Change In Deformation Mode Research Articles

The size-dependent non-localized deformation in a metallic alloy

The tensile behaviors of Cu50Zr50 metallic glass (MG) films with the same cooling history as a function of film thickness are systematically studied by molecular dynamics simulations. It clearly reveals that with decreasing film thickness, a transition from the localized deformation to the non-localized deformation indeed occurs in Cu50Zr50 MG films, which might be a general phenomenon for all MGs. The thickness-dependent deformation mode change observed here, is then reasonably explained by the Griffith crack-propagation criterion.

The Anvil Effect in the Spherical Indentation Testing on Sheet Metals

Spherical indentation hardness tests are widely used in manufacturing industry to obtain material property of the workpiece. For the indentation test on sheet metals, significant error can occur in measuring the penetration depth due to the anvil effect. This anvil effect is manifested with decreasing specimen thickness and increasing indentation depth. This paper critically reviews the experimental observations of the anvil effect, and studies the mechanism behind it. The experimental results and numerical simulations suggest that, when the thickness of the specimen is reduced, a change in deformation mode is experienced during the indentation process. A correction method for the anvil effect is proposed and an empirical equation is derived to determine the effective indentation depth. The experimental and numerical investigations have important implications for the interpretation of the indentation anvil effect; and the correction method for anvil effects can be used in hardness measurement to obtain accurate material property.

OS0121-236 CRSSの寸法依存性を考慮したMg基LPSO相単結晶のマイクロピラーに対する転位-結晶塑性FEM解析

In this study, a finite element analysis based on a dislocation-based crystal plasticity model considering a size dependence of a critical resolved shear stress (CRSS) is performed to reproduce a size dependence of yield stress in a micropillar composed of single crystal of Mg-based LPSO phase. The CRSS model in which a size-dependent term is proportional to the inverse of a pillar diameter is applied and a numerical parameter is identified from experimental results. We consider four initial crystal orientations where a basal slip, prismatic slip, pyramidal slip and kink deformation are likely to occur. The results show that the applied model reproduces the size dependence of the yield stress. In addition, in the initial orientation where a kink deformation is tend to occur, the deformation mode changes according to the specimen size.

Enhancing yield strength by suppressing detwinning in a rolled Mg–3Al–1Zn alloy with {101¯2} twins

Abstract A remarkable improvement in the tensile yield strength of rolled Mg–3Al–1Zn alloy with pre-existing { 10 1 ¯ 2 } twins was achieved by inducing double twinning through compression deformation. The resulting formation of { 10 1 ¯ 2 }–{ 10 1 ¯ 2 } double twins inside pre-existing { 10 1 ¯ 2 } twin bands effectively suppressed the detwinning of pre-existing twins during subsequent deformation. This in turn led to a change in the dominant deformation mode from detwinning to slip, significantly increasing the yield strength. The mechanisms behind this behavior are considered to be a combination of the redistribution of residual tensile stress within pre-existing twins, refinement of pre-existing twins by double twins, a change in texture due to double twinning and a loss of coherency at twin boundaries.

Influence of yttrium content on phase formation and strain hardening behavior of Mg–Zn–Mn magnesium alloy

The aim of this study was to identify the effect of yttrium (Y) addition on the phase development and strain hardening behavior of an extruded Mg–Zn–Mn (ZM31) magnesium alloy. The addition of a small amount (0.3wt.%) of Y in the alloy led to the formation of icosahedral quasicrystalline I (Mg3YZn6) phase. Both I-phase and W-phase (Mg3Y2Zn3) were present in the extruded ZM31+3.2Y alloy, while long period stacking ordered (LPSO) X-phase (Mg12YZn) and Mg24Y5 were observed in the extruded ZM31+6Y alloy. The Y addition significantly refined grains in the extruded state. The presence of I-phase in the extruded ZM31+0.3Y alloy increased hardness, compressive yield strength, and Stage B strain hardening rate. The extruded ZM31+3.2Y alloy exhibited a lower hardness and Stage B hardening rate due to the formation of W-phase. Both extruded ZM31+0.3Y and ZM31+3.2Y alloys showed a yield point phenomenon with an initial negative strain hardening rate. The extruded ZM31+6Y alloy had a high hardness and compressive yield strength without Stage B hardening, suggesting a change of major deformation mode from twinning to slip mainly due to the role of LPSO X-phase. After solution treatment and aging, the hardness and compressive yield strength gradually increased with increasing Y content, while the strain hardening exponent and the extent of Stage B strain hardening decreased due to the dissolution of I- and W-phases and the presence of LPSO X-phase.

