Multiple Pop-ins Research Articles

Integrated experimental and computational study on the effect of hydrogen in mechanical responses of pure tungsten

Hydrogen irradiation profoundly changes the microstructure and morphological characteristics of tungsten, ultimately resulting in the modifications to its mechanical properties. Unfortunately, the specific mechanisms through which hydrogen irradiation influences the mechanical behavior of tungsten have not been clearly elucidated. Therefore, this study aims to further improve the understanding of the impact of hydrogen irradiation on the mechanical responses and its microstructure dependence in tungsten by employing two advanced techniques: nano-indentation experiments and Molecular dynamics (MD) simulations. The experimental findings revealed that deuterium (D) exposure decreased the maximum shear stress required for the incipient plasticity and led to the occurrence of multiple pop-ins. Particularly noteworthy was the most remarkable change in mechanical behaviors observed prominently in the recrystallized tungsten after D exposure. The microstructure dependence of irradiation-induced change in mechanical responses was investigated by two series of MD simulation, indentation and dislocation glide simulation, focusing on the independent effects of each irradiation-induced defect type (interstitial, substitutional, vacancy, vacancy-hydrogen (Vac-H) cluster, void-hydrogen (Vo-H) cluster) on dislocation activities. Based on the MD simulations, it was eventually revealed that Vo-H clusters were the most influential agent affecting dislocation activities. They not only lowered the pop-in stress by heterogeneous dislocation nucleation but also caused strain serration by a continuous pinning/unpinning process during dislocation glide. Consequently, we concluded that the dominant presence of d-induced Vo-H clusters facilitated the onset of plastic yielding and led to multiple pop-in phenomena in recrystallized tungsten, whereas inserted hydrogen caused minimal change in mechanical responses by interacting with pre-existing dislocation in cold-rolled tungsten.

Cleavage fracture micromechanisms in simulated heat affected zones of S690 high strength steels

High strength steels are widely used for structural applications, where a combination of excellent strength and ductile-to-brittle transition (DBT) properties are required. However, such a combination of high strength and toughness can be deteriorated in the heat affected zone (HAZ) after welding. This work aims to develop a relationship between microstructure and cleavage fracture in the most brittle areas of welded S690 high strength structures: coarse-grained and intercritically reheated coarse-grained HAZ (CGHAZ and ICCGHAZ). Gleeble thermal simulations were performed to generate three microstructures: CGHAZ and ICCGHAZ at 750 and 800 °C intercritical peak temperatures. Their microstructures were characterised, and the tensile and fracture properties were investigated at − 40 °C, where cleavage is dominant. Results show that despite the larger area fraction of martensite-austenite (M-A) constituents in ICCGHAZ 750 °C, the CGHAZ is the zone with the lowest fracture toughness. Although M-A constituents are responsible for triggering fracture, their small size (less than 1 μm) results in local stress that is insufficient for fracture. Crack propagation is found to be the crucial fracture step. Consequently, the harder auto-tempered matrix of CGHAZ leads to the lowest fracture toughness. The main crack propagates transgranularly, along {100} and {110} planes, and neither the necklace structure at prior austenite grain boundaries of ICCGHAZs nor M-A constituents are observed as preferential sites for crack growth. The fracture profile shows that prior austenite grain boundaries and other high-angle grain boundaries (e.g., packet and block) with different neighbouring Bain axes can effectively divert the cleavage crack. Moreover, M − A constituents with internal sub-structures, which have high kernel average misorientation and high-angle boundaries, are observed to deflect and arrest the secondary cracks. As a result, multiple pop-ins in load-displacement curves during bending tests are observed for the investigated HAZs.

A systematic study on microstructure evolution, mechanical stability and the micro-mechanical response of tensile deformed medium manganese steel through interrupted tensile test

