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Nanoindentation tests were performed on high purity aluminum and its alloys to clarify the effects of the types of additional elements and the effects of interactions between additional elements and grain boundaries or existing dislocations on local mechanical properties. The samples used were ultrahigh purity 5N-Al and high purity Al-Fe, Mn, Cu, Zr and Zn alloys processed by solution annealing or friction stir process (FSP). The results of nanoindentation hardness revealed that the amount of each element and the misfit had a significant effect on deformation resistance of plastic deformation in solution annealed samples. On the other hand, the results for the FSPed samples showed that the existing dislocations introduced by FSP have a significant effect on the plastic resistance. From the values of the nanoindentation hardness ratio of the grain boundary to the grain interior, it is concluded that the segregation effect of the additional elements at the grain boundary on the plastic resistance is small in solution annealed samples and FSPed samples. The results of critical load at pop-in revealed that elements with large misfit act as dislocation sources in solution annealed samples. For the FSPed samples showed that the density of existing dislocations introduced by FSP has a significant effect on the critical load for dislocation generation. [ABSTRACT FROM AUTHOR]

Different scale microstructures at different scales were obtained by one and two-step isothermal transformation. The deformation behavior and stability of retained austenite under the action of nanoindentation stress was investigated in these different microstructures. The results demonstrate that the boundary fraction and density of the K–S relationship increase significantly after two-step isothermal transformation, promoting the nucleation of bainite and resulting in a refined microstructure. Changes in the nano-hardness as a function of indentation depth and morphology confirmed the quantity of RA transformation and the composite deformation/transformation behavior of blocky martensite/retained austenite (M/RA). A higher dislocation density is enriched in RA with higher KAM with the two-step isothermal process. This resulted in higher stability to delay the occurrence of the pop-in phenomenon. The stability of RA is highly dependent on the crystal orientation, with (001) oriented RA having an increased propensity to undergo mechanical transformation to martensite, with additional pop-ins generated during loading.

In this study, nanoindentation tests at three different strain rates within 100 nm indentation depth were conducted on an a-plane ( 11 2 ¯ 0 ) ZnO single crystal to investigate the effect of strain rate on its nano-scale mechanical behavior. The load–indentation-depth curves, pop-in events, hardness and Young's moduli of an a-plane ( 11 2 ¯ 0 ) ZnO single crystal at different strain rates were investigated at the nano-scale level. The results indicated that, with the indentation depth increasing, the load increased gradually at each maximum indentation depth, hma, during the loading process. A distinct pop-in event occurred on each loading curve except that corresponding to the hmax of 10 nm. The applied load at the same indentation depth increased with the increasing strain rate during the nanoindentation of the a-plane ( 11 2 ¯ 0 ) ZnO single crystal. The higher strain rate deferred the pop-in event to a higher load and deeper indentation depth, and made the pop-in extension width larger. The hardness showed reverse indentation size effect (ISE) before the pop-in, and exhibited normal ISE after the pop-in. Both the hardness and the Young's modulus of the a-plane ( 11 2 ¯ 0 ) ZnO single crystal increased with the increasing strain rate, exhibiting the positive strain-rate sensitivity. [ABSTRACT FROM AUTHOR]

The correlations among microstructure, surface morphology, hardness, and elastic modulus of Bi2Te3 thin films deposited on c-plane sapphire substrates by pulsed laser deposition are investigated. X-ray diffraction (XRD) and transmission electron microscopy are used to characterize the microstructures of the Bi2Te3 thin films. The XRD analyses revealed that the Bi2Te3 thin films were highly (00l)-oriented and exhibited progressively improved crystallinity when the substrate temperature (TS) increased. The hardness and elastic modulus of the Bi2Te3 thin films determined by nanoindentation operated with the continuous contact stiffness measurement (CSM) mode are both substantially larger than those reported for bulk samples, albeit both decrease monotonically with increasing crystallite size and follow the Hall—Petch relation closely. Moreover, the Berkovich nanoindentation-induced crack exhibited trans-granular cracking behaviors for all films investigated. The fracture toughness was significantly higher for films deposited at the lower TS; meanwhile, the fracture energy was almost the same when the crystallite size was suppressed, which indicated a prominent role of grain boundary in governing the deformation characteristics of the present Bi2Te3 films. [ABSTRACT FROM AUTHOR]

In this study, nanoindentation tests at three different strain rates within 100 nm indentation depth were conducted on an a-plane (112¯0) ZnO single crystal to investigate the effect of strain rate on its nano-scale mechanical behavior. The load–indentation-depth curves, pop-in events, hardness and Young’s moduli of an a-plane (112¯0) ZnO single crystal at different strain rates were investigated at the nano-scale level. The results indicated that, with the indentation depth increasing, the load increased gradually at each maximum indentation depth, hma, during the loading process. A distinct pop-in event occurred on each loading curve except that corresponding to the hmax of 10 nm. The applied load at the same indentation depth increased with the increasing strain rate during the nanoindentation of the a-plane (112¯0) ZnO single crystal. The higher strain rate deferred the pop-in event to a higher load and deeper indentation depth, and made the pop-in extension width larger. The hardness showed reverse indentation size effect (ISE) before the pop-in, and exhibited normal ISE after the pop-in. Both the hardness and the Young’s modulus of the a-plane (112¯0) ZnO single crystal increased with the increasing strain rate, exhibiting the positive strain-rate sensitivity.

