Small Volume Of Material Research Articles

Application of spherical indentation and the materials knowledge system framework to establishing microstructure-yield strength linkages from carbon steel scoops excised from high-temperature exposed components

Industrial power generation turbines operate at elevated temperatures for prolonged periods of time (around 100,000 h) which leads to significant microstructure evolution and mechanical property changes. Due to physical and structural constraints of operational turbines, only small scoop samples can be excised from heat-exposed steel components. Scoop samples at various service time intervals provide valuable information on microstructure changes, for example on graphite formation in carbon steels, with increasing service time. However, mechanical evaluation of such small material volumes poses significant challenges using conventional tests. A novel spherical microindentation technique is applied to evaluate a library of scoop samples ranging between 0 and 99,000 h of service. Furthermore, microstructure and yield strength data for the different exposure periods is used to construct a structure-property (S-P) linkage using the MKS homogenization approach that employs spatial correlations, principal component analysis, and regression techniques. The accuracy of the extracted S-P linkage was assessed on new samples that were not included in the calibration set.

Influence of some Test Parameters on Automated Ball Indentation Test Results

Automated Ball Indentation (ABI) technique has the potential to evaluate tensile and fracture properties of materials by testing a small volume of material. ABI testing involves various parameters such as surface finish of test specimen, maximum depth of indentation, number of loading-unloading cycles and material of indenter. As ABI test is yet to be standardized, it is necessary to assess and understand the consequences of the various test parameters. In this investigation, the influence of the above test parameters on the tensile properties of 316LN SS has been investigated at 298 K. It was found that by increasing the maximum depth of indentation, number of loading-unloading cycles and surface finish of test specimen at the cost of time and efforts, ABI results were not altered significantly. The selection of indenter material has to be done judiciously, according to test temperature.

Comparison of three approaches to determine the projected area in contact from finite element Berkovich nanoindentation simulations in tungsten

Nanoindentation nowadays is a standard method for the mechanical characterization of thin films and small volumes of material. One of the most meaningful parameters determined in nanoindentation experiments and simulations is the hardness of the tested material. For its determination, the knowledge of the exact value of the projected area in contact between the indenter and the specimen is essential. Inaccurate results for the projected area will result in noticeable errors in hardness. The determination of this area in finite element (FE) nanoindentation simulations is challenging because it cannot be determined directly and phenomena like pile-up and geometric imperfections of the indenter have to be considered. Hence, a new method, namely the triangulation method has been developed. It provides a reliable way to determine the projected area in FE-simulations, even under the occurrence of material pile-up. It is based on the nodes in contact between the indenter and the specimen as well as on the coordinates of the nodes. With this information, a Delaunay triangulation and Alpha shapes can be used to calculate the projected area. The triangulation method is compared to two established methods, one following the Oliver-Pharr analysis and the other one based on the computed true area in contact between specimen and indenter. The three methods are applied to results from an elastic-plastic FE simulation. Bland-Altman plots are used to compare the results of the three methods and to validate the triangulation method.

Modeling and Simulation of Nanoindentation

Nanoindentation is a hardness test method applied to small volumes of material which can provide some unique effects and spark many related research activities. To fully understand the phenomena observed during nanoindentation tests, modeling and simulation methods have been developed to predict the mechanical response of materials during nanoindentation. However, challenges remain with those computational approaches, because of their length scale, predictive capability, and accuracy. This article reviews recent progress and challenges for modeling and simulation of nanoindentation, including an overview of molecular dynamics, the quasicontinuum method, discrete dislocation dynamics, and the crystal plasticity finite element method, and discusses how to integrate multiscale modeling approaches seamlessly with experimental studies to understand the length-scale effects and microstructure evolution during nanoindentation tests, creating a unique opportunity to establish new calibration procedures for the nanoindentation technique.

