Methods of actual indenter shape determination

Knowledge of the actual indenter shape (area function) is fundamental for the correct evaluation of measured indentation data. Various methods and models were developed for the determination of indenter area function. Nevertheless, these methods usually do not have clear geometrical interpretation and/or their application is based on some severe assumption so they cannot be applied generally. Basically, the shape of the indenter can be determined by two different procedures – indentation into reference materials with known mechanical properties or reconstruction of the tip by direct imaging methods, e.g. AFM. The aim of this study is the comparison of these two procedures applied for characterization of Berkovich and spherical diamond indenters. Several materials were used for reference indentation measurements and it was found that the area function obtained on studied materials significantly differs. The area function determined by this technique in fact does not correspond to the actual indenter shape but it characterizes the convolution of actual indenter shape and material surface and deformation characteristics. It means that this technique is appropriate only while testing the materials with the properties close to the reference sample. Please click Additional Files below to see the full abstract

DEFORMATION OF FE3SI SINGLE-CRYSTALS UNDER NANOINDENTATION

Knowledge of the complex deformation behavior in the anisotropic materials is one of essential issues in materials science and it is crucial for the applications of a given material. In this study, mechanical response of Fe3(wt.%)Si single crystal to nanoindentation with spherical indenter was investigated. Hardness and indentation Young´s modulus were determined experimentally and by finite element modelling. Observed pop-in phenomenon, shape of the residual imprints and origin of the slip lines were explained on the basis of resolved shear stress computed by finite element model

COMPARISON OF TWO TECHNIQUES FOR EVALUATING SPHERICAL INDENTATION DATA

Flow curves of 15Kh2MFA, Sv 08Kh19N10G2B and 08Kh18N10T steels used for fabrication of WWER-440 nuclear reactor pressure vessel and core internals were obtained using the automated ball indentation (ABI) test technique and compared with flow curves evaluated from the same measured load-displacement data and widely used Oliver-Pharr method. Differences in results obtained by both studied methods do not exceed 12 % and are attributed to the amount of material pile-up

Nanoindentation induced reversible plasticity detected by acoustic emission

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Nanomechanical characterization of high pressure torsion processed HfNbTaTiZr high entropy alloy

High entropy alloys (HEAs) are a new material class in which the configurational entropy of a multicomponent solid solution phase is maximized so that the entropy of mixing stabilizes disordered solid solution phases against the possible intermetallic phases development. Generally, to achieve high entropy of mixing, the alloys contain typically five or more major elements in equimolar concentrations. The composition of HEAs is generally based on 3d transition metals, refractory metals, light metals, lanthanide transition metals, precious metals, brasses and bronzes. The HEAs exhibit promising structural and mechanical properties in wide range of applications. Mechanical properties of such alloys can further be improved by grain refinement especially by severe plastic deformation. However, studies of ultrafine grained HEAs are rather scarce in the literature. An increase of strength with decreasing grain size was achieved in the probably most investigated HEA i.e. Cantor alloy (equiatomic CoCrFeMnNi with fcc structure) processed by high pressure torsion (HPT) [1]. Much less attempts were made to process in such a way HEAs with bcc structure. Recently HfNbTaTiZr bcc HEA was successfully nanostructured by HPT straining [2]. It was reported that grain refinement by HPT resulted in a significant enhancement of the strength of this bcc HEA, keeping excellent ductility during room temperature straining. Nevertheless, there is still a lack of information about the development of microstructure and physical properties of this refractory metal HEA subjected to severe plastic deformation processing. Recent investigations [3] revealed that thermodynamically stable system of HfNbTaTiZr alloy at room temperature is a mechanical mixture of Zr, Hf rich hcp phase and Ta, Nb rich bcc phase. The decomposition of the solid solution after long-term annealing obviously leads to the deterioration of mechanical properties (loss of ductility and decrease of strength). The difference in hardness of both phases is relatively small and both are softer than the random solid solution. On the other hand, considerable contribution to the solid solution strengthening can arise from atomic size misfit (phase separation on the nano-meter scale) which is provoked by the high density of vacancies introduced by HPT. This work thus aims on the relationship between phase (de)composition, microstructure, lattice defects, and length-scale-dependent material response of HfNbTaTiZr HEA after different thermal treatment and HPT straining. The microstructure and phase composition evolution were characterized by the electron microscopy and X-ray diffraction. The length-scale-dependent material response was characterized by indentation at various indentation depths. The contributions of different hardening mechanisms were separated and attributed to distance between dislocation pinning defects so that the differences between thermal treatment (diffusion) and HPT (straining) ‑induced hardening could be explained. Acknowledgement: This research was carried out in frame of the project CZ.02.1.01/0.0/0.0/15_003/0000485 (European Regional Development Fund). [1] A. Heczel et al. Defect structure and hardness in nanocrystalline CoCrFeMnNi high-entropy alloy processed by high-pressure torsion, J. Alloy. Comp. 711 (2017) 143-154. [2] J. Čížek et al. Strength enhancement of high entropy alloy HfNbTaTiZr by severe plastic deformation J. Alloy. Comp. 768 (2018) 924-937. [3] B. Schuh et al. Thermodynamic instability of a nanocrystalline, single-phase TiZrNbHfTa alloy and its impact on the mechanical properties, Acta Mater. 142 (2018) 201-212

