Gerhard Wilde

Grain boundaries in ultrafine grained materials processed by severe plastic deformation and related phenomena

Grain boundaries in ultrafine grained (UFG) materials processed by severe plastic deformation (SPD) are often called “non-equilibrium” grain boundaries. Such boundaries are characterized by excess grain boundary energy, presence of long range elastic stresses and enhanced free volumes. These features and related phenomena (diffusion, segregation, etc.) have been the object of intense studies and the obtained results provide convincing evidence of the importance of a non-equilibrium state of high angle grain boundaries for UFG materials with unusual properties. The aims of the present paper are first to give a short overview of this research field and then to consider tangled, yet unclear issues and outline the ways of oncoming studies. A special emphasis is given on the specific structure of grain boundaries in ultrafine grained materials processed by SPD, on grain boundary segregation, on interfacial mixing linked to heterophase boundaries and on grain boundary diffusion. The connection between these unique features and the mechanical properties or the thermal stability of the ultrafine grained alloys is also discussed.

The competing crystalline and amorphous solid solutions in the Ag–Cu system

Abstract Upon nonequilibrium processing using vapor quenching or mechanical alloying, the supersaturated fcc solid solution predominates over the amorphous solution in the Ag–Cu system. The thermodynamic and kinetic origins of this phase selection are explored. The enthalpy of formation of both solutions has been determined as a function of composition using calorimetry measurements and molecular dynamics (MD) simulations. The enthalpy of the fcc solution is found to be lower than that of the competing amorphous phase. The preference for the fcc crystalline state is enhanced by the low kinetic barrier to crystallization of the amorphous solution, which occurred during quenching even when high cooling rates were employed or during annealing at low temperatures. Consequently, the retention of an amorphous Ag–Cu solution required kinetic trapping using ultrahigh quenching rates achievable only under stringent vapor deposition conditions or in MD simulations. However, transmission electron microscopy revealed the presence of local regions of amorphous Ag–Cu after cold rolling of multilayers of elemental Ag and Cu foils at room temperature. This result of partial amorphization demonstrates the possibility of mechanically driven solid-state amorphization in a system with a positive heat of mixing.

Thermodynamic properties of Pd40Ni40P20 in the glassy, liquid, and crystalline states

Bulk specimens of the easy glass‐forming alloy Pd40Ni40P20 have been undercooled consistently into the glassy state at cooling rates as low as 10 K/min applying the melt‐fluxing technique in boron trioxide. Due to this low cooling rate, heat capacity measurements could be performed in a commercial heat‐flow differential calorimeter, covering for the first time the entire undercooling regime of a liquid metal from the melting temperature down to the glass transition temperature. Based on the measured specific heat data of the undercooled liquid and the crystalline state, the differences in the thermodynamic functions enthalpy, entropy, and Gibbs free energy are determined in dependence on temperature. The entropy balance yields a value of T0=500±5 K for the ideal glass transition temperature of this metallic system. The experimental values are compared to the corresponding thermodynamic functions, derived from commonly applied Gibbs free energy approximations for the undercooled liquid.

Glass formation versus nanocrystallization in AN Al92Sm8 alloy

Solid state glass formation is often viewed as a non-equilibrium process resulting from the destabilization of crystalline phases when the maximum metastable solubility is exceeded. In this work, glassy Al{sub 92}Sm{sub 8} alloys which have been obtained by both rapid quenching and deformation mixing techniques have been studied as a representative of a marginal glass forming material. The samples were examined to explore whether the nucleation of nanocrystals can be avoided by choosing an appropriate reaction pathway and whether the changes in the reaction pathway can influence the kinetic stability of the amorphous phase.

Nanomagnonic devices based on the spin-transfer torque

Magnonics is based on signal transmission and processing by spin waves (or their quanta, called magnons) propagating in a magnetic medium. In the same way as nanoplasmonics makes use of metallic nanostructures to confine and guide optical-frequency plasmon-polaritons, nanomagnonics uses nanoscale magnetic waveguides to control the propagation of spin waves. Recent advances in the physics of nanomagnetism, such as the discovery of spin-transfer torque, have created possibilities for nanomagnonics. In particular, it was recently demonstrated that nanocontact spin-torque devices can radiate spin waves, serving as local nanoscale sources of signals for magnonic applications. However, the integration of spin-torque sources with nanoscale magnetic waveguides, which is necessary for the implementation of integrated spin-torque magnonic circuits, has not been achieved to date. Here, we suggest and experimentally demonstrate a new approach to this integration, utilizing dipolar field-induced magnonic nanowaveguides. The waveguides exhibit good spectral matching with spin-torque nano-oscillators and enable efficient directional transmission of spin waves. Our results provide a practical route for the implementation of integrated magnonic circuits utilizing spin transfer.