Pop-in is a widely observed phenomenon in nanoindentation. In this paper, dislocation evolution in pop-in processes is analysed in detail through molecular dynamics (MD) simulations. We found that a large number of dislocations nucleate homogeneously at the initiation of pop-in, followed by extensive dislocation propagation, which is the dominant mode of plastic deformation during pop-in. Moreover, we noted that establishing the correct dislocation evolution mechanisms of pop-in can serve to explain the overshoot phenomenon observed in nanoindentation experiments. Through our MD analysis on the obtained dislocation structures, therefore, we were able to propose a model that can predict the total length of dislocations associated with the plastic processes underneath a spherical indenter. In addition, the Taylor model was used to verify that our proposed dislocation length model sits well with the MD simulated force-displacement curves of nanoindentation. In fact, the MD simulated linear relation between critical force and indentation depth during pop-in is consistent with the Hertzian and Taylor models. Our MD simulations, therefore, can provide significant insight into the experimentally observed pop-in phenomena. [ABSTRACT FROM AUTHOR]

Nanoindentation is an attractive characterization technique, as it not only measures the local properties of a material but also facilitates understanding of deformation mechanisms at submicron scales. However, because of the complex stress-strain field and the small scale of the deformation under the nanoindenter, the results can be easily influenced by artifacts induced during sample preparation. In this work, a systematic study was conducted to better understand the influence of sample preparation methods on the nanoindentation results of ductile metals. All experiments were conducted on a steel (Fe-22Mn-0.65C, wt%) with twinning-induced plasticity (TWIP), which was selected for its large grain size and sensitivity to different surface preparation methods. By grouping the results obtained from each nanoindent, chemical polishing was found to be the best sample preparation method with respect to the resulting mechanical properties of the material. In contrast, the presence of a deformation layer left by mechanical polishing and surface damage induced by focused ion beam (FIB) scanning were confirmed by the dislocation-nucleation-induced pop-in events of nanoindentation. [ABSTRACT FROM AUTHOR]

Spherical tip nanoindentation experiments on two typical face-centered cubic high-entropy alloys (HEAs), CoCrFeNi and CoCrFeMnNi in as-solutionized and aged conditions were performed using tips of two different radii. Large datasets of the strength at the first pop-in were obtained and their statistical nature were analyzed to gain insights into the micromechanisms responsible for the onset of incipient plasticity in HEAs that are notionally monophasic. In all the cases examined, the probability density distributions were bimodal in nature. The deconvoluted distributions were utilized to estimate the activation volumes of the underlying deformation mechanisms. They show that when the probed material's volume is relatively small, heterogeneous dislocation nucleation aided by monovacancies occurs at lower indentation stresses; this followed by homogeneous dislocation nucleation at high loads, resulting in strengths corresponding to the theoretical strengths. When a substantially larger volume is sampled, by using a larger radius tip, either the preexisting dislocation mediated ones at low stresses or vacancy cluster /grain boundary aided heterogeneous dislocations nucleation at higher stresses become dominant. Increasing the chemical short-range order in the alloy via high temperature aging leads to overall strengthening of the alloy by enhancing stress required for the homogeneous dislocation nucleation. Implications of such plurality of mechanisms with overlapping strength distributions at HEA's disposal in imparting high strength-ductility combinations are discussed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Mechanical properties of gallium nitride (GaN) single crystals upon carbon ion irradiation are examined using nanoindentation analysis at room temperature. Pop-in events in the load-depth curves are observed for unirradiated and irradiated GaN samples. A statistical linear relationship between the critical indentation load for the occurrence of the pop-in event and the associated displacement jump is exhibited. Both the slope of linear regression and the measured hardness increase monotonically to the ion fluence, which can be described by logistic equations. Moreover, a linear relationship between the regression slope as a micromechanical characterization and the hardness as a macroscopic mechanical property is constructed. It is also found that the maximum resolved shear stress of the irradiated samples is larger than that of the unirradiated samples, as the dislocation loops are pinned by the irradiation-induced defects. Our results indicate that the nanoindentation pop-in phenomenon combined with a statistical analysis can serve as a characterization method for the mechanical properties of ion-irradiated materials.

