Amorphous Shear Bands Research Articles

Increase in interplanar distances and formation of amorphous shear bands in deformed diamond

Amorphous transformation-deformation bands have been observed in diamond under cyclic loading (at planetary ball milling) near the graphite-diamond equilibrium line at temperatures below the Debye temperature. The maximum stresses and temperature in the diamond do not exceed 6 GPa and 420 K, respectively. These bands add to the set of plastic deformation mechanisms in diamond. TEM methods and conventional X-rays were used to investigate the mechanisms of plastic deformation of diamond. The observed mechanisms include amorphous deformation bands, bond breaking, twin formation, and phase transitions of diamond-lonsdaleite and diamond-onions. The increased interplanar distances provide evidence of the plastic deformation mechanism due to bond breaking. The distances of 0.222 and 0.255 nm are significantly larger than the initial value of 0.206 nm, which is characteristic of d111 diamond. This increase in distance cannot be attributed to lattice expansion caused by point defects, but rather to the breakage of interatomic bonds.

Amorphous shear bands in crystalline materials as drivers of plasticity.

Traditionally, the formation of amorphous shear bands in crystalline materials has been undesirable, because shear bands can nucleate voids and act as precursors to fracture. They also form as a final stage of accumulated damage. Only recently were shear bands found to form in undefected crystals, where they serve as the primary driver of plasticity without nucleating voids. Here we have discovered trends in materials properties that determine when amorphous shear bands will form and whether they will drive plasticity or lead to fracture. We have identified the materials systems that exhibit shear-band deformation, and by varying the composition, we were able to switch from ductile to brittle behaviour. Our findings are based on a combination of experimental characterization and atomistic simulations, and they provide a potential strategy for increasing the toughness of nominally brittle materials.

An atomistic study of plastic deformation of SmCo5 by amorphous shear bands

Recently, SmCo5 was found to be capable of forming amorphous shear bands to induce dislocation-free plastic deformation at large strains. The formation of shear band is energetically more favorable compared to cracking and the small density variation in shear bands is not likely to induce high local stress that assists crack opening. However, the related mechanism of crystalline-to-amorphous transition during shear-band formation at the atomic level remains unclear. In this paper, the shear deformation of SmCo5 is investigated by molecular dynamics simulations and we discuss the behavior of shear-band formation by exploring the atomic packing in the interfacial zone between crystal matrix and shear band. The stress-strain curves of SmCo5 show anisotropy with respect to formation of amorphous shear band. The analysis of atomic packing indicates that the behavior of Sm is critical to the atomistic amorphization path and thus accounts for the mechanical anisotropy of the material. Temperature calculations show that the material is not subject to shear melting during deformation that would otherwise lead to embrittlement.

Mechanical properties of samarium cobalt: A molecular dynamics study

Sm-Co permanent magnets are usually very brittle which makes them vulnerable to even mild impacts. Recently, it was reported by simulation and confirmed by experiment that SmCo5 can be plastically deformed via amorphous shear bands activated by external stress. Since most Sm-Co magnets mainly consist of the Sm2Co17 phase, it is necessary to study how the Sm2Co17 phase can be deformed compared with the SmCo5 phase (existing also in Sm-Co magnets). In addition, the mechanical properties of the two-phase composites with an interface should be investigated. In this paper, we show that the Sm2Co17 phase can also be plastically deformed under tensile loading due to the formation of amorphous shear bands. We find that the interfacial stress between SmCo5 and Sm2Co17 significantly affects the formation of amorphous shear bands. The formation energies of amorphous shear bands are lower than the energies needed for transgranular, intergranular and interphase cleavage. The two-phase samples notched at the interface are also deformed by tensile loading. When the amorphous shear band is initiated in the Sm2Co17 phase, the sample quickly necks toward failure. In contrast, when amorphous shear band is initiated in the SmCo5 phase, the sample can accommodate more strain. These results help us understand the intrinsic mechanical properties of Sm-Co magnets.

Shear band formation during nanoindentation of EuB6 rare-earth hexaboride

Research on rare-earth hexaborides mainly focuses on tuning their electronic structure from insulating-to-metallic states during high pressure experiments. However, the structural evolution that contributes to their mechanical failure is not well understood. Here, we examine the pressure-induced structural evolution of a model rare-earth hexaboride, EuB6, during nanoindentation. Transmission electron microscopy reveals that nanoscale amorphous shear bands, mediated by dislocations, play a decisive role in deformation failure. Density functional theory calculations confirm that amorphous bands evolve by breaking boron-boron bonds within B6 octahedra during shear deformation. Our results underscore an important damage mechanism in hard and fragile hexaborides at high shear pressures.

