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TEM study on the effect of machining on the microstructure of a soft magnetic Ni76Fe17Cu5CrMo2 alloy
(1991)
Low-cycle fatigue experiments under combined axial-torsional loading have been carried out on alloy 800 H tubular specimens at room temperature. In comparison with proportional loading, an extra cyclic hardening effect produced by nonproportional loading was observed. The microstructure study highlights the fact that the dislocation arrangement under proportional loading is significantly different from that under nonproportional loading. Mechanical twinning was found in specimens cycled under axial loading and nonproportional loading. It is suggested that mechanical twinning depends not only on shear stress but also on normal stress on the plane of maximum shearing. The extra hardening can be interpreted in terms of the deformation microstructure. Fatigue cracking was initiated generally at the specimen surfaces along the plane of maximum shearing, but under nonproportional loading cracks were found also in the bulk of the specimens. Transcrystalline crack propagation was observed in the specimens after proportional and nonproportional LCF tests.
An extended model of the tension-compression asymmetry of single crystal nickel-base superalloys
(1996)
Interrupted LCF-tests at 850 deg C for Inconel 738LC have been carried out and the specimens have been intensively examined by REM to quantify the evolution of fatigue damage. Surface crack nucleation at oxidatized grain boundaries and coalescence of neighboring flaws occurred during the whole experiment. An anomalous fast single crack growth is observed just after nucleation, followed by a relative stagnation of the propagation in absence of coalescence. A statistical model is proposed that describes these pheonmena. It leads to an integro-differential equation for the crack density f(c,n) after n cycles, similar to the Bolzmann equation which describes the collision of molecules in a dilute gas.
The role of grain size and ferrite substructure in respect to mechanical properties of an HSLA steel
(1986)
Several heat treatments were applied to an HSLA steel on type StE460 (German standard) to produce four ferrite substructures of different strength and at least two different grain sizes respectively. Whereas the ferrite substructure has a strong influence on yield strength the effect of grain size was negligible though probably to some extent masked by different pearlite volume fractions. The different strength levels could be explained by regarding the arrangement of dislocations and vanadium carbonitride particles and their mutual interaction. Increasing both grain size and pearlite volume fraction leads to a remarkable shift of transition temperature T sub 27J which was further enhanced by increasing ferrite strength. 16 ref.--AA
Cross-sectional transmission electron microscopy, in combination with energy dispersive X-ray spectroscopy and focused beam microdiffraction, was applied to study the solid-state reactions taking place during contact formation of the system Ge(115 nm)/Pd(50 nm)In0.53Ga0.47As. In order to get information about the sequence of the different processes, rapid thermal, annealing experiments in the range 225400 °C were performed. The following features were observed: at 225 °C Pd reacted with the substrate forming the quaternary phase PdxIn0.53Ga0.47As (x 4), and with the Ge-layer forming mainly PdGe and Pd2Ge. Between PdxIn0.53Ga0.47As and In0.53Ga0.47As, a 5 nm thick amorphous Pd-In-Ga-As layer remained, indicating that the first reaction step was solid-state amorphization. After annealing at 350 °C, PdxIn0.53Ga0.47As disappeared and regrowth of In0.53Ga0.47As occured. Finally, at 400 °C, residual Ge from the amorphous top layer diffused to the interface and grew epitaxially on the regrown In0.53Ga0.47As, thus separating the IIIV compound semiconductor from the Pd-Ge reaction products. The interface remained flat, while only about 10 nm of the active In0.53Ga0.47As layer had been modified during the annealing processes.
Transmission electron microscopy study of rapid thermally annealed Pd/Ge contacts on IN0.53Ga0.47As
(1996)
8 wt% Y2O3-ZrO2 spray powder and thermally sprayed as thermal barrier coatings on outer air seals were characterized by Raman microprobe spectroscopy (RMS) and transmission electron microscopy to investigate the lateral distribution, composition, structure and grain size of ZrO2 phases and to separate ZrO2 and Y2O3. RMS of transfer layers on ZrO2-coated shrouds after rub tests against SiC laser-tipped metallic blades showed the formation of metallic oxides MeIMeII2O4 of spinel type. Wear tracks were partly covered by graphite and spinel-type oxides. In some cases, the latter were better detectable after laser enhanced cleaning by burning off the covering carbon layer.
