Onset Of Plastic Deformation Research Articles

Optical Properties of Tensilely Strained Ge Nanomembranes.

Group-IV semiconductors, which provide the leading materials platform of micro- electronics, are generally unsuitable for light emitting device applications because of their indirect- bandgap nature. This property currently limits the large-scale integration of electronic and photonic functionalities on Si chips. The introduction of tensile strain in Ge, which has the effect of lowering the direct conduction-band minimum relative to the indirect valleys, is a promising approach to address this challenge. Here we review recent work focused on the basic science and technology of mechanically stressed Ge nanomembranes, i.e., single-crystal sheets with thicknesses of a few tens of nanometers, which can sustain particularly large strain levels before the onset of plastic deformation. These nanomaterials have been employed to demonstrate large strain-enhanced photoluminescence, population inversion under optical pumping, and the formation of direct-bandgap Ge. Furthermore, Si-based photonic-crystal cavities have been developed that can be combined with these Ge nanomembranes without limiting their mechanical flexibility. These results highlight the potential of strained Ge as a CMOS-compatible laser material, and more in general the promise of nanomembrane strain engineering for novel device technologies.

Effect of mechanical loading on the galvanic corrosion behavior of a magnesium-steel structural joint

Here a time dependent numerical model aimed to investigate the role of mechanical deformation on the corrosion behavior of galvanic joint is developed. The influence of mechanical loading on the corrosion behavior of the AE44 (Magnesium alloy) and mild steel galvanic joint immersed in a 1.6 wt% NaCl solution is explored across a wide range of combined mechanical and electrochemical conditions. It is shown that the onset of plastic deformation during mechanical loading greatly accelerates the galvanic corrosion behavior. The overall numerical approach developed here provides a robust framework for understanding the role of mechanical deformation on the corrosion behavior.

Onset of plastic deformation in non-equiatomic fcc CoCrFeMnNi high-entropy alloys under high-rate loading

Onset of plastic deformation in non-equiatomic fcc CoCrFeMnNi high-entropy alloys under high-rate loading

High strength NiTiHf shape memory alloys with tailorable properties

Microstructure of NiTiHf shape memory alloys can be engineered to have high strength and operate at high stress levels for a large temperature window. Nanoprecipitation is well-known method to improve the strength of materials but it can be employed to NiTiHf alloys to substantially alter their phase transformation characteristics (martensite morphology, transformation strain, hysteresis and stress). The martensitic transformation of Ni-rich Ni51.2Ti28.8Hf20 was severely suppressed in the solution treated condition (900 °C-3h/water quench) and after aging at low temperatures, while the transformation temperatures were greater than 100 °C after 650 °C-3h aging. Generation of nanosize precipitates (∼20 nm in size) after 3 h aging at 450 °C and 550 °C improved the strength of the material, resulting in a near perfect dimensional stability during isobaric thermal cycling at stress levels of greater than 1500 MPa, with work output of 20–30 J cm−3. Superelastic behavior with 4% recoverable strain was demonstrated at low temperatures (−20 to 40 °C) after aging at 450 °C-3h and at elevated temperatures (120–160 °C) after aging at 550 °C-3h, with stresses reaching 2 GPa without the onset of plastic deformation. A clear relationship between thermal treatments, microstructure, mechanical and shape memory properties will be shown.

Isolation and characterization of gum from <i>Chrysophyllum albidum</i> fruits as pharmaceutical excipient

