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Bulk Metallic Glasses (BMG) show extraordinarily high values of strength and elastic strain. Despite the, in most cases, macroscopically brittle behaviour (especially in tension and bending), extremely high fracture toughness values are reported. Even though it is sometimes stated that huge plastic zones and strain softening make K-based approaches (ASTM E399) or J-integral based approaches (ASTM E1820) difficult for BMG, changes in material behaviour or intrinsic metallic glass materials properties are still inferred by many authors in order to explain the results. In addition, the limited dimensions of fully glassy bulk ingots mean that the available specimen sizes differ from those required by the common standards of fracture toughness testing. In this context, reported results from fracture toughness of prominent Pd-, Pt- and Zr-based BMG are analysed. An in-depth discussion of the size dependent effects on the mechanical loading conditions during fracture toughness testing is provided. As a result, it is shown that the differences and peculiarities of characterized BMG toughness mainly relate to geometrical/mechanical effects of small scale sample testing. They are not due to an intrinsic material property or to a size dependent ramification of intrinsic material behaviour.

Austenitic Fe-Mn-C-based alloys are considered as promising candidates for biodegradable vascular implants due to their high strength, ductility and mechanical integrity during degradation.The present study demonstrates that microalloying with S and B is an effective method to further enhance the degradation rate and the mechanical properties of a Fe-30Mn-1C twinning-induced plasticity (TWIP) alloy without deteriorating the biocompatibility. For studying the microstructural changes due to S or B addition, the alloys were analysed by X-ray diffraction (XRD) as well as scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX), wavelength dispersive X-ray analysis (WDX) and electron backscatter diffraction (EBSD). Thereby precipitates of (Fe0.3Mn0.7)S and (Fe,Mn)23(C3B3) types were detected in the austenitic matrix. These precipitates have a distinct influence not only on the mechanical properties under tensile load but also on the occurring corrosion mechanism. This was displayed by potentiodynamic polarization measurements and immersion tests in simulated body fluid (SBF) and associated SEM as well as X-ray photoelectron spectroscopy (XPS) investigations. In vitro cytotoxicity analyses with L929 fibroblast cells indicated that microalloying with S and B does not affect the cytocompatibility.Thus, the novel alloy modifications show a high potential for future application as biodegradable implant material. Keywords: Fe-based alloy, Biodegradable, Microstructure, Mechanical properties, Corrosion behaviour, Cytocompatibility

Ti and Ti-TiB composite materials were produced by selective laser melting (SLM). Ti showed an α΄ microstructure, whereas the Ti-TiB composite revealed a distribution of needle-like TiB particles across an α-Ti matrix. Hardness (H) and reduced elastic modulus (Er) were investigated by nanoindentation using loads of 2, 5 and 10 mN. The results showed higher H and Er values for the Ti-TiB than Ti due to the hardening and stiffening effects of the TiB reinforcements. On increasing the nanoindentation load, H and Er were decreased. Comparison of the nanoindentation results with those derived from conventional hardness and compression tests indicated that 5 mN is the most suitable nanoindentation load to assess the elastic modulus and hardness properties. The wear resistance of the samples was related to their corresponding H/Er and H3/Er2 ratios obtained by nanoindentation. These investigations showed that there is a high degree of consistency between the characterization using nanoindentation and the wear evaluation from conventional wear tests. © 2017 Elsevier B.V.

