Band 92, Heft 1

Eine neue Untersuchung am Schwert des Cid Campeador aus dem 11. Jahrhundert v. Chr. versucht aus Gefügekenndaten eine Altersbestimmung zu erreichen. Dabei wird die Problematik der metallphysikalischen Vorgänge über lange Zeiten bei Raumtemperatur offenkundig. Bereits seit längerem diskutiert wird die Stabilität von Martensit, wie er an Eisenfunden aus „Ferrum Noricum“ des Magdalensberges (Kärnten, um Christi Geburt) als typisch für diese Werkstoffqualitat beobachtet wird. Ein jüngeres Problem ist die Härtestabilität in kupferzeitlichen Werkzeugen, wie dem Beil des Mannes vom Hauslabjoch (Ötzi), das an der Klinge gedängelt und geschliffen vorliegt. Es wird diskutiert, welche zeitlichen Vorgänge bei Raumtemperatur wirksam sein konnen und welche Möglichkeiten es gibt, Metallgegenstände, vor allem solche aus Eisen, mit physikalischen Methoden zu datieren. Dabei ist für Kupfer ein Zeitraum von maximal 8000 Jahren besonders interessant, für Eisen sind die letzten 4000 Jahre aus der Sicht der Metallphysik in Betracht zu ziehen. Einige Probleme der Zeit in der Metallkunde werden hier aus verschiedener Sicht diskutiert.

Via three-dimensional atomistic computer simulations, the influence of a lattice misfit between precipitates and matrix on the precipitate size distribution has been investigated during phase separation in a binary alloy. The elastic interactions are modeled by springs connecting nearest neighbor atoms. The difference in size between the two species of atoms, specified by the misfit parameter δ , leads to effective long range interactions. Performing simulations with five different values of δ we find a broadening of the scaled precipitate size distribution for δ > 0.5% in comparison with the case of no misfit. Additionally, the size of the largest precipitate in the system increases with increasing misfit. We also observe a percolated network-like precipitate structure for a concentration of solute atoms as low as 20 at % at large misfit. We interpret these effects as due to an increased tendency of the precipitates to coalesce, which is caused by their nonspherical shape, their alignment along the 〈100〉 directions and their regular arrangement.

The phase relationships for Ga–Ni–Sb alloys were determined by means of differential thermal analyses along ten selected isopleths through the ternary system. From these the sequence of invariant reactions involving the liquid phase was derived in terms of a Scheil diagram. Furthermore, a projection of the liquidus surface was constructed for the entire composition range. It contains a total of nine invariant four-phase reactions, seven of the transition type and two of the eutectic type. Together with the isothermal sections at 900, 600, and 500 °C that were presented earlier this results in a full description of the phase equilibria in the ternary Ga–Ni–Sb system as a function of composition and temperature.

The binary Al–Gd and the ternary Al–Gd–Mg systems were calculated using the Calphad method. It is demonstrated that previous interpretation of ternary liquidus temperatures below 700 °C must be related to other phase equilibria. The actual ternary liquidus temperatures are much higher, up to some 600 °C above the previous interpretation in literature. They are widely governed by the high-melting compounds Al 2 Gd and Al 3 Gd with liquidus surfaces stretching far into the ternary system. A small number of key experiments in this work confirmed the calculated liquidus temperature and the phase relations. The available experimental data in literature fit excellently with the calculation in the binary Al–Gd system. In the ternary Al–Gd–Mg system, which is shown in several sections of the phase diagram, a good agreement can be observed too, considering the necessary reinterpretation of the liquidus temperatures suggested by Rokhlin et al. Ternary solubilities were not found experimentally. The ternary compound Al 4 GdMg (τ) forms in a ternary peritectic reaction at 761 °C.

The diffusional mobilities of Cr, Ni, and Si in fcc Cr–Ni–Si alloys have been assessed as a function of temperature and composition by means of a CALPHAD (CALculation of PHAse Diagrams) type of procedure. Optimized kinetic parameters are presented. Comparisons between the calculated and measured diffusion coefficients show that most of the experimental information is satisfactorily accounted for by the present modeling. The obtained parameters can predict reasonably concentration profiles in binary Ni–Si and ternary Cr–Ni–Si diffusion couples.

The specific volume changes due to isothermal decomposition of hypereutectoid Fe–C austenite into cementite and carbon depleted austenite above the A 1 temperature, and into ferrite and cementite below the A 1 temperature, have been calculated quantitatively, using lattice parameter data as function of carbon content and temperature for the phases involved. Corresponding dilatometric experiments have been performed. The measured relative length changes by isothermal decomposition of austenite in Fe–C alloys of different carbon content, above and below the A 1 temperature, are in good agreement with the calculated results if a slightly modified set of lattice constants for cementite is used.

The excess enthalpies of liquid alloys in the ternary systems Ag–Sb–Te and Ag–Bi–Te were determined in a heat flow calorimeter for five sections Ag y Bi 1– y Te with y = 0.2, 0.4, 0.5, 0.6, and 0.8 at 1173 K, for the section Ag 0.5 Bi 0.5 –Te at 1073 K, and to complete recent measurements at 1073 and 1173 K for the section Ag 0.5 Sb 0.5 –Te. The thermodynamic functions of the systems Ag–Sb, Ag–Bi and Ag–Te were reoptimized with the Lukas programs using the association model. Ternary interactions were taken into account for the analytical description of the excess enthalpies of the liquid Ag–Sb–Te and Ag–Bi–Te alloys. The enthalpy surfaces of the systems are determined by valleys of exothermic minima between the congruently melting compounds in the binary tellurium systems. The values of exothermic enthalpies decrease with increasing temperature, due to the dissociation of the telluride associates in the binary systems.