An experimental investigation of the scratch behaviour of polymers – 2: Influence of hard or soft fillers

Variation in the scratch behaviour of polymers with incorporation of different types and amounts of fillers was investigated both in terms of scratch visibility and scratch hardness. Scratch tests were carried out by a microscratch tester. The effect of fillers on the scratch deformation mode and hence on scratch visibility was evaluated by examining recorded penetration depth and acoustic emission traces together with optical microscope images of the groove left on the surface. Scratch hardness was determined by using a recently proposed scratch model. Adequacy of the model to interpret the local phenomenon induced by addition of fillers was questioned.It was demonstrated that incorporation of hard fillers can enhance the scratch hardness of polymers as well as their basic mechanical properties but leads to an increase in scratch visibility due to a change in scratch deformation mode from ductile ploughing to a brittle failure with crack formation. On the contrary, addition of soft fillers causes a reduction in scratch hardness but it also reduces scratch visibility since it improves the recovery characteristics of the polymer.

A transition from localized shear banding to homogeneous superplastic flow in nanoglass

A promising remedy to the failure of metallic glasses (MGs) by shear banding is the use of a dense network of glass-glass interfaces, i.e., a nanoglass (NG). Here we investigate the effect of grain size (d) on the failure of NG by performing molecular dynamics simulations of tensile-loading on Cu50Zr50 NG with d = 5 to 15 nm. Our results reveal a drastic change in deformation mode from a single shear band (d ∼ 15 to 10 nm), to cooperative shear failure (d ∼ 10 to 5 nm), to homogeneous superplastic flow (d ≤ 5 nm). Our results suggest that grain size can be an effective design parameter to tune the mechanical properties of MGs.

From Quantity To Quality: Massive Molecular Dynamics Simulation of Nanostructures under Plastic Ddeformation in Desktop and Service Grid Distributed Computing Infrastructure

The distributed computing infrastructure (DCI) on the basis of BOINC and EDGeS-bridge technologies for high-performance distributed computing is used for porting the sequential molecular dynamics (MD) application to its parallel version for DCI with Desktop Grids (DGs) and Service Grids (SGs). The actual metrics of the working DG-SG DCI were measured, and the normal distribution of host performances, and signs of log-normal distributions of other characteristics (CPUs, RAM, and HDD per host) were found. The practical feasibility and high efficiency of the MD simulations on the basis of DG-SG DCI were demonstrated during the experiment with the massive MD simulations for the large quantity of aluminum nanocrystals ($\sim10^2$-$10^3$). Statistical analysis (Kolmogorov-Smirnov test, moment analysis, and bootstrapping analysis) of the defect density distribution over the ensemble of nanocrystals had shown that change of plastic deformation mode is followed by the qualitative change of defect density distribution type over ensemble of nanocrystals. Some limitations (fluctuating performance, unpredictable availability of resources, etc.) of the typical DG-SG DCI were outlined, and some advantages (high efficiency, high speedup, and low cost) were demonstrated. Deploying on DG DCI allows to get new scientific $\it{quality}$ from the simulated $\it{quantity}$ of numerous configurations by harnessing sufficient computational power to undertake MD simulations in a wider range of physical parameters (configurations) in a much shorter timeframe.

The influence of surface microstructure on the scratch characteristics of Kevlar fibers

In this work, nanoindentation and nanoscratching experiments are combined with atomic force microscopy to investigate the relationships between contact geometry, apparent friction, and deformation modes of two grades of Kevlar® (Dupont) fiber—Kevlar KM2 and Kevlar 49. Changes in the relative angle between the scratching probe and the fiber surface, often termed as the attack angle, result in changes in deformation mode, which correlate with the changes in the apparent friction. As attack angle increases, the observed deformation modes of the fiber surface change from a smoothing of the surface, often termed as ironing, to fibrillation, in which the fibrils break and coalesce in front of the progressing probe. A mixture of these two modes occurs at intermediate attack angles. When fibrillation occurs, material pile-up forms in front of the progressing probe. This pile-up introduces an additional component to the frictional response that is largely responsible for an increase in apparent friction with an increasing attack angle and/or scratch length. The level of friction associated with fibrillation is measured to be up to approximately three times higher than previously reported for Kevlar yarn–yarn friction. Fibrillation of Kevlar KM2 occurs at larger attack angles as compared to Kevlar 49, which is believed to be related to a near-surface region of reduced modulus and hardness previously observed in KM2 fibers. A detailed discussion of the measured response is given based on the interactions between the scratching probe and the fibrillar network and the resulting deformation mechanisms.

