Elastic-plastic-brittle transitions of potassium dihydrogen phosphate crystals: characterization by nanoindentation

Potassium dihydrogen phosphate (KDP) crystals are widely used in laser ignition facilities as optical switching and frequency conversion components. These crystals are soft, brittle, and sensitive to external conditions (e.g., humidity, temperature, and applied stress). Hence, conventional characterization methods, such as transmission electron microscopy, cannot be used to study the mechanisms of material deformation. Nevertheless, understanding the mechanism of plastic-brittle transition in KDP crystals is important to prevent the fracture damage during the machining process. This study explores the plastic deformation and brittle fracture mechanisms of KDP crystals through nanoindentation experiments and theoretical calculations. The results show that dislocation nucleation and propagation are the main mechanisms of plastic deformation in KDP crystals, and dislocation pileup leads to brittle fracture during nanoindentation. Nanoindentation experiments using various indenters indicate that the external stress fields influence the plastic deformation of KDP crystals, and plastic deformation and brittle fracture are related to the material’s anisotropy. However, the effect of loading rate on the KDP crystal deformation is practically negligible. The results of this research provide important information on reducing machining-induced damage and further improving the optical performance of KDP crystal components.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-020-00320-3     

Dislocation Inelastic Effects in Zinc Single Crystals

Dislocation internal friction,elastic modulusdefect and their ratio r have been studied in an-nealed and plastically deformed Zn single crystals inthe temperature range of 80 to 300 K in a widerange of oscillation amplitudes.The results ob-tained are discussed within the present notion aboutthe nature of dissipative elastic oscillation losses insolids.     

High-Throughput Nanoindentation Mapping of a Microsegregated CoCrFeNi Multi-Principal Element Alloy (MPEA): Challenges and Limitations

Equimolar CoCrFeNi is a popular multi-principal element alloy, generally obtained by melt-aided routes. The dendritic microstructure of as-cast alloys is cooling-rate dependent and thus may show different macromechanical behaviors. Knowledge about the nanomechanics of chemically/structurally distinct zones is important for explaining these differences and facilitating material's design through processing conditions. To this aim, high-throughput nanoindentation mapping is a potentially powerful tool. However, results are possibly biased by the indentation size effect (ISE) and plastically deformed surfaces induced by inadequate sample preparation. This work is aimed at giving some guidelines for map acquisition and sample preparation based on data from various nanoindentation techniques (i.e., high-throughput nanoindentation, continuous stiffness, and quasi-static measurements) collected on differently polished surfaces. It is shown that conventional metallographic preparation leads to a plastically deformed layer that penetrates deep into the surface (11–17 μm). Electropolishing is efficient in removing this layer. However, difficulties may arise due to preferential corrosion in a multiphase system wherefore polishing conditions needs optimization. Nanoindentations of adequately prepared surfaces of CoCrFeNi result in an important ISE, which affects measurements at depths lower than ca 2000 nm. This must be taken into account when performing high-throughput nanoindentation mapping of this material.     

Growth of unidirectional potassium dihydrogen orthophosphate single crystal by SR method and its characterization

Unidirectional <100> potassium dihydrogen orthophosphate (KDP) single crystals were grown for the first time by Sankaranarayanan-Ramasamy (SR) method. The <100> oriented seed crystals were mounted at the bottom of the glass ampoules and the crystal of 15 mm diameter, 65 mm height were grown by SR method. The grown crystals were characterized by high-resolution X-ray diffraction (HRXRD), etching studies. The HRXRD analysis indicates that the crystalline perfection is excellent without having any very low angle internal structural grain boundaries. Dislocation density is less in SR grown KDP compared to conventional method grown KDP.     

A new view of the onset of plasticity during the nanoindentation of aluminium

In nanoscale contact experiments, it is generally believed that the shear stress at the onset of plasticity can approach the theoretical shear strength of an ideal, defect-free lattice, a trend also observed in idealized molecular dynamics simulations. Here we report direct evidence that plasticity in a dislocation-free volume of polycrystalline aluminium can begin at very small forces, remarkably, even before the first sustained rise in repulsive force. However, the shear stresses associated with these very small forces do approach the theoretical shear strength of aluminium (approximately 2.2 GPa). Our observations entail correlating quantitative load-displacement measurements with individual video frames acquired during in situ nanoindentation experiments in a transmission electron microscope. We also report direct evidence that a submicrometre grain of aluminium plastically deformed by nanoindentation to a dislocation density of approximately 10(14) m(-2) is also capable of supporting shear stresses close to the theoretical shear strength. This result is contrary to earlier assumptions that a dislocation-free volume is necessary to achieve shear stresses near the theoretical shear strength of the material. Moreover, our results in entirety are at odds with the prevalent notion that the first obvious displacement excursion in a nanoindentation test is indicative of the onset of plastic deformation.     

