Volume 92, Issue 6

Methods for applying first principles (FP) calculations to CALPHAD modeling, are discussed, with emphasis on easily calculated quantities that can be used to estimate input parameters for CALPHAD optimizations. Estimations of vibrational entropies, and melting points, from chemical systematics of measured elastic constants, or via semiempirical methods based on FP calculations are reviewed. Some strategies for including higher-order (clusters larger than pairs) short range order correlations in CALPHAD calculations are considered.

The discussion on thermodynamic modelling of solution that started at the Ringberg meeting 1996 is continued with some revisions and some details. In particular the methods to implement chemical ordering for a multicomponent system, also on interstital sublattices and the contributions due to strain energies are discussed.

A summary of the discussions of the group concerned with the modelling of oxide and oxynitride phases during the “Workshop on Thermodynamic Modelling and Applications” held at Schloss Ringberg, November 14 –20, 1999, is presented. The group focussed its efforts on thermodynamic modelling with a view to creating databases for practical applications. Particular attention was paid to techniques of estimation of entropies, to the proper choice of model parameters and their sequence of optimization in the compound energy formalism (CEF), to the description of magnetic and volumetric properties of solutions within the CEF, to the treatment of short-range ordering and long-range ordering, and to the distribution of ions on different crystallographic sites (i.e. different sublattices) in oxides and silicates.

The thermodynamic modelling of carbide, nitride and carbonitride phases has been reviewed and recommendations made. A comprehensive list of assessed phases has been made with a bibliography. Recommended, estimated values for metastable phases have been tabulated. A new method of predicting phase stability of ordered phases has been outlined. This involves the coupling of CVM (cluster variation method) and mean field approximation calculations. Application to the Nb-C system is discussed.

Formation of a “constrained liquid” as the boundary film with a constant thickness and segregation of the surplus “free liquid” into the joint pockets in the microstructure of the liquid-phase sintered ceramics has been ascribed to an additional interaction in the system. The parallel tangent construction has been derived as condition of true thermodynamic equilibrium between the constrained and the free liquid. Similarly constrained equilibrium has been considered for amorphous boundary phase and suggested as possible explanation of its apparent stability.

The phase relations on the B-rich side of stoichiometric TiB 2 were experimentally studied. It was found that a peritectic four-phase reaction liquid + AlB 12 ⇔ AlB 2 + TiB 2 occurs at a temperature between 1073 and 1173 K. Part of the Al-rich corner of the Al –Ti–B phase diagram was calculated and isothermal sections at 973, 1073, 1173 and 1273 K are presented. The peritectic four-phase reaction was calculated to occur at 1150 K in agreement with the experimental results. The Al-rich part of the binary Al –B system was reassessed. A eutectic reaction liquid ⇔ AlB 2 + (Al) was confirmed to occur at 932.9 K and 0.055 at.% B and a peritectic reaction liquid +AlB 12 ⇔ AlB 2 concluded to happen at 1150 K (slightly above the four-phase reaction) and 0.60 at.% B, which is at a lower temperature than previously reported.

The superplastic formability of the AZ31 magnesium alloy is evaluated in the temperature range between 300 and 525 °C under biaxial tensile stress through argon-blowing testing. During superplastic forming, the cellular precipitates originally existing in the as-received AZ31 alloy dissolve into solid solution. Superplastic deformation is found to play a part in the dissolution of these cellular precipitates. In the highly deformed specimens, two types of cavities are observed. After superplastic forming, the microhardness of this AZ31 alloy decreases distinctively. However, the corrosion behavior is improved in the case of deformed specimens when the number of cavities has not reached to a significant level. Both the decrease of microhardness and the increase of corrosion resistance can be attributed to the dissolution of cellular precipitates after superplastic forming.

Three pure elemental powder mixtures of Zn–22%Al – 18 %Cu, Zn–5 %Al–11 %Cu, and Zn–27%Al –3%Cu (in wt.%) were mechanically alloyed by steel-ball milling processing. The mechanical alloying characteristics were investigated using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy techniques. It was explored that mechanical alloying started with the formation of phases from pure elemental powders, and this was followed by mechanical milling-induced phase transformation. During mechanical alloying, phases stable at the higher temperatures formed at the near room temperature of milling. Nano-structure Zn–Al-based alloys were produced by mechanical alloying.