Characterization of mechanically alloyed FeAlSi intermetallic powders

Powder metallurgy is very promising material production technology which allows to prepare the alloys that could hardly be manufactured by other processing route. Basic prerequisite to obtain the product of desired properties is the high quality of initial primary commodities, i.e. powders in the case of powder metallurgy. One of the available methods of powder preparation is so called mechanical alloying which starts from blended powder mixtures and allows production of homogeneous materials by severe deformation in a high-energy ball charge. This technology is especially suitable for brittle materials such as intermetallic alloys being developed for high-temperature and corrosive environments applications [1]. Please click Additional Files below to see the full abstract

INDENTATION SIZE EFFECT IN HIGH PRESSURE TORSION PROCESSED HIGH ENTROPY ALLOY

High entropy alloy HfNbTaTiZr in as cast conditions and after high pressure torsion straining was characterized by nanoindentation. The length-scale dependent material response (indentation size effect) was characterized by indentation at various indentation depths. Hardness dependence on the characteristic length (depth of penetration) indicated decomposition of disordered high entropy alloy in the as cast sample, which probably occurred during slow cooling after casting. Subsequent severe plastic deformation by high pressure torsion led on the other hand to the short-range disorder of (originally partially decomposed as cast) structure. Further hardening was generated during high pressure torsion by the mechanisms of grain refinement and increasing dislocation density

FRACTURE BEHAVIOR OF FeAlSi INTERMETALLICS

The study is devoted to the intermetallic alloy FeAl20Si20 (wt.%) with the potential applications in high temperature aggressive environments. The samples of the same chemical composition were prepared by spark plasma sintering from the different mechanically alloyed powders (pure elements and pre-alloyed powders). Differences in mechanical properties were characterized. Whereas no significant differences were found in hardness and Young´s modulus, fracture resistance was higher for the samples from pre-alloyed powders in which Palmqvist and lateral cracks were observed (contrary to the sample made of pure elements where only Palmqvist cracks were identified)

Conditions for long-term durability of dissimilar metal welds of power plants – environmental effect

Heterogenní svarové spoje v energetických zařízeních představují uzly, které mohou mít za určitých podmínek sníženou životnost. Příčiny změny užitných vlastností a způsoby poškození heterogenních svarových spojů lze rozdělit do dvou skupin podle teploty aplikace. Svarové spoje exponované při teplotách t < 300–350 °C podléhají poškození s významnějším vlivem prostředí. V příspěvku bude podána obecná charakteristika svarových spojů, s ohledem na specifické prostředí jaderných elektráren.Dissimilar metal welds in power plants equipment may have reduced service life under certain conditions. The causes of the change in utility properties and damage types of heterogeneous weld joints can be divided into two groups according to the application temperature. Welds exposed at temperatures t < 300 – 350 °C are subject to significant environmental damage. The paper will present general characteristics of dissimilar metal welds with regard to the specific environment of nuclear power plants

Characterization of dissimilar metal welds

V příspěvku je studován heterogenní svarový spoj potrubí z uhlíkové a austenitické ocele. Ve svařeném potrubí byly na rozhraní základní materiál – svarový kov nalezeny trhliny, které vznikly a šířily se mechanismem korozního praskání. Trhlina se šířila podél linie ztavení v oblasti promísení obou kovů. V této oblasti byly identifikovány změny v chemickém složení a zákalné struktury s výrazně vyšší tvrdostí oproti svařovaným kovům.TK01030108 (TA ČR)Heterogeneous dissimilar metal welds of carbon and austenitic stainless steel used in the piping systems were studied in this paper. The cracks were found in the welded pipeline at the interface between the base carbon steel and austenitic filler material. The stress corrosion cracking was identified as the cracking mechanism. The crack propagated along the fusion line in the mixing zone (area with higher diffusion) of both metals. In this area, changes in chemical composition and quenching structures with significantly higher hardness compared to the base metals were identified