Volume 93, Issue 8

The intrinsic barriers against the glide of dislocations lying along low-index crystallographic directions, the so-called Peierls barriers , are overcome by the thermally activated and stress-assisted generation of pairs of kinks of opposite sign in the dislocations. In plastically deformed Al four low-temperature mechanical relaxation processes resulting from this are identified. In a sequence of falling temperatures, the corresponding internal-friction peaks are the original Bordoni peak ( a 0 〈110〉/2 screw dislocations on {111} slip planes; a 0 = edge length of the elementary cube), Niblett’s ‘subsidiary peak’ (60°-dislocations on {111}), the Lax–Filson peak (edge dislocations on {111}), and the Kosugi–Kino peak (30°-dislocations on {111}). In other face-centred cubic (fcc) metals (e. g. Cu, Pd) the relaxation processes of the dislocations lying along 〈110〉 as well as those lying along 〈112〉 overlap, giving rise to just two peaks, namely very broad Bordoni and Niblett–Wilks peaks. In the body-centred cubic (bcc) transition metals, the Peierls barriers of a 0 〈111〉/2 screw dislocations are exceptionally high. Beyond the microstrain regime, overcoming them by the formation of kink pairs determines the flow stress up to the knee-temperature T K . The theory of the kink-pair formation enthalpy H kp (σ*) (effective stress σ* = difference between applied resolved shear stress σ and long-range internal stress σ M ) has been tested and verified with great accuracy on various high-purity bcc metals using the Ackermann–Mughrabi cyclic-deformation method as well as stress-relaxation measurements. The ‘hump’ in the flow-stress–temperature relationship of high-purity bcc metals follows from the theory without further assumptions. The comparison experiment–theory shows that the kink-pair generation takes place on {110} planes below the temperature T ≤ of the lower bend of the flow-stress–temperature curve and on {112} in the range T ˇ < T ≤ T K . $\check{T}<T \leq T_{\mathrm{K}}.$This change-over in the plane of the elementary slip steps is attributed to a first-order transition in the screw-dislocation core structure. The core transformation at Ť $\check{T}$accounts not only for the striking difference of the slip-line pattern below and above that temperature but also for several hitherto ill-understood phenomena in bcc metals such as alloy softening , resulting from ‘constitutional overheating’ of the transformation, and anomalous slip , resulting from the interaction of 〈111〉{112} slip systems with different slip directions. The {112} slip is also responsible for the tension–compression asymmetry of bcc metals, which, however, cannot be explained entirely in terms of the familiar twinning–antitwinning asymmetry of {112} slip but involves also stress components other than the effective resolved shear stress σ*. In 1971 it was proposed that the generation of kink-pairs in a 0 〈111〉/2 screw dislocations gives rise to the so-called γ-relaxation in the internal friction of bcc metals. The agreement of the activation enthalpy of this process with twice the enthalpy of formation of isolated kinks in screw dislocations, H k , as deduced from flow-stress measurements not only verifies this proposal but also demonstrates that in the ‘classical’ γ-relaxation the kink-pair generation takes place on {112} planes. However, recent measurements on Nb and Ta have shown that annealing may lead to a modified γ-relaxation with a smaller activation enthalpy. A tentative suggestion is that here the kink-pair generation takes place on {110} planes as a consequence of ‘constitutional overheating’ of the screw dislocation cores.

The fatigue behavior of iron-based materials has been investigated by rotating bending testing, employing positron annihilation spectroscopy to probe defects on the atomic level. Positron annihilation spectra have been recorded at various stages of material fatigue. The defect density has been determined by analysing the line shape of the Doppler broadening of the annihilation radiation in the photo peak. The line shape parameter ( S parameter), a measure of the defect density, showed a linear relation to the logarithm of the number of loadings, thus from only a small number of loadings it is possible to determine the remaining useful life of the sample. Furthermore, along the longitudinal sample axis spatially resolved line-scans are taken using the Bonn Positron Microprobe. Due to the special sample geometry, the stress gradient allows to obtain the S parameter for different values of the applied load using the very same sample. This leads to a way to determine a complete Wöhler diagram using a non-destructive method for just one sample.