A method to quantitatively measure the elastic modulus of materials in nanometer scale using atomic force microscopy

A method is proposed for quantitatively measuring the elastic modulus of materials using atomic force microscopy (AFM) nanoindentation. In this method, the cantilever deformation and the tip-sample interaction during the early loading portion are treated as two springs in series, and based on Sneddon's elastic contact solution, a new cantilever-tip property α is proposed which, together with the cantilever sensitivity A, can be measured from AFM tests on two reference materials with known elastic moduli. The measured α and A values specific to the tip and machine used can then be employed to accurately measure the elastic modulus of a third sample, assuming that the tip does not get significantly plastically deformed during the calibration procedure. AFM nanoindentation tests were performed on polypropylene (PP), fused quartz and acrylic samples to verify the validity of the proposed method. The cantilever-tip property and the cantilever sensitivity measured on PP and fused quartz were 0.514?GPa and 51.99?nm?nA(-1), respectively. Using these measured quantities, the elastic modulus of acrylic was measured to be 3.24?GPa, which agrees well with the value measured using conventional depth-sensing indentation in a commercial nanoindenter.     

Room temperature nanoindentation creep of nanoscale Ag/Fe multilayers

This paper deals with room temperature indentation creep behavior of nanoscale Ag/Fe multilayers. The constant-load nanoindentation test results reveal that all the multilayers exhibit steady-state creep after transient creep occurring at first 150 s and decreasing periodicity leads to a decrease in the stress exponent and an increase in creep rate. The dependence of the stress exponent and creep rate on the periodicity indicates that the creep process is dominated by dislocation glide-climb mechanism and the increasing fraction of grain boundaries and interfaces provide effective diffusion paths for the creep climb that determines the whole creep rate.     

Depth sensing indentation of linear viscoelastic-plastic solids: A simple method to determine creep compliance

Viscoelastic solids may deform plastically under indentation. This leads to an overestimation of the creep compliance when the analytical solution for indentation of linear viscoelastic materials [Lu H, Wang B, Ma J, Huang G, Viswanathan H. Measurement of creep compliance of solid polymers by nanoindentation. Mech Time-Depend Mater 2003;7(3-4):189-207] is used for its determination. Using finite element analysis, in this work it is shown that the plastic and viscoelastic deformation processes occur simultaneously, even during holding of a constant indentation load. A simple procedure to separate the viscoelastic response from the plastic response is proposed, which involves spherical indentations at different loads. To illustrate the proposed method, indentation tests are conducted on a polymer, polymethylmethacrylate (PMMA), and its viscoelastic properties are determined.     

Microstructural characterization of calcium flouride single crystals deformed in steady state

Calcium fluoride single crystals have been deformed in compression to conditions of steady-state deformation in the temperature range 590 to 907° C (0.53 to 0.72T/T _m). The deformation microstructures have been characterized using cold-stage transmission electron microscopy. The microstructure of deformed samples is seen to consist of dislocation tangles, networks and subgrain boundaries. Dislocation structures in the subgrain boundaries have been characterized and the effect of the temperature of deformation on the subgrain boundary structure has been established. The flow stress,σ, during steady-state deformation, has been found to be proportional tod ^−1.14, whered is the subgrain size. The steady-state deformation behaviour is believed to be controlled by the mechanisms of obstacle-limited glide of dislocations and power-law creep. During characterization of the deformation microstructures, regions of non-uniform cell or subgrain boundary structure have been observed. It has been suggested that such regions arise from either non-uniform deformation or recovery and recrystallization. Despite the presence of such regions, subgrain strengthening appears to be a viable means of improving the flow stress of calcium fluoride single crystals.     

A critical appraisal of the nanoindentation creep 'nose' effect in Ni thin films

The creep behaviour of polycrystal Ni thin films under the same maximum load (Pmax = 8000 microN) and different unloading periods (ranging from 1 to 250 s) has been investigated at room temperature using nanoindentation tests. A 'nose' has been observed in the unloading segment of the load-penetration depth curve when the holding time at peak load is short and/or the unloading rate is small, and when the peak load is sufficient high. When a 'nose' presents, the apparent unloading stiffness Su, defined as dP/dh, is negative, and the reduced modulus can no longer be calculated from the Oliver-Pharr method. Taking such uncertainties into account, a critical appraisal is proposed for ranking creep propensities exhibited during nanoindentation under specified conditions.     