This study describes the morphology, physicochemical and compressional characteristics of a natural gum derived from the fruits of Chrysophyllum albidum. Preliminary phytochemical screening and physicochemical properties of Chrysophyllum albidum gum (in comparison with tragacanth gum) were determined while scanning electron microscopy (SEM), Fourier transform infra red (FTIR) and differential scanning calorimetry (DSC) were used to characterize the morphology of the gum. The compressional behaviour of the gum in comparison with that of corn starch BP was analyzed using density measurements and indices of plasticity from the Heckel and Kawakita equations. Chrysophyllum albidum gum contains saponins, polysaccharide, flavonoids, tannins and non- reducing sugar. The pH of 1% w/v suspension of Chrysophyllum albidum was 4.13. The angle of repose of Chrysophyllum albidum gum was high and indicative of a slightly poor flow. FTIR Spectra reveal absorption peaks between 1638 and 1454 cm-1 while the absorption bands between 800 and 1200 cm-1 represent the fingerprint region for carbohydrates. The SEM images reveal that the particles are round and regular in shape while the DSC thermogram shows a gradual increase in heat flow which suggests that Chrysophyllum albidum gum exhibits only the amorphous structure. Chrysophyllum albidum is a Type C material which exhibits a slower onset of plastic deformation than corn starch but had a lower overall amount of plastic deformation during the compression process. The results establish the fundamental characteristic of Chrysophyllum albidum gum and explain the rationale behind its use as raw material in pharmaceutical formulationsKeywords: Chrysophyllum albidum; Gum; Excipients; Compressional properties

Investigation into Corrosion Pit-to-Fatigue Crack Transition in 7075-T6 Aluminum Alloy

Transition of corrosion pit to crack under fatigue condition was investigated in high-strength 7075-T6 aluminum alloy. The pit was formed at the edge of a hole in a specimen. Specimen was subjected to a constant stress during the pit formation. Two types of corrosion pit were considered: corner-pit and through-pit. Two sizes were tested for each pit type. Also, the baseline data of cycles to initiate a 250-µm-long crack were established when the corrosion pit was created without any applied stress on the specimen, i.e., Sappl = 0. The cycles to initiate a 250-µm-long crack initially decreased with increasing Sappl relative to the baseline value and then increased with increasing Sappl such that this increase was significant with higher value of Sappl. The transition between this increase and decrease occurred when the Sappl was greater or less than a value which caused the onset of plastic deformation at the root of the pit, respectively. Microstructural analysis showed that this decrease in cycles to initiate the crack was due to microcracks at the pit front which developed at the lower level of Sappl, and the increase was due to plastic deformation at the higher levels of Sappl.

The Mechanical Properties of Minimally Processed RDX

We report for the first time the mechanical properties of RDX crystals in a conventionally processed, sub-millimeter form that have had no additional mechanical processing. Nanoindentation of RDX powders was used to measure the elastic modulus (19.1±1.9 GPa), hardness (0.741±0.098 GPa), and yield point (onset of plastic deformation) on the as-grown faces of seven different RDX crystals, selected to provide random orientations. Properties within each crystal showed narrow distributions, while the range of properties across all crystals is indicative of testing a variety of orientations. The elastic modulus and hardness are within the range of other published reports on bulk and mechanically polished RDX. The distribution in yield point behavior, with the onset of plasticity occurring between 0.1 and 0.7 GPa, indicates that powders of RDX likely contain a significant number of dislocation sources in the as-processed condition, suggesting that deformation sources are prevalent in the energetic component of plastic bonded explosives prior to incorporating into pressed forms.

Microstructural evolution of a ductile metastable β titanium alloy with combined TRIP/TWIP effects

Herein, we investigated the microstructural evolution of a metastable β Ti-12(wt%)Mo alloy under tension, exhibiting combined phase transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effects. The deformed samples were analyzed by X-ray diffraction (XRD), optical microscopy (OM) and transmission electron microscopy (TEM) until fracture. Lamella-like {112}〈111〉 nano-twinning was observed at the onset of plastic deformation. Upon straining, {332}〈113〉 twinning and stress induced martensitic (SIM) β->α″ transformation were both identified locally in the deformed β matrix as primary mechanisms. Secondary mechanisms such as 2nd twinning and 2nd SIM, were then observed in the primary β twins. At the end of the uniform plastic deformation, a complex deformation microstructure was observed, showing highly disturbed matrix induced by a high volume fraction of twins and occurrence of multiples secondary mechanisms. Resultant macroscopic mechanical properties of the material display a combination of very high ductility (45%), high strength and superior strain-hardening behavior.