The paper describes a detailed analysis of diffusion bonding in a binary titanium aluminide alloy of composition Ti-45Al (at.%). The study involves scanning electron microscopy coupled with chemical microanalysis and electron backscattered diffraction (EBSD). Diffusion bonding leads to the formation of a thin layer of a 2 (Ti 3 Al) phase at the bonding interface and to a remarkable redistribution of the phase constitution. The micromechanisms behind these phenomena will be discussed with regard to the phase transformations and diffusion processes involved.

Recent breakthroughs in computer simulation of the dislocation dynamics of glide and cross-slip give strong hints for a nanotheory of nucleation of persistent slip bands (PSBs) as the recovery process controlling cyclic saturation in metal fatigue. At the atomic scale, the intense slip localization associated with PSB nucleation is determined by thermally activated annihilation of screw dislocation dipoles of critical height depending on the availability of jogs. The transformation of matrix walls to ‘condensed’ PSB walls and, hence, the very persistence of PSBs are explained by the line tension of superjogged screw dislocations threading condensed walls as a result of the yield events during PSB nucleation. The new theory extends the mesoscopic saturation theories of Winter, Mughrabi and Brown to the saturation process of nanoscale PSB nucleation. The theory seems capable of further extension to explain simple alloying effects. For pure face-centred cubic metals, very simple analytical nanomodels of yield and recovery expose the theory to stringent tests by quantitative nanoscale characterization and atomic-scale simulation.

Copper single crystals and polycrystalline very-low-carbon steel (0.036 wt.% C) were fatigue loaded at low and very low amplitudes. The resulting dislocation structures and surface features were studied and the results correlated. Similarities of the face- and body-centred cubic materials as to the resulting dislocation and surface structures are outlined. In addition, the question of an eventual effect of the loading frequency on the resulting microstructural changes is shortly discussed.

The effect of Hf additions on the solidification characteristics and castability of the Ni-base superalloy IN792 has been studied. A quenching technique was employed to freeze-in the microstructure during solidification. The crack length in tube-shaped demonstration castings was evaluated. Hf additions were found to reduce the hot-tearing susceptibility significantly. Hf also caused a more gradual decrease of the liquid fraction with temperature in the last stages of solidification, which was observed to improve castability in other alloys previously and which can be correlated with lower strains and strain rates in the vulnerable period where hot tears form. It was also found that the γ/γ′ rosette structure, often referred to as γ/γ′ eutectic in the literature, actually forms by a discontinuous precipitation mechanism from the solid. The amount of γ/γ′ observed in the interdendritic area, therefore, does not reflect the amount of remaining liquid in the last stages of solidification.

Magnesium/ceramic composites with an interpenetrating microstructure were made by pressure infiltrating an open cellular ceramic preform. The low-density ceramic preform with a porosity of 82% was prepared by foaming a low-viscous polysiloxane/alumina suspension which formed an SiO 2 /Al 2 O 3 microcomposite ceramic foam after thermal treatment in air at 1200 °C. Foaming was achieved by an in situ blowing process during curing of the polysiloxane precursor at 200 – 300 °C. The open cellular structure was characterized by a mean strut thickness of 130 μm, an average cell diameter of 800 μm and a high pore connectivity. The ceramic preforms were infiltrated with AZ 31 magnesium alloy by squeeze casting at 680 °C applying a maximum pressure of 86 MPa. Interface bonding was achieved by the formation of spinel and cordierite in a reaction layer at the metal/ceramic interface. The dense metal/ceramic composite possesses significantly higher elastic modulus, yield strength and creep resistance than the monolithic alloy at room temperature and 135 °C. The experimental results suggest that filler-loaded preceramic polymers have a high potential for optimization of lightweight metal cast components.

A pair-controlled study was performed to compare the biomechanical behaviour of implant-reinforced fractured humeri under bending and under torsional loading, respectively. Special interest is related to the fixation potential of a prototype version of an intramedullary nail for stabilisation of subcapital humeral fracture. This nail is especially designed in terms of application for less invasive surgery purposes. In this study, two different modifications of this prototype nail are directly compared with respect to the stabilisation potential in direct comparison to conventionally applied implant techniques (wire technique and plate fixing). First results indicate that fixing of subcapital fracture by this intramedullary nail type provides a good compromise concerning the stabilisation under both loading types, bending and torsion, respectively.