In the current work, the microstructure evolution, mechanical stability of austenite and the micro-mechanical response of tensile deformed Fe-0.18C-4Al-7Mn (Wt%) steel are investigated. Interrupted tensile tests on the annealed samples (750 °C, 1 h) were performed at a tensile elongation of 5, 10, 20, 30, 40 and 60% to understand the deformation behavior and transformation-induced plasticity (TRIP) effect. Strain-induced martensitic transformation (SIMT) of reverted austenite is explored using X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) analysis. The coefficient of mechanical stability of austenite (k-parameter) is quantified through the interrupted tensile tests using XRD analysis. Instrumented nanoindentation test was conducted on individual phases of the interrupted tensile-tested samples to determine their hardness and modulus values. The scanning electron microscopy (SEM) analysis reveals the gradual decrease in width and increase in length of austenite-martensite islands with an increase in the percentage of elongation. Evolution of dislocation lines, dislocation tangles and dislocation accumulations occur at ferrite grain boundaries and martensite lath boundaries during tensile deformation. The volume fraction of austenite decreases, and the transformation of austenite to martensite and dislocation density of austenite increase with the increase in the tensile elongation. Austenite is found to be mechanically more stable at 30% of tensile elongation due to the low austenite transformation ratio. Nanoindentation results show that the hardness increases with an increasing percentage of deformation. Multiple pop-ins are observed in load (P)–indentation depth (h) curves due to dislocation interaction at the interphase boundary.

Unveiling the grain boundary-related effects on the incipient plasticity and dislocation behavior in nanocrystalline CrCoNi medium-entropy alloy

• The incipient plasticity and dislocation behavior in nanocrystalline (NC) CrCoNi alloy were systematically investigated and compared with its coarse-grained (CG) counterpart and NC Ni. • Multiple pop-ins appear in the P-h curves of NC and CG CrCoNi alloys but disappear in the curves of NC Ni. • The incipient plasticity is mediated by dislocation nucleation in NC CrCoNi alloy but by GBs in NC Ni. • Chemically complex GBs in NC CrCoNi alloy facilitate dislocation nucleation but impede dislocation propagation. • Spatial restrictions from GBs in NC CrCoNi alloy induce size-dependent pop-in behaviors. The incipient plasticity and dislocation behavior in a nanocrystalline (NC) CrCoNi medium-entropy alloy were systematically investigated in terms of pop-in events during instrumental nano-indentation tests. Quantitative statistical analysis and molecular dynamic simulations were performed to reveal the effects of grain boundaries (GBs) on initial stages of plastic deformation . Multiple pop-in events appeared during loading on the NC CrCoNi. The first pop-in that represents the initial yielding was identified to be controlled by dislocation nucleation, which is in sharp contrast to the continuous elastic-plastic transition mediated by GB mechanisms in NC pure metals. This can be attributed to the sluggish kinetics of the chemically complex GBs (CCGBs) in the NC CrCoNi that hinders diffusive GB activities but facilitates dislocation nucleation. Subsequent pop-ins were also found to be closely related to the extra dragging effects imposed by the CCGBs on dislocation propagation in the NC alloy . Moreover, the extremely small grain sizes and the consequent high-volume fraction of GBs in the NC alloy severely restrict the lengths of dislocation source and the radii of dislocation loop, giving rise to a higher critical stress, smaller activation volume and lower pop-in width as compared with its coarse-grained counterpart. These results provide new insights into the onset of nano-plasticity in concentrated multi-principal element alloys.

Hydrogen suppression of dislocation cell formation in micro and nano indentation of pure iron single crystals

The influence of hydrogen on dislocation mobility in pure single crystal iron subjected to micro- and nanoindentation testing perpendicular to the (100) plane has been analysed using electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) and simulated using the self-consistent kinetic Monte-Carlo (SCkMC) method. The nanoindentation load curve for the hydrogen free sample has two major constant load pop-ins, and multiple smaller pop-ins, whereas the hydrogen charged material has only one pop-in but with a longer loading release. A well-defined cell structure was observed below the indenter in the hydrogen free condition, but the dislocations were homogenously distributed in the hydrogen charged sample. The SCkMC simulations showed that it is difficult for dislocations to glide out of the primary slip plane after hydrogen charging compared with the hydrogen free sample under the present loading conditions.

Reveal the Deformation Mechanism of (110) Silicon from Cryogenic Temperature to Elevated Temperature by Molecular Dynamics Simulation.

Silicon undergoes a brittle-to-ductile transition as its characteristic dimension reduces from macroscale to nanoscale. The thorough understanding of the plastic deformation mechanism of silicon at the nanoscale is still challenging, although it is essential for developing Si-based micro/nanoelectromechanical systems (MEMS/NEMS). Given the wide application of silicon in extreme conditions, it is, therefore, highly desirable to reveal the nanomechanical behavior of silicon from cryogenic temperature to elevated temperature. In this paper, large-scale molecular dynamics (MD) simulations were performed to reveal the spherical nanoindentation response and plastic deformation mechanism of (110)Si at the temperature range of 0.5 K to 573 K. Special attention was paid to the effect of temperature. Multiple pop-ins detected in load/pressure-indentation strain curves are impacted by temperature. Four featured structures induced by nanoindentation, including high-pressure phases, extrusion of α-Si, dislocations, and crack, are observed at all temperatures, consistent with experiment results. The detailed structure evolution of silicon was revealed at the atomic scale and its dependence on temperature was analyzed. Furthermore, structure changes were correlated with pop-ins in load/pressure-indentation strain curves. These results may advance our understanding of the mechanical properties of silicon.