Size effects and ductility of Al-based metallic glass

This paper reports the results of compression tests conducted in situ in a transmission electron microscope of taper-free nanosized pillars of Al86Ni9Y5 metallic glass with diameters ranging between 110 and 900 nm. A critical size was found, below which the deformation mode changes from intermittent shear banding to homogeneous flow. The threshold size for the transition from brittle to ductile behavior is 300 nm in Al86Ni9Y5. Such a high plasticity has not been found previously in metallic glasses of these dimensions. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Stress-state-dependent deformation behavior in Ni–Nb metallic glassy film

The transition in deformation mode from highly localized to non-localized deformation was investigated in Ni60Nb40 glassy film by monitoring the reduction in thickness during film/substrate co-bending. It is revealed that in addition to the film thickness, the mode of plastic deformation depends on the stress state. With the reduction in thickness of thin film, tensile stress can efficiently suppress the change in deformation mode from highly localized to non-localized deformation in comparison with compressive stress. A mechanism for the stress-state-dependent deformation mode change in glassy alloys is discussed on the basis of the pressure/stress effect of plastic deformation and Griffith’s crack-propagation criterion. This study provides distinct evidence of the deformation mode change in metallic glassy film via the variation in stress state, and also sheds light on the deformation mechanism of glassy alloys.

Effect of structural relaxation on plastic flow in a Ni–Nb metallic glassy film

The effect of structural relaxation on the deformation behavior of Ni60Nb40 glassy films was systematically investigated. The film and Ti substrate were co-bent and morphology evolution of the film during bending was monitored. With a decrease in thickness there was a mode change from highly localized to non-localized deformation, with the critical thickness dependent on annealing temperature. During co-bending only a proportion of the stored elastic energy in the metallic glassy film can transfer to the shear band due to the presence of the Ti substrate, which increases with annealing temperature due to the decrease in interfacial adhesion between the film and the substrate. Thus the decrease in critical thickness and the “thickness window” for deformation mode change caused by annealing arises from the enhanced hardness/Young’s modulus and the reduced interfacial adhesion.

Crystal plasticity analysis of constitutive behavior of 5754 aluminum sheet deformed along bi-linear strain paths

Sheet specimens of aluminum alloy 5754 were deformed along a series of bi-linear, equal-biaxial and uniaxial, strain paths while simultaneously measuring stress–strain behavior. The Visco-Plastic Self-Consistent crystal plasticity model, which incorporates texture evolution, was used to simulate the flow stress and hardening behavior based in part on the measured crystallographic texture before deformation. The predicted crystallographic texture qualitatively captures the texture measured in the experiments. Including latent hardening of multiple slip planes allowed the model to explain the decrease in flow stress when changing from equal-biaxial to uniaxial deformation. However, the model captures neither the details of the drop in flow stress nor the magnitude of the plastic hardening after the change in deformation mode. This is likely due to room temperature recovery between the two segments of testing.

Is the compression of tapered micro- and nanopillar samples a legitimate technique for the identification of deformation mode change in metallic glasses?

In situ transmission electron microscopy compression was conducted on amorphous ZrCu nanopillars, 140 nm in diameter and with a taper angle. The aspect ratio of nanopillars is over 6. Experimental observations and simulations both demonstrate that the deformation is localized near the top of the tapered metallic glass pillar, in sharp contrast to pillars of classic Mises plastic solids, which exhibit typical barreling behavior in compression. Localized shear bands appear to be absent, but they form inside the tapered samples.

Mechanisms on Peel Adhesive Strength of Polyethylene/Polypropylene Laminates -Change of Deformation Mode of Bulk Polyethylene Due to the Improvement of the lnterfacial Adhesion-

Mechanisms on Peel Adhesive Strength of Polyethylene/Polypropylene Laminates -Change of Deformation Mode of Bulk Polyethylene Due to the Improvement of the lnterfacial Adhesion-

Prediction of Compressive Strength for Unidirectional Fiber Reinforced Plastics in Considering Effect of Strain-Rate Dependency on Mechanical Properties of Constituent Materials

This study presents an analytical model to predict compressive strength of unidirectional FRP. Proposed model considers the effect of strain-rate dependency on mechanical properties of constituent materials. The model is based on the elastic foundation model and the microbuckling model of fiber which has initial misalignment in matrix. Compressive deformation of unidirectional FRP is considered by dividing into fiber microbuckling region and plastic kinking region. Additionally, to take into consideration the change in compressive deformation mode accompanying fiber volume fraction or fiber microbuckling, A mode function is introduced. The predictions from the proposed model are compared with experimental results of unidirectional E-glass/Epoxy and T700SC/Epoxy evaluated by using the conventional split Hopkinson pressure bar method. Incorporating strain-rate dependency on compressive modulus of reinforcement calculated from composite mixture law, the predictions are found to be in good agreement with experimental results of strain-rate dependency on compressive strength. Accuracy of the prediction is improved by changing the mode function.

