Large Strain Bursts Research Articles

Effect of Microstructure on Oxidation and Micro-mechanical Behavior of Arc Consolidated Mo-Ti-Si-(B) Alloys

AbstractThe present study deals with the development and characterization of Mo-35Ti-10Si and Mo-35Ti-10Si-2B (wt.%) alloy for ultra-high temperature applications beyond the temperature limit of existing super alloys. The microstructural characterization using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), electron back scattered diffraction (EBSD), x-ray diffraction (XRD) revealed that the Mo-35Ti-10Si-2B alloy was consisted of three phases, namely, (Mo, Ti)ss, (Mo, Ti)5SiB2 and (Ti, Mo)5Si3; whereas, Mo-35Ti-10Si alloy was found to be consisting of (Mo, Ti)ss, and (Mo,Ti)3Si phases. Since quantification of boron is difficult by EDS, Particle Induced Gamma-ray Emission (PIGE), a nuclear reaction analysis technique was used for chemical composition analysis of boron. The oxidation behavior of the Mo-35Ti-10Si-2B alloy in the temperature regime of 825-1250 °C was studied in detail and compared with boron-free Mo-35Ti-10Si alloy. Mo-35Ti-10Si-2B alloy exhibited superior oxidation behavior at intermediate temperatures of 825 °C, and excellent oxidation resistance at higher temperatures between 1000 and 1250 °C due to the formation of the protective borosilica and double oxide layers (TiO2 and duplex borosilica-TiO2), respectively. High-temperature oxidation mechanisms were discussed using detailed microstructural cross section analysis of the oxidized alloy samples. The micro-mechanical behavior of constitutive phases of the Mo-35Ti-10Si-2B alloy were studied by microhardness, nano-indentation and micropillar compression testing. The micropillar compression of (Mo, Ti)ss phase showed fairly ductile behavior with the evidence of activation of dislocation in the form of slip lines revealed through the post-deformation fractography. Deformation studies of (Mo, Ti)5SiB2 and (Ti, Mo)5Si3 phases were also carried out which showed large strain bursts indicating possibility of activation of dislocation activities even at room temperatures imparting low level of ductility.

Ultra-Large Compressive Plasticity of E-Ga2O3 Thin Films at the Submicron Scale.

Gallium oxide (Ga2O3) usually fractures in the brittle form, and achieving large plastic deformability to avoid catastrophic failure is in high demand. Here, ε-Ga2O3 thin films with columnar crystals and partial unoccupied Ga sites are synthesized, and it is demonstrated that the ε-Ga2O3 at the submicron scale can be compressed to an ultra-large plastic strain of 48.5% without cracking. The compressive behavior and related mechanisms are investigated by in situ transmission electron microscope nanomechanical testing combined with atomic-resolution characterizations. The serrated plastic flow and large strain burst are two major deformation forms of ε-Ga2O3 during compression, which are attributed to the dislocation nucleation and avalanches, formation of new grains, and amorphization. The ultra-large compressive plasticity of ε-Ga2O3 thin films at the submicron scale can inspire new applications of Ga2O3 in micro- or nano- electronic and optoelectronic devices, especially those that require impact resistance during processing or service.

Plasticity of the Nb-rich μ-Co7Nb6 phase at room temperature and 600°C

The µ phase is a common precipitation phase in superalloys and it exists in a wide composition and temperature range. As such, we study the influences of composition and temperature on its plasticity by micropillar compression tests and transmission electron microscopy. The micropillars of the μ-Co7Nb6 phase deform plastically by basal slip at room temperature and 600 °C. At room temperature, the Co-49Nb and Co-52Nb micropillars show high yield stresses and an abrupt large strain burst at the onset of yielding regardless of orientation, whereas the Co-54Nb micropillars oriented for basal slip yield at much lower stresses and show intermittent small strain bursts during plastic deformation. While the Co-49Nb micropillars deform by full dislocation slip on the basal plane at room temperature, the Co-54Nb micropillars deform by partial dislocation slip on the basal plane. At 600 °C, the Co-49Nb micropillars oriented for basal slip show stable and continuous plasticity and their critical resolved shear stresses decrease dramatically. In contrast to the full dislocation slip at room temperature, the plastic deformation of the Co-49Nb micropillars occurs via partial dislocation slip on the basal plane at 600 °C. Based on the geometric γ-surfaces for all potential basal slip planes, we explore where and why the glide of full and partial dislocations on the basal plane occurs in the μ-Co7Nb6 phase.