Introduction: Underlay technique myringoplasty is most commonly used technique to repair tympanic membrane perforation by temporalis fascia graft and Trans-tympanic pop-in technique is an another technique that allows temporalis fascia graft placement medial to tympanic membrane remnant through the perforation without the need for tympano-meatal flap elevation. This study was undertaken to find the prevalence of perforated graft in underlay and Pop-in Technique Myringoplasty. Methods: This descriptive cross-sectional study was done at Manipal teaching hospital, Pokhara, Nepal and comprises of 86 patients between January 2014 and June 2015. Patients undergoing Underlay Trans-canal approach by tympano-meatal flap elevation and Trans-canal, Trans-tympanic pop-in technique were included. Sample size calculation was done and convenient sampling method was applied. Point estimate at 95% CI was done for binary data along with frequency and proportion. The descriptive statistical analysis was done. Results: The prevalence of perforated graft was 15 (17.4%) at 95% Confidence Interval (39.75-60.25%). In underlay technique there were 8 (18.60 %) perforation and 35 (81.39%) were intact while in pop-in technique there were 7 (16.27%) perforation and 36 (83.72%) were intact. The postoperative mean Pure Tone Average (PTA) of underlay was 9.53 and pop-in was 8.31. The mean Pure Tone Average (PTA) gain after underlay technique was 16.095 and pop in technique was 16.87. Conclusions: Trans-tympanic pop-in myringoplasty gives similar hearing & graft uptake results when compared with tympano-meatal flap method of underlay myringoplasty done by trans-canal.

There are two types of pop-in mode that have been widely observed in nanoindentation experiments: the single pop-in, and the successive pop-in modes. Here we employ the molecular dynamics (MD) modelling to simulate nanoindentation for three face-centred cubic (FCC) metals, including Al, Cu and Ni, and two body-centred cubic (BCC) metals, such as Fe and Ta. We aim to examine the deformation mechanisms underlying these pop-in modes. Our simulation results indicate that the dislocation structures formed in single crystals during nanoindentation are mainly composed of half prismatic dislocation loops. These half prismatic dislocation loops in FCC metals are primarily constituted of extended dislocations. Lomer–Cottrell locks that result from the interactions between these extended dislocations can resist the slipping of half dislocation loops. These locks can build up the elastic energy that is needed to activate the nucleation of new half dislocation loops. A repetition of this sequence results in successive pop-in events in Al and other FCC metals. Conversely, the half prismatic dislocation loops that form in BCC metals after first pop-in are prone to slip into the bulk, which sustains plastic indentation process after first pop-in and prevents subsequent pop-ins. We thus conclude that pop-in modes are correlated with lattice structures during nanoindentation, regardless of their crystal orientations. [ABSTRACT FROM AUTHOR]

Spherical nanoindentation was performed on tungsten single crystal to investigate the effects of tip radius on the pop-in stress. The results show that the pop-in stress decreases as the tip radius increases. A statistical model with randomly distributed dislocation sources captures experimentally observed source-controlled indentation size effect. The discrepancy between model and experimental data especially for a large tip was corrected by considering the collective activation of multiple dislocation sources. Our results provide a quantitative insight into the understanding of the transition from source-controlled plasticity to bulk plasticity under spherical nanoindentation. Unlabelled Image [ABSTRACT FROM AUTHOR]

High–speed nanoindentation rapidly generates large datasets, opening the door for advanced data analysis methods such as the resources available in artificial intelligence. The present study addresses the problem of differentiating load–displacement curves presenting pop-in, slope changes, or instabilities from curves exhibiting a typical loading path in large nanoindentation datasets. Classification of the curves was achieved with a deep learning model, specifically, a convolutional neural network (CNN) model implemented in Python using TensorFlow and Keras libraries. Load–displacement curves (with pop-in and without pop-in) from various materials were input to train and validate the model. The curves were converted into square matrices (50 × 50) and then used as inputs for the CNN model. The model successfully differentiated between pop-in and non-pop-in curves with approximately 93% accuracy in the training and validation datasets, indicating that the risk of overfitting the model was negligible. These results confirmed that artificial intelligence and computer vision models represent a powerful tool for analyzing nanoindentation data.

