Pop-in Behavior Research Articles

Water-enhanced plasticity of calcium fluoride

Calcium fluoride (CaF2) ionic crystals have been observed to be sensitive to humid environments during ultra-precision machining, but further discussion about detailed mechanisms of the corresponding water-induced mechanical behaviors is still lacking. This work aims to perform a systematic investigation to understand the effect of water on the deformation and correlated plastic enhancement of CaF2 under indentation. Micro-indentation experiments were employed to observe different mechanical responses including the pop-in behavior and surface pile-up to identify the water-enhanced plasticity of CaF2 preliminarily. Then, atomic simulations were performed to reproduce the indentation process and confirm the close relationship between the indentation behaviors and water. The dislocation activities and crystal slip of both simulations with and without water were captured to analyze the plastic flow induced by hydration. Furthermore, Raman spectroscopy measurement was also adopted in this study to explain the corresponding enhancing mechanisms of plastic behaviors from the perspective of surface effect.

Strain-rate insensitive photoindentation pop-in behavior in ZnS single crystals at room temperature

Strain-rate insensitive photoindentation pop-in behavior in ZnS single crystals at room temperature

Mechanical and microstructural analysis of tungsten exposed in JET deuterium plasmas

A tungsten Langmuir probe exposed in the JET divertor during the ITER-like wall campaigns (ILW) has been studied to evaluate changes in mechanical properties and microstructure. The tip of the probe that was exposed to plasma was cross-sectioned and polished for post mortem analysis. Analysis involved a comparison with a non-exposed probe to determine the effect of plasma exposure on material microstructure and mechanical properties. Visually the probe appeared to have melted and re-solidified during its time in the vessel. Secondary electron (SE) images of cross sections showed the formation of bubbles near the exposed surface that ranged from 50 µm to sub-micron sized. Electron backscatter diffraction (EBSD) revealed that the average grain size had increased from 33 µm to 570 µm. The investigation also showed that hardness had increased from 5.2 to 6.1 GPa and pop-in behaviour was supressed after exposure. This was initially attributed to the uptake of deuterium (D) but nuclear reaction analysis (NRA) indicated that no deuterium remained in the sample and hinted that some other type of defect was modifying the mechanical properties.

Fatigue crack growth mechanism in ferroelectric ceramics under alternating electric field: An investigation by digital image correlation technique

The deformation of crack tip in PZT ceramics under cyclic electric field loading was characterized by Digital Image Correlation (DIC) technique. Two other specimens without speckles were used to observe the physical processes during the crack propagation in-situ. The deformation of the region of interest is elucidated. Near the U-shaped notch, the DIC results directly reveal the existence of incompatible deformation. However, in the front of the newly formed crack, no incompatible deformation was observed. Additionally, in-situ observation reveals that, the crack growth occurs at the stage of crack closure when the crack propagates steadily. The observed pop-in behavior of the crack during the first cycle of electric field loading is rationalized by the interaction between the incompatible domain switching zones with and without U-shaped notch, which is consistent with the explanation of Westram et al. The subsequent crack propagation after the pop-in cannot be rationalized by the model exploited by Zhu and Yang, Westram et al., and Liu and Fang, which is thought to be related to three different factors – arcing, wedging and grain-to-grain interaction. The mechanism for the appearance and the disappearance of the incompatible domain switching zone near the U-shaped notch and around the tip of the newly formed crack is discussed based on the electrical boundary condition (EBC) of the crack surfaces we proposed recently.

Effect of irradiation temperature on the nanoindentation behavior of P92 steel with thermomechanical treatment

The nanoindentation behavior of P92 steel with thermomechanical treatment under 3.5 MeV Fe13+ ion irradiation at room temperature, 400 and 700 °C was investigated. Pop-in behavior is observed for all the samples with and without irradiation at room temperature, while the temperature dependence of pop-in behavior is only observed in irradiated samples. The average load and penetration depth at the onset of pop-in increase as the irradiation temperature increases, in line with the results of the maximum shear stress. Irradiation induced hardening is exhibited for all irradiated samples, but there is a significant reduction in the hardness of sample irradiated at 700 °C in comparison to the samples irradiated at room temperature and 400 °C. The ratio of hardness to elastic modulus for all samples decreases with increasing penetration depth except for samples at 700 °C. With the increasing of irradiation temperature, the ratio of the irreversible work to the total work gradually decreases. In contrast, it increases for samples without irradiation.