Deformation mechanisms at multiple pop-ins under spherical nanoindentation of (1 1 1) Si

The pop-in events and the deformation mechanisms of (1 1 1) Si under spherical nanoindentation were investigated using molecular dynamics simulations. The simulations result successfully reproduced the plastic deformation mediated by high-pressure phase transformations and dislocation burst, and the highly desirable brittle fracture of silicon. Quadruple occurrences of the pop-ins were observed in loading curves. The one-to-one relationship between the multiple pop-ins and the deformation mechanisms was established. Two fundamental processes, the asynchronous occurrences of high-pressure phase transformation and extrusion of α-Si, were identified from several deformation modes to be responsible for the multiple pop-ins. Moreover, the dislocation burst also contributes to the plastic deformation, and the cracks were observed to contribute to the brittle deformation, but contribute nothing to the pop-ins.

Pop-in effects and dislocation nucleation of c-plane single-crystal ZnO by Berkovich nanoindentation

In the present study, the Berkovich nanoindentation-induced multiple pop-ins phenomena are observed in the load–displacement curves of c-plane single-crystal ZnO. The results indicated that no evidence of phase transformation is revealed by using the cross-sectional transmission electron microscopy (XTEM) and the selected area diffraction (SAD) analyses. Instead the XTEM observations suggest that these “instabilities” might be resulted from the sudden nucleation of dislocations propagating along the slip systems lying on the {0001} basal planes and the {101¯1} pyramidal planes. Therefore, based on this scenario, an energetic estimation of dislocation nucleation is made. Further, the cathodoluminescence (CL) spectroscopy was examined as a function of applied indentation loading for the c-plane single-crystal ZnO. The results showed the near band-edge luminescence intensity falling exponentially with the applied nanoindentation load and reduced to about 5% of that obtained in pristine area as the maximum indentation load up to 100mN. It suggests that the CL technique may serve as an effective alternative for studying the near-surface plasticity semiconductors.

Incipient plasticity of single-crystal tantalum as a function of temperature and orientation

The nanocontact plastic behaviour of single-crystalline Ta (1 0 0), Ta (1 1 0) and Ta (1 1 1) was studied as a function of temperature and indentation rate. Tantalum, a representative body centred cubic (BCC) metal, reveals a unique deformation behaviour dominated by twinning and the generation of stacking faults. Experiments performed at room temperature exhibit a single pop-in event, while at 200 °C, above the critical temperature, a transition to multiple pop-ins was observed. The experimental results are discussed with respect to the orientation as well as temperature and correlated to the defect structures using both anisotropic finite element and MD simulations. The serrated flow observed at 200 °C is related to differences in the quasi-elastic reloading originating from changes in the defect mechanism.

Dislocation Energetics and Pop-Ins in AlN Thin Films by Berkovich Nanoindentation.

Nanoindentation-induced multiple pop-ins were observed in the load-displacement curves when the mechanical responses of AlN films grown on c-plane sapphire substrates were investigated by using Berkovich indenters. No evidence of phase transformation is revealed by cross-sectional transmission electron microscopy (XTEM) and selected area diffraction (SAD) analyses. Instead XTEM observations suggest that these “instabilities” resulted from the sudden nucleation of dislocations propagating along the slip systems lying on the {0001} basal planes and the pyramidal planes commonly observed in hexagonal compound semiconductors. Based on this scenario, an energetic estimation of dislocation nucleation is made.

Determination of fracture toughness and energy dissipation of SiO2-poly(methyl metacrylate) hybrid films by nanoindentation

In this paper we have applied different methods based on nanoindentation techniques to measure the toughness of SiO2-poly methyl-methacrylate hybrid films on organic acrylic substrates. The hybrid films were deposited by the Sol–Gel method from precursor solutions containing tetraethyl-orthosilicate, methylmethacrylate and 3-trimethoxysilyl propyl methacrylate (TMSPM) as the coupling agent. The influence of TMSPM content in the hybrid precursor solution on the fracture behavior of the hybrid films was studied. The classical indentation crack length method was applied from nanoindentation tests to determine the stress intensity factor by direct measurement of crack length from atomic force microscopy images. A second method, based on the pop-in analysis allowed the separation of crack formation from film delamination from multiple pop-ins. Finally, a third method based on energy methods is also reported and discussed. The amount of TMSPM in the precursor solution showed a strong influence on the toughness of the hybrid films.

