Ewald Bischoff

Effect of adatom surface diffusivity on microstructure and intrinsic stress evolutions during Ag film growth

The effect of the adatom surface diffusivity on the evolution of the microstructure and the intrinsic stress of thin metal films was investigated for the case of growth of polycrystalline Ag films on amorphous SiO2 (a-SiO2) and amorphous Ge (a-Ge) substrates, with high and low Ag adatom surface diffusivity, respectively. The surface diffusivity of the deposited Ag adatoms on the a-Ge substrate is suppressed also after coalescence of Ag islands due to the continuous (re)segregation of Ge at the surface of the growing film as evidenced by in-situ XPS. An assessment could be made of the role of adatom surface diffusivity on the microstructural development and the intrinsic stress evolution during film growth. As demonstrated by ex-situ TEM and ex-situ XRD, the Ag films grown on the a-SiO2 and a-Ge substrates possess strikingly different microstructures in terms of grain shape, grain size, and crystallographic texture. Nevertheless, the real-time in-situ stress measurements revealed a compressive → tensile → ...

Faceting of ∑3 and ∑9 grain boundaries in copper

Faceting is a well documented phenomenon known both for surfaces and interfaces, particularly, grain boundaries (GBs). Faceting can be considered as a phase transition when the original surface or GB dissociates onto flat segments whose energy is less than that of the original surface or GB. For the investigation of GB faceting a cylindrical Cu bicrystal with an island grain was grown by the Bridgman technique. Grain 1 in this bicrystal is completely surrounded by grain 2. The dissociation Σ9 → Σ3 + Σ3 proceeds during the growth of the bicrystal. The twins appear instead of {111}1/{115}2 or (110)Σ9CSL facet. GB faceting was studied at 1293 K, 1073 K, and 873 K. The profiles of the GB thermal groove were analysed by atomic force microscopy. Wulff-Herring plots and GB phase diagrams have been constructed for the Σ3, Σ9 and Σ9 + Σ3 GBs. With increasing temperature the facets with low-density CSL-planes disappear in the GB shape. GB roughening phase transition can be responsible for this phenomenon.

Determination of Carbide and Matrix Compositions in High-Speed Steels by Analytical Electron Microscopy

Procedures for the analysis of the blocky (“primary”) carbides and of the matrix in high-speed tool steels with scanning transmission electron microscopy-energy dispersive X-ray (STEM-EDX) are described. Results for the alloys AISI Tl, M2, and M7 and for an experimental, Nballoyed material are reported and compared to determinations by quantitative metallography in scanning electron microscopy (SEM), combined with compositional analysis by SEM-EDX. With proper precautions, good accuracy can be obtained with either method, and the results obtained by the two routes are in good agreement.

Unusual Precipitation of Amorphous Silicon Nitride upon Nitriding Fe-2at.%Si Alloy

Silicon-nitride precipitation in ferritic Fe–2at.%Si alloy was investigated upon nitriding in NH3/H2 gas mixtures, using light microscopy, hardness measurements, scanning and transmission electron microscopy, X-ray diffraction and electron-probe microanalysis for microstructural characterisation. Surprisingly, ideally weak nitriding behaviour occurred upon nitriding thick (1 mm) recrystallised Fe–2at.%Si alloy specimens. This phenomenon can be attributed to the onset of silicon-nitride precipitation only after a certain degree of nitrogen supersaturation has been established at all depths in the specimen. Silicon-nitride precipitates formed inside the ferrite grains and also along the ferrite grain boundaries. The precipitates were amorphous and had a stoichiometry of Si3N4. The amorphous nature of the tiny precipitates has a thermodynamic origin. Nitride precipitation occurred very slowly due to the very large volume misfit of the nitride with the matrix. An anomalous non-monotonous hardness change occurred with increasing nitriding time, which was ascribed to the initially fully elastic accommodation of precipitate/matrix misfit. The nitrogen uptake rate increased upon continued nitriding as a result of “self-catalysis”. The possible, favourable application of amorphous silicon-nitride precipitates as grain-growth inhibitors in the production of grain-oriented electrical steel is discussed.