In situ electrochemical nanoindentation of a nickel (111) single crystal: hydrogen effect on pop-in behaviour

Abstract The hydrogen effect on dislocation nucleation in Ni single crystals with (111) surface orientation has been examined with the aid of a specifically designed nanoindentation setup for in situ electrochemical experiments. The effect of the electrochemical potential on the indent load – displacement curve, especially the unstable elastic-plastic transition (pop-in), was studied in detail. The experiments allowed the exclusion of the surface from hydrogen effects. The observations showed a pop-in load drop from an average value of 250 to 100 μN due to in situ hydrogen charging, which is reproducibly observed within sequential hydrogen charging and discharging. Clear evidence is provided that hydrogen atoms facilitate homogeneous dislocation nucleation.

Understanding the hydrogen effect on pop-in behavior of an equiatomic high-entropy alloy during in-situ nanoindentation

• In-situ electrochemical nano-indentation test was applied on Cantor alloy. • The pop-in behavior was investigated under different H charging conditions. • Both pop-in width and load were reversibly reduced by H. • The H reduction effect on pop-in width is more noticeable than that of pop-in load. • An energy balance model was used showing H reduced dislocation mobility. The variations in the pop-in behavior of an equiatomic CoCrFeMnNi high-entropy alloy under different hydrogen charging/discharging conditions were characterized via in-situ electrochemical nanoindentation. Results show that hydrogen accumulatively reduces both pop-in load and width, among which the reduction of pop-in width is more noticeable than that of pop-in load. Moreover, the hydrogen reduction effect on both pop-in load and width is reversible when hydrogen is degassed during anodic discharging process. Particularly, the hydrogen-reduced pop-in width was studied in detail by a comprehensive energy balance model. It is quantitatively shown that the dissolved hydrogen enhances lattice friction, leading to an increased resistance to dislocation motion. As a result, fewer dislocations can be generated with a higher hydrogen concentration, causing a smaller pop-in width. This is the first time that the pop-in width indicated dislocation mobility under hydrogen impact is quantitively revealed.

Experimental Analysis of HSLA Steel Welded Joint Fracture Behaviour

In order to assess the integrity of welded pressure vessel containing a crack, the behavior of three-point bending specimens made of HSLA steel welded joint are tested at -40°C, and their fracture behaviour analyzed, as a conservative measure of crack resistance. Specimens were made of low-alloyed steel NIONIKRAL 70, welded by SAW process. Surface notches were machined by the electro-erosion method in the parent metal (PM), heat affected zone (HAZ) and weld metal (WM). The highest resistance to cracking was in PM, and the lowest one in WM, due to so-called pop-in behaviour.

The Fabrication and Indentation of Cubic Silicon Carbide Diaphragm for Acoustic Sensing.

In this study, 550 nm thick cubic silicon carbide square diaphragms were back etched from Si substrate. Then, indentation was carried out to samples with varying dimensions, indentation locations, and loads. The influence of three parameters is documented by analyzing load-displacement curves. It was found that diaphragms with bigger area, indented at the edge, and low load demonstrated almost elastic behaviour. Furthermore, two samples burst and one of them displayed pop-in behaviour, which we determine is due to plastic deformation. Based on optimum dimension and load, we calculate maximum pressure for elastic diaphragms. This pressure is sufficient for cubic silicon carbide diaphragms to be used as acoustic sensors to detect poisonous gasses.

A Molecular Dynamics Simulations Study of the Influence of Prestrain on the Pop-In Behavior and Indentation Size Effect in Cu Single Crystals

The pop-in effect in nanoindentation of metals represents a major collective dislocation phenomenon that displays sensitivity in the local surface microstructure and residual stresses. To understand the deformation mechanisms behind pop-ins in metals, large scale molecular dynamics simulations are performed to investigate the pop-in behavior and indentation size effect in undeformed and deformed Cu single crystals. Tensile loading, unloading, and reloading simulations are performed to create a series of samples subjected to a broad range of tensile strains with/without pre-existing dislocations. The subsequent nanoindentation simulations are conducted to investigate the coupled effects of prestrain and the presence of resulting dislocations and surface morphology, as well as indenter size effects on the mechanical response in indentation processes. Our work provides detailed insights into the deformation mechanisms and microstructure-property relationships of nanoindentation in the presence of residual stresses and strains.