Effect of solute content and temperature on the deformation mechanisms and critical resolved shear stress in Mg-Al and Mg-Zn alloys

The influence of solute atoms (Al and Zn) on the deformation mechanisms and the critical resolved shear stress for basal slip in Mg alloys at 298 K and 373 K was ascertained by micropillar compression tests in combination with high-throughput processing techniques based on the diffusion couples. It was found that the presence of solute atoms enhances the size effect at 298 K as well as the localization of deformation in slip bands, which is associated with large strain bursts in the resolved shear stress (τRSS)-strain (ε) curves. Deformation in pure Mg and Mg alloys was more homogeneous at 373 K and the influence of the micropillar size on the critical resolved shear stress was much smaller. In this latter case, it was possible to determine the effect of solute content on the critical resolved shear stress for basal slip in Mg-Al and Mg-Zn alloys.

Grain refinement and amorphization in nanocrystalline NiTi micropillars under uniaxial compression

Microscale plastic deformation mechanisms of nanocrystalline NiTi micropillars with an average grain size (GS) of 65 nm are investigated by uniaxial compression. Cuboidal NiTi micropillars with uniform cross-sectional area along the height are manufactured by focused ion beam. It is found that the plastic deformation of the micropillars under compression is realized by localized shear band formation with large strain bursts, leading to significant amorphization and GS reduction from 65 nm to as small as 11 nm in the shear band. The experiment indicates that microfabrication by severe plastic deformation could be a promising manufacturing technology.

Interpreting strain burst in micropillar compression through instability of loading system

Understanding the mechanism of intermittent plastic flow in single crystal at micron to nano scale is vitally important to the fields where the reliability of system is paramount. Jumps in the stress-strain curves (strain bursts) are usually interpreted from the perspective of internal processes of material such as dislocation avalanches or intermittent activation of dislocation sources. However, the responses obtained by the sensors are also severely affected by the loading system, and this effect is often neglected in understanding the essence of strain burst. Thus, after analyzing the detailed deformation process of the loading system and the pillar during the period of strain burst, the theoretical model with the loading system that is assumed to be linearly elastic and the pillar connected in series is established. Also, the constitutive relations of micro-pillars are obtained by molecular dynamics (MD) in which the plastic flow stresses fluctuate. Based on the model, the critical condition of the instability of loading system is derived. It is shown that strain burst is the response obtained by the sensors during the period of instability of loading system caused by strain softening of the pillar and whether or not strain burst occurs depends on the degree of strain softening of pillar and the ratio of initial stiffness of the pillar to the effective stiffness of the machine. The size of strain burst is also predicted under the assumption that the strain softening stage continues to the point where elastic strain energy released by the loading system is equal to energy absorbed by the pillar and then this softening stage immediately turns into a linear elastic stage. In addition, the mechanism behind large strain burst is also revealed.

Micro-mechanical characterization of micro-architectured tungsten coatings

The current work provides a detailed analysis of the microstructure, mechanical properties, and out-of-plane compression failure modes of tungsten micro-architectured coatings. These coatings are candidates for extremely high temperature applications and for plasma facing components. The current analysis reveals that these coating are multi-layered coatings composed of columnar grains with many pre-existing voids/cracks along grain boundaries. By means of ex situ and in situ microcompression experiments of microcrystals fabricated into these coatings, the mechanical response and possible failure modes have been investigated. These experiments show a size-dependent response in both the flow stress and deformation modes. Smaller microcrystals show a higher strength and large strain burst followed by catastrophic failure dominated by intergranular fracture assisted plastic buckling of the limited number of grains that comprise the microcrystal. Larger microcrystals also show intergranular fracture, however, a continuous hardening response is driven by restricting the lateral movement of the columnar grains by other grains in the crystal. In addition, three-dimensional finite element method simulations were performed to further shed light on the influences of microstructural configurations on the post-yielding behavior, as well as providing a predictive tool of the deformation of these coatings. This work underscores the importance of microstructural design and control of the grain aspect ratio and grain boundary cohesive strength in an effort to extend the life of such micro-architectured coatings.

Micro-plasticity of medium Mn austenitic steel: Perfect dislocation plasticity and deformation twinning

The micro-scale plastic deformation behavior of an austenitic Fe-1.2%C-7.0%Mn (in wt%) steel was studied by means of nano-indentation and in situ compression of micro-pillars with selected crystallographic orientations. Transmission electron microscopy analysis reveals that the partial dislocation mediated twinning is preferred in a [001]-oriented single grain under compression. Twinning leads to large strain bursts during the nano-indentation and the micro-pillar compression. Perfect dislocation slip is the dominant deformation mechanism in a [111]-oriented single grain under compression. The observations offer strong support for the hypothesis that deformation twinning is a plasticity enhancing mechanism activated during the deformation of medium Mn steel.