Hydrogen effect on dislocation nucleation in a vanadium (1 0 0) single crystal as observed during nanoindentation

The effect of hydrogen on dislocation nucleation in a vanadium (1 0 0) single crystal has been examined by means of nanoindentation . For electrochemically doped samples to different hydrogen concentrations within the α-phase, the pop-in load decreased with the increase in hydrogen concentration and multiple pop-ins were observed on the load–displacement curves. The interaction between the dissolved hydrogen atoms and the newly formed dislocation loops, resulting in the reduction of their line energy, is evaluated and explained based on the novel thermodynamic defactant concept.

Analysis of the indentation size effect in brittle materials from nanoindentation load–displacement curve

In this paper, we investigated the relationships between crack initiation/propagation, stress induced transformation and indentation size effect (ISE) using Universal hardness. Preparing three kinds of specimens, 8 mol%Y 2O 3–ZrO 2 single crystal (single 8Y-FSZ), 12 mol%CeO 2–ZrO 2 polycrystals (12Ce-TZP) and fused quartz, we were able to research the above relationships separately. The load dependence of universal hardness in the three specimens was studied using nanoindentation equipment and found that single 8Y-FSZ and 12Ce-TZP showed the ISE. To reveal the origin of the ISE in these materials, we analyzed the relationship between load–displacement curve and Universal hardness ISE. Adapting the second-order differential ( d 2 P/ dh 2) to the load–displacement curve showed that a small displacement change (micro multiple pop-ins) would occur in each deformation process of the single 8Y-FSZ and 12Ce-TZP. The origin of the micro multiple pop-ins is considered to be crack initiation/propagation in single 8Y-FSZ and stress induced transformation in 12Ce-TZP, respectively, and these micro multiple pop-ins should result in the Universal hardness ISE.

Temperature dependence of the plastic flow behavior of tantalum

Nanoindention has been used to study the plastic flow behavior of bcc Ta(001) in the temperature range 25–200°C. Most notably, it is found that the shape of the load–displacement curves changes with increasing temperature. Only one large discontinuity marking the onset of plasticity is observed in room-temperature experiments, whereas multiple pop-ins separated by elastic reloading segments were observed at 200°C. Detailed analysis of the load–displacement curves indicates that the lattice resistance decreases with increasing temperature, consistent with the mobilization of screw dislocations by thermal activation. Contributions from the oxide layer and/or thermal drift can be excluded.

Nucleation of dislocations during nanoindentation in MgO

This study investigates the incipient plasticity during a nanoindentation test with a spherical indenter in magnesium oxide single crystals. The load–displacement curves show multiple pop-ins, interspaced by elastic deformation, and reveal the transition from an initial elastic deformation regime to a continuous elastoplastic one. Nanoindentations have been stopped after the first pop-in and the dislocation structures have been three-dimensionally characterized by AFM observations of the surface deformation and by nanoetching. The dislocation pattern shows two activated slip systems. The resolved shear stress has been calculated just before the pop-in, for all of the activated slip systems. The final positions of the dislocations generated during the first pop-in reveal that they were all nucleated at the point of maximum resolved shear stress.

Atomistic modeling of dislocation activity in nanoindented GaAs

The mechanical behavior of GaAs was investigated by nanoindentation with the aid of molecular dynamics (MD) analysis based on the Tersoff potential. Particular attention was devoted to the evolution characterization of dislocation activity during deformation. The transition from elastic-to-plastic deformation behavior was clearly observed as a sudden displacement excursion occurring during the load–displacement curves of larger loads (single pop-in), faster impact velocity and higher temperature (multiple pop-ins). Even for an ultra-small penetration depth (<3nm), the MD simulation shows that GaAs deforms plastically and a good description is given in the results. The plastic deformation occurs due to the anticipated change in the twinning and/or dislocation motion. Dislocation nucleations occurred inside the material near the top of the surface and generated loops in the {111} slip planes. The MD analysis of the deformation behavior shows an agreement with that of previous atomic force microscopy (AFM) and transmission electron microscopy (TEM) experiments.