In materials science and engineering, a significant amount of research has been carried out using indentation techniques in order to characterize the mechanical properties and microstructure of a broad range of natural and engineered materials. However, there are many unresearched or partly researched areas, such as, for example, the investigation of the shape of the indentation load–displacement curve, the associated mechanism in porous materials with clastic texture, and the influence of the texture on the constitutive behavior of the materials. In the present study, nanoindentation is employed in the analysis of the mechanical behavior of a benchmark material composed of plaster of Paris, which represents a brand of highly porous-clastic materials with a complex structure; such materials may find many applications in medicine, production industry, and energy sectors. The focus of the study is directed at the examination of the influence of the porous structure on the load–displacement response in loading and unloading phases based on nanoindentation experiments, as well as the variation with repeating the indentation in already indented locations. Events such as pop-in in the loading phase and bowing out and elbowing in the unloading phase of a given nanoindentation test are studied. Modulus, hardness, and the elastic stiffness values were additionally examined. The repeated indentation tests provided validations of various mechanisms in the loading and unloading phases of the indentation tests. The results from this study provide some fundamental insights into the interpretation of the nanoindentation behavior and the viscoelastic nature of porous-clastic materials. Some insights on the influence of indentation spacing to depth ratio were also obtained, providing scope for further studies.

Pop-in and pop-out effects in silicon (Si) have long been known. They were evidenced in the indentation loading-unloading curves as a sudden displacement discontinuity. They consist in a sudden contraction (pop-in) or a sudden expansion (pop-out) of the material underneath the indenter in a short period of time and are attributed to Si phase transformations that take place during the nanoindentation procedure. In this paper, first we provide a statistic analysis of such pop-in/pop-out events depending on the maximum indentation load and second we examine the dependence of their appearance on the indentation loading-unloading rate.

The attractive strain burst phenomenon, so-called “pop-in”, during indentation-induced deformation at a very small scale is discussed as a fundamental deformation behavior in various materials. The nanoindentation technique can probe a mechanical response to a very low applied load, and the behavior can be mechanically and physically analyzed. The pop-in phenomenon can be understood as incipient plasticity under an indentation load, and dislocation nucleation at a small volume is a major mechanism for the event. Experimental and computational studies of the pop-in phenomenon are reviewed in terms of pioneering discovery, experimental clarification, physical modeling in the thermally activated process, crystal plasticity, effects of pre-existing lattice defects including dislocations, in-solution alloying elements, and grain boundaries, as well as atomistic modeling in computational simulation. The related non-dislocation behaviors are also discussed in a shear transformation zone in bulk metallic glass materials and phase transformation in semiconductors and metals. A future perspective from both engineering and scientific views is finally provided for further interpretation of the mechanical behaviors of materials.

Abstract An unusual room temperature mechanical behavior of pure tungsten is investigated by focusing on three specimens prepared with different microstructures: as-received (hot-rolled), recrystallized, and cold-rolled specimens. Contrary to ordinary expectations in metallic materials, only the cold-rolled specimen exhibits significant plastic deformation during tensile testing, with improved strength and ductility, while the recrystallized and the as-received specimens fail in the elastic region. We further provide an explanation of such characteristics by systematically utilizing experimental and theoretical analysis at a small scale: nano-indentation tests clarify the inherent mechanical response inside grains and provide a statistical distribution of the maximum shear stress during pop-in events, corresponding to onset of plastic yielding; atomistic simulations provide information on the overall fracture strength of various grain boundaries. By comparing the maximum shear stress and the grain boundary fracture stress, we could explain that the distinctive plasticity in the cold-rolled specimen is caused by the movement of pre-existing dislocations before grain boundary fracture. This contrasts the recrystallized and as-received specimens, where grain boundary fracture occurs prior to the nucleation of dislocations or activation of dislocation sources. Graphical abstract Image 1 Highlights • Distinctive plasticity of cold rolled tungsten was investigated at room temperature. • The more pre-existing dislocations, the lower shear stress for plastic yielding. • Molecular dynamic simulations provide the fracture stress of grain boundaries, ∼16 GPa. • Cold rolled tungsten has lower stress for plastic yielding than the fracture stress. • Ductility was enhanced by the dislocation activation prior to intergranular fracture. [ABSTRACT FROM AUTHOR]

Abstract A 0.1C5Mn3Al steel with laminated ferrite/martensite and layered ferrite/martensite microstructures was processed by hot-rolling, cold forging and subsequent annealing. Tensile tests revealed that hot rolled sample (laminated) and cold forged + 650 °C annealed sample (layered) showed similar tensile strengths but different elongations. It is confirmed that laminated structure is good for retaining elongation. Furthermore, nanoindentation tests were performed at locations at or near interfaces and in the ferrite or martensite matrix, revealing that the nanohardness difference across the interface is important in affecting the elongation of the material. The underlying mechanisms were discussed with regarding to the dislocation-interface interactions. Graphical abstract fx1 [ABSTRACT FROM AUTHOR]