Rolled-up nanotechnology has received significant attention to self-assemble planar nanomembranes into 3D micro and nanotubular architectures. These tubular structures have been well recognized as novel building blocks in a variety of applications ranging from microelectronics and nanophotonics to microbatteries and microrobotics. However, fabrication of multiwinding microtubes with precise control over the winding interfaces, which is crucial for many complex applications, is not easy to achieve by existing materials and technologies. Here, a dry rolling approach is introduced to tackle this challenge and create tight windings in compact and highly symmetric cylindrical microstructures. This technique exploits hydrophobicity of fluorocarbon polymers and the thermal expansion mismatch of polymers and inorganic films upon thermal treatment. Quality parameters for rolled-up microtubes, against which different fabrication technologies can be benchmarked are defined. The technique offers to fabricate long freestanding multiwinding microtubes as well as hierarchical architectures incorporating rolled-up wrinkled nanomembranes. This work presents an important step forward toward the fabrication of more complex but well-controlled microtubes for advanced high-quality device architectures. �� 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper presents results about the influence of the selective laser melting (SLM) process parameters on a FeCr4Mo1V1W8C1 (wt%) alloy regarding microstructure and mechanical behavior. Tailored parameter variation studies were performed to obtain crack-free and highly dense SLM parts. The microstructure was studied using scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy, and scanning transmission electron microscopy. Additionally, the mechanical properties were investigated by compression and tensile tests. The obtained microstructure is composed of complex nanoscale carbides, retained austenite, and martensite. Caused by the fast directional cooling during SLM, a completely dendritic solidification aligned in building direction occurs. Non-equilibrium segregation leads to an orderly phase arrangement of complex carbides at the boundary of the dendrites surrounded by retained austenite and martensite in the center of the dendrites. A strong work hardening behavior was observed, based on an austenite-to-martensite phase transformation (TRIP effect). This effect accounts for the outstanding mechanical properties such as compression strength of 6000 MPa, a 0.2% tensile yield strength of 560 MPa, and an ultimate tensile strength of over 1000 MPa. These findings reveal that SLM is advantageous for the processing of ultra-high-strength FeCrMoVWC tool steel.

This work presents the investigation of the solidification and cooling behaviour of a newly developed Fe92.65Cr4.2V2.1B0.05C1 wt% (FeCrVBC) tool steel with special emphasis to phase formations and transformations. JMatPro simulations were compiled to predict the characteristic temperatures of phase crystallizations and transformations under equilibrium conditions. Differential thermal analysis (DTA) was used to cool down samples from 1500 °C to room temperature with various cooling rates (5, 10, 20, 40, and 50 K/min), whereby differences in the number and position of the resulting peaks occurred. To correlate the peaks with specific phase reactions, the microstructure of the variously cooled samples was analysed by means of optical (OM) and scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), wavelength-dispersive X-ray spectroscopy (WDS), electron backscatter diffraction (EBSD) and microhardness tests. The systematic analyses show that the microstructure evolution of the FeCrVBC alloy strongly depends on the cooling rate. For low cooling rates up to 10 K/min, the austenite-ferrite transformation and a marked formation of M7C3 and M23(C,B)6 secondary carbides were observed. At rates of 20 K/min and above, the austenite-martensite transformation takes place, accompanied with a strong increase of the hardness in dendritic areas. By cooling with rates of 40 K/min and higher, the precipitation of M7C3 or M23(C,B)6 secondary carbides cannot be observed anymore, whereas primary M3(C,B) besides MC carbides were detected. Furthermore it was determined, that the very small content of boron (0.05 wt%) strongly influences the (trans-)formation of special carbides, shown by the comparison to its base alloy without boron, the formerly investigated Fe92.7Cr4.2V2.1C1 wt% (FeCrVC). Consequently, strongly depending on the formed carbides the materials hardness and wear resistance can be significantly affected by boron addition. Concluding, the obtained knowledge about the thermal behaviour of the FeCrVBC alloy and the strong impact of a small addition of boron offers the possibility for tailored adjustment of phases and respective properties for a load-adapted tool design.

In the present work, the wetting behaviour of the Cu–GaX (X = 10–30 wt.%) liquid alloys on 304L steel was investigated in vacuum at temperatures between 1123 and 1373 K. Contact angle and surface tension measurements were performed using the sessile drop setup. In general, the Cu–GaX (X = 10–30 wt.%) alloys show good or even excellent wetting on 304L steel. However, the wetting mechanism strongly depends on the Ga content. While for Ga-poor alloys secondary wetting occurs, there is a partial dissolution of steel grains combined with an austenite-to-ferrite transformation for larger Ga contents. Cu penetrates along the grain boundaries of the substrate and provides a path for Ga diffusion into the steel. The dissolution of steel grains at the interface is suggested to be responsible for slowing down of the spreading at elevated temperatures. Understanding these interrelations between microscopic and macroscopic effects is important for the development of new Cu-based liquid alloys containing Ga, especially for applications involving steels. Keywords: Brazing, Reactive wetting, Gallium, Copper, Steel, Phase transformation