Numerical Determination of Fatigue Threshold from CTOD

The experimental procedure to obtain valid and comparable values of ΔKth is laborious and time consuming. The objective of this work is to determine ΔKth using a numerical approach. The CTOD is plotted versus load, and the fatigue threshold corresponds to the onset of plastic deformation. A parametric study was developed in order to understand the effect of material parameters on ΔKth.

Interface and strain effects on the H-sorption thermodynamics of size-selected Mg nanodots

This work deals with the thermodynamics of hydride formation in 3-D nanoconfined Mg. Two ensembles of nearly monodisperse Mg nanodots (NDs) with different diameters (60 and 320 nm), were grown by the template nanopatterning method, using ultra-thin alumina membranes (UTAMs) with ordered porosity as evaporation masks. Multilayer NDs consisting of 30 nm Mg, 5 nm Ti and 5 nm Pd were deposited on UTAM-coated glass substrates by molecular beam epitaxy. The lateral surface of the NDs is constituted by native MgO. The morphology of the NDs was characterized by field emission scanning electron microscopy and atomic force microscopy. Hydride formation and decomposition was studied at low temperature (363–393 K) by means of optical hydrogenography. Compared to bulk Mg, the plateau pressure for hydrogen absorption in NDs exhibits an upward shift, which is larger for small NDs. Differently, the desorption plateau pressure is almost the same for the two NDs size and is lower than for bulk Mg. These hydrogen sorption features are discussed in the frame of a model that takes into account both interface energy and elastic strain energy in the constrained nanodots. The onset of plastic deformation, marked by a high pressure hysteresis between hydrogen absorption and desorption isotherms, limits the extent of hydride destabilization that can be achieved by elastic strain engineering.

Fundamental limitations at the determination of initial yield stress using nano-indentation with spherical tips

Abstract Nano-indentation with spherical indenters can be used to determine the local initial yield stress with an approach consisting of i) detection of the onset of plastic deformation applying a loading-partial-unloading (LPU) procedure and ii) calculation of the spatial distribution of the von Mises stress in the sample for the state of the system where plastic deformation just starts. The spatial maximum of the von Mises stress is taken as a measure of the initial yield stress. This approach, however, is subject to a fundamental restriction due to the fact that the plastic deformation starts within the volume of the sample and only limited information about this is available at the specimen surface. Since the displacement resolution of the indentation device is always limited, this leads to a fundamental overestimation of the initial yield stress. In this paper, the limitation of the approach is investigated by using finite element simulations of spherical LPU experiments for various sample parameters and sphere radii. With reference to the displacement resolution of the used nano-indentation device, the overestimation of the initial yield has been quantified and used to correct experimental results. The comparison of these corrected yield stresses to reference values clearly shows that the initially obtained dependence on the sphere radius was due to the fundamental overestimation of the yield stresses determined via spherical nano-indentation (Fig. 5).

The influence of hold time on the onset of plastic deformation in silicon

The transformation of diamond-cubic silicon to the metallic β-Sn phase is known to be “sluggish,” even when the critical pressure (∼11 GPa) for the transformation is reached. In this letter, we use nanoindentation to apply pressures to just above the critical threshold. In this regime, the sample displays purely elastic behavior at zero hold time. As the hold time at maximum load is increased up to 180 s, the percentage of indents that plastically deform also increase. Interestingly, the indents deform via one of two distinct processes: either via a phase transformation to a mixed bc8/r8-Si end phase, or by initiation of crystalline defects. Raman spectroscopy and cross-sectional transmission electron microscopy are used to show that the two deformation mechanisms are mutually exclusive under the indentation conditions presented here, and elastic modelling was utilized to propose a model for this mutually exclusive behavior. Hence, this behavior enhances the potential for application of the exotic bc8/r8-Si end phase.

On the critical thickness for non-localized to localized plastic flow transition in metallic glasses: A molecular dynamics study

Molecular dynamics simulations were employed to investigate the specimen thickness-dependent tensile behavior of a series of CuxZr100-x (x=20, 40, 50, 64 and 80at.%) metallic glass (MG) films, with a particular focus on the critical thickness, tc, below which non-localized plastic flow takes place. The simulation results reveal that while the transition occurs in all the alloys examined, tc is sensitive to the composition. We rationalize tc by postulating that the strain energy stored in the sample at the onset of plastic deformation has to be sufficient for the formation of shear bands. The composition-dependence of tc was found to correlate with the average activation energy of the atomic level plastic deformation events.