High-entropy metal carbide nanowires

The development of high-entropy ceramic nanomaterials has significant scientific and technological potential, yet studies on these materials are rare. Here, we successfully synthesize (Hf0.25Ta0.25Nb0.25Ti0.25)C high-entropy metal carbide (HEC-1) nanowires—a class of high-entropy ceramic nanomaterials—via a facile bamboo-based carbothermal method with an Fe-Ni catalyst. The growth of HEC-1 nanowires occurs through a classical vapor-liquid-solid mechanism based on the solubility of metal, carbon, and HEC-1 in the Fe-Ni alloy. After high-temperature treatment, HEC-1 nanowires show good thermal stability without morphological evolution below 1,600°C, while they evolve into “pearl-necklace”’-like nanostructures or particles above 1,600°C due to the Rayleigh instability mechanism. After high-pressure treatments, HEC-1 nanowires are broken into nanorods above 1 GPa, in which brittle fracture without any dislocations, slip bands, or amorphous shear bands is directly observed at nano and atomic scales.

Mitigating the formation of amorphous shear band in boron carbide

Boron carbide is super-strong and has many important engineering applications such as body armor and cutting tools. However, the extended applications of boron carbide have been limited by its low fracture toughness arising from anomalous brittle failure when subjected to hypervelocity impact or under high pressure. This abnormal brittle failure is directly related to the formation of a tiny amorphous shear band of 2–3 nm in width and several hundred nm in length. In this Perspective, we discuss mitigating the amorphous shear bands in boron carbide from various strategies including microalloying, grain boundary engineering, stoichiometry control, and the addition of a second phase. Combined with recent theoretical and experimental studies, we discuss strategies that can be applied in synthesizing and producing boron carbide-based materials with improved ductility by suppressing the formation of the amorphous shear band.

Amorphous material in experimentally deformed mafic rock and its temperature dependence: Implications for fault rheology during aseismic creep and seismic rupture

Amorphous materials are frequently observed in natural and experimentally produced fault rocks. Their common occurrence suggests that amorphous materials are of importance to fault zone dynamics. However, little is known about the physico-chemical impact of amorphous materials on fault rheology. Here we present deformation experiments on mafic fault rock, where amorphous material forms due to intense mechanical wear during the experiments. The experiments are run at temperatures from 300 to 600 °C, confining pressures of 0.5 or 1.0 GPa, and at constant displacement rates of (d˙ax) 2 ·10−7, 2 ·10−8 or 2 ·10−9 ms−1, resulting in bulk strain rates (γ˙) of ≈3 ·10−4, 3 ·10−5 and 3 ·10−6 s−1. At these conditions, the mafic rock material undergoes intense brittle deformation and cataclastic flow, but sample strength significantly decreases with increasing temperatures – a feature commonly attributed to viscous deformation processes. Microstructural analyses show that after an initial stage of homogeneous cataclastic flow, strain localizes into narrow (2–10 μm wide) ultra-cataclastic bands that evolve into amorphous shear bands. With the data presented in this research paper, we argue that the temperature sensitivity recorded in the mechanical data is caused by viscous deformation of the amorphous material. We suggest that with the formation of amorphous materials during brittle deformation, fault rheology becomes significantly temperature-sensitive. This has important implications for our understanding of fault strength and weakening due to the presence of amorphous materials. In addition, weak material along faults will lead to stress concentrations that may trigger seismic rupture.

Amorphization and Plasticity of Olivine During Low‐Temperature Micropillar Deformation Experiments

AbstractExperimentally quantifying the viscoplastic rheology of olivine at the high stresses and low temperatures of the shallow lithosphere is challenging due to olivine's propensity to deform by brittle mechanisms at these conditions. In this study, we use microscale uniaxial compression tests to investigate the rheology of an olivine single crystal at room pressure and temperature. Pillars with nominal diameters of 1.25 μm were prepared using a focused ion beam milling technique and were subjected to sustained axial stresses of several gigapascal. The majority of the pillars failed after dwell times ranging from several seconds to a few hours. However, several pillars exhibited clear evidence of plastic deformation without failure after 4–8 hr under load. The corresponding creep strain rates are estimated to be on the order of 10−6 to 10−7 s−1. The uniaxial stresses required to achieve this deformation (4.1–4.4 GPa) are in excellent agreement with complementary data obtained using nanoindentation techniques. Scanning transmission electron microscopy observations indicate that deformation occurred along amorphous shear bands within the deformed pillars. Electron energy loss spectroscopy measurements revealed that the bands are enriched in Fe and depleted in Mg. We propose that inhomogeneities in the cation distribution in olivine concentrate stress and promote the amorphization of the Fe‐rich regions. The time dependence of catastrophic failure events suggests that the amorphous bands must grow to some critical length scale to generate an unstable defect, such as a shear crack.

