K. Krištiaková

Free volume microstructure of tetramethylpolycarbonate at low temperatures studied by positron annihilation lifetime spectroscopy: a comparison with polycarbonate

The annihilation of positrons in amorphous tetramethylpolycarbonate (TMPC) has been investigated in the temperature range from 30 to 300 K. The dependences of the mean lifetime of the ortho-positronium τ3 and its relative intensity I3 on temperature can be described by the empirical relations: τ3 = 2.08(1 + 4.8 × 10−4 T) (ns) and I3 = 20.2(1 + 7.9 × 10−4T) (%). These dependencies are interpreted within the framework of the microstructural free volume concept. The comparison with the polycarbonate (PC) reveals that both values τ3 and I3 are larger for TMPC, indicating larger free volume realized through higher number (concentration) of larger free volume entities. The temperature dependencies of τ3 and I3 for TMPC exhibit a different character and a weaker dependence on the temperature. This different behaviour of the free volume is due to a different packing efficiency of chains and a different dominant mechanism of the free volume generation, as well as a decrease in the flip dynamics of the phenyl rings. The implications of different free volume microstructures on some mechanical properties (e.g. relaxation and fracture) as well as on the transport properties of gases in TMPC and PC are discussed.

Direct evidence of free-volume relaxation and the crossover effect in Ni25Zr55Al20 metallic glass

Abstract The changes in positron annihilation lifetime in a thermally treated Ni 25 Zr 55 Al 20 alloy were observed. The glass transition temperature T g lies around 700 K. Isothermal annealing was used to treat samples sequentially in the amorphous (at 640 K) and the undercooled liquid (at 740 K) states. This provides insight into the microstructural processes taking place via determination of the evolution of the positron capture site density and distribution. The observed shifts in the positron lifetime distribution are analyzed in terms of relaxation of the free volume V f , i.e. chemical and topological short-range ordering (CSRO, TSRO) which are known to be influenced in an important manner by the relaxation processes. The upper limits of the activation energies of the two relaxation processes (2.2 and 2.6 eV) are determined. For the first time, the “crossover” effect in the free-volume distribution has been observed.

Short range ordering in the melt and its manifestation in glassy Fe-Co-B. Investigation by positron annihilation lifetime

Abstract Six sets of ribbons of Fe 64 Co 21 B 15 metallic glass obtained by rapid quenching of the melt having different temperature and time history, selected with respect to the temperature-time evolution of melt viscosity, have been investigated by positron annihilation spectroscopy. The interest has been focused on the correlation between the expected melt precursor structure and the presence of ordered units (clusters, associations) in the amorphous matrix. Ribbons have been annealed for suitable times at temperatures below and above the crystallization temperature. Such selective thermal treatment of the samples has enabled to accentuate or anneal-out the effects of the presence of small ordered structural units as reflected by the positron annihilation lifetimes and positron affinities and by changes of electron structure obtained from electrical resistivity measurements. Fine structure of the amorphous matrix has been determined by analyzing the positron annihilation lifetime spectra with the use of a numerical Laplace transform method which provides continuous distributions of the position annihilation lifetimes. Differences in local ordering have been found for samples from master alloys with different melt histories.

Higher order analysis of the distribution of crystallization processes in metallic glasses

Abstract Metallic glassy alloys prepared by rapid quenching are expected to exhibit, unlike conventional polycrystalline matter, several specific features. The existence of disorder (frozen metastable state), chemical SRO, structural SRO (clusters), local heterogeneity, occurrence of ordered, yet noncrystalline, groups of atoms in the amorphous matter necessarily must be reflected in the complexity of processes controlling crystallization. Experimental observations on crystallization of amorphous alloys interpreted via classical thermodynamic analyses (which result usually in a single value, sum of values or prescribed activation energy distribution) provide further indication that these processes are numerous and should be more realistically described by a distribution reflecting the thermodynamic distribution of the initial state of the structure. These processes are by no means sufficiently described by standard simple theoretical distributions. Using a numerical Laplace inversion method, it has been possible to determine the distribution of processes controlling crystallization of metallic glasses in time and parametrically against temperature without assumptions concerning the form of the distributions. The results obtained from the changes of electrical resistivity in the course of isothermal crystallization of Fe–Co–B metallic glass have been analysed with respect to the expected inherent complexity of the system.

Positron annihilation studies of the dynamics of the evolution of free volume in amorphous alloy Ni25Zr55Al20

Abstract The dynamics of microstructural changes taking place in amorphous as well as in the undercooled-liquid states (UCL) have been studied for NiZrAl alloys. The samples have been prepared by planar flow casting. The glass transition and crystallization temperatures have been determined from both dilatation and resistivity measurements to be around 700 and 800 K, respectively. An annealing regime has been selected to treat the samples sequentially in the amorphous and UCL states, i.e., at temperatures above (740 K) and below (640 K) the glass transition temperature. The values of the positron lifetimes, τ, and the intensities of the lifetime components, I, indicate that positrons annihilate at different sites; on annealing, the formation of positron capture sites is observed which gradually change their properties, finally relaxing to a state dependent on the annealing sequence. The change of the positron lifetime distribution can be attributed to a free volume relaxation.

Influence of high pressure annealing on free volume in polycarbonate studied by positron annihilation

Abstract The method of positron annihilation was used to study changes of free volume in polycarbonate. Annealing at 180 °C and pressure 800 MPa causes significant changes of free volume (~23%) compared to nonannealed polycarbonate. The free volume compressibility was estimated to be ~3.10 −5 MPa −1 . The distribution of free volume has changed. This work discusses the influence of free volume on molecular dynamics and the transportation mechanism of free radical decay.

Cluster Structure of the Amorphous State and (NANO)Crystallization of Rapidly Quenched Iron and Cobalt Based Systems

Micromechanisms and energetics of transitions from metastable to more stable state were investigated in complex metastable disordered systems prepared by rapid quenching from the melt from the viewpoint of spatially (structurally) correlated distribution of transformation rates of individual microprocesses controlling the transition process. Using a novel, model-independent method for determination of continuous distributions of process rates it was possible to obtain information on distributions of true activation energies of these microprocesses. Detailed analysis of subdistributions of microprocesses active at each stage a of transition yielded also the information on temperature dependence of the activation energies.

Investigation of the Ni-Zr-Al alloy in the undercooled liquid region

Abstract NiZrAl system exhibits a broad temperature interval between the glass transition and crystallization temperature, giving thus a possibility of studying microstructural changes taking place both in amorphous and in the undercooled-liquid states. The changes in positron annihilation lifetime spectra in thermally treated Ni 25 Zr 55 Al 20 alloy, as observed previously (3), indicating that positrons annihilate at places with different properties, have been supported by resistivity measurements. Sequential annealing above and below T g is found to activate different micromechanisms controlling microstructural atomic rearangements and affecting the properties of the alloy; the influence of such treatment on nucleation mechanism is discussed.