Large strain burst induced by martensitic transformation in austenitic micropillars

The present work investigates, for the first time, the effects of martensitic transformation as well as orientation on the stress-strain behaviour of austenitic micropillars. It is found that large strain burst can be induced by martensitic transformation, and its magnitude depends on the orientation of pillars. Pillars with an orientation close to [1 0 0] show a much larger strain burst than the ones with an orientation close to [1 1 0]. This is because [1 0 0] pillars have a much higher fraction of transformed martensite to which the magnitude of strain burst is directly correlated.

Strongly correlated breeding of high-speed dislocations

Under very high stresses, dislocations can be accelerated to approach the speed of shear wave over a distance as short as 101 nm. Our atomistic simulations demonstrate that dislocations with such high speeds often react in counter-intuitive manners that are beyond textbook descriptions of conventional dislocation behavior. A high-speed dislocation can “rebound” when hitting a free surface rather than simply annihilate. When two high-speed dislocations collide, they can “penetrate” through each other. An individual dislocation can even spontaneously generate multiple dislocations via self-dissociation. These anomalous mechanisms lead to rapid proliferation of dislocations that are strongly correlated both spatially and temporally, and as such may play a role in high-stress and high-strain-rate plastic deformation; a potentially related case is nanoscale pristine single crystals, which often yield via a large strain burst at ultrahigh stresses.

Evaluation of critical resolved shear strength and deformation mode in proton-irradiated austenitic stainless steel using micro-compression tests

Micro-compression tests were applied to evaluate the changes in the strength and deformation mode of proton-irradiated commercial austenitic stainless steel. Proton irradiation generated small dots at low dose levels and Frank loops at high dose levels. The increase in critical resolved shear stresses (CRSS) was measured from micro-compression of pillars and the Schmid factor calculated from the measured loading direction. The magnitudes of the CRSS increase were in good agreement with the values calculated from the barrier hardening model using the measured size and density of radiation defects. The deformation mode changed upon increasing the irradiation dose level. At a low radiation dose level, work hardening and smooth flow behavior were observed. Increasing the dose level resulted in the flow behavior changing to a distinct heterogeneous flow, yielding a few large strain bursts in the stress–strain curves. The change in the deformation mode was related to the formation and propagation of defect-free slip bands. The effect of the orientation of the pillar or loading direction on the strengths is discussed.

Dislocation-nucleation-controlled deformation of Ni3Al nanocubes in molecular dynamics simulations

The strength of Ni3Al nanocubes under compression, as well as the underlying dislocation mechanisms, are analysed in molecular dynamics simulations. For this purpose, the bond-order parameters analysis is extended to multi-atomic systems in order to identify the intrinsic planar defects in Ni3Al. We benchmarked different interatomic potentials and compared the results with experimental ones. The different potentials resulted in different elastic responses under compression but all yielded abruptly at a compressive stress of about 7–8 GPa, followed by a large strain burst. The nanocubes yielded by nucleating Shockley partial dislocations at the vertices on planes, leaving a structure of faulted planes. The mechanical response was found to be size independent, which we attribute to the cubical shape of the nanoparticle and the lack of stress gradients at its vertices.

Mechanical Responses of Copper Bicrystalline Micro Pillars with Σ3 Coherent Twin Boundaries by Uniaxial Compression Tests

Two types of bicrystalline micro pillars (BCMs) involving ­3 coherent twin boundaries along the pillar axis and single-crystalline micro pillars of two grains constituting the BCMs were fabricated in an oxygen-free copper sample by focused ion beam milling. These pillars were compressed using a nanoindenter to investigate slip transfer across grain boundary (GB). Referring their stress­strain (SS) curves, the possible slip systems were resolved from scanning electron microscope images after deformation and GB interaction criteria with the three largest Schmid factors (SFs) of each grain and the geometric relationship of intersections between the slip planes and the GB. The quite different SS behaviors were observed in the two types of BCMs, of which one was the unstable elastic-almost perfect plastic deformation due to large strain bursts and the other was work-hardening accompanying several small strain bursts. The slip transfer analyses suggest that the slip directions of both grains with the maximum SF are almost parallel to GB plane to the former behavior, and the combination of three slip systems from both grains produces the terraced hardening with repeated slipping across GB and piling-up against GB to the latter. [doi:10.2320/matertrans.MA201312]

Average yielding and weakest link statistics in micron-scale plasticity

Micron-scale single-crystalline materials deform plastically via large intermittent strain bursts that make the deformation process non-deterministic. Here we investigate this stochastic phenomenon by analyzing the plastic response of an ensemble of specimens differing only in the initial arrangement of dislocations. We apply discrete dislocation dynamics simulations and microcompression tests on identically fabricated Cu single-crystalline micropillars. We find that a characteristic yield stress can be defined in the average sense for a given specimen ensemble, where the average and the variance of the plastic strain start to increase considerably. In addition, in all studied cases the stress values at a given strain follow a Weibull distribution with similar Weibull exponents, which suggests that dislocation-mediated plastic yielding is characterized by an underlying weakest-link phenomenon. These results are found not to depend on fine details of the actual set-up; rather, they represent general features of micron-scale plasticity.