An evaluation of the capability of data conversion of impression creep test

High temperature power plant components are now working far beyond their operative designed life. Therefore, establishing their in-service material properties is a matter of significant concern. Reliable secondary creep data can be produced by impression creep test (ICT), which requires a small volume of material that can be scooped from in-service critical components. This paper presents an overview of ICT to highlight the capability in determining the minimum creep strain rate (MSR) data by use of conversion relationships that relates uniaxial and impression creep tests data. Some existing ICT data are critically reviewed. Furthermore, this paper presents some new impression creep test data and their correlation with uniaxial data, obtained from a number of steels at different stresses and temperatures. The presented data, in terms of MSR against the reference uniaxial stress, are used to assess the capability and reliability of the ICT method.

Development and application of a heated in-situ SEM micro-testing device

Understanding temperature-dependent deformation behaviour of small material volumes is a key issue in material science, especially the deformation behaviour of bcc metals at elevated temperatures is of particular interest for small-scale structural applications. Therefore, a custom-built heating device consisting of independently resistive-heated sample and indenter, and adaptable to existing micro-indenters, is presented. Key parameters of material selection, design of components and temperature control are outlined. Testing temperatures ranging from room temperature up to ∼300°C are reached with low drift and without active cooling. To demonstrate the functionality, a variety of in-situ SEM micromechanical experiments were conducted at room temperature and 230°C, respectively. Examples of micro-pillar compression on single crystalline and ultrafine-grained Chromium, as well as notched cantilever fracture experiments on ultrafine-grained Chromium show assets of this powerful tool, allowing more detailed insights into temperature-dependent deformation and fracture behaviour.

Mechanical properties of hybrid composites prepared by ice-templating of alumina

Ceramic-polymer hybrid composites are often designed for its high strength and low density. Ice-templating (freeze casting) is a promising method for preparation of such composites. However, the most of the reported mechanical properties were gained from a small volume of material. In this work 70cm3 of the lamellar composite with lamella length, up to 70mm was prepared by ice-templating followed by polymer infiltration. The volume of ceramic (alumina) in starting suspensions was varied from 25 to 45vol.% and the same manufacturing process was applied. The fracture toughness and flexural strength were determined on prepared beams from plates by loading in bending. The fractographic analysis conducted on the fracture surfaces and obtained mechanical properties demonstrated that an optimal strength/density ratio lies between 51 and 55% of alumina volume fraction. The density ranking from 2.6 to 2.8cm−3 of these composites results in values of Weibull strength above 110MPa.

Determination of Creep Damage Properties from a Miniature Three Point Bending Specimen Using an Inverse Approach

In this work, a novel method for determining the creep damage properties which can be used to represent the full life until failure from a miniature bending creep test specimen is developed based on a mathematical analysis and an inverse approach. Using the Kachanov-Rabotnov creep damage model, a mathematical expression for the deflection of a simply supported, rectangular miniature thin beam creep test specimen, under three-point bending (TPB), is derived. The outputs of these equations are iterated numerically using a MATLAB program. The time-dependent deflection curves are computed as the virtue TPB tests at various loads. The accuracy of the mathematical solutions is evaluated by the corresponding results obtained from finite element analysis. On this basis, an inverse method is then developed to obtain the creep and damage constants using a MATLAB optimisation scheme, where the primary creep is neglected. The results obtained for a power plant Grade 91 Cr steel is used for demonstration. The inverse approach developed has potential applications for assessing the high temperature material strength as part of a NDT procedure and for deriving the full life creep damage constitutive properties from a small volume of material, in particular, for various microstructure regions within a heat-affected zone of weldments, e.g. of power plant pipelines and aero-engine components.

Small Punch Testing of Powder Bed Direct Laser Deposits

Additive Layer Manufacturing (ALM) technologies, such as Powder Bed Direct Laser Deposition (PB-DLD), have gained increasing popularity within the aerospace industry due to the advantages they hold over conventional processing routes. Among the advantages are the ability to produce more sophisticated cross-sectional geometries, a decrease in production lead times and an improvement to the buy-to-fly ratio. However, build quality and microstructural characteristics have a dependency on the process variables such as build direction. In order to understand the influence of grain size and build orientation on tensile behaviour, the Small Punch Tensile (SPT) testing technique has been applied to variants of the nickel based superalloy C263, manufactured using the PB-DLD method. The test technique utilises miniaturised samples, requiring only small volumes of material and is therefore a desirable test method to employ. SPT testing has characterised the mechanical properties between vertically and horizontally built PB-DLD C263 in comparison with the cast material derivative. Differences in mechanical performance between each variant have been revealed and found to be associated with microstructural variations. The deformation behaviour across each material variant have been exposed by interrupted tests. SPT results have also been accompanied by fractography, fracture energy calculations along with comparisons with uniaxial data.