Highlights • Pop-ins can be simulated by finite element analyses using the nodal release method. • When a pop-in is caused by a split, the load mode bending is severer than or equal to tension. • In the case of a split and a load-drop not exceeding 8%, the pop-in can be regarded as insignificant. Abstract During a fracture toughness test on a welded joint, a pop-in accompanied by a load drop may occur prior to final rupture. The currently used standards for conditions to determine whether pop-ins are insignificant vary, and are not prescribed based on adequate physical grounds. Moreover, these standards can be too severe in considering the occurrence of pop-ins as equivalent to a fatal rupture in welded structures. Research to formulate reasonable conditions to assess pop-ins is important to determine whether it is necessary to remake or repair the relevant welded structure. This study reports fracture toughness tests on specimens with pop-ins caused by brittle cracks featuring arrest behavior. The results were simulated through finite element analysis by using a nodal release method, which was implemented when a pop-in occurred owing to split formation, to determine whether the pop-in was significant. From analytical results based on a load mode featuring bending and tension, it was found that the crack opening stress distribution under the bending mode at a load drop of 8% or less was larger than or almost equal to that under the tensile mode. Therefore, if the opening of a brittle crack is arrested during three-point bending fracture toughness tests, it would also be arrested in the welded structures under tensile load. In other words, the results showed that such a pop-in could be regarded as insignificant. [ABSTRACT FROM AUTHOR]

Abstract Early stage irradiation effects on incipient plasticity are quantitatively investigated in single-crystalline molybdenum using nanoindentation pop-in tests. Defects produced under low-dose ion irradiations, even when they are hardly detected by ion-channeling technique, can significantly reduce the critical stress for the elastic-plastic transition, through acting as heterogeneous dislocation nucleation sources. The density and strength of defects are derived using a unified model convoluting homogeneous and heterogeneous mechanisms. In addition to the increased defect density, defect strength is found to decrease with increasing irradiation dose, suggesting a growth in defect size, which is further evidenced by combined analyses between pop-in and hardness tests. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

The correlations between the microstructure and nanomechanical properties of a series of thermal annealed Co thin films were investigated. The Co thin films were deposited on glass substrates using a magnetron sputtering system at ambient conditions followed by subsequent annealing conducted at various temperatures ranging from 300 °C to 800 °C. The XRD results indicated that for annealing temperature in the ranged from 300 °C to 500 °C, the Co thin films were of single hexagonal close-packed (hcp) phase. Nevertheless, the coexistence of hcp-Co (002) and face-centered cubic (fcc-Co (111)) phases was evidently observed for films annealed at 600 °C. Further increasing the annealing temperature to 700 °C and 800 °C, the films evidently turned into fcc-Co (111). Moreover, significant variations in the hardness and Young’s modulus are observed by continuous stiffness nanoindentation measurement for films annealed at different temperatures. The correlations between structures and properties are discussed.

In this work, structural and mechanical properties of hydrogen-charged metallic glass are studied to evaluate the effect of hydrogen on early plasticity. Hydrogen is introduced into samples of a Zr-based (Vit 105) metallic glass using electrochemical charging. Nanoindentation tests reveal a clear increase in modulus and hardness as well as in the load of the first pop-in with increasing hydrogen content. At the same time, the probability of a pop-in occurring decreases, indicating that hydrogen hinders the onset of plastic instabilities while allowing local homogeneous deformation. The hydrogen-induced stiffening and hardening is rationalized by hydrogen stabilization of shear transformation zones (STZs) in the amorphous structure, while the improved ductility is attributed to the change in the spatial correlation of the STZs.

The outstanding functional properties of single crystals used in many engineering applications often rely on their surface quality. The associated grinding process in single crystals is known to introduce surface or sub-surface defects (cracks), which may compromise the functionality and/or structural integrity of the final device. The small size of such defects often yields relatively high strength values, but also usually large scatter which implies low reliability. The aim of this work is to analyze the onset of surface contact damage in single crystals with respect to crystal orientation and elastic properties. LiTaO 3 and LiNbO 3 anisotropic single crystal samples are investigated using nanoindentation techniques and focused ion beam based sub-surface analyses. Experimental findings show that the onset of damage is correlated to weaker cleavage planes. At this stage also traces of plastic deformation on the contact surface due to twinning are observed. Further load increase revealed contact cracks in both materials; their morphology and extension being related to the orientation of the cleavage planes and elastic properties of the crystals. Our results advance the understanding of damage in anisotropic materials such as LiTaO 3 and LiNbO 3 , and can generally be utilized to assess the onset of damage in other brittle materials. [ABSTRACT FROM AUTHOR]