Amorphous shear bands in SmCo5

Formation of thin amorphous shear bands has been recently shown to enable plastic flow in crystalline SmCo5 in the absence of dislocations. To bring insights into the criteria for when this mechanism can be active, here we analyze energetics and atomic structure of deformation-induced shear bands in SmCo5. We find that the formation energy of amorphous shear bands is significantly lower than cleavage energies on different crystallographic planes. The density of the atoms in the shear bands decreases by less than 2% due to the amorphization, which indicates a very small dilation. Analysis of local ordering shows that the amorphous shear bands are more disordered than a melt-quenched sample and the overall fraction of icosahedra is much lower than usually reported in metallic glasses, such as Cu–Zr. The influence of amorphous shear bands on the evolution of grain orientations is also discussed.

Improved Ductility of B12 Icosahedra-based Superhard Materials through Icosahedral Slip

Boron carbide (B4C) is superhard but suffers from brittle failure because shear stress leads to the formation of amorphous shear bands in which the icosahedral clusters fracture, leading to amorphous regions with higher density than the crystal that results in tension-induced cavitation and brittle failure. On the basis of our previous studies of related systems, we speculated that replacing the C–B–C chains with Si2 and replacing cage C with Si might reduce or eliminate this amorphous shear band formation responsible for brittle failure. We consider (B10Si2)Si2, using density functional theory to examine its shear deformation. We find that the stress accumulated as shear increases is released by slip of the planes of icosahedra through breaking and then reforming the Si–Si chain bonds without fracturing (B10Si2) icosahedra. This is because the (B10Si2) icosahedra are more stable than the chain under highly stressed conditions. This chain disruption deformation mode prevents amorphous shear band formation...

Nucleation of amorphous shear bands at nanotwins in boron suboxide.

The roles of grain boundaries and twin boundaries in mechanical properties are well understood for metals and alloys. However, for covalent solids, their roles in deformation response to applied stress are not established. Here we characterize the nanotwins in boron suboxide (B6O) with twin boundaries along the planes using both scanning transmission electron microscopy and quantum mechanics. Then, we use quantum mechanics to determine the deformation mechanism for perfect and twinned B6O crystals for both pure shear and biaxial shear deformations. Quantum mechanics suggests that amorphous bands nucleate preferentially at the twin boundaries in B6O because the twinned structure has a lower maximum shear strength by 7.5% compared with perfect structure. These results, which are supported by experimental observations of the coordinated existence of nanotwins and amorphous shear bands in B6O, provide a plausible atomistic explanation for the influence of nanotwins on the deformation behaviour of superhard ceramics.

Effects of Thermo-mechanical Processing on the Mechanical Properties and Shape Recovery of the Nanostructured Ti50Ni45Cu5 Shape Memory Alloy

The main purpose of this research is to evaluate effects of the thermo-mechanical treatment of cold rolling followed by annealing on microstructure and shape recovery of the nanostructured shape memory Ti50Ni45Cu5 alloy. Several samples were produced by the copper-boat vacuum induction melting technique. The as-solidified samples were homogenized and hot rolled at 900̊C. The hot rolled samples were then cold rolled up to 70% thickness reduction followed by annealing at 400 ̊C for 1h. The microstructures and critical temperatures of the alloys were characterized by scanning electron microscopy, X-ray diffraction and differential scanning calorimetry, respectively. According to the results, the average grain size was decreased by increasing the thickness reduction. Microstructure of the sample with 70% thickness reduction had a bimodal size distribution of Nano-sized austenite grains within the amorphous matrix. The dislocation density was increased up to 1013mm−2 in the specimen with 70% reduction. During the annealing process, the softening processes and crystallization of the amorphous shear bands occurred together, resulting in a nanostructured austenitic microstructure. The crystallization temperature was about 330̊C. The shape recovery percentage was increased by cold rolling.

Non-localized deformation in metallic alloys with amorphous structure

In bulk or ribbon metallic alloys with monolithic amorphous structure, plastic deformation at room temperature is dominated by highly localized shear banding. Here we report suppressed shear localization under tension in a metallic glass (MG) embedded with pre-existing amorphous shear bands using large-scale atomistic simulations. It is demonstrated that pre-deformation lowers material strength and triggers enhanced strain fluctuation before sample yielding, leading to highly dispersed plastic shearing in the entire sample. Furthermore, the transition in deformation mode from highly localized shear banding to non-localized plastic deformation is associated with the competition between the yield strength of the material and the critical stress required for the formation of mature shear bands in the load-bearing materials. Given that a sufficient amount of glassy heterogeneities can be introduced into MGs under experimental conditions, the obtained results could provide a promising strategy for designing tensile ductile MGs with pure amorphous structure at room temperature.