Gas-to-gas heat exchanger design for high performance thermal energy storage

The mathematical modelling and optimization of a gas-to-gas heat exchanger with a non-constant cross sectional area is presented. The design of the cross sectional area of the heat exchanger analyzed is based on an hexagonal mesh, which would be highly impractical to fabricate in a conventional way but could be built relatively easily through modern manufacturing techniques. The geometric configuration proposed allows attaining a high exergy efficiency and a significant cost reduction, measured in terms of volume per unit of exergy transfer. The relationship that exists between the overall exergy efficiency of the heat exchanger and its cost is thoroughly explained throughout the study.The results obtained from the modelling demonstrate the premise that it is possible to realize designs for heat exchangers that are highly exergy-efficient and very cheap, owing to the small volume of material required, if the constrains imposed by the limitations of traditional manufacturing methods are set aside. Furthermore, the study reveals a very important fact: the volume of material in a heat exchanger increases in quadratic proportion to its characteristic dimension, which implies that scaling up the geometry has a strong impact on its cost-effectiveness.

Investigation of plastic strain rate under strain path changes in dual-phase steel using microstructure-based modeling

Micromechanical-based finite element simulations were carried out to investigate the transient plastic strain rate evolutions of ferrite and martensite dual-phase steel during strain path changes. A representative volume element (RVE) was generated through a three-dimensional (3D) reconstruction of microstructure images which were acquired from sequential polishing of a small material volume. The 10 × 10 × 10 μm3 3D RVEs consisted of martensite islands embedded in a ferrite base matrix. Each phase was assumed to exhibit distinct mechanical properties but the grain and phase boundary effects were ignored in this work. The effective mechanical properties for the constituent phases were assumed to be well defined by the von Mises or Hill 1948 yield criteria, the associated flow rule, and an empirical isotropic hardening equation based on chemical composition. This model was applied to investigate the transient behavior of the r-value (Lankford coefficient) in uniaxial tension when the loading direction changed. In addition to monotonic tension, compression-tension, and tension-orthogonal tension, sequences were considered. The simulation results captured well in a qualitative manner the experimental r-value evolutions in terms of a temporary transition and asymptotic limit. The evolutions of stress states in ferrite and martensite were analyzed to explain the r-value behavior that resulted from three factors: (1) r-value differences between ferrite and martensite, (2) martensite configuration-induced stress state in phases, and (3) stress partitioning and its evolution during non-proportional loading. Finally, an analytical relationship between the stress evolution in the constituent phases and the relevant r-value changes is suggested.

Mechanical properties determination of AM components

Characterisation of engineering materials and components is a crucial part for design and save service life utilization. Due to components processing technologies and exploitation conditions local properties can significantly vary from location to location over larger components as well as over small material volumes with gradual material changes such as welds, coatings or additively manufactured parts. The current paper is dealing with local properties characterisation for additively manufacture (AM) components by micro tensile test (M-TT). Components produced by additive manufacturing techniques yield properties variation in dependence of the considered location within the component regarding to direction in relation to deposition process. Properties vary over the thickness, length, angle or contacts with the supporting structures necessary for a successful components production by additive manufacturing techniques. The properties differences are mainly related to varying heating/reheating and cooling conditions at various locations of usually very complex parts produced mainly by these technologies. The standard testing procedures fail to characterize such local properties of complex shaped objects due to large size requirements on specimens. Therefore, new techniques have to be established for such detailed local characterizations. Results of miniaturized tensile tests application for local properties and orientations are shown here.