The nano-scale deformation behaviors and indentation-induced fracture features of the (111)-oriented yttrium-stabilized zirconia single crystal (YSZ(111)) were investigated by Berkovich and micro-Vicker indentations, respectively. The load-displacement curves in the Berkovich nano-indentation experiments evidently exhibited indentation-induced single “pop-in” phenomenon during loading, indicating that the nano-scale deformation in the YSZ(111) crystals is due primarily to the activities of dislocation nucleation and propagation. Based on this scenario, the number of nanoindentation-induced dislocation loops giving rise to the pop-in event was estimated to be around 2 × 10 5 with a critical radius of ∼2 nm. The hardness and Young's modulus of YSZ(111) single crystal obtained by the continuous contact stiffness measurements (CSM) mode were 22.3 ± 1.1 GPa and 270.6 ± 8.5 GPa, consistent with those reported previously in the literature. In addition, the fracture toughness of YSZ(111) single crystal was estimated to be about 1.4–1.6 MPa m 1/2 . [ABSTRACT FROM AUTHOR]

Mechanical properties that are considered to be deterministic in the macro-scale have been shown to be stochastic in the sub-micron length scale. The origin of such stochastic responses is not well understood. This work examines the potential influence of grain boundaries and grain orientations on the stochastic nature of pop-in and hardness measurement in annealed high purity polycrystalline Cu samples during low load nanoindentation. Statistical analysis on pop-in load and hardness showed that variations of these measurements depend on crystal orientations and is influenced by the indenter probe size. Analysis on the pop-in load statistics showed that pop-ins are likely initiate from an atomic sized precursor that leads to dislocation generation or expansion. Variation in hardness measurements near an arbitrary chosen grain boundary and the apparent grain boundary hardening effect observed may be related to the higher density of dislocations at and near the grain boundary. [ABSTRACT FROM AUTHOR]

Fatigue pre-cracking performance and fracture toughness in polycrystalline tungsten (W) and molybdenum (Mo) have been investigated in relation to grain boundary (GB) configuration with respect to the crack advance direction. Sub-sized, single edge notched bend (SENB) specimens with three different orientations, R-L (ASTM notation) for a forged Mo rod and L-S and T-S for a rolled W plate, were pre-cracked in two steps: fully uniaxial compression fatigue loading to provoke crack initiation and its stable growth from the notch root, and subsequent 3-point bend (3PB) fatigue loading to extend the crack. The latter step intends to minimize the influence of the residual tensile stresses generated during compression fatigue by moving the crack tip away from the plastic zone. It is shown that fatigue pre-cracking performance, especially pre-crack extension behavior, is significantly affected by the specimen orientation. The R-L orientation, giving the easiest cracking path, permitted crack extension completely beyond the plastic zone, while the L-S and T-S orientations with the thickness cracking direction of the rolled plate sustained the crack lengths around or possibly within the plastic zone size due to difficulty in crack advance through an aligned grain structure. Room temperature fracture toughness tests revealed that the 3PB fatigued specimens exhibited appreciably higher fracture toughness by about 30% for R-L, 40% for L-S and 60% for T-S than the specimens of each orientation pre-cracked by compression fatigue only. This indicates that 3PB fatigue provides the crack tip front out of the residual tensile stress zone by crack extension or leads to reduction in the residual stresses at the crack tip front. Strong dependence of fracture toughness on GB configuration was evident. The obtained fracture toughness values are compared with those in the literature and its strong GB configuration dependence is discussed in connection with the appearance of pop-in. [ABSTRACT FROM AUTHOR]

The deformation behaviors and fracture features of GaP(100) single-crystal are investigated by using nano- and micro-scale indentation techniques. The hardness and Young’s modulus were measured by nanoindentation using a Berkovich diamond indenter with continuous contact stiffness measurements (CSM) mode and the values obtained were 12.5 ± 1.2 GPa and 152.6 ± 12.8 GPa, respectively. In addition, the characteristic “pop-in” was observed in the loading portion of load-displacement curve, which was caused by the nucleation and/or propagation of dislocations. An energetic estimation methodology on the associated nanoindentation-induced dislocation numbers resulting from the pop-in events was discussed. Furthermore, the Vickers indentation induced fracture patterns of GaP(100) single-crystal were observed and analyzed using optical microscopy. The obtained fracture toughness KC of GaP(100) single-crystal was ~1.7 ± 0.1 MPa·m1/2, which is substantially higher than the KIC values of 0.8 MPa·m1/2 and 1.0 MPa·m1/2 previously reported for of single-crystal and polycrystalline GaP, respectively.

Relying on nanoindentation technology, we investigated the elastic-to-plastic transition via first pop-in event and estimated the corresponding shear stress for incipient plasticity, i.e., yielding in the three typical orientations, i.e., X-112°, Y-36°, and Y-42° planes. The occurrence of incipient plasticity exhibited a stochastic distribution in a wide range for the three orientations. Accordingly, the obtained values of yield stress were uniform and scattered in the range from about 4 to 7 GPa for LiTaO3 single crystal. The orientation effect on yield stress at the nano-scale was revealed to be insignificant in LiTaO3 single crystal. The yield stresses were 5.44 ± 0.41, 5.74 ± 0.59, and 5.34 ± 0.525 GPa for the X-112°, Y-36°, and Y-42° planes, respectively. The activation volumes of dislocation nucleation were computed based on the cumulative distribution of yield stress, which were 12 Å3, 8 Å3, and 9 Å3 for the X-112°, Y-36°, and Y-42° planes. The results indicated that point-like defects could be the source of plastic initiation on the surface of LiTaO3 single crystal.

