H. J. Fecht

Nanocrystalline metals prepared by high-energy ball milling

This is a first systematic report on the synthesis of completely nanocrystalline metals by high-energy deformation processes. Pure metals with body-centered cubic (bcc) and hexagonal close-packed (hcp) structures are subjected to ball milling, resulting in a decrease of the average grain size to ≈9 nm for metals with bcc and to ≈13 nm for metals with hcp crystal structures. This new class of metastable materials exhibits an increase of the specific heat up to 15 pct at room temperature and a mechanically stored energy determined as up to 30 pct of the heat of fusion after 24 hours of high-energy ball milling. The grain boundary energy as determined by calorimetry is higher than the energy for fully equilibrated high-angle grain boundaries.

Containerless processing in the study of metallic melts and their solidification

AbstractThe study of metallic melts, their physical properties, and their solidification behaviour is of relevance to understanding of fundamental principles and to metallurgical applications. Many, if not most, of these properties are extremely sensitive to contamination of the melt by impurities. Consequently, containerless processing techniques have become an important tool in the investigation of these systems. In this review, first the most frequently used containerless techniques are discussed, such as free fall facilities (drop tubes, drop towers) and levitation facilities (acoustic, electrostatic, electromagnetic) including potential applications in the microgravity environment of space. These are correlated with the research areas which make use of containerless processing. Specifically, the measurement of thermophysical properties, nucleation experiments, crystal growth, and the solidification of nonequilibrium states are discussed. In all these cases, the major emphasis is on the study of the u...

Metastable phase formation in the Zr‐Al binary system induced by mechanical alloying

We have studied the sequence of phase transformations induced in the Zr-Al binary system by mechanical alloying of mixed Zr and Al powders. The structure of these materials has been studied by transmission electron microscopy and by x-ray diffraction measurements. Three different metastable phases have been found experimentally with variation of the initial composition xAl: (1) a nanocrystalline supersaturated solid solution of alpha-Zr for xAl<=0.15, (2) an amorphous phase for 0.15<xAl<=0.4, and (3) a metastable face-centered-cubic phase for xAl=0.5 with a grain size of 4 nm. The crystallization reaction of the amorphous phase was monitored by differential scanning calorimetry, and the kinetics of the reaction have been examined as well. A possible explanation based on thermodynamic arguments is given for the defect-driven vitrification of the crystalline Zr phase.

Synthesis and properties of nanocrystalline metals and alloys prepared by mechanical attrition

Abstract The grain size in powder samples can be reduced to nanometer scales during heavy cyclic mechanical deformation as produced in a standard ball mill. For pure bcc and hcp metals, intermetallic compounds and solid solutions, nanocrystalline materials can be synthesized at temperatures close to room temperature with a grain size ranging from 5 to 15 nm. During this process three different stages have been observed with (i) the deformation being localized in shear bands, (ii) formation of small angle grain boundaries separating the individual grains and (iii) formation of large angle grain boundaries with a completely random orientation of the nanosized grains. Thermal analysis of these samples reveals excess energies of up 40% of the heat of fusion and excess heat capacities of up to 20% in comparison to the undeformed state thus exceeding by far any values determined for conventional deformation processes and the energy of grain boundaries in fully equilibrated polycrystalline samples. These thermophysical data are in agreement with a theoretical model adopting a free volume approach for the grain boundaries based on the universal equation of state at negative pressure.

Thermodynamic properties and stability of grain boundaries in metals based on the universal equation of state at negative pressure

Abstract Adopting a free volume approach at negative pressure of the universal equation of state, the thermodynamic properties of unrelaxed grain boundaries are determined for f.c.c., b.c.c. and h.c.p. metals including the excess energy, excess enthalpy, the bulk modulus, the Gruneisen parameter and consequently, the excess entropy and Gibbs free energy. A mechanical instability of a grain boundary is predicted based on the Gibbs stability criteria at an excess volume of about 0.45, corresponding to a scaled atomic separation of 0.7. The calculations suggest that grain boundaries with an excess volume close to this mechanical instability condition are entropy stabilized and can become more stable than boundaries with small excess volume at elevated temperatures. Taking the entropy of sublimation as an upper limit of the grain boundary entropy in order to prevent an entropy catastrophe, this state is separated by the single crystal state at elevated temperatures by an energy barrier on the order of 1 eV.

