Wolfgang Sprengel

Multiphase diffusion in the CoNb and NiNb systems: Part I. Solid-solid phase equilibria and growth of intermetallic phases

Abstract Diffusion couples of cobalt or nickel and niobium were employed to study solid-state diffusion reaction (multiphase diffusion). The formation and growth of intermetallic phases was investigated by means of optical metallography and electron probe microanalysis. Parabolic layer growth of Co7Nb2 and Co2Nb— the dominant phases in the diffusion zone of CoNb pairs—was observed. The growth constants were determined at various temperatures. Slow growth of the CoNb compound was also observed, but the layer thickness never exceeded a few μm. In the diffusion zones of NiNb pairs the two expected intermetallic phases NiNb and Ni3Nb were present. The phase diagrams of CoNb and NiNb have been carefully redetermined for the temperature regions of solid-solid equilibrium.

Element-specific study of local segmental dynamics of polyethylene terephthalate upon physical aging.

Time-dependent relaxation processes upon physical aging below the glass transition temperature have been studied in polyethylene terephthalate by high-precision dilatometry (DLT), differential scanning calorimetry (DSC), and element-specific positron and positronium (Ps) annihilation spectroscopy. The macroscopic volume change observed by DLT can be described by the Kohlrausch-Williams-Watts decay function, whereas changes in the relaxation enthalpies evaluated by DSC and free volumes probed by positron and Ps annihilation spectroscopy are reproduced by two superimposed exponentials. The multi-method approach reveals three kinds of relaxation processes with characteristic relaxation times: (a) fast Arrhenius-type β relaxation involving the instantaneous local segmental densification along with the exclusion of oxygen atoms from free volumes, (b) macroscopically observable non-Arrhenius-type α relaxation originated from a distribution of relaxation times due to the heterogeneous dynamics of solid-state- and liquid-state-like local segments, and (c) extremely slow Arrhenius-type α relaxation as the consequence of a uniform relaxation time solely due to the thermal dependence of nanometer-scale solid-state-like local segments.

Reversible atomic processes as basic mechanisms of the glass transition

Reversible formation and disappearance of vacant spaces (vacancy-type defects) in bulk Zr57Cu15.4Ni12.6Nb5Al10 glass are directly evidenced by high-resolution, time-differential dilatometry studies. The vacancy kinetics are strongly temperature-dependent, with an effective migration enthalpy of HVM = 3.34 eV. This may explain the strong temperature dependence of glass properties such as viscosity. The results presented here are of general importance for understanding amorphous condensed matter and biomaterials and for the technical development of amorphous steels.

Dilatometry: a powerful tool for the study of defects in ultrafine-grained metals

Vacancies, dislocations, and interfaces are structural defects that are deliberately introduced into solids during grain refinement processes based on severe plastic deformation (SPD). Specific combinations of these defects determine the improved mechanical properties of the obtained ultrafine-grained materials. High-precision, non-equilibrium dilatometry, i.e., measurement of the irreversible macroscopic length change upon defect annealing, provides a powerful technique for the characterization and the study of the kinetics of these defects. It is applied to determine absolute concentrations of vacancies, to characterize dislocation processes, and to assess grain boundary excess volume in pure, FCC and BCC ultrafine-grained metals processed by SPD.

Investigation of Structural Vacancies in Titanium Monoxide by Electron-Positron Annihilation

Local atomic environment of vacancies in nonstoichiometric titanium monoxide ranging in composition from TiO0.74 to TiO1.26 was studied by electron-positron annihilation. Analysis of the Doppler broadening spectra of the annihilation gamma line for titanium and liquid oxygen showed that positrons in titanium monoxide are trapped by titanium vacancies. Experiments revealed that the lifetime of positrons in ordered and disordered titanium monoxide TiOy increases with increasing oxygen content y and varies from 184 to 210 ps. Data on the valence electron density permitted the prediction that the lifetime of free positrons in stoichiometric titanium monoxide is about 140 ps and the lifetime of positrons localized in an oxygen vacancy is about 170 ps. The method used to analyze the gamma-line Doppler broadening spectra makes it possible to determine the type and number of atoms around a vacancy and to investigate order-disorder phase transformations in nonstoichiometric compounds.