Dislocation Multiplication by Single Cross Slip for FCC at Submicron Scales

The operation mechanism of single cross slip multiplication (SCSM) is investigated by studying the response of one dislocation loop expanding in face-centered-cubic (FCC) single crystal using three-dimensional discrete dislocation dynamic (3D-DDD) simulation. The results show that SCSM can trigger highly correlated dislocation generation in a short time, which may shed some light on understanding the large strain burst observed experimentally. Furthermore, we find that there is a critical stress and material size for the operation of SCSM, which agrees with that required to trigger large strain burst in the compression tests of FCC micropillars.

Sample size effects on the large strain bursts in submicron aluminum pillars

In situ transmission electron microscope compression testing of submicron Al pillars shows two sample size regimes with contrasting behavior underlying the large strain bursts. For small pillars, the bursts originate from explosive and highly correlated dislocation generation, characterized by very high collapse stresses and nearly dislocation-free post-collapse microstructure. For larger pillars, the bursts result from the reconstruction of jammed dislocation configurations, featuring relative low stress levels and retention of dislocation network after bursts.

In-situ characterization of the dislocation-structure evolution in Ni micro-pillars

The high strength of micro-crystals is determined in the early flow regime, where a transition from elastic to plastic flow is obscured if compared to stress–strain data from bulk single crystals. In the present work we therefore focus on the evolution of dislocation structures in Ni micro-pillars during early deformation by employing in-situ Laue micro-diffraction. It will be shown that substantial changes in the lattice fine structure, such as multiple subgrain formation and significant rotational gradients, can be resolved prior to the onset of large strain generation. The results reveal more pronounced effects for smaller sample dimensions and also suggest that most of the evolving dislocation structure is formed prior to the occurrence of large strain bursts. A clear increase in dislocation density as a function of strain is observed, which we discuss in the context of size-dependent strain hardening and exhaustion hardening.

Size effect in compression of single-crystal gold microparticles

Single-crystal Au microparticles on a sapphire substrate were deformed under compression. Most of microparticles yield with a large strain burst, and there is a strong dependence of the yield strength on microparticle size. With the help of molecular dynamics simulations and finite-element analysis we conclude that the deformation is dislocation nucleation-controlled and that the stress levels reached at the onset of plasticity approach the theoretical shear strengths of Au. A significant size effect is identified in both the experimentally measured and computed strength of the microparticles; the smaller microparticles yield at higher compressive stresses. We propose a stress-gradient nucleation model relating this size effect to stress gradients along the slip plane.

Deformation behavior of silver submicrometer-pillars prepared by nanoimprinting

Scaling in material properties is of great importance in microsystems and microelectronics where the device dimensions continuously shrink. Recently, compression tests of micron-sized pillars produced using a focused-ion beam (FIB) have become standard in the investigation of scaling effects. The influence of the fabrication process on the mechanical properties of the samples has, however, not been conclusively resolved. In this study, 130 silver pillars were fabricated using a novel embossing technique that does not pose the issues associated with FIB milling, i.e. surface amorphization and gallium contamination. Displacement-controlled compression tests on pillars with diameters of 130–3000 nm reveal for submicrometer samples an inverse proportionality of flow strength to diameter, which is associated with a wide strength distribution and a deformation taking place in large discrete strain bursts. The largest pillars show instead near bulk-like behavior. Unlike studies on other fcc materials, mechanical twinning is also an important deformation mechanism in Ag due to its very low stacking fault energy. In addition, occasional preexisting twin boundaries constituted preferential planes for localized deformation if favorably sloped. Despite the absence of ion-induced artifacts, scaling laws and absolute values of strength are comparable to those obtained via FIB milling.

Twining and Slip Activity in Magnesium <11-20> Single Crystal

AbstractUniaxial μ-compression tests have been performed on single crystal Mg with a <11-20> compression direction, an orientation unfavorable for basal slip. Results show that the early stages of deformation proceed via both twinning and dislocation plasticity. Twinning leads to a reorientation of the crystal favorable for basal slip, typically with the <2-1-1-3> aligned with the compression direction. At a critical strain a large strain burst occurs, and is associated with both rapid propagation of the twin and the activation of basal slip within the twin. Such a mechanistic picture of the deformation behavior is revealed through SEM, EBSD and TEM characterization of the deformation structures.