Calorimetric, thermomechanical, and rheological characterizations of bulk glass-forming Pd40Ni40P20

Calorimetric, thermomechanical, and rheological properties of undercooled liquid Pd40Ni40P20 were determined within a wide temperature range above the glass transition. The concept of the limiting fictive temperature was applied to the entire set of measurements to compare the different properties adequately. It was found that an equilibrium state of a sample that is defined by its calorimetric glass temperature corresponds to a similar equilibrium state for the specific volume and for the shear viscosity as well. The Kauzmann temperature as one of the most important material characteristics concerning the glass transition could be determined with high accuracy leading to the evaluation of the free volume persistent in the samples. Viscosity values of the liquid extending over a range of about nine orders in magnitude could be described best by the free volume theory evaluated by Cohen and Grest, provided that experimentally obtained parameters were used for the calculations. The comparison between nonequilibrium measurements at isochronous heating and model calculations in the framework of bimolecular reaction kinetics shows that good agreement can be achieved using thermodynamic parameters that have been obtained from equilibrium measurements. However, systematic deviations indicate also the limitations of the model that are related to the intrinsic dynamic heterogeneity of the vitreous state.Calorimetric, thermomechanical, and rheological properties of undercooled liquid Pd40Ni40P20 were determined within a wide temperature range above the glass transition. The concept of the limiting fictive temperature was applied to the entire set of measurements to compare the different properties adequately. It was found that an equilibrium state of a sample that is defined by its calorimetric glass temperature corresponds to a similar equilibrium state for the specific volume and for the shear viscosity as well. The Kauzmann temperature as one of the most important material characteristics concerning the glass transition could be determined with high accuracy leading to the evaluation of the free volume persistent in the samples. Viscosity values of the liquid extending over a range of about nine orders in magnitude could be described best by the free volume theory evaluated by Cohen and Grest, provided that experimentally obtained parameters were used for the calculations. The comparison between nonequ...

Glass formation and primary nanocrystallization in Al-base metallic glasses

Abstract Aluminum-rich metallic glasses containing transition metals and rare earth elements have been found to yield finely mixed microstructures of Al nanocrystals embedded in an amorphous matrix and exhibit enhanced fracture strength with several percent strain. Upon primary crystallization of melt spun ribbons, this novel microstructure comprised of a high particle density (>10 20 m −3 ) of Al nanocrystals (20 nm) in an amorphous matrix develops and offers exceptional strength (1500 MPa) and high temperature stability (533 K). Numerical modeling based upon the size distribution of the Al nanocrystals after isothermal annealing is applied to study the nucleation kinetics in the metallic glasses. In addition to the kinetic study of primary nanocrystallization, the glass transition temperature ( T g ) has been assessed in Al–7at.% Y–5at.% Fe and Al–8at.% Sm alloys. In usual calorimetric measurements, the thermal response of the primary crystallization often obscures the observation of the signal corresponding to the glass transition. As a result, T g is often assumed to be near the onset of the primary crystallization reaction ( T x Al ). However, it has been demonstrated by modulated-temperature calorimetry that this assumption does not apply strictly to the metallic glasses under study. The thermal stabilization of the microstructure by the occurrence of diffusion field impingement allows for the observation of the glass transition of the remaining amorphous phase in the matrix by modulated differntial scanning calorimetry (DSC). The reliable assessment of the glass transition temperature provides not only a fundamental basis for the kinetics analysis, but also an important parameter in designing suitable annealing treatments that allow for the development of desired microstructures to yield optimized properties.

Transition between Periodic and Quasiperiodic Structures in Al-Ni-Co

Abstract A series of Al–Ni–Co alloys forming stable decagonal (D-ANC) quasicrystals was studied in as-cast and annealed states. It was shown that under certain conditions periodic structures with pseudodecagonal (PD) symmetry can be produced at the same compositions as stable decagonal quasicrystals. Different variants of D-ANC and PD were observed in a compositional range of 70–72.5 at.% Al and 13–18 at.% Co. As-cast D-ANC can be transformed to single-phase PD of the same local composition. Single-phase PDs can be transformed to D-ANC of the same composition by heating to a temperature higher than the formation temperature of these PDs. The transition between PD and D-ANC was studied in more detail in Al 7 1 Ni 14.5 Co 14.5 and Al 70 Ni 1 5 Co 1 5 by electron microscopy, powder X-ray diffractometry and differential thermal analysis. The results of this study do not confirm the thermodynamic stability of this PD structure.

Primary crystallization in amorphous Al-based alloys

Abstract An important characteristic of the new marginal glass forming alloys, that include amorphous Al-based alloys, is the development of a high number density of Al nanocrystals during initial devitrification. The observed nanocrystal densities can range from 1021 to 1023 m−3 and display a remarkable thermal stability that is reflected in a wide separation of 75 °C or more between the primary and final crystallization reactions. Isothermal crystallization studies based upon nanocrystal size distribution and microcalorimetry heat flow measurements in an Al–8at.% Sm alloy confirm that the nanocrystal dispersions can develop by a heterogeneous mechanism that appears to be related to a site density retained from melt quenching. Annealing experiments at temperatures spanning the glass transition as well as the use of incorporated nucleation catalysts and deformation treatments have revealed new aspects of the crystallization kinetics and strategies for the control of the nanocrystal density during microstructure evolution.

Hypercooling of completely miscible alloys

Calorimetric measurements and undercooling investigations were performed on the high‐melting, completely miscible binary systems (Co, Ni, Fe)‐Pd. The alloys, characterized by phase diagrams with concavous liquidus and solidus lines, exhibit heat‐of‐fusion values considerably lower than calculated assuming ideal solution behavior. As a consequence, these metallic systems offer the possibility to achieve the hypercooling regime at a reduced extent of undercooling. Investigations on the undercoolability of the liquid alloys indicate the surmounting of the calculated hypercooling limit. Time‐resolved radiation thermometry experimentally proved the appearance of complete isenthalpic solidification. Metallographic investigations of samples solidified from different levels of undercooling revealed the corresponding stages of microstructural evolution.