A conceptual approach for noncontact calorimetry in space

A concept is developed and described which allows to measure the heat capacity and the effective thermal conductivity of stable and undercooled liquid metals and alloys in an electromagnetic levitation apparatus. We propose to use an ac pulse heating method which is used nowadays as a standard technique for precision measurement of low temperature heat capacities. The ideal process parameters including the drop diameter D, temperature T, and frequency of measurement ω can be optimized when the following relations hold for the external and internal relaxation time constants τ1 and τ2, respectively: ωτ1≳10 and ωτ2<0.1. Then heat capacity data can be obtained with an accuracy of better than 1% with D about 5 to 10 mm, T between 1200 and 1800 K and ω between 0.1 and 1 Hz for typical metals and alloys.

Thermodynamic properties and crystallization kinetics of glass‐forming undercooled liquid Au‐Pb‐Sb alloys

The heat capacities of liquid and crystalline Au-Pb-Sb alloys in the glass-forming composition range were measured with droplet emulsion and bulk samples. Based on the measured Cp data, the entropy, enthalpy, and Gibbs free-energy differences between the eutectic solid mixture and undercooled liquid were determined as a function of temperature over ~60% of the undercooling range below the liquidus temperature and compared with theoretical predictions. The results indicate an isentropic temperature at 313 (±5) K, which agrees well with experimental data for the glass transition. The thermodynamic evaluation was applied further to develop a kinetics analysis of the nucleation undercooling response during cooling. Use of different approximations for the Gibbs free energy leads to a variation of the prefactor terms of six orders of magnitude for classical nucleation theory and, consequently, large variation in calculated transformation diagrams which is more pronounced with increasing undercooling. Extrapolations into the glass-forming temperature range and the effects of viscosity, transient nucleation, and estimated Kauzmann temperatures on the crystallization kinetics at high undercooling have been evaluated. This analysis reveals the importance of using measured values of thermophysical properties, even if they represent a limited temperature range at modest undercooling, rather than model approximations in order to obtain reliable evaluations of crystallization kinetics at high undercooling in the glass-forming temperature range.

Mechanism of Achieving Nanocrystalline AIRu By Ball Milling

We investigated through X- ray diffraction and transmission electron microscopy the crystal refinement of the intermetallic compound AIRu by high- energy ball milling. The deformation process causes a decrease of crystal size to 5–7 rum and an increase of atomic level strain. This deformation is localized in shear bands with a thickness of 0.5 to 1 micron. Within these bands the crystal lattice breaks into small grains with a typical size of 8–14 rum. Further deformation leads to a final nanocrystalline structure with randomly oriented crystallite grains separated by high- angle grain boundaries.

Phase selection during crystallization of undercooled liquid eutectic lead-tin alloys

Abstract During rapid solidification substantial amounts of undercooling are in general required for formation of metastable phases. Crystallization at varying levels of undercooling and melting of metastable phases were studied during slow cooling and heating of emulsified PbSn alloys. Besides the experimental demonstration of the reversibility of metastable phase equilibria, two different principal solidification paths have been identified and compared with the established metastable phase diagram and predictions from classical nucleation theory. The results suggest that the most probable solidification path is described by the “step rule” resulting in the formation of metastable phases at low undercooling, whereas the stable eutectic phase mixture crystallizes without metastable phase formation at high undercooling.

Free energy functions of undercooled liquid glass-forming Au–Pb–Sb alloys

Abstract Based on measured specific heat data, the entropy, enthalpy and Gibbs free energy functions of crystalline, liquid and undercooled liquid AuPbSb alloys were determined and compared with theoretical predictions. Models proposed for the free energy difference between the undercooled liquid and the stable crystalline phase agree with the experimental results at low and modest undercooling. At high undercooling the models are found to be inconsistent with results of nucleation kinetics measurements, the extrapolated isentropic temperature, and the minimum cooling rates necessary to avoid crystallization in drop tower experiments.