Plant macrofossil, pollen and invertebrate analysis of a mid-14th century cesspit from medieval Riga, Latvia (the eastern Baltic): Taphonomy and indicators of human diet

The paper presents the results of an integrated environmental analysis on the fill of an exceptionally well-preserved mid-14th century cesspit from the historic centre of Riga (Latvia, eastern Baltic). Palynological, plant macrofossils and invertebrate analysis yielded important new information about the use of plants by the indigenous community living within the medieval city, including their socio-economic status. The taphonomy of the botanical and invertebrate data is considered to largely reflect the input of undigested food waste and human faecal material with a subordinate component derived through the input of cereal waste-products. The results show that the diet of the indigenous community was based largely on cereal products, most probably bread and porridge, supplemented by a limited range of locally cultivated and/or collected vegetables and spices. Documentary sources emphasise that bee-keeping was an important element of the local economy of Riga. Elevated levels of lime pollen in the cesspit samples are taken as possible evidence for the consumption of honey, most likely of a local origin. This study also serves to demonstrate the significant quantity of information that can be gleaned from a relatively small volume of material.

Forming of Miniature Components from Powders by Combining Field-activated Sintering and Micro-Forming

Abstract Micro-FAST is a process concept which scales down conventional FAST to the micro-scale process (dealing with miniature an micro-sized components) and it combines the sintering process with a micro-forming process to enable shaping components under coupled multi-fields actions and hence, to achieve high-density, near-net-shaped components with high efficiency. The main techniques developed for overcoming the barriers for the applications of FAST at the miniature/micro-scales include: (i). Directly pressing/forming loose powders in the die without using binders; (ii). Combining heating and shaping to enable complex shapes/features; and (iii). Dedicatedly controlling fusion bonding and material’s plastic flow to enable high-quality forming. Forming from powders without using binders significantly shortened the process cycle, which also led to high-purity of the parts formed; Combining forming and sintering has led to high-density components produced as well as achieving complex-shaped components; Large current density (e.g., >100~400 KA/cm -2 ) enables very high heating rates and using small volumes of materials results in high cooling rates, leading to much shorter forming/sintering cycles which enables consolidation of micro/nanocomposites into bulk-sized components while also preserving their micro/nanostructures.

SPD Processed Materials Mechanical Properties Determination with the Use of Miniature Specimens

The main reason why new technologies and treatment procedure are being developed is to attain special mechanical properties. However, these developments are nowadays done on a small material volume either using some laboratory simulators, applying sever plastic deformation procedures or chemical composition screening for multicomponent alloys development by laser or electron beam melting. In all these application a small volume of the material assessed is available and standard procedures for crucial mechanical properties determinations are not applicable. Thus small size techniques should be applied. There has been extensively used small punch test technique (SPT) for those cases in recent years. This technique is mainly based on the evaluation using correlation between standard and SPT tests for considered material. In cases when insufficient material volume is available, those correlations cannot be established and thus comparative evaluation only can be carried out. This kind of evaluation is insufficient for the contemporary purposes, when full material potential is to be utilized. Therefore, procedures providing results directly comparable with standard specimens are being developed. Fundamental properties are those determined from tensile tests. The current paper is presenting application of developed miniature tensile test specimen method to materials after SPD processes. Quasi static properties determination is shown here for Magnesium and Titanium alloys for ECAP and Rotary Swaging SPD techniques. The results obtained from testing can be used not only for a direct material properties assessment and comparison, but also as input data for FEM codes, significantly increasing the materials considered application potential assessment.

A Brief History of Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA–ICP–MS)

Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) has been used for more than 30 years to determine the elemental composition of natural and synthesized objects. A focused laser beam ablates a small volume of target material, and the aerosol produced is transferred in a gas stream to an ICP–MS for elemental and/or isotopic analysis. Through the increasing use of deep ultraviolet lasers and ultra-sensitive mass spectrometers, the technique has evolved towards higher sampling resolution and to generating 2-D (and 3-D) images of compositional variations. The future is likely to see femtosecond lasers and simultaneous mass spectrometers in common use, making new research areas possible.