The complete nickel-indium system was thermodynamically assessed at a total pressure of 1 bar from room temperature up to liquidus temperatures. The Gibbs energies of five condensed solution phases and five stoichiometric compounds were modeled using all available T–x phase diagram and thermodynamic data. For the Ni-rich solid solution with face-centered cubic structure as well as the liquid phase a substitutional approach was chosen to describe the Gibbs energies. Sublattice models were applied to account for the description of the thermodynamics of three nonstoichiometric intermetallic compounds. Three sublattices were chosen according to the crystallographic structures of the ζ-Ni 2 In and ζ’-Ni 13 In 9 phases, whereas two sub-lattices were selected for the δ-NiIn phase. The gas phase was treated as thermodynamically ideal. All computations were carried out with the Gibbs energy minimization program ChemSage and its routine for parameter optimization.

The onset of the oxidation of epitaxial Cr films and NiAl single crystals will be discussed, that was characterized by surface sensitive X-ray diffraction techniques. From X-ray reflectivity measurements atomic-scale information on the oxide thickness, its roughness and the metal/oxide interfacial morphology can be obtained. As an example, time-resolved reflectivity measurements during the oxidation of Cr are presented, which allow the determination of an activation barrier for Cr ion diffusion through the growing oxide layer. The structure and defects of the growing Cr 2 O 3 layers are in addition visualized in cross-section high-resolution transmission microscopy images. In the case of the oxidation of NiAl (110) the formation of Al 2 O 3 overlayers was observed, leaving behind a chemically disturbed substrate. Here, so-called crystal truncation rod measurements provide information about the oxidation-induced defect formation close to the NiAl/Al 2 O 3 interface. In the case of the oxidation of NiAl (110) Al anti-site atoms are present at the interface, which are forbidden in the Ni –Al volume phase diagram.

To investigate the misorientation dependence of fracture behavior and Sb segregation level for different [001] symmetric tilt boundaries in Cu–2.0 mass% Sb alloy bicrystals, tensile tests at several temperatures from 77 to 743 K and energy-dispersed X-ray spectroscopy analyses were performed. The degree of embrittlement of grain boundaries becomes weaker as the test temperature increases up to about 400 K, while it becomes stronger again at higher temperatures. The fracture stress at 77 K and Sb segregation level vs. misorientation angle curves display several peaks and cusps at the same angles, and the positions of these peaks and cusps are in agreement with those of cusps in the boundary energy of pure Cu vs. misorientation curve. Grain boundaries with higher energy are more sensitive to Sb segregation to cause grain-boundary embrittlement.

A structural analysis of a nonstandard Mg-10 wt.% Al alloy has been performed. Structural examinations were carried out for the alloy either in the as-cast state and after T6 heat treatment involving solutioning and artificial ageing. In the cast alloy, the presence of the degenerated eutectic (Mg) + γ was disclosed, which occurred as a result of a strong segregation during solidification. The T6 heat treatment resulted in a homogenization of the solid solution and the formation of a large number of secondary γ′ precipitates with a characteristic lamellar morphology. Transmission electron microscopy allowed the determination of two crystallographic orientation relationships between the γ′ and the (Mg) phases: ( 0001 ) M g ( 1 1 ¯ 0 ) γ ′ , [ 2 1 ¯ 1 ¯ 0 ] M g [ 111 ] γ ′ $\begin{equation} (0001)_{\mathrm{Mg}}\left\|(1 \overline{1} 0)_{\gamma^{\prime}},[2 \overline{1} \overline{1} 0]_{\mathrm{Mg}}\right\|[111]_{\gamma^{\prime}} \end{equation}$and ( 0001 ) M g ( 1 1 ¯ 0 ) γ ′ , [ 01 1 ¯ 0 ] M g [ 113 ] γ ′ . $ \begin{equation} (0001)_{\mathrm{Mg}}\left\|(1 \overline{1} 0)_{\gamma^{\prime}},[01 \overline{1} 0]_{\mathrm{Mg}}\right\|[113]_{\gamma^{\prime}}. \end{equation}$