In situ spectroscopic study of the plastic deformation of amorphous silicon under non-hydrostatic conditions induced by indentation.

Indentation-induced plastic deformation of amorphous silicon (a-Si) thin films was studied by in situ Raman imaging of the deformed contact region of an indented sample, employing a Raman spectroscopy-enhanced instrumented indentation technique. Quantitative analyses of the generated in situ Raman maps provide unique, new insight into the phase behavior of as-implanted a-Si. In particular, the occurrence and evolving spatial distribution of changes in the a-Si structure caused by processes, such as polyamorphization and crystallization, induced by indentation loading were measured. The experimental results are linked with previously published work on the plastic deformation of a-Si under hydrostatic compression and shear deformation to establish a sequence for the development of deformation of a-Si under indentation loading. The sequence involves three distinct deformation mechanisms of a-Si: (1) reversible deformation, (2) increase in coordination defects (onset of plastic deformation), and (3) phase transformation. Estimated conditions for the occurrence of these mechanisms are given with respect to relevant intrinsic and extrinsic parameters, such as indentation stress, volumetric strain, and bond angle distribution (a measure for the structural order of the amorphous network). The induced volumetric strains are accommodated solely by reversible deformation of the tetrahedral network when exposed to small indentation stresses. At greater indentation stresses, the increased volumetric strains in the tetrahedral network lead to the formation of predominately five-fold coordination defects, which seems to mark the onset of irreversible or plastic deformation of the a-Si thin film. Further increase in the indentation stress appears to initiate the formation of six-fold coordinated atomic arrangements. These six-fold coordinated arrangements may maintain their amorphous tetrahedral structure with a high density of coordination defects or nucleate as a new crystalline β-tin phase within the a-Si network.

The influence of stacking fault energy on the microstructural and strain-hardening evolution of Fe–Mn–Al–Si steels during tensile deformation

Abstract Understanding the relationship between the stacking-fault energy (SFE), deformation mechanisms, and strain-hardening behavior is important for alloying and design of high-Mn austenitic transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The present study investigates the influence of SFE on the microstructural and strain-hardening evolution of three TRIP/TWIP alloys (Fe–22/25/28Mn–3Al–3Si wt.%). The SFE is increased by systemically increasing the Mn content from 22 to 28 wt.%. The Fe–22Mn–3Al–3Si alloy, with a SFE of 15 mJ m−2, deforms by planar dislocation glide and strain-induced ehcp-/αbcc-martensite formation which occurs from the onset of plastic deformation, resulting in improved work-hardening at low strains but lower total elongation. With an increased SFE of 21 mJ m−2 in the Fe–25Mn–3Al–3Si alloy, both mechanical twinning and ehcp-martensite formation are activated during deformation, and result in the largest elongation of the three alloys. A SFE of 39 mJ m−2 enables significant dislocation cross slip and suppresses ehcp-martensite formation, causing reduced work-hardening during the early stages of deformation in the Fe–28Mn–3Al–3Si alloy while mechanical twinning begins to enhance the strain-hardening after approximately 10% strain. The increase in SFE from 15 to 39 mJ m−2 results in significant changes in the deformation mechanisms and, at low strains, decreased work-hardening, but has a relatively small influence on strength and ductility.

A new method to texture dense Mn+1AXn ceramics by spark plasma deformation

This work explored a new method to texture Mn+1AXn phases by deforming discs of dense ceramics Maxthal 211® (nominally Ti2AlC) and Maxthal 312® (nominally Ti3SiC2) in a spark plasma facility. The disc diameter increased by 40% without crack formation. After deformation, the MAX grains exhibited clear preferential crystallographic orientation, whereby the c-axis was aligned to the compression direction. The fracture toughness increased both parallel and perpendicular to the textured top surface compared with the as-sintered materials. The onset of plastic deformation was observed near the brittle-to-plastic transformation temperature, a fact that could be associated with the change in elastic response.

