C. Borchers

Microstructural and macroscopic properties of cold sprayed copper coatings

Cold spraying is a coating technique in which the formation of dense, tightly bonded coatings occurs only due to the kinetic energy of high velocity particles of the spray powder. These particles are still in the solid state as they impinge on the substrate. This study correlates optimized deposition parameters with the corresponding microstructure as well as mechanical and conductive behavior of cold sprayed copper coatings in order to explain possible bonding mechanisms. In addition, the performance of cold sprayed copper coatings is compared to that of cold rolled copper and to coatings prepared by thermal spray methods.

Atom probe tomography characterization of heavily cold drawn pearlitic steel wire

Atom Probe Tomography (APT) was used to analyze the carbon distribution in a heavily cold drawn pearlitic steel wire with a true strain of 6.02. The carbon concentrations in cementite and ferrite were separately measured by a sub-volume method and compared with the literature data. It is found that the carbon concentration in ferrite saturates with strain. The carbon concentration in cementite decreases with the lamellar thickness, while the carbon atoms segregate at dislocations or cell/grain boundaries in ferrite. The mechanism of cementite decomposition is discussed in terms of the evolution of dislocation structure during severe plastic deformation.

Application of cold drawn lamellar microstructure for developing ultra-high strength wires

Abstract Composite materials having lamellar structure are known to have a good combination of high strength and ductility. They are widely used in the fields of automobiles, civil engineering and construction, machines and many other industries. An application of lamellar microstructure for developing ultra-high strength steel wires was studied and discussed. Based on the experimental results, the relationships between the strength increase and microstructure development during the cold wire drawing were studied to reveal the strengthening mechanism. As cold drawing proceeds, the wire strength extremely increases, the microstructure changes from large single crystal lamellar structure to very fine polycrystalline lamellar one which has nano-sized grains, high dislocation density and amorphous regions. From the results obtained, it is concluded that heavy cold drawing technique is an effective method for lamellar composite to get high strength wires. Furthermore, formation process of the best microstructure for producing the ultra-high strength wires was also discussed.

High resolution microstructure analysis of the decomposition of Cu90Co10 alloys

Abstract Homogeneous CuCo alloys with a Co concentration up to 10 at.% Co are prepared by rapid quenching. This allows for the first time the investigation of the decomposition process of the Cu90Co10 supersaturated solid solution on a nanometer scale using a combination of atom probe/field ion microscopy (AP/FIM) analyses and transmission electron microscopy. Annealing of a Cu90Co10 alloy at 440°C for various times leads to a compositional modulated microstructure. The composition profiles determined by AP/FIM analyses clearly exclude a classical nucleation and growth behavior and instead suggest a spinodal type decomposition whereby the compositions of the precipitates continuously increase. In contrast, at the grain boundaries of the Cu90Co10 alloy heterogeneous nucleation of pure Co particles is observed.

Microstructure development during rapid solidification of highly supersaturated Cu-Co alloys

Abstract We report a detailed microstructure analysis of rapidly quenched Cu 1− x Co x (0.1 ⩽ × ⩽ 0.5) alloys. The chemical homogeneity of the alloys was investigated on nanometer scale using a combination of AP/FIM- and TEM-studies. Cu-Co alloys with a Co concentration up to 10 at.% Co can be prepared homogeneously by rapid quenching. For larger Co contents, the microstructure is determined by a competition between polymorphic solidification, nucleation of the Co-rich solid solution from the melt, and spinodal decomposition of the unstable supersaturated solid solution. The microstructures are discussed in terms of the kinetics of the Cu-Co system, provided quantitatively by recent thermodynamic calculations.

Atomic scale investigation of redistribution of alloying elements in pearlitic steel wires upon cold-drawing and annealing

A local electrode atom probe has been employed to analyze the redistribution of alloying elements including Si, Mn, and Cr in pearlitic steel wires upon cold-drawing and subsequent annealing. It has been found that the three elements undergo mechanical mixing upon cold-drawing at large strains, where Mn and Cr exhibit a nearly homogeneous distribution throughout both ferrite and cementite, whereas Si only dissolves slightly in cementite. Annealing at elevated temperatures leads to a reversion of the mechanical alloying. Si atoms mainly segregate at well-defined ferrite (sub)grain boundaries formed during annealing. Cr and Mn are strongly concentrated in cementite adjacent to the ferrite/cementite interface due to their lower diffusivities in cementite than in ferrite.

Carbon-defect interaction during recovery and recrystallization of heavily deformed pearlitic steel wires

Cold-drawn pearlitic steel wires are known to exhibit increasing strength with increasing elongation and are therefore highly interesting for a wide field of engineering applications. When isochronal heat treatment is performed at different temperatures, the tensile strength as well as the electrical resistivity decrease well before microstructural changes are observed. An Arrhenius analysis of both processes yield mean activation energies of about 0.3 eV. This is construed as interaction between carbon atoms and defects in ferrite, mainly vacancies and vacancy clusters.

Influence of hydrogen loading on the microstructure of niobium-palladium multilayers

Abstract Nb—Pd multilayers were prepared by laser deposition. Samples were loaded electrolytically with hydrogen up to 0.3 H/Nb, others were submitted to four complete loading—unloading cycles and remained loaded. The microstructure of these samples as well as that of as-prepared samples were studied by high resolution electron microscopy. After several hydrogen loading and unloading cycles the change in microstructure consists mainly of a new arrangement of dislocations. Further transmission electron microscopy studies reveal that β-NbH grows in columns parallel to the growth direction of the films after phase separation. X-ray studies show that after unloading from the p-NbH phase the stress in the film changes to the reverse mode. This is valid for completely loaded as well as partially loaded specimens.

Size distributions of nanoscopic holes in Ti/h-BN and Ti/B nanocomposites

Positron annihilation spectroscopy was employed for defect studies of “Ti”-based nanocomposites prepared by high-energy ball milling and consisting of Ti nanoparticles separated by hexagonal boron nitride (h-BN) or boron (B) additive. The size distribution of nanoscopic holes in nanocomposites was determined directly from measurement of ortho-positronium (Ps) lifetimes. Chemical environment of defects was characterized using coincidence Doppler broadening. It was found that size of nanoscopic holes is reduced with increasing milling time in H2/He atmosphere and also probability of Ps formation in holes decreases. At the same time the Ti content in the vicinity of holes increases. This can be explained by (i) increased intermixing of Ti particles with h-BN or B additive and by (ii) filling the nanoscopic holes with absorbed hydrogen. Analysis of obtained results showed that both these processes take place during milling of nanocomposites. In addition, it was found that the effect of filling the nanoscopic ...

Size and Structure of Palladium Clusters Determined by XRD and HREM

Abstract Scherrer formula is often applied to X-ray-diffraction profiles for the size determination of small size clusters. However, for small clusters this often leads to conflicting results in comparison to other methods. A series of Pd-clusters of different size is studied by X-ray diffraction analysis and transmission electron microscopy. The influence of size and structure on the results is presented and discussed in comparison with theoretical calculations. It will be shown that the different structure of small size systems are one main origin of the conflicts. The structure problem can be overcome by using Fourier Transform of the X-ray diffraction pattern. The importance of the knowledge of the cluster structure was demonstrated by showing its strong influence on the hydrogen solubility.