Frictional Anisotropy of Oblique Nanocolumn Arrays Grown by Glancing Angle Deposition

Frictional anisotropy of oblique Ti nanocolumns grown by glancing angle deposition (GLAD) was investigated by ramp-scratch experiments in forward (in the direction of the column tilt) and reverse (against the column tilt) directions using rectangular and conical loading tips. Strong anisotropy in friction was generally observed, wherein the resistance or friction coefficient in reverse scratching was higher than that in forward scratching, and the reverse resistance was sensitive to the tip shape. In forward scratching, the nanocolumns bend with the column tilt direction, designated as the W-mode deformation. On the other hand, in reverse scratching, the nanocolumns deflect against the column tilt in high normal force, designated as the A-mode deformation. The deformation mode changes from the W-mode to the A-mode as the normal force increases in reverse scratching. The mechanism of the frictional anisotropy was discussed on the basis of a simple deformation model of column by finite element method. The anisotropy can be explained by the difference in the deformation mode. That is, compressive deformation in the longitudinal direction and large deflection against the column tilt induce higher friction resistance in reverse scratching than in forward scratching brought about by compliant bending deformation with the column tilt.

Improvements in the Superelasticity and Change in Deformation Mode of β-Type TiNb24Zr2 Alloys Caused by Aging Treatments

The superelasticity and deformation behaviors of β-type TiNb24Zr2 subjected to aging treatment were investigated in this study. As the aging time increased, the precipitation of isothermal ω phase was found to restrain the formations of twinning and stress-induced α″ martensite. The superelastic recovery of the sample first improves and then deteriorates with extended aging times. A maximum and stable superelastic recovery of 4.3 pct is obtained for an aging time of 7.2 ks.

Observation of yielding and strain hardening in a titanium alloy having high oxygen content

Plastic deformation behavior and its relation to tensile properties were investigated in an attractive β-type titanium alloy (Ti–29Nb–13Ta–4.6Zr) with the oxygen content of 0.1–0.7 mass% subjected to hot rolling and solution treatment after hot rolling. Hereafter, Ti–29Nb–13Ta–4.6Zr is abbreviated to TNTZ. With the increase of oxygen content, the tensile strength and 0.2% proof stress of all the samples increase, however, their elongation indicates special change, which is contradictory to that reported conventionally. The elongation firstly decreases and then increases with the increase in the oxygen content. Therefore, TNTZ with high strength and high ductility due to the addition of high oxygen content (0.7 mass%) is obtained. Remarkable yielding phenomenon and strain hardening are observed in TNTZ, which can be explained by the interaction between oxygen atoms and a lot of screw and edge dislocations leading to the easy activation of the multiple slip systems. The deformation behavior changes with the addition of oxygen in TNTZ. The plastic deformation mode changes from the deformation-induced martensite transformation to slip mechanism. It is realized that there is a specific compositional area of oxygen in which the TNTZ exhibits strain hardening and high strength, and appropriate Young's modulus value.

Effects of size on the strength and deformation mechanism in Zr-based metallic glasses

We report results of uniaxial compression tests on Zr 35Ti 30Co 6Be 29 metallic glass nano-pillars with diameters ranging from ∼1.6 μm to ∼100 nm. The tested pillars have nearly vertical sidewalls, with the tapering angle lower than ∼1° (diameter >200 nm) or ∼2° (diameter ∼100 nm), and with a flat pillar top to minimize the artifacts due to imperfect geometry. We report that highly-localized-to-homogeneous deformation mode change occurs at 100 nm diameter, without any change in the yield strength. We also find that yield strength depends on size only down to 800 nm, below which it remains at its maximum value of 2.6 GPa. Quantitative Weibull analysis suggests that the increase in strength cannot be solely attributed to the lower probability of having weak flaws in small samples – most likely there is an additional influence of the sample size on the plastic deformation mechanism.