Shear amorphization of boron suboxide

We report for the first time the shear-induced local amorphization of boron suboxide subjected to nanoindentation. The amorphous bands have a width of ∼1–3 nm and a length of 200–300 nm along the ( 0 1 ¯ 1 1 ) crystal plane. We show direct experimental evidence that the amorphous shear bands of boron suboxide are driven from the coalescence of dislocation loops under high shear stresses. These observations provide insights into the microscopic deformation and failure of high-strength and lightweight ceramics.

Atomic structure of amorphous shear bands in boron carbide

Amorphous shear bands are the main deformation and failure mode of super-hard boron carbide subjected to shock loading and high pressures at room temperature. Nevertheless, the formation mechanisms of the amorphous shear bands remain a long-standing scientific curiosity mainly because of the lack of experimental structure information of the disordered shear bands, comprising light elements of carbon and boron only. Here we report the atomic structure of the amorphous shear bands in boron carbide characterized by state-of-the-art aberration-corrected transmission electron microscopy. Distorted icosahedra, displaced from the crystalline matrix, were observed in nano-sized amorphous bands that produce dislocation-like local shear strains. These experimental results provide direct experimental evidence that the formation of amorphous shear bands in boron carbide results from the disassembly of the icosahedra, driven by shear stresses.

Experimental and Numerical Atomistic Investigation of the Third Body Formation Process in Dry Tungsten/Tungsten-Carbide Tribo Couples

The third body in tungsten/tungsten-carbide sliding systems is studied using a combination of experiments and atomistic simulations. Ex situ X-ray photoelectron spectroscopy and focused ion beam analysis of the structural and chemical changes near the surfaces reveals that sliding of tungsten against tungsten-carbide results in plastic deformation of the W surface, leading to grain refinement, and the formation of a mechanically mixed amorphous layer on the WC counter body. Molecular dynamics simulations of W/WC sliding couples exhibit the formation of a nanoscale amorphous W/WC interface. The infrequent occurrence of atomic jamming events in the interface resulted in the emission of dislocations into the W bulk and the generation of amorphous shear bands in the WC counter body in agreement with the different third bodies observed in W and WC after the experiments.

Shock induced amorphization as the onset of spall

Spall formation in the glass-forming alloy Cu–Ti was studied via molecular dynamics simulations. It is shown that spall initiation is a combined process where void nucleation is accompanied by local amorphization. The amorphous regions nucleate at the surfaces of the voids at a critical stress and then grow, allowing the voids to grow faster in the mechanically less stable amorphous region. Dislocations are emitted from the amorphous regions and form shear bands between the amorphous regions. Subspall events result in the formation of a damaged layer, including voids, amorphous regions, and shear bands. The simulations are consistent with recent experimental observation of intergranular amorphous bands in shocked boron carbide.

TEM of nanostructured metals and alloys

Nanostructuring has been used to improve the mechanical properties of bulk metals and alloys. Transmission electron microscopy (TEM) including atomic resolution is therefore appropriate to study these nanostructures; four examples are given as follows. (1) The early stages of precipitation at RT were investigated in an Al–Mg–Si alloy. By high resolution TEM it is shown that the precipitates lie on (0 0 1) planes having an ordered structure. (2) In Co alloys the fronts of martensitic phase transformations were analysed showing that the transformation strains are very small thus causing no surface relief. (3) Re-ordering and recrystallization were studied by in situ TEM of an Ni 3 Al alloy being nanocrystalline after severe plastic deformation. (4) In NiTi severe plastic deformation is leading to the formation of amorphous shear bands. From the TEM analysis it is concluded that the amorphization is caused by plastic shear instability starting in the shear bands.

TEM of nanostructured metals and alloys

Nanostructuring has been used to improve the mechanical properties of bulk metals and alloys. Transmission electron microscopy (TEM) including atomic resolution is therefore appropriate to study these nanostructures; four examples are given as follows. (1) The early stages of precipitation at RT were investigated in an Al–Mg–Si alloy. By high resolution TEM it is shown that the precipitates lie on (0 0 1) planes having an ordered structure. (2) In Co alloys the fronts of martensitic phase transformations were analysed showing that the transformation strains are very small thus causing no surface relief. (3) Re-ordering and recrystallization were studied by in situ TEM of an Ni3Al alloy being nanocrystalline after severe plastic deformation. (4) In NiTi severe plastic deformation is leading to the formation of amorphous shear bands. From the TEM analysis it is concluded that the amorphization is caused by plastic shear instability starting in the shear bands.