This paper demonstrates the successful processing of a high-strength Fe85Cr4Mo8V2C1 tool steel by selective laser melting (SLM). Based on tailored parameter variation studies, crack-free and highly dense FeCrMoVC parts (99.99% relative density) are obtained by this processing method. The applied localized high heat inputs during very short interaction times result in a fine, homogeneous microstructure composed of martensite, austenite, and carbides. This combination of phases together with the morphology of the microstructure results in excellent mechanical properties, such as a microhardness of 900 HV 0.1 (ca. 67 HRC) and a compressive strength of about 3800 MPa combined with a fracture strain up to 15%. Therefore, the SLM processing of this alloy enables a further increase of the hardness and compressive strength compared to the as-cast state.Altogether, processing the FeCrMoVC steel by SLM as one key additive manufacturing technology shows high potential for advanced tool design. Keywords: Selective laser melting, Steel, Microstructure, Hardness, Mechanical properties

In this work a systematic investigation of the influence of the cooling rate on the microstructure and properties of a newly developed Fe92.7Cr4.2V2.1C1 (FeCrVC) tool steel is presented. By applying a tailored casting process and sufficiently high cooling rates excellent mechanical properties are obtained for the presented alloy already in the as-cast state. Since no subsequent heat treatment is required, the cooling parameters applied during the casting process play a key role with respect to the evolving microstructure and resulting properties. In the present publication the effect of the cooling rate on the microstructure and properties of as-solidified FeCrVC was investigated. By using differential scanning calorimetry (DSC), several samples were heated up and cooled with continuous rates of 3–50 K/min. The received DSC data was used to investigate the alloy’s solidification and phase transformation behavior. Subsequently, these samples were studied regarding their properties and microstructure by different analysis methods (EDX/WDX, EBSD, XRD). With increasing cooling rates the liquidus and solidus temperature are lowered, whereas the solidification interval is enlarged. A higher cooling rate is accompanied by a lower solidification time which results in a refinement of the dendritic microstructure. Furthermore, with rising cooling rates the microhardness increased. This provides the opportunity to make predictions from the applied cooling parameters upon the hardness and vice versa and enables one to draw first conclusions on the mechanical properties of the FeCrVC alloy.

Starting from the bulk glass-forming composition Mg 59.5 Cu 22.9 Ag 6.6 Gd 11 , the development of a bulk glassy matrix composite alloy with high strength and ductility was attempted by adding a large amount of Sc to induce the formation of the MgSc intermetallic compound. However, copper-mould casting of the modified composition, Mg 54.7 Cu 11.5 Ag 3.3 Gd 5.5 Sc 25 , as rods with 3 and 5 mm diameter, yields besides the expected MgSc, a second (CuSc) and a third (Mg 3 Gd-type) intermetallic. These compounds are surrounded by a Mg-rich matrix, possibly composed of glassy regions and a Mg-based terminal solid solution. Although the obtained microstructure deviates largely from the expected one, the new alloy exhibits very high strength, i.e. 790 MPa, and visible plastic deformation, i.e. 0.9%. Shear dimples were found in MgSc.

New Ti–Cu–Ni shape memory bulk metallic glass composites were obtained by carefully controlling the cooling rate upon quenching. This allows for the formation of a metastable microstructure consisting mainly of ductile, spherical martensitic Ti(Ni,Cu) precipitates embedded in an amorphous matrix also containing a small volume fraction of TiCu and Ti2Cu precipitates. These composites exhibit large ductility and high strength combined with a strong work-hardening behaviour. A deformation mechanism is proposed with the help of experimental observations and finite-element simulation. The simulation results demonstrate that stress concentrations occur around the precipitates, which promotes a heterogeneous stress distribution and the formation of multiple shear bands. Additionally, different transformation temperatures were observed for martensitic precipitates depending on whether they are completely or partially embedded in the amorphous matrix.