Probing hydrogen effect on nanomechanical properties of X65 pipeline steel using in-situ electrochemical nanoindentation

The hydrogen effect on a X65 carbon steel was investigated using in-situ electrochemical nanoindentation approach. The alterations in elastic behavior, pop-in load, and hardness under hydrogen-free and hydrogen-charged conditions in both ferrite and bainite were compared and discussed. The results demonstrated a non-affected elastic behavior by hydrogen in both microconstituents. The homogeneous and heterogeneous dislocation nucleation are proposed as the dominant mechanisms for pop-in behavior in ferrite and bainite, respectively. In addition, the reduction of pop-in load by hydrogen in both microconstituents indicates a hydrogen-enhanced dislocation nucleation in both homogenous and heterogeneous manners. Moreover, a hydrogen-induced hardness increment was detected in both microconstituents, which is related to the hydrogen-enhanced lattice friction on dislocations. Also, the more prominent hardness increment in bainite was caused by its significantly more trapping sites.

Stress-dependent incipient plasticity of a face-centered-cubic-based Al0.3CoCrFeNi multi-principal element alloy with nano-scaled phase separation

The stress-dependent incipient plasticity was probed by measuring the first pop-in behavior in a face-centered-cubic-based Al0.3CoCrFeNi multi-principal element alloy with nano-scaled phase separation. A large number of indentation measurements revealed that the operation of a bimodal distribution for the maximum shear stress was associated with different dislocation nucleation sites. This bimodal distribution phenomenon can be used to characterize the structural heterogeneity of the alloy. The activation volume and the dislocation nucleation rate were calculated to make a further investigation to the different dislocation nucleation mechanisms.

A unified statistical model for indentation pop-in: Effects of indenter radius and microstructure density on the transition from homogeneous nucleation to heterogeneous nucleation to bulk plasticity

Indentation pop-in, i.e. a sudden displacement burst on the measured load-penetration depth curves, has been known as the onset of elastic-plastic deformation for crystalline materials. Depending on the indenter radius or density of pre-existing dislocation nucleation sources, the evolution of pop-in shear stress can generally be categorized into three deformation stages, i.e. the homogeneous dislocation nucleation stage, heterogeneous dislocation nucleation stage and bulk plasticity stage. For the former, the fluctuation of pop-in stress is dominated by the thermally activated nucleation process. For the middle, a size-dependent pop-in behavior with stress fluctuation is informed over a wide range of indenter radius and microstructure density. For the later, the materials strength is determined by the critical resolved shear stress for the motion of existing dislocations. Here, we find two additional transition stages between the adjacent deformation stages that can be effectively addressed by a novel competition mechanism, and the transition points are affected by the density of dislocation nucleation sites. All these critical features are well characterized by a unified statistical model proposed in this work. Moreover, a mechanistic model with closed-form formulas is developed for the size-dependent pop-in behavior during the heterogeneous dislocation nucleation stage. By comparing both the calibrated and predicted theoretical results with different sets of experimental data, good agreements are achieved that can rationally verify the proposed model. This work presents a theoretical framework to characterize the fundamental pop-in mechanisms, and provides an avenue towards the prediction of transition points distinguishing different plasticity deformation regimes.

Effect of orientation and loading rate on the incipient behavior of Ti60(AlCrVNb)40 medium entropy alloy

