Uta Klement

Thermal stability of nanocrystalline Ni

Abstract Thermal stability of electroplated nanocrystalline Ni of 10 and 20 nm grain size was investigated by differential scanning calorimetry (DSC). The temperature dependence and heat release ΔH during grain growth have been determined by linear anisothermal measurements (linear heating at 10 K min −1 ). The corresponding change in microstructure has been monitored in the temperature range between 373 K and 693 K using transmission electron microscopy (TEM). The TEM and DSC studies identified three exothermic reactions: “nucleation” and abnormal grain growth (353–562 K), normal grain growth (562–593 K) and growth towards equilibrium (643–773 K). The grain growth behaviour, and the similar heat releases ΔH = 18 J g −1 , and 16 J g −1 measured for the 10 nm and 20 nm Ni nanocrystals respectively in the DSC experiments may be related to the observed sulphur segregation at grain boundaries and triple junctions.

Thermal stability of nanostructured electrodeposits

As a result of their unique, but well-behaved structure-property relationships, porosity free nanostructured electrodeposits in the form of thin and thick coatings, free-standing sheet, foil or wire, and complex shapes, are rapidly finding applications in many different areas. Because of the large driving force for grain growth in these materials, however, their thermal stability may be a critical issue for some applications. This paper reviews previous grain growth studies of nanostructured electrodeposits. Thermal stability has been evaluated using several different experimental approaches. Calorimetric studies of nanocrystalline nickel based electrodeposits have shown a general trend of increasing thermal stability by alloying with either P or Fe. There is, however, little agreement between the various studies in terms of suggested growth mechanisms. Indeed, because of the large range of annealed structures obtained from different annealing treatments, several distinct growth mechanisms have been suggested. Recent grain growth studies of nanostructured Ni electrodeposits, covering a much broader range of annealing conditions than used before, have shown that the multiple types of previously reported annealed structures are in fact the product of a multi-staged growth process.

Alloy design for intrinsically ductile refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs), comprising group IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (Cr, Mo, W) refractory elements, can be potentially new generation high-temperature materials. However, most existing RHEAs lack room-temperature ductility, similar to conventional refractory metals and alloys. Here, we propose an alloy design strategy to intrinsically ductilize RHEAs based on the electron theory and more specifically to decrease the number of valence electrons through controlled alloying. A new ductile RHEA, Hf0.5 Nb 0.5 Ta 0.5Ti1.5Zr, was developed as a proof of concept, with a fracture stress of close to 1 GPa and an elongation of near 20%. The findings here will shed light on the development of ductile RHEAs for ultrahigh-temperature applications in aerospace and power-generation industries.

On the orientations of abnormally grown grains in nanocrystalline Ni and Ni–Fe

The scanning electron microscopy–based electron backscatter diffraction technique has been used to determine grain orientations of abnormally grown grains upon annealing in nanocrystalline Ni and Ni–20 at.% Fe electrodeposits. The results show that in nanocrystalline Ni and Ni–Fe, the first grown grains that can be detected are 〈411〉 oriented with respect to the normal direction (〈411〉//ND). Upon annealing, further grain growth occurs and the dominant orientation of the abnormally growing grains changes from 〈411〉//ND to 〈111〉//ND. Twinning is found to be the mechanism responsible for the orientation change and is for the first time described in connection with abnormal grain growth in nanocrystalline materials. This means that well‐known models for the formation of annealing twins (initially introduced in connection with recrystallization) also seem to apply in nanocrystalline materials.

Thermal Stability of Electrodeposited Nanocrystalline Ni-and Co-Based Materials

The attractive properties associated with nanocrystalline materials are to a large extent a result of their high intercrystalline volume fraction. However, the intrinsic instability of the nanostructured state may compromise the gain in properties by the occurrence of grain growth during exposure at elevated temperatures. Thermal stability is therefore a fundamental materials issue for nanocrystalline materials. The aim of this project is to obtain a deeper and more fundamental understanding of microstructural development and texture in nanocrystalline materials upon annealing. This is achieved by applying a combination of techniques such as transmission electron microscopy, electron backscatter diffraction in the scanning electron microscope, 3D atom probe, calorimetry, and X-ray diffraction. In Co-P, the effect of solutes together with the allotropic phase transformation is investigated. Already in the as-prepared state, P is found at grain boundaries and further segregation occurs upon annealing. This segregation stabilizes the nanocrystalline structure by the combination of a thermodynamic effect (reduction in driving force for grain growth) and a kinetic effect (reduction in grain boundary mobility). Transmission electron microscopy and 3D atom probe reveal that precipitation takes place upon annealing. The P atoms in the grain boundaries are consumed during the formation of Co2P and CoP precipitates and grain growth can take place. In a material with higher P content, the thermal stability is slightly reduced due to earlier grain boundary saturation and precipitation. Nanocrystalline Ni and Ni-Fe are also analyzed. They have a lower thermal stability; initial grain growth occurs abnormally and is followed by normal grain growth at higher temperatures. In addition to chemical and morphological influences, the texture development during grain growth is investigated. It is found that during abnormal grain growth the initial //ND changes to a more favorable //ND orientation by twinning. Hence, the texture development is not a result of oriented nucleation. Furthermore, a Co-based nanocomposite is investigated with respect to the influence of the µm-sized boron carbides. Thermal stability is observed not to be affected by the presence of the carbides. In the thesis the investigated materials are also compared with other nanocrystalline electrodeposits and the stabilizing mechanisms discussed in a broader perspective.