A Two-Tilt Analysis of Electron Diffraction Patterns from Transition-Iron-Carbide Precipitates Formed During Tempering of 4340 Steel

Fine-scale transition-iron-carbide precipitates in a lath martensitic microstructure of 4340 steel tempered at 200 °C for 1 h were examined via imaging and electron diffraction techniques with a transmission electron microscope. Region-to-region variations were eliminated by analyzing a small volume of material (about 0.03 μm3) at two tilt conditions. Geometric analyses showed that measured interplanar spacings compared favorably with accepted values from both epsilon-carbide and eta-carbide phases (within 1%), whereas measured interplanar angles were within 1−2% of accepted values. Centered-dark-field imaging identified precisely which reflections were produced from a single group of small precipitates (each about 10 nm in diameter). Consistent indexing schemes are provided for epsilon- and eta-carbide, including the proper angular relationship between the two tilt conditions. An interzonal angle of 17.9o ± 0.1o was determined for both candidate phases. The orientation relationship between the observed transition-iron-carbide phase and martensite was determined, and confirmed previous results reported for both carbide phases. Diffracted intensities of several reflections were estimated and compared favorably with those calculated from structure factors derived from idealized crystal structures of both transition-iron-carbide phases. All results are shown to be near-equally consistent with a hexagonal epsilon-carbide phase and an orthorhombic eta-carbide phase.

The fate of mafic and ultramafic intrusions in the continental crust

Geochemical and petrological data indicate that the bulk continental crust results from the fractionation of basaltic magmas followed by the foundering of residual mafic cumulates. Structural and geological evidence for foundering has been elusive and it is argued that it lies in the shapes of mafic intrusions that have been preserved in the crust. Numerical calculations of visco-elasto-plastic deformation induced by a dense intrusive body in continental crust have been carried out for a wide range of physical conditions. Three regimes are defined on the basis of the amount of dense material that remains at the original emplacement level as well as on the shape of the residual body. With strong encasing rocks, the intrusion deforms weakly in a sagging regime characterized by downwarping of the floor. At the other extreme, the intrusion sinks through weak surroundings, leaving behind a very small volume of material. In an intermediate regime, the intrusion does not sink wholesale and undergoes a dramatic change of shape. A residual body is preserved with a shape that depends on the aspect ratio of the initial intrusion. For aspect ratios of order one, the residual body is funnel-shaped above a thin and deep vertical extension. For the small aspect ratios that typify large igneous complexes such as the Bushveld, South Africa, the residual body is characterized by thick peripheral lobes with inward-dipping igneous layers and a thinner central area that has lost some of the basal cumulates. The transitions between these regimes depend on the rheology and temperature of encasing rocks.

Effect of Strain History of Steel Sheets on the Mechanical Characteristics of Individual Microstructural Components by Depth Sensing Indentation

The macroscopic mechanical properties of steel are highly dependent upon microstructure, crystallographic orientation of grains and distribution of each phase present, etc. Nanomechanical testing using depth sensing indentation (DSI) provides a straightforward solution for quantitatively characterizing each of phases in microstructure because it is very powerful technique for characterization of materials in small volumes. Measuring the local properties of each microstructure component separately in multiphase materials gives information that is valuable for the development of new materials and for modelling. The work experimentally analyses the effect of strain history on the mechanical properties of individual components in steel sheets by depth sensing indentation. The measurements were carried out on broken tensile specimens.

Small Two-Bar Specimen Creep Testing of Grade P91 Steel at 650°C

AbstractCommonly used small creep specimen types, such as ring and impression creep specimens, are capable of providing minimum creep strain rate data from small volumes of material. However, these test types are unable to provide the creep rupture data. In this paper the recently developed two-bar specimen type, which can be used to obtain minimum creep strain rate and creep rupture creep data from small volumes of material, is described. Conversion relationships are used to convert (i) the applied load to the equivalent uniaxial stress, and (ii) the load line deformation rate to the equivalent uniaxial creep strain rate. The effects of the specimen dimension ratios on the conversion factors are also discussed in this paper. This paper also shows comparisons between two-bar specimen creep test data and the corresponding uniaxial creep test data, for grade P91 steel at 650°C.