On the measurement of pile-up corrected hardness based on the early Hertzian loading analysis

Nanoindentation has been used to gain insight into the elastic/plastic contact responses of material at very small scales. The Oliver and Pharr's analysis (W.C. Oliver and G.M. Pharr, J. Mater. Res. 7 (1992) 1564) on the nanoindentation curve, however, can be meaningless when plastically deformed material piles around the indented points. This study suggests a measuring methodology of the real contact area enlarged by the material pile-up and its corresponding mechanical properties; the pile-up corrected contact area can be calculated inversely from the reduced modulus formulation with input information of the independently determined Young's modulus based on the Hertzian loading analysis. This contact correction relaxed overestimates in the elastic modulus and hardness interpreted from the nanoindentation curve and yielded actual mechanical properties comparable to the literature values of a (100) tungsten monocrystal. In addition, theoretically estimated upheaval amount of the contact boundary in this study was nearly consistent with the average pile-up height measured from an atomic-force microscope.     

Plastic deformation of polyethylene crystals by dislocation motion

Solvent-free polyethylene single crystals were plastically deformed by depositing the crystals on carbon replicas obtained from the surfaces of deformed copper single crystals. The observations by transmission electron microscopy suggest that plastic shear parallel to the molecular chains of the polyethylene crystals ([001]-direction) occurs by the movement of dislocations the Burgers vector of which is parallel to the [001]-direction.     

Annihilation of deformation-produced vacancy clusters in alkali halides at room temperature

The aging of plastically deformed crystals of NaCl and KCl at room temperature for a long time results in a bulk density increase as evidenced by thermal gradient flotation measurements. The effect is explained by the annihilation of deformation-induced clusters on dislocations. No density variation was found in LiF and NaF crystals. A model has been suggested for the formation of vacancy clusters by disrupture of unrelaxed dislocation dipoles. From the model it follows that cluster size and mobility should decrease in the homologic row of alkali halides, which is consistent with experimental evidence. The cluster diffusion coefficient has been estimated to be 10^?16 cm²s^?1 at room temperature.     

Creep behaviour of injection moulded polyamide 6/organoclay nanocomposites by nanoindentation and cantilever-bending

The creep behaviour of injection moulded PA 6/organoclay nanocomposites was studied by depth-sensing nanoindentation and DMA cantilever-bending. The glass transitions of PA 6 and its nanocomposites were decreased below room temperature upon saturation with water so that the materials could be tested in the rubbery regime. For nanoindentation creep on the skin and core regions of injection moulded samples, whilst organoclay improves the creep resistance of PA 6, the enhancement is due to the decrease of the initial compliance at zero time but the time-dependent creep is actually increased. In contrast, for cantilever-bending creep, organoclay reduces the creep compliance and the time-dependent creep in PA 6. It is suggested that the organoclay imparts a constraint effect on the PA 6 molecular chains, restricting their mobility in the bulk compared to the surface and hence improving their resistance to creep. A modified Halpin-Tsai equation was used to model their creep behaviour under these two loading configurations and compared to experimental data.     

喷射沉积8009/SiCP铝合金的陶粒轧制成形性能研究

针对喷射沉积铝合金板坯含有一定量的孔隙、直接轧制时易出现板坯表面横裂及边裂的问题,探讨了一种新型的准等静压轧制工艺-陶粒轧制.主要研究在陶粒轧制工艺过程中传压介质对喷射沉积8009/SiCP铝合金板坯轧制成形性能的影响.结果表明,在陶粒轧制过程中合理设计、选取及控制传压介质,避免了板坯轧制变形过程中裂纹的形成,有效地提高了喷射沉积铝合金板坯的轧制成形性能.     

Influence of monotonic and cyclic predeformation on fatigue crack propagation of high-strength aluminum alloys

Regarding the plastically deformed volume ahead of a propagating fatigue crack two types of plastic zones can be distinguished, the K_max-zone which is generated during the load increase upon maximum loading in a cycle and where monotonie tensile deformations occur, and the Δ K-zone, where the crack tip material is reversed plastically deformed.