In this study, the effect of sputter cleaning and TiN coating after sputter cleaning on tensile and fatigue behaviour of GGG 40 grade nodular cast iron was studied. It was found that sputter cleaning and TiN coating after sputter cleaning by arc physical vapour deposition technique improved the static and dynamic strength properties (tensile strength and endurance limit) of GGG 40 grade nodular cast iron material at the same amount and these properties were increased about 20 MPa. From this result, it is concluded that TiN coating did not cause a further increase in these properties after sputter cleaning.

The nucleation of iron nitrides during gaseous nitriding has been investigated using light microscopy and X-ray diffraction. Initially, the nucleation of γ′-Fe 4 N 1– x on a pure iron surface starts at grain boundaries meeting the surface, from where the nitride grains grow laterally into the iron grains. On prolonged nitriding, immediate nucleation at the surface of iron grains becomes possible. Calculated incubation times for the nucleation of γ′-Fe 4 N 1– x during nitriding are generally longer than those observed experimentally in the present work. The incubation time is reduced dramatically after oxidation of the iron surface prior to nitriding. Furthermore, a more uniform distribution of iron-nitride nuclei over the surface is obtained after preoxidation. The enhanced nucleation of iron nitrides on oxidised iron is discussed in terms of the kinetics of the surface reactions, and the development of a metastable precursor for nitride formation.

An activated nickel catalyst (Raney-type nickel) was produced by leaching nickel-aluminium alloys in alkali solution. The structure of this leached residue was investigated using transmission electron microscopy and X-ray diffraction. The leaching process selectively dissolves aluminium from the alloy and the remaining atoms form a nonequilibrium body-centred cubic phase of disordered nickel and aluminium atoms. This phase exists over a wide range of compositions. The further loss of aluminium leads to a rearrangement of the remaining nickel atoms into the stable face-centred cubic structure of nickel. This transformation occurs by small adjustments of nickel atoms over short distances resulting in crystallites in the size range of a few nanometers.

Electrical conductivity and thermo-electromotive force measurements were performed for a liquid binary Pb 83 Mg 17 eutectic and Pb 83 Mg 17– x M x alloys ( x ≤ 3 at.%; M = Al, Ga, Ni, Bi, Cu) at temperatures up to 1300 K under pressures of argon gas (up to 50 MPa) using the four-probe method. It was revealed that small amounts of a third component affect significantly the electrophysical properties. The influence of these additions on the electrical conductivity and thermo-electromotive force is discussed briefly. Possible applications of such alloys have been proposed.

Interfacial reactions were investigated for a novel P/M (powder metallurgy) 5056 aluminium alloy metal matrix composite reinforced with particulate intermetallic Ni 3 Al. Composites were fabricated by extrusion of a blend of matrix alloy powder and intermetallic particles produced by self-propagating high temperature synthesis. Extrusion of 5056/15 vol.% Ni 3 Al at 400 °C produced bars of excellent quality, whereas extrusion of 5056/15 vol.% Ni 3 Al at 420 °C and of 5056/25 vol.% Ni 3 Al at 380 °C resulted in severe interfacial reactions and volume expansion during extrusion. The same reactions occurred in the good quality composite during subsequent heat treatment at 500 and 520 °C. The 5056 Al alloy with its high (~5 %) Mg content exhibited a higher reactivity with Ni 3 Al than Al alloys with low Mg content. The interfacial reaction products were identified to be Al 3 Ni and Al 3 Ni 2 . The interfacial reactions consumed Al so raising the Mg content of the residual matrix. This gave the possibility for the matrix to become partially molten at lower temperatures than expected from its nominal composition. Segregation of Mg near the reinforcement particles is accelerated by the presence of a high dislocation density in the matrix generated during extrusion.

Melt-spun Al-rich metallic glasses and nanostructured composites were synthesized under various quenching rates. Differential scanning calorimetry (DSC), X-ray diffraction and transmission electron microscopic techniques were employed to study the microstructure and devitrification of these samples in more details. The microstructure obtained is strongly sensitive to the quenching rates used. A reduction of the quenching rate from 50 to 9 m/s changed the structure from an amorphous single phase to the coexistence of α-Al nanocrystals and amorphous phase, and finally to a combination of α-Al, Al 3 (Ni,Fe), Al 11 Ce 3 and amorphous matrix. Upon devitrifying, a two-stage crystallization reaction was observed but the glass transition was not detected in these samples. The absence of glass transition was presumably due to the formation of pre-existing nuclei. Isothermal and non-isothermal DSC measurements indicated that the first crystallization was associated with the growth of α-Al nanocrystals controlled by the volume diffusion with a small activation energy of 1.3 –1.7eV. The second crystallization was derived from the growth of Al 3 (Ni,Fe) and Al 11 Ce 3 nanophases.