Thin amorphous alumina layers (10 to 40 nm thick) are processed on sputtered aluminum thin film (500 nm) by atomic layer deposition (ALD) at low temperature (85 °C). Global methodology combining quantitative experimental observations of fracture and numerical modeling is proposed to obtain the fracture strength of ALD thin film on Al layer. First, mechanical properties of the multilayer specimen are characterized by Berkovich nanoindentation, then fracture of ALD alumina is studied through spherical indentation with various tip radius. Spherical indentation load driven-displacement curves display a plateau (pop-in) at a critical load and critical indentation depth. A statistical approach is used to determine pertinent/fracture parameters from pop-in displacement. Careful SEM and AFM observations of indentation imprint exhibit circumferential cracking in agreement with the assumption that the pop-in event is predominantly controlled by the fracture of the oxide layer on the soft Al film. Finally, a numerical model calibrated with experimental results is used in order to predict both the mechanical response prior to the oxide fracture and a value of fracture strength for ALD alumina thin films. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of applied stress on mechanical properties in metallic glasses, nanoindentation tests were conducted on elastically bent Zr-Cu-Ag-Al metallic glasses with two different structure states. From spherical P-h curves, elastic modulus was found to be independent on applied stress. Hardness decreased by ~8% and ~14% with the application of 1.5% tensile strain for as-cast and 650 K annealed specimens, while it was slightly increased at the compressive side. Yield stress could be obtained from the contact pressure at first pop-in position with a conversion coefficient. The experimental result showed a symmetrical effect of applied stress on strengthening and a reduction of the contact pressure at compressive and tensile sides. It was observed that the applied stress plays a negligible effect on creep deformation in as-cast specimen. While for the annealed specimen, creep deformation was facilitated by applied tensile stress and suppressed by applied compressive stress. Strain rate sensitivities (SRS) were calculated from steady-state creep, which were constant for as-cast specimen and strongly correlated with applied stress for the annealed one. The more pronounced effect of applied stress in the 650 K annealed metallic glass could be qualitatively explained through the variation of the shear transformation zone (STZ) size. [ABSTRACT FROM AUTHOR]

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. • Although M-A constituents act as crack initiators, the matrix is the decisive feature on fracture toughness. • M-A constituents are rarely observed in the main crack path and, then, do not act as favourable routes for propagation. • M − As with high-angle boundaries and high plastic strain deflect and arrest cracks, resulting in multiple pop-ins. [ABSTRACT FROM AUTHOR]

The indentation-induced deformation mechanisms in InP(100) single crystals were investigated by using nanoindentation and cross-sectional transmission electron microscopy (XTEM) techniques. The results indicated that there were multiple “pop-in” events randomly distributed in the loading curves, which were conceived to arise primarily from the dislocation nucleation and propagation activities. An energetic estimation on the number of nanoindentation-induced dislocations associated with pop-in effects is discussed. Furthermore, the fracture patterns were performed by Vickers indentation. The fracture toughness and the fracture energy of InP(100) single crystals were calculated to be around 1.2 MPa·m1/2 and 14.1 J/m2, respectively.

The nanomechanical properties and nanoindentation responses of bismuth selenide (Bi2Se3) thin films are investigated in this study. The Bi2Se3 thin films are deposited on c-plane sapphire substrates using pulsed laser deposition. The microstructural properties of Bi2Se3 thin films are analyzed by means of X-ray diffraction (XRD). The XRD results indicated that Bi2Se3 thin films are exhibited the hexagonal crystal structure with a c-axis preferred growth orientation. Nanoindentation results showed the multiple “pop-ins” displayed in the loading segments of the load-displacement curves, suggesting that the deformation mechanisms in the hexagonal-structured Bi2Se3 films might have been governed by the nucleation and propagation of dislocations. Further, an energetic estimation of nanoindentation-induced dislocation associated with the observed pop-in effects was made using the classical dislocation theory.