In order to achieve similar deformation structures as they are present within these two types of plastic zones a predeformation was applied on the bulk material. The deformation structure of the K_max-zone was simulated by defined cold rolling and the cyclic predeformation structure by reversed cyclic plastic deformation. The crack propagation behaviour of the monotonically and cyclically predeformed material and of the completely non-predeformed material was observed. The tests showed that predeformation influenced the crack propagation rate: an increase in the predeformation led to an increase in the crack propagation rate.

In tests with variations in the loading conditions, e.g. a change from a high to a low loading level, considerable sequence effects were observed. The tests with predeformed and non-predeformed materials showed less retardation for the monotonically as well as for the cyclically predeformed material. The ranges in the crack lengths where retardation occurred after the decrease in the loading level, were smaller in the predeformed material. The test results give a clear indication of the significance of the plastic zone behaviour regarding sequence effects after variations in the loading conditions.     

Plasticity in pentacene thin films

We have investigated the structure, defects and plasticity of thermally evaporated thin films of the organic molecular semiconductor pentacene using X-ray Diffraction (XRD), Optical microscopy (OM), Transmission Electron Microscopy (TEM), Electron Diffraction (ED), and High Resolution Electron Microscopy (HREM). Using XRD the degree of (001) texturing present in the as-grown films was characterized. The nature of pentacene plasticity and deformation-induced molecular alignment was investigated using rubbing and scratching techniques, as well as nanoindentation. Rubbing of the bulk powder produced thin oriented films, and a deformation length scale dependence was seen. Under stress pentacene crystals initially fail by cracking, until they reach a critical size of about one micron, when they tend to plastically deform into thin sheets. Alignment of thermally evaporated films was achieved under a controlled load scratch, and the degree of molecular orientation inside the scratched region was directly imaged using HREM. Finally, using nanoindentation we measured pentacene's plastic hardness to be 0.25 GPa at a loading rate 0.05 mN/s. A loading rate dependence of the hardness and stiffness was measured, with thin films behaving harder and stiffer at faster indentation rates.     

Investigation of the nanomechanical properties of nylon 6 and nylon 6/clay nanocomposites at sub-ambient temperatures

The nanomechanical properties of nylon 6, nylon 6/exfoliated clay and nylon 6/non-exfoliated clay nanocomposites have been investigated from room temperature to ?10 °C in a controlled environment with humidity less than 1% RH. The hardness, elastic modulus and creep resistance of nylon 6 were improved in the nanocomposites across the temperature range. However, the effective reinforcement of the clay depended on the temperature due to the change between the glassy and transition states in the nylon. The exfoliated clay nanocomposite showed the greater improvements than in the non-exfoliated clay nanocomposite at all testing temperatures due to the improved constraint of the polymer chains by the clay platelets in the exfoliated structure. The surface mechanical properties of nylon 6 and the nanocomposites were also found to be highly sensitive to the moisture level during the tests; increasing the humidity in the room temperature tests resulted in a dramatic decrease in hardness and stiffness due to plasticisation by water molecules. The kinetics of the re-humidification process on nylon 6 were studied by monitoring the change in nanoindentation response. Analysis of the indentation creep revealed a significant change in the strain rate sensitivity when the humidity of the near-surface region probed by nanoindentation was in the vicinity of the glass transition.     

Rate equation for recovery,precipitation, transient creep,and sintering

Abstract

The empirical rate equations of solid state reactions such as recovery of deformed metals, clustering, and transient creep are all of the same type, referred to here as recovery kinetics. For two systems, transient creep in fcc metal crystals and the first stage of tempering of Fe–C and Fe–Ni–C martensite, the rate equations were analysed 30 years ago using the increment technique. These results can be used to deduce the common physical principles which govern the mechanisms operating in the various systems. From this basis, it can be concluded that the densification kinetics in the intermediate stage sintering of certain ceramic green pellets is also a type of recovery kinetics. Initial experimental results are shown to support this conclusion.

MST/1048     

Observations of contact damage in MgO and LiF crystals by cathodoluminescence

Cathodoluminescence (CL) studies in a scanning electron microscope of the static and dynamic contact damage in MgO and LiF crystals are described. The main luminescence for both MgO and LiF was found to be associated with the plastically deformed zone at and around the contact site, although there were differences of details in the CL behaviour of the two materials. It was also found that in MgO the intensity of luminescence from screw dislocations was markedly higher than that from the edge dislocations for all possible orientations of the specimen. It is proposed that this simple and rapid technique can be used for assessing the mechanical state of a surface.