Structural aspects of elasto-plastic deformation of a Zr-based bulk metallic glass under uniaxial compression

The structural rearrangements occurring during compressive deformation of a plastically deformable Zr52.5Ti5Cu18Ni14.5Al10 bulk metallic glass have been investigated in situ using high energy synchrotron X-rays. It was found that in the elastic regime, the atomic distances at both short and medium range order vary linearly with macroscopic stress where the atomic bonds in short range order appear significantly stiffer than medium range order. Upon elastic loading, a small fraction of bonds in the first shell is broken in the loading direction whereas some new bonds are formed in the transverse direction. Atomic strain–stress correlation at medium range order deviates from linearity at the onset of plastic deformation which was correlated to the activation of irreversible STZs. This was confirmed by quantifying the amount of atomic shear strain value during loading. The length scale of 12.5Å indicated the largest shear strain and is thought to be the most effective length scale in the formation of STZs. The typical fracture angle of this BMG was explained by the orientation of maximum atomic shear strain at the onset of major shear band formation.

Finite Element Analysis of Plastic Deformation During Impression Creep

Finite element (FE) analysis of plastic deformation associated with impression creep deformation of 316LN stainless steel was carried out. An axisymmetric FE model of 10 × 10 × 10 mm specimen with 1-mm-diameter rigid cylindrical flat punch was developed. FE simulation of impression creep deformation was performed by assuming elastic-plastic-power-law creep deformation behavior. Evolution of the stress with time under the punch during elastic, plastic, and creep processes was analyzed. The onset of plastic deformation was found to occur at a nominal stress about 1.12 times the yield stress of the material. The size of the developed plastic zone was predicted to be about three times the radius of the punch. The material flow behavior and the pile-up on specimen surface have been modeled.

The Compaction, Mechanical and Disintegration Properties of Modified Pennisetum glaucum (Poaceae) Starch in Directly Compressed Chloroquine Tablet Formulations

Pennisetum glaucum L (Millet) starch is a potential source of direct-compression excipient for use in tableting but has not been commercially explored. The aim of this study was to investigate the compaction properties of pregelatinized freeze (PFDMS) and oven-dried (PODMS) millet starch in directly compressed Chloroquine tablet formulation and compare with the native starch (NMS) and Avicel®. Tablets were directly compressed using the starch in drug-diluent ratios of 1:2, 1:4 and 1:9 and evaluated by compaction, mechanical and disintegration properties. The result showed that PFDMS and PODMS starch samples had higher flow properties in comparison to the native starch and PFDMS also gave the fastest onset of plastic deformation. Tablets formulated with PFDMS and PODMS showed acceptable mechanical and disintegration properties comparable with Avicel® in the order Avicel® > PFDMS > PODMS > NMS. The pregelatinized starch samples can be substituted for Avicel® for direct compression. Key words: Pennisetum glaucum starch, Pregelatinisation, Tablets, Direct compression

ATOMISTIC SIMULATION OF TORSIONAL VIBRATION AND PLASTIC DEFORMATION OF FIVE-FOLD TWINNED COPPER NANOWIRES

In this paper, atomistic simulations have been conducted to investigate the torsional mechanical behaviors of five-fold twinned nanowires (FTNs), including the torsional vibration properties, elasto-plastic deformation behaviors and activation process of the first partial dislocation nucleation. Simulation results show that the fundamental torsional vibration frequency is inversely proportional to the wire length and is independent of the wire radius. Provided that an effective shear modulus of FTNs is used, the classic elastic torsional theory may be applicable to nanoscale. Furthermore, it is found that the plastic deformation of FTNs is dominated by partial dislocation activities. The normalized critical torsional angle corresponding to the onset of plastic deformation increases with the decrease of the wire radius and temperature, while it is almost independent of the wire length and loading rate. In addition, the activation energy of the first partial dislocation nucleation is about several electric voltages and decreases with the increase of the wire radius and applied torsional load.