Nanocrystallization mechanism of amorphous Fe78B13Si9

The nanocrystallization mechanism of an amorphous alloy is discussed based on the kinetics of open nanospaces in Fe78B13Si9. There already exists a high concentration of Fe-enriched fluctuated sites with open nanospaces in the amorphous matrix. The structural and compositional fluctuation helps transient short-range Fe diffusion in the metastable amorphous matrix with an increase of temperature, triggering highly concentrated α-Fe nucleation. Along with the growth of α-Fe nucleus, Fe atoms are transferred from the intergranular amorphous phase to Fe-based nanocrystallites. The nanocrystallization of α-Fe is achieved through nucleation by short-range Fe diffusion and its growth by nanovoid-mediated long-range Fe diffusion.

Self- and Solute Diffusion, Interdiffusion and Thermal Vacancies in the System Iron-Aluminium

Starting from fundamental aspects of thermal vacancies and solid-state self-and solute diffusion, this paper reviews procedures for tracer-and interdiffusion studies and of the major techniques for vacancy studies by dilatometry and positron annihilation in metals. Equilibrium vacancy and diffusion studies performed on pure iron and aluminium are mentioned at first. We also comment some peculiarities of solute diffusion in aluminium. Positron annihilation and differential dilatometry studies for Fe-Al alloys with various compositions are summarized and new experimental studies by the authors are reported for vacancy migration in Fe61Al39. All these studies indicate a relatively high fraction of thermal vacancies with relatively low mobility in this type of iron-aluminides as compared to pure metals. Tracer diffusion of iron and of several substitutional solutes such as Co, Ni, Cr, Mn, Zn, and In in Fe-Al from the Münster laboratory are summarized. The diffusion studies of Fe-Al cover various alloy composition between Fe3Al and FeAl and several structures such as A2, B2 and D03. Interdiffusion coefficients obtained from diffusion couples between Fe-Al alloys are discussed together with Fe tracer diffusion data. The Darken-Manning equation is used to deduce Al diffusivities therefrom. The latter are hardly accessible to radiotracer experiments due to a lack of a suitable Al tracer. Diffusion of Al is slightly faster than diffusion of Fe indicating diffusion mechanisms with coupled jumps of Fe and Al atoms.

A high-stability non-contact dilatometer for low-amplitude temperature-modulated measurements

Temperature modulated thermophysical measurements can deliver valuable insights into the phase transformation behavior of many different materials. While especially for non-metallic systems at low temperatures numerous powerful methods exist, no high-temperature device suitable for modulated measurements of bulk metallic alloy samples is available for routine use. In this work a dilatometer for temperature modulated isothermal and non-isothermal measurements in the temperature range from room temperature to 1300 K is presented. The length measuring system is based on a two-beam Michelson laser interferometer with an incremental resolution of 20 pm. The non-contact measurement principle allows for resolving sinusoidal length change signals with amplitudes in the sub-500 nm range and physically decouples the length measuring system from the temperature modulation and heating control. To demonstrate the low-amplitude capabilities, results for the thermal expansion of nickel for two different modulation frequencies are presented. These results prove that the novel method can be used to routinely resolve length-change signals of metallic samples with temperature amplitudes well below 1 K. This high resolution in combination with the non-contact measurement principle significantly extends the application range of modulated dilatometry towards high-stability phase transformation measurements on complex alloys.

Direct measurement of vacancy relaxation by dilatometry

A model is proposed for directly determining the volume of lattice vacancies by means of dilatometric measurements of the anisotropic irreversible length change which occurs during annealing of lattice vacancies at grain boundaries of shape-anisotropic crystallites. The model is tested using nanocrystalline Ni after the high-pressure torsion deformation which exhibits excess concentration of lattice vacancies and elongated crystallite shape. Different length changes upon annealing parallel and perpendicular to the elongation axis occur from which a vacancy volume can be derived.

In Situ Real-Time Monitoring of Aging Processes in an Aluminum Alloy by High-Precision Dilatometry

A detailed understanding of the atomistic processes governing the age hardening response of light metal alloys is of vital importance for the optimization of their properties. While a static characterization of metastable precipitates is possible, e.g., by advanced microscopic techniques, the kinetic aspects of the underlying formation of phases are by far more difficult to assess. In this work we present isothermal, high-stability laser dilatometric measurements, during the natural and artificial aging of a commercial aluminum alloy (Al-Mg-Si), with a rate resolution below 1 nm/h. The results of this case study show that the presented dilatometric technique allows for direct monitoring of the precipitation process with an unprecedented accuracy on the volume as well as on the time scale.