An attempt was undertaken to obtain a better understanding of the tribological properties of two wear-resistant coatings on tool steel by structural and microchemical analysis of wear particles using a transmission electron microscope. Coatings were deposited by physical vapor deposition and plasma-assisted chemical vapor deposition techniques and tribological properties were derived from reciprocating sliding tests of the coatings against alumina balls. Three types of wear particles were identified by electron diffraction and energy dispersive X-ray spectroscopy: nanocrystalline rutile (TiO2), nanocrystalline graphite and microcrystalline graphite. Low coefficients of friction, of the order of 0.2, were attributed to the formation of solid lubricant films of sub-stoichiometric TiO2-x Magnéli phases and/or graphite.
XTEM and TFXRD investigations of ohmic Ti/Al/Ti/Au/WSiN contacts on AlGaN/GaN HFET layer systems
(2002)
The microstructural features of the high-temperature-stable ohmic contact system Ti/Al/Ti/Au/WSiN on AlGaN/GaN were investigated using analytical transmission electron microscopy and thin film x-ray diffraction. For two typical rapid thermal annealing steps at 750 °C (non-ohmic behaviour) and 850 °C (ohmic behaviour) the intermetallic phases at the metal-semiconductor interface are presented. Increased annealing leads to the transformation of an Al2Au-AlAuTi phase mixture to a mixture of Al2Au-Al3Au8 phases and the formation of Ti-Al-nitride layers at the interfaces. In light of these results the electrical contact properties are discussed.
X-ray photoelectron spectroscopy (XPS) and cross-sectional transmission electron microscopy (XTEM) were used to study the formation of AlN films by N+2 ion implantation of aluminium at energies of 3 keV and 100 keV. In both cases, a two-stage mechanism was found, comprising first the oriented precipitation of small particles of the hexagonal AlN-phase, followed by growth and coalescence finally forming a continuous AlN-layer while increasing the implantation dose from 1×1017 cm-2 to 2×1017 cm-2. The results of both methods are in excellent agreement and furthermore provide complementary information concerning chemical composition and binding energies as well as microstructural details.
Different wear-resistant coatings produced by physical vapour deposition (PVD) were characterized with the aid of cross-sectional transmission electron microscopy (XTEM). All coating systems were optimized by the producers and exhibited good properties with respect to their special applications. The microstructure, texture and chemical composition of binary and ternary systems produced by the arc process [TiN, CrN, Cr2N (Ti, Cr)N on steel substrates] and magnetron sputtering process [TiN, CrN on steel substrates, (Ti, Al)N on Si-substrate] were investigated. All coatings had a more or less columnar microstructure, which was interrupted by interlayers in some cases. Whereas arc coatings always did show some kind of substrate modification, the latter was not observed after magnetron sputtering. Electron diffraction normally revealed a mono-phase fcc structure, except at sites very near to the interface. Only for the systems CrN and (Ti, Cr)N were different additional phases observed at low nitrogen partial pressures.
The formation of a third body layer on a conventional friction material during braking tests was studied in some detail using TEM, SEM, and X-ray photoelectron spectroscopy (XPS) techniques. Plate shaped micro-contact areas representing a compositional mix of all components of the tribocouple and exhibiting a nanocry stalline microstructure were identified after a run-in period.
Multi-layer ohmic contact systems such as Au/Pt/Ti/Pt on III-V double layers such as n-InGaP/p+-GaAs are of considerable technological relevance, e.g. for heterojunction bipolar transistors. The paper shows that such contacts can be effectively reacted through the InGaP layer and exhibit very good contact resistances (0.1 O mm) to the base layer if the thickness of the first Pt layer is properly matched to the thickness of the contacted InGaP layer. Interdiffusion and phase formation associated with the annealing processes are studied by cross-sectional analytical transmission electron microscopy, thin-film x-ray diffraction and Auger electron spectroscopy depth profiling. Thermal ageing experiments up to 400 °C show good electrical stability. Device related reliability tests do not show any degradation effect related to this novel base contact.
Patches of white etching layers on rail surfaces were investigated using sophisticated techniques like cross-sectional transmission electron microscopy (XTEM) and synchroton X-ray diffraction. Optical microscopy failed to resolve the microstructure, but in the TEM submicron grains with high dislocation densities and occasional twins, which are characteristic features of high carbon martensite, were observed. The martensitic structure was confirmed by evaluation of synchroton X-ray diffraction line profiles. The latter technique also allowed to determine dislocation densities of the order of 1012 cm-2 and residual compressive stresses of about 200 MPa.