Phase transformation of mixed Cr1-xAlxN nitride precipitates in ferrite

Nitriding of specimens with the composition Fe–1.5 wt% Cr–1.5 wt% Al (Fe–1.6 at.% Cr–3.1 at.% Al) at 853 K leads to the formation of mixed, ternary Cr1− x Al x N nitride platelets precipitated in the cubic, rock-salt structure type obeying a Bain-type orientation relationship with the ferrite matrix. Upon subsequent annealing (at 973 K) the mixed, ternary nitrides transform into the two equilibrium, binary nitrides, namely CrN of cubic, rock-salt structure type in the Bain orientation relationship with the ferrite matrix and AlN of hexagonal, wurtzite structure type, obeying a Pitsch–Schrader orientation relationship with the ferrite matrix. At the same time, the mobile excess nitrogen, dissolved in the ferrite matrix, diffuses towards the originally not nitrided core, where relatively coarse, cubic CrN and hexagonal AlN precipitates develop. The microstructure and (local) composition changes were analysed using X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray and electron probe microanalysis techniques. It was found that the transformation proceeds by Al depletion of the original mixed Cr1− x Al x N precipitates.

Examination of phase transformations in the system Fe-N-C by means of nitrocarburising reactions and secondary annealing experiments; the α+ε two-phase equilibrium

Abstract The until now controversially discussed existence of the two-phase equilibrium α-Fe + ∊-Fe3(N, C)1+x in the ternary system Fe – N – C was proven by the investigation (optical microscopy, X-ray diffraction, Electron Backscatter Diffraction) of compound-layer microstructures generated by gaseous nitrocarburising of pure iron and subsequent secondary annealing experiments under static inert-gas atmosphere. The invariant transition reaction γ-Fe4N + Fe3C → α-Fe + ∊-Fe3(N, C)1+x could be shown to proceed at a (single) temperature (at constant pressure of 1 atm) between 833 K and 843 K.

Octapod-shaped, nanosized, amorphous precipitates in a crystalline ferrite matrix

The development of uniquely octapod-shaped nanosized amorphous silicon–nitride precipitates in a ferrite matrix was observed upon nitriding of Fe–4.5at.%Si alloy. The legs of the amorphous precipitate are oriented along ⟨1 1 1⟩-directions of the ferrite. The occurrence of such peculiarly shaped amorphous silicon–nitride precipitates, which experience a volume misfit of more than 100% with the surrounding ferrite, was attributed to precipitate growth influenced by long-range diffusion within the evolving highly anisotropic stress field around the developing precipitates after nucleation.

Grain boundary faceting close to the Σ3 coincidence misorientation in copper

The faceting of a cylindric tilt grain boundary (GB) in Cu bicrystal has been studied. The crystal lattices of both grains give rise to a superlattice called coincidence site lattice (CSL) with inverse density of coincidence sites Σ = 3. The (100) C S L , (210) C S L, (130) C S L facets and the non-CSL 82°9R facet develop upon annealing at 0.95 T m , where T m is the melting temperature. The ratio of GB energy σ G B and surface energy σ s u r of the specimen was measured applying atomic force microscopy from the profile of the GB thermal groove formed upon additional annealing. The influence of deviation from the exact coincidence misorientation θ Σ , Δθ = ‖θ - θ Σ ‖, has been studied on the basis of electron backscattering diffraction patterns. The Wulff-Herring diagrams were constructed using measured σ G B / σ s u r values, revealing stable and metastable GB facets. T m is lower than the roughening temperature for the (100) C S L and 9R facets facets in Cu.

Growth of electromigration-induced hillocks in Al interconnects

Electromigration-induced hillock growth in polycrystalline Al segments was extensively investigated. Hillocks composed of columnar grains grew near the anode by epitaxial Al addition at the interface between the Al and underlying TiN layer, which pushed up the original Al film. The hillocks rotated away from their initial (111) out-of-plane orientation in a manner consistent with the physical rotation of the hillock surface. Wedgelike and rounded hillocks were observed, and their formation is explained by the interaction between grain extrusion and grain growth. Trends elucidated by review of both thermal- and electromigration-induced hillock studies can be explained by the mechanisms identified in this work.

Preliminary investigations of a correlation between electron energy loss and morphometric analyses on ultrathin cryosections from normal and neoplastic gastric tissues

Electron energy loss spectroscopy (EELS) has been used to measure the ratios of C, N, O, P and Ca in ultrathin cryosections from normal and neoplastic gastric tissues. First results show a correlation between the EELS, and morphometric data in cells from these tissues. We have found that ultrathin, freeze-dried cryosections, with an average thickness of up to 75 nm, are stable enough for EELS-analysis in a 200 KV electron microscope with an adapted Gatan-EELS-Spectrometer.