The effect of low-temperature on the active deformation mechanism is studied in pure copper. For this purpose, cryogenic wire drawing at liquid nitrogen temperature (77 K) was performed using molybdenum disulfide lubrication. Microstructural investigation and texture analysis revealed severe twin formation in the cryogenically drawn copper, with a broad twin size distribution. The spacing of the observed deformation twins ranges from below 100 nm, as reported in previous investigations, up to several micrometers. The extent of twin formation, which is significantly higher when compared to other cryo-deformation techniques, is discussed with respect to the state of stress and the texture evolution during wire drawing. � 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In the present work, the mechanical behaviour at different strain rates of a high strength tool steel with the composition Fe88.9Cr4.3V2.2C4.6 (at.%), manufactured under relatively high cooling rates and highly pure conditions, was studied. The applied preparation conditions promote a microstructure composed of martensite, retained austenite and a fine network of special carbides already in the as-cast state. This special phase combination yields a structural material with superior mechanical properties under quasi-static loading, exhibiting an extremely high engineering compression strength of almost 4000 MPa combined with a large compression strain of about 20% due to deformation-induced martensite formation. However, the knowledge of the performance of this material under dynamic loading conditions is also very important for its possible application as a novel kind of tool steel. For this reason, compression tests in the strain rate range of 10−3–103 s−1 were performed and the transformation-induced plasticity (TRIP effect) was investigated. The analysis of the transformation behaviour of the retained austenite reveals that the rate of martensite formation decreases with increasing strain rate. Furthermore, an increase of the offset yield stress and the compression strength with increasing strain rate together with a remarkable plasticity was detected.

The process of the preparation of graphitic nanoplatelets via chemical intercalation and thermal exfoliation of graphite was considered. The dependence of the properties of the final product on the microstructure of the raw material and on different process parameters was analyzed. It is shown that the microstructure of the raw material has considerable influence on the structural transformations during the different preparation steps. The observed differences can be understood assuming that intercalation is initiated at the edges of graphite crystallites by the formation of intercalate nuclei. Essential differences of the nucleation time appear for unequal types of microstructure and give rise to different intercalation states. It is also shown that the sub-process of washing and drying the samples after intercalation is most critical and has significant influence on the characteristics of the final products. The results suggest that graphite fibres with sub-micron diameter and orientation of the crystallographic c-axis along the fibre axis will be particularly favourable for the preparation of graphitic nanoplatelets.

Dendrites are one of the major microstructural constituents of peritectic alloys. In the present work, the effect of melt convection on the secondary dendritic arm spacing (SDAS) and volume fraction of properitectic α-Fe was investigated during solidification of stoichiometric Nd–Fe–B alloys using the forced crucible rotation technique. The resulting microstructure of the alloy in consideration of melt convection has been investigated using scanning electron microscopy and optical microscopy. The average SDAS was determined for each sample from the whole cross-section of the cylindrical test samples using image analyzing software LEICA QWIN. A detailed statistical analysis of the spacing distribution was performed on the basis of the variation of SDAS values, averaged from about 80 to 120 dendrites in different zones. The α-Fe volume fraction, measured by vibrating sample magnetometer (VSM), reduces with increasing crucible rotation frequency. Similarly, the SDAS values decrease with increasing rotation frequency. These results are explained from the viewpoint of a reduced melt convection state under steady forced crucible rotation leading to a reduced effective mass transfer coefficient.

Microstructures of the solidified Fe–Gd–Mo ternary alloys have been examined using optical and scanning electron microscopy. The nature and chemical composition of the different phases present have been determined by electron probe X-ray microanalysis and X-ray diffraction. These results have been utilized to construct a liquidus projection of the Fe–Gd–Mo system, taking into account the thermal effects detected in the course of solidification with the use of differential thermal analysis. Six ternary peritectic reactions and two ternary eutectic reactions have been found to occur within the liquidus projection. These observations have been used to establish the solidification sequences of the Fe–Gd–Mo alloys studied.

This paper discusses the experimental realisation of two types of X-ray interferometer based on pinhole diffraction. In both interferometers the beam splitter was a thin metal foil containing micrometer pinholes to divide the incident X-ray wave into two coherent waves. The interference pattern was studied using an energy-dispersive detector to simultaneously investigate in a large spectral range the diffraction properties of the white synchrotron radiation. For a highly absorbing pinhole mask the interference fringes from the classical Young's double-pinhole experiment were recorded and the degree of coherence of X-rays could be determined. In the case of low absorption of the metal foil at higher X-ray energies (> 15 keV) the interference pattern of a point diffraction interferometer was observed using the same set-up. The spectral refraction index of the metal foil was determined.