The influence of orientation and loading rate on the pop-in behavior of Ti60(AlCrVNb)40 medium entropy alloy (MEA) under nanoindentation was investigated. The crystal structure of Ti60(AlCrVNb)40 is a single body centered cubic phase, confirmed by both X-ray diffractometer and transmission electron microscopy. Nanoindentation experiments were carried out on the (100), (110) and (111) orientations and over loading rates of 10–500 μN/s. The incipient plasticity showed a clear orientation and loading rate dependences. Specifically, the (110) data exhibit the lowest pop-in load and pop-in displacement, and the associated loads of all orientations increase with increasing loading rate. In addition, the spread of the load at pop-in for (110) is narrower compared to those for (100) and (111), which is caused by the lower influence from defects on (110). The activation volume was estimated to be ~0.7 b3, which falls between those of conventional metals (~0.3 b3) and high entropy alloys (HEAs) (~1.7 b3), suggesting that dislocation activities in MEAs is more difficult than that in traditional metals but easier than that in HEAs. Such a medium activation volume implies that the onset of yielding is associated with a mixture of two mechanisms: heterogeneous nucleation of dislocation through atomic-sized precursors and/or expansion of dislocation loop by pre-existing dislocation multiplication. The crystalline orientation effects can be rationalized by the different densities and dislocation configurations resulted from the different numbers of activated slip systems and Schmid factors for different loading planes.

Does the stacking fault energy affect dislocation multiplication?

This paper investigates the interdependence of the stacking fault energy (SFE) and the stress required for dislocation multiplication. For this purpose copper-aluminum (Cu-Al) alloys with varying Al content are investigated by spherical nanoindentation. The load-displacement curve shows a characteristic pop-in, which is interpreted as elastic-plastic transition and correlated to the dislocation source nucleation and/or activation stress. The statistical analysis of the pop-in behaviour documents a strong dependence of the maximum shear stress beneath the indenter on the Al content, which cannot be explained by a variation in shear modulus. Instead, the pop-in stress clearly increases with increasing stacking fault energy.

Effect of Hydrogen Charging on Pop-in Behavior of a Zr-Based Metallic Glass

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.

Indentation pop-in behavior of CoCrFeNiAl0.3 high-entropy alloy

The pop-in behavior of face-centered cubic (FCC) CoCrFeNiAl0.3 high-entropy alloy (HEA) under different loading rates was investigated by nanoindentation with spherical indenter. Experimental results show the distinct three-stage feature of the first pop-in, namely the elastic stage before pop-in, linear plastic stage and deceleration plastic stage for the pop-in evolution. Rate effect was analysed and critical shear stress required for the first pop-in event is obtained. A rheological model was proposed to characterize the pop-in behavior of CoCrFeNiAl0.3 HEA under different loading rates, and the related model parameters are given.

Pop-in behavior and elastic-to-plastic transition of polycrystalline pure iron during sharp nanoindentation

This study analyzes the elastic-to-plastic transition during nanoindentation of polycrystalline iron. We conduct nanoindentation (Berkovich indenter) experiments and electron backscatter diffraction analysis to investigate the initiation of plasticity by the appearance of the pop-in phenomenon in the loading curves. Numerous load–displacement curves are statistically analyzed to identify the occurrence of pop-ins. A first pop-in can result from plasticity initiation caused by homogeneous dislocation nucleation and requires shear stresses in the range of the theoretical strength of a defect-free iron crystal. The results also show that plasticity initiation in volumes with preexisting dislocations is significantly affected by small amounts of interstitially dissolved atoms (such as carbon) that are segregated into the stress fields of dislocations, impeding their mobility. Another strong influence on the pop-in behavior is grain boundaries, which can lead to large pop-ins at relatively high indentation loads. The pop-in behavior appears to be a statistical process affected by interstitial atoms, dislocation density, grain boundaries, and surface roughness. No effect of the crystallographic orientation on the pop-in behavior can be observed.

Dependence of pop-in behavior of a high-entropy alloy FeCoCrMnNi on tip radius

Our previous work has established that the dislocation nucleation during the onset of plasticity, the so-called pop-in, in face-centered cubic single-phased high-entropy alloy FeCoCrMnNi is controlled by vacancy defects. This implies that the tip radius would affect the pop in behavior as the number of vacancy, the available sites for dislocation nucleation, within the stressed volume is proportional to radius. To verify this in current work, a wide range of nanoindenter tips with radius across from 200 to 2013 nm were used. It was found that when tip radius is smaller than 638 nm, the pop-in or displacement burst size increases linearly with it, and that when tip radius is larger than 638 nm, the pop—in size became essentially constant. These experimental findings confirm the effect of tip radius on pop-in behavior. A theoretical model based on image force has been developed to rationalize the above observations.

Impact of crystal orientation on pop-in behavior of magnesium

Impact of crystal orientation on pop-in behavior of magnesium