The chemical and microstructural changes occurring during braking simulation tests at the surface of a conventional brake pad material were investigated mainly by scanning and transmission electron microscopy and surface analytical techniques. It can be shown that patches of a third body material develop, comprising a compositional mix of all constituents of the pad and iron oxides from the disk. Milled debris particles still have the crystal structure of barite, the major phase of the pad material, but the grain size is reduced drastically to the nanometer scale. The major wear mechanism is delamination of filler particles from the organic binder, supported by local degradation of the phenolic resin during asperity heating. Quartz crystals are preserved thereby adopting the function of primary contact areas.
Mechanical and thermal response of a nickel-base superalloy upon grinding with high removal rates
(1997)
Optical and electron metallography have been used to investigate the development of different microstructures in Armco Fe after rolling reductions between 0 and 90%. Simultaneously, the orientation distribution functions have been calculated from pole figures obtained by X-ray diffraction. Different stages of texture development could be assigned to typical microstructures. Strain inhomogeneities, such as microbands and shear bands, caused some broadening of the texture and a slight deviation of one skeleton line from the predicted fibre texture component. The decomposition of orientations near {111} < 121 > , and the strengthening of {211} < 011 > after high rolling reductions, was not due to mechanical twinning but could rather be explained by assuming {211} < 111 > slip. 30 ref.--AA
The surfaces of a brake pad and rotor were investigated after a run-in period during which a stable coefficient of friction had developed. The Focused Ion Beam Technique (FIB) was used to reveal tribologically induced surface films and for cross-sectional preparation of superficial layers. Additional information was obtained by TEM/EDS of thin lamellae prepared with the FIB and by surface analytical methods (GDOS, XPS and RS). Microscopic contact areas of the pad showed bright contrast in Scanning Ion Microscopy (SIM). This was attributed to severe plastic deformation finally leading to a nanocrystalline microstructure. Metallic particles of the pad, the so-called primary contact areas, were mostly covered with a smooth oxide layer of less than 1 µm thickness. Above this layer a thin (100 nm) partly amorphous film was often observed. The film was not only restricted to the metal particles, but also spread over adjacent regions, suggesting that secondary plateaus had formed. Similar layers and films, although with slightly different composition and structure, were also observed at the surface of the rotor.
The development of two metallizations based on the solid-phase regrowth principle is presented, namely Pd/Sb(Zn) and Pd/Ge(Zn) on moderately doped In0.53Ga0.47As (p=4×1018 cm-3). Contact resistivities of 23×10-7 and 67×10-7 ?cm2, respectively, have been achieved, where both systems exhibit an effective contact reaction depth of zero and a Zn diffusion depth below 50 nm. Exhibiting resistivities equivalent to the lowest values of Au-based systems in this doping range, especially Pd/Sb(Zn) contacts are superior to them concerning metallurgical stability and contact penetration. Both metallizations have been successfully applied for contacting the base layer of InP/In0.53Ga0.47As heterojunction bipolar transistors.
A rather new tool, the focused ion beam (FIB) technique, was used to characterise superficial layers at micro-contact areas of a commercial brake pad. The friction material was a polymer matrix composite (PMC) with approximately 50% metal content (semi-metallic) and the counter part was a cast iron rotor. Though the contact areas were not visible as topographic features, they could be identified with the aid of their increased secondary electron emission during FIB-scanning of the surface after tribological activation. Target preparation of micron-sized cross-sections with the FIB enabled the study of superficial layers at predetermined sites at high magnification.
Depending on the constituent of the pad, one, two or three layers were identified. The three layer structure comprised: (i) a 100 nm thick friction film containing nanocrystalline metal oxides and an amorphous phase which was enriched with sulphur, (ii) a nanocrystalline friction layer of compacted wear debris accommodating surface roughness and (iii) a severely deformed layer if the supporting constituent was a metal particle. Though the majority of loose wear particles was iron oxide, the friction film which adhered tightly to the pad surface contained a large amount of copper and sulphur, whereas zinc was transferred to the cast iron rotor.
Ti/TiN multilayer coatings with multilayer periods in the range 550 nm and a final thickness of 2 µm were deposited on steel substrates by cyclic modulation of nitrogen gas flow into the chamber of a PVD sputtering device. Coating characterization was performed by cross-sectional transmission electron microscopy, glancing-angle X-ray diffraction and instrumental indentation testing. Individual a-titanium and titanium nitride layers were always observed, although for the finer microstructures, the TiN layers were thicker than the Ti layers by a factor three. The plastic hardness of the films increased steadily with decreasing layer spacing, following a HallPetch relationship. Finally, a hardness value of 42 GPa was reached, which is similar to that of a thick TiN monolayer, prepared under the same coating conditions.