Starting from a general expression for the surface tension γ of a binary mixture, a criterion is derived which indicates if and at which temperature the temperature derivative of the surface tension, d γ /d T , changes sign. An explicit and simple equation for this temperature is obtained by using the ideal solution model.

Micro-Vickers hardness measurement has been used to determine the composition of functionally graded materials (FGMs). Materials used in the present study are Al-SiC, Al–Shirasu and Al–Al 3 Ni FGMs, which are fabricated by a centrifugal method. The micro-hardness of the aluminum matrix for Al–SiC FGM markedly increases as the volume fraction of SiC particles increases. However, the tendency is not so clear in the cases of Al–Shirasu and Al–Al 3 Ni FGMs. Plastic strain energy due to thermal misfits between the matrix and dipersed particles is calculated based on an elasto-plastic analysis by applying a shell theory after Lee et al. The hardness is correlated theoretically with the composition gradient through the mean plastic strain energy (responsible for dislocation density). It is concluded that the composition gradient of metal-based FGMs can be determined from micro-hardness measurements.

In order to determine the temperature dependence of the misfit of the nickel-based superalloy CMSX-4, the lattice parameters of its constituents, γ′ phase and a nickel solid solution, have been determined by neutron diffraction measurements and compared with the lattice parameters of the two-phase superalloy of earlier experiments. From these data, the thermal expansion coefficients and the unconstrained misfit is calculated. As a second method, the macroscopic volume change was determined by dilatometer measurements and compared with that of the neutron scattering experiments. It can be concluded that both, the differences in macroscopic thermal expansion as well as the concentration changes between the phases, are able to explain the slightly increasing more negative misfit of the superalloy for temperatures up to 1100 K, whereas at even higher temperatures concentration shifts are solely responsible for the misfit changes.

Das Schädigungs- und Bruchverhalten der ausscheidungshärtbaren Aluminiumknetlegierung AA7075 wird im quasistatischen und Hochgeschwindigkeits-Zugversuch bis zu Umformgeschwindigkeiten von ε̇ ≤ 5500 s –1 untersucht, und die Ergebnisse bei Raumtemperatur werden den Ergebnissen bei einer erhöhten Versuchstemperatur von 150 °C gegenübergestellt. Durch unterschiedliche Wärmebehandlungen kann der Einfluss von Partikelgröße, -anteil und -verteilung ermittelt werden. Versuche an ungekerbten Zugstäben erlauben die Untersuchung des Materialverhaltens, das unter dynamischer Beanspruchung mit Hilfe eines konstitutiven Modells beschrieben wird. Dabei ist im Zugversuch neben der gesteigerten Geschwindigkeitsempfindlichkeit des Materials und dem adiabatischen Charakter des Umformprozesses sukzessive strukturelle Schädigung bei der Formulierung des Stoffgesetzes zu berücksichtigen. Mit Hilfe von Versuchen an unterschiedlich gekerbten Zugstäben wird der Spannungszustand in der Probe variiert und der Einfluss der Mehrachsigkeit auf die Bruchverformung untersucht. Durch Kombination von Experimenten und Finite-Element-Simulation wird die Versagensgrenze für duktilen Bruch formuliert und der Einfluss einer Geschwindigkeits- und Temperaturerhöhung auf die lokale plastische Verformung beim Bruch als Funktion des Mehrachsigkeitsgrads ermittelt. Mit Hilfe der Versagensgrenze kann schließlich der Bruchentstehungsort im gekerbten Stab bestimmt werden.

Two approaches are used to analyse X-ray or neutron scattering by crystals with different dislocation ensembles. Analogy between the above concepts is discussed. The microscopical approach takes into account the detailed displacement field due to different types of dislocation arrangement in the crystal. Frequently a phenomenological concept operating with the ideas of displacements, strains and stresses arising in the materials under deformation is applied. Unfortunately, the phenomenological concept cannot take into account that similar stress distributions may have significantly different microscopic reasons. The comparison of the results obtained by both phenomenological and microscopic descriptions of scattering enables us to understand the connection between different types of strains and stresses in the crystal with the type of the defects in it.

The paper presents creep curves of copper single crystals of three crystallographic orientations, obtained at two applied stresses at each of the temperatures 773 and 873 K. The curves indicate a creep strength increasing in the sequence of the crystal loading axes [010], [110], and [111]. The effect of the crystal orientation can be correlated with the potential activity of slip systems estimated from the Schmid factors. The slip lines usually indicate an activity of less systems than the Schmid factor values suggest. Buckling of the specimens in compressive creep is connected with an inhomogeneity and asymmetry of slip. This is probably also a reason of the fact that the observed dislocation structures contain all types of dislocations including those which should not be active under the applied uniaxial load.

In predicting the physical and mechanical anisotropy of rolled sheets, an overall texture through thickness must be taken into account, through for materials with inhomogeneous texture. To know the average texture thickness the whole of a sheet, a crystallite orientation distribution function (ODF) on the rolling direction (RD)-basis was calculated from incomplete pole figures measured by X-ray diffraction in the RD-section perpendicular to the rolling direction, and subsequently transformed into that on the normal direction basis, based on orthorhombic sample symmetry. The calculations for materials with texture gradient were in good agreement with the average of ODFs determined at various locations through the thickness. This method, therefore, appears to be very suitable for evaluating the average ODF of sheet materials. The ODF on the RD-basis before texture transformation also revealed that the β-fibre of {211}<011> – {634}<123> – {111}<112> for body-centred cubic metals was formed in cold rolled ultra-low carbon steel. Furthermore, the textures of a laminated sheet consisting of alternately stacked nickel and aluminium were successfully analysed by using this method.