifying HV of diamond-like materials approaching or potentially exceeding the hardness of the diamond indenter, leading to debate about methodology and the very definition of hardness. Elasticity measurements on such materials are equally challenging. Here we used a high-precision, GHz-ultrasonic interferometer in conjunction with a newly developed optical contact micrometer and 3D optical microscopy of indentations to evaluate elasticity-hardness relations in the ultrahard range (HV > 80 GPa) by examining single-crystal boron-doped diamond (BDD) with boron contents ranging from 50–3000 ppm. We observe a drastic elastic-mechanical softening in highly doped BDD relative to the trends observed for superhard materials, providing insight into elasticity-hardness relations for ultrahard materials.

Thermal rejuvenation in metallic glasses

Structural rejuvenation in metallic glasses by a thermal process (i.e. through recovery annealing) was investigated experimentally and theoretically for various alloy compositions. An increase in the potential energy, a decrease in the density, and a change in the local structure as well as mechanical softening were observed after thermal rejuvenation. Two parameters, one related to the annealing temperature, Ta/Tg, and the other related to the cooling rate during the recovery annealing process, Vc/Vi, were proposed to evaluate the rejuvenation phenomena. A rejuvenation map was constructed using these two parameters. Since the thermal history of metallic glasses is reset above 1.2Tg, accompanied by a change in the local structure, it is essential that the condition of Ta/Tg ≥ 1.2 is satisfied during annealing. The glassy structure transforms into a more disordered state with the decomposition of icosahedral short-range order within this temperature range. Therefore, a new glassy structure (rejuvenation) depending on the subsequent quenching rate is generated. Partial rejuvenation also occurs in a Zr55Al10Ni5Cu30 bulk metallic glass when annealing is performed at a low temperature (Ta/Tg ~ 1.07) followed by rapid cooling. This behavior probably originates from disordering in the weakly bonded (loosely packed) region. This study provides a novel approach to improving the mechanical properties of metallic glasses by controlling their glassy structure.

Influence of chemistry, interfacial width, and non-isothermal conditions on spatially heterogeneous activated relaxation and elasticity in glass-forming free standing films.

We employ the Elastically Collective Nonlinear Langevin Equation (ECNLE) theory of activated relaxation to study several questions in free standing thin films of glass-forming molecular and polymer liquids. The influence of non-universal chemical aspects on dynamical confinement effects is found to be relatively weak, but with the caveat that for the systems examined, the bulk ECNLE polymer theory does not predict widely varying fragilities. Allowing the film model to have a realistic vapor interfacial width significantly enhances the reduction of the film-averaged glass transition temperature, Tg, in a manner that depends on whether a dynamic or pseudo-thermodynamic averaging of the spatial mobility gradient is adopted. The nature of film thickness effects on the spatial profiles of the alpha relaxation time and elastic modulus is studied under non-isothermal conditions and contrasted with the corresponding isothermal behavior. Modest differences are found if a film-thickness dependent Tg is defined in a dynamical manner. However, adopting a pseudo-thermodynamic measure of Tg leads to a qualitatively new form of the alpha relaxation time gradient where highly mobile layers near the film surface coexist with strongly vitrified regions in the film interior. As a consequence, the film-averaged shear modulus can increase with decreasing film thickness, despite the Tg reduction and presence of a mobile surface layer. Such a behavior stands in qualitative contrast to the predicted mechanical softening under isothermal conditions. Spatial gradients of the elastic modulus are studied as a function of temperature, film thickness, probing frequency, and experimental protocol, and a rich behavior is found.

Ductilization of bulk metallic glassy material and its mechanism

Bulk metallic glass has aroused intensive interest due to its unique atomic structure and properties, while its structural application is restricted by the shortcomings of its mechanical properties-room temperature brittleness and strain softening. To make up for these shortcomings, various approaches have been proposed, including tailoring intrinsic parameters such as elastic modulus and structural heterogeneity, and changing stress state or defect concentration. Bulk metallic glass composites with ex-situ added or in-situ formed crystallites have been fabricated, series of bulk metallic glasses and their composites with good mechanical properties have been designed, especially TRIP (Transformation-induced plasticity)-reinforced bulk metallic glass composites with large tensile ductility and work-hardening. In this paper, we review the ductilization of bulk metallic glass and its composites, as well as the related mechanism. Particularly, fabrication, properties, structure control and the ductilization mechanism of TRIP-reinforced bulk metallic glass composite are introduced in detail. A perspective of the challenges of ductilization of bulk metallic glassy materials is also mentioned briefly.

Novel features of the Mullins effect in filled elastomers revealed by stretching measurements in various geometries.

Stretching experiments with various geometries are performed using a custom-built tensile tester to reveal the intriguing features of the mechanical softening phenomena of filled elastomers in loading-unloading cycles, commonly known as the Mullins effect. The dissipated energy (D), residual strain (εr), and dissipation factor (Δ; the ratio of D to input strain energy) in the loading-unloading cycles are evaluated as a function of the maximum stretch in cyclic loading (λm) using three types of extension, i.e., uniaxial, planar, and equibiaxial extension, for silica-filled elastomers with various filler contents, and with or without a silane coupling agent. The dissipated energy D and εr increase with an increase in λm, and they depend on the type of extension when compared at the same λm: D and εr increase in the order of equibiaxial, planar, and uniaxial extension. In contrast, the values of Δ obtained for various degrees and types of extension are collapsed into a single curve when the first invariant of the deformation tensor (I1,m) corresponding to λm is employed as a variable: Δ steeply increases with an increase in I1,m in the small deformation regime of I1,m < 3.2, while Δ levels off in the large deformation regime of I1,m > 3.5. The plateau values of Δ increase with an increase in filler content. The characteristic dependence of Δ on I1,m in each of the small and large deformation regimes is expected to reflect the destruction process of the inherent structures, including filler networks and the filler-polymer interface, and the friction between the fillers and the rubber matrix, respectively.