This work reports the loading rate effect on nanohardness of sapphire. The intrinsic nanoscale contact deformation resistance of sapphire increased with the loading rates following empirical power law dependence with a positive exponent. The results showed a significant enhancement (e.g., ~66%) of the nanohardness of sapphire with the increase in loading rates from 10 to 10,000 μN s −1 . These results were explained mainly in terms of the maximum shear stress generated underneath the nanoindenter, dislocation density and critical resolved shear stress of the sapphire. [ABSTRACT FROM AUTHOR]

A set of undoped Al x Ga 1 − x N epilayers with different thicknesses were grown on (0001) sapphire substrates using metal-organic chemical vapor deposition (MOCVD) technique, using the same gas-phase composition of trimethylaluminum (TMAl) and trimethylgallium (TMGa). Giving that the first growth stage is crucial for the successful fabrication of Al x Ga 1-x N epilayers; thus, SiN nano-mask, as an alternative method to nanoimprint lithography (NIL) which is used for patterning substrates, was deposited on sapphire substrate. Indeed, SiN treatment initiates the three-dimensional (3D) growth mode to efficiently decrease the threading dislocation density. It was found that depending on the film surface coalescence degree, the physical and nanomechanical properties are thoroughly dependent. As the non-coalescent surface is present, the porosity effects are dominant. As soon as the film thickness attains the surface coalescence, the Al incorporation is enhanced and convoyed with it point defects. Despite the decrease of the threading dislocations along with the film thickness evolution, it was observed that the physical properties are slightly decreased and the nanomechanical properties are improved. This is likely related to point defect interaction with nucleated dislocations. Moreover, the defects did not severally affect the plastic deformation capacity (ductility) of Al x Ga 1-x N films, where high indentation force was applied and no failure was revealed. Therefore, the balance between Al x Ga 1 − x N physical and nanomechanical performances devotes to manufacture a wider range of efficient UV devices. • Different Al x Ga 1 − x N films were grown on sapphire substrate using MOCVD process. • First study of mechanical thickness Al x Ga 1-x N dependent. • Strong correlation between physical and mechanical properties. • Improvement of mechanical features with the increase of Al incorporation. • Balance between physical and mechanical features devotes to efficient UV LEDs. [ABSTRACT FROM AUTHOR]

In this study, nanoindentation tests at three different strain rates within 100 nm indentation depth were conducted on an a-plane (112¯0) ZnO single crystal to investigate the effect of strain rate on its nano-scale mechanical behavior. The load-indentation-depth curves, pop-in events, hardness and Young's moduli of an a-plane (112¯0) ZnO single crystal at different strain rates were investigated at the nano-scale level. The results indicated that, with the indentation depth increasing, the load increased gradually at each maximum indentation depth, h , during the loading process. A distinct pop-in event occurred on each loading curve except that corresponding to the ma of 10 nm. The applied load at the same indentation depth increased with the increasing strain rate during the nanoindentation of the a-plane (112¯0) ZnO single crystal. The higher strain rate deferred the pop-in event to a higher load and deeper indentation depth, and made the pop-in extension width larger. The hardness showed reverse indentation size effect (ISE) before the pop-in, and exhibited normal ISE after the pop-in. Both the hardness and the Young's modulus of the a-plane (112¯0) ZnO single crystal increased with the increasing strain rate, exhibiting the positive strain-rate sensitivity.h max of 10 nm. The applied load at the same indentation depth increased with the increasing strain rate during the nanoindentation of the a-plane (112¯0) ZnO single crystal. The higher strain rate deferred the pop-in event to a higher load and deeper indentation depth, and made the pop-in extension width larger. The hardness showed reverse indentation size effect (ISE) before the pop-in, and exhibited normal ISE after the pop-in. Both the hardness and the Young's modulus of the a-plane (112¯0) ZnO single crystal increased with the increasing strain rate, exhibiting the positive strain-rate sensitivity.

The pop-in phenomenon, quite common in fracture mechanics tests of welded joints, corresponds to a brittle crack initiation grown from a local brittle zone (LBZ) that is arrested in reaching the higher toughness material that surrounds this LBZ. A methodology to obtain a high percentage of pop-in occurrence in laboratory testing is necessary to study the pop-in significance. Such a method is introduced in this work and includes the consumable combination and welding procedures for the SMAW welding process to generate artificial LBZ. In order to find out the influence of the loading state upon the pop-in phenomenon, laboratory CTOD tests were performed using two specimen configurations: some single edge-notched specimens were loaded on a three-point bending (SE(B)) fixture while others were tested in tensile load (SE(T)). A higher frequency of pop-in occurrence was observed in the SE(B) geometry.

Multiple pop-in events which indicate a gradual transformation from a metastable austenite grain to martensites in steel were detected during nanoindentation. It was observed by means of an automotive mapping technique with TEM that the partial volume of prior austenite had transformed into several martensite blocks with different variants. From a finite element calculation combined with phenomenological approach for martensitic transformation, it was confirmed that each variant corresponded to those for which the transformation strain effectively accommodates external stress. [ABSTRACT FROM AUTHOR]