Dielectric Properties of SiC Nanowires With Different Chemical Compositions

The investigated SiC nanowires were prepared by the “shape memory process” technique. Depending on the processing parameters, nanowires with different chemical compositions, i.e., with varying amount of Si, C, and O were obtained. The permittivity of the SiC nanowires was measured in the frequency range between 1 and 18 GHz, which revealed that the permittivity, both real and imaginary parts, depends mostly on the C content of the nanowires. A higher C concentration in the nanowires gives rise to a higher permittivity.

EBSD and EDX analysis at the cladding–substrate interface of a laser clad railway wheel

Abstract Electron backscatter diffraction and energy-dispersive X-ray spectrometry were used to investigate the intermixed interface produced during laser cladding of a Co–Cr–Mo alloy on a steel substrate. A multi-component system and rapid solidification conditions together lead to a complex microstructure at the interface. The solidification of the cladding starts with the formation of an interface layer, which is about 75lm in thickness and consists of randomly oriented equiaxed grains of Co–Cr–Fe solid solution and martensite. Orientation analysis of the grains in the interface layer revealed that some grains have a special orientation relationship with the former austenite grains in the heat affected zone but the cladding is not formed by epitaxial growth on the substrate. Intermixing of the materials at the interface is providing a strong bond between the substrate and the cladding. For a grain from the interface layer to emerge as columnar grain in the cladding, it was determined that its <001> crystallographic direction is not supposed to deviate more than 25° from the sample normal direction.

Microwave absorbing properties of ferrite-based nanocomposites

A study of the microwave absorbing properties of polymer (epoxy) based nanocomposites is presented. The ferrite nanoparticles employed as filler materials were produced by a co-precipitation method, which was designed for production of large amounts at low cost. The absorbing properties of different kinds of ferrite nanoparticles, soft (manganese) and hard (cobalt) magnetic nanoparticles, are compared. In addition, the impact of high and low densities of the respective ferrite type has been investigated. Our analysis of the microwave absorbing properties is made over a wide frequency band including both MHz and GHz regions, which is of high interest for a number of different applications both military and civilian.

EBSD characterization of carbide–carbide boundaries in WC–Co composites

A sample of WC‐6wt%Co was investigated for grain boundary character distribution and occurrence of coincidence site lattice (CSL) boundaries on a statistical basis. For this purpose orientation measurements of the grains were carried out using electron back‐scattered diffraction (EBSD). The dominant misorientation relationships were determined by complementary EBSD data representation tools such as orientation maps, misorientation angle distribution histograms and the sectioned three‐dimensional misorientation space. It was found that the grain boundary character distribution of the material is nearly random and the CSL boundaries are not present in statistically significant amounts. It was also found that the amount of binder phase does not play a role in the formation of special boundaries. The paper focuses on the methodology of characterizing grain boundaries in a hexagonal material using EBSD.

Predicting the solid solubility limit in high-entropy alloys using the molecular orbital approach

High-entropy alloys (HEAs) are currently at the research frontier of metallic materials. Understanding the solid solubility limit in HEAs, such a highly concentrated multicomponent alloy system, is scientifically intriguing. It is also technically important to achieve desirable mechanical properties by controlling the formation of topologically or geometrically closed packed phases. Previous approaches to describe the solid solubilities in HEAs could not accurately locate the solubility limit and have to utilize at least two parameters. Here, we propose to use a single parameter, the average energy of d-orbital levels, Md, to predict the solid solubility limit in HEAs. It is found that Md can satisfactorily describe the solid solubilities in fcc structured HEAs containing 3 d transition metals, and also in bcc structured HEAs. This finding will greatly simplify the alloys design and lends more flexibility to control the mechanical properties of HEAs. When 4 d transition metals are alloyed, Md alone cannot...