Volume 97, Issue 11

The coefficient of 65 Zn heterodiffusion in the Mg 17 Al 12 intermetallic compound (β-phase) and the eutectic alloy Mg-33.4 wt.% Al was measured in the temperature region 598 – 698 K using serial sectioning and residual activity methods. The diffusion coefficient of 65 Zn in the intermetallic can be written as Δ I = 1.5 × 10 – 2 m 2 s –1 × exp( – 154.3 kJ mol –1 / RT ). At temperatures T ≥ 648 K, where the mean diffusion path is greater than the mean interlamellar distance in the eutectic, the effective diffusion coefficient Δ eff = 2.7 × 10 – 2 m 2 s –1 exp(–155.1 kJ mol – 1 / RT ) was evaluated. At two lower temperatures, the diffusion coefficients 65 Zn in interphase boundaries were estimated: Δ b (623 K) = 1.6 × 10 –12 m 2 τ – 1 and Δ b (598 K) = 4.4 × 10 – 13 m 2 τ –1 . The specific (Mg)/β interfacial energy γ = 140 mJ m – 2 was assessed from measurement of the time dependence of interlamellar distance during isothermal anneal.

The thermodynamic properties of the Al –Na and Na–Al – H systems have been assessed by combining the “calculation of phase diagram” approach with first-principles predictions. The Gibbs energies of the individual phases were thermodynamically modeled, where the model parameters were obtained from best fit optimizations to combined experimental and first-principles predicted finite temperature data. The first-principles thermodynamic predictions were based upon density functional theory ground state minimizations and direct method lattice dynamics. The predictions proved to be important adjuncts to the assessments whenever experimental measurements were lacking or not feasible. It was shown that the phase stability conditions of sodium alanates, NaAlH 4 and Na 3 AlH 6 , were well described with the present models.

The thermodynamic parameters of the systems La 2 O 3 – Al 2 O 3 and La 2 O 3 –Y 2 O 3 are assessed using the Calphad technique. The available phase equilibrium data and calorimetric measurements for the LaAlO 3 , perovskite phase have been used. The calculated phase diagrams are in reasonable agreement with experimental results. However, experimental data for the La 2 O 3 –Y 2 O 3 system demonstrated large uncertainty and new experimental study of solid state reactions would be useful to verify the calculated diagram. Experimental investigations of thermodynamic properties are necessary for both systems to verify the calculated phase diagrams and thermodynamic properties.

Phase equilibria and homogeneity ranges of the intermetallic phases in the Al –Mo system have been determined. The microstructures of samples of fixed composition and of diffusion couples were investigated by light-optical and scanning electron microscopy, the crystallographic structures and lattice constants were established by X-ray diffraction and the compositions of the phases were determined by electron probe microanalysis. In addition, transition temperatures were determined by differential thermal analysis. The homogeneity ranges of the Al-rich intermetallic phases are narrow, with about 0.1 to 0.4 at.% widths. The phases and phase equilibria as established in previous studies have been confirmed with the exception of the phase Al 3+ x Mo1– x , which has not been detected. The congruent melting point of Al 8 Mo 3 was found at 1546 ± 3 °C and the Al-rich phase boundary for AlMo 3 has been determined between 800 and 1200 °C.

Electron backscatter diffraction and energy-dispersive X-ray spectrometry were used to investigate the intermixed interface produced during laser cladding of a Co – Cr –Mo alloy on a steel substrate. A multi-component system and rapid solidification conditions together lead to a complex microstructure at the interface. The solidification of the cladding starts with the formation of an interface layer, which is about 75 μm in thickness and consists of randomly oriented equiaxed grains of Co–Cr –Fe solid solution and martensite. Orientation analysis of the grains in the interface layer revealed that some grains have a special orientation relationship with the former austenite grains in the heat affected zone but the cladding is not formed by epitaxial growth on the substrate. Intermixing of the materials at the interface is providing a strong bond between the substrate and the cladding. For a grain from the interface layer to emerge as columnar grain in the cladding, it was determined that its <001> crystallographic direction is not supposed to deviate more than 25° from the sample normal direction.

The thermodynamic properties of liquid Ag–Sb –In alloys were determined using solid oxide galvanic cells with zirconia electrolyte. The following galvanic cells were employed: in the temperature range from 973 to 1200 K. (I) Re+ kanthal, Agx− Iny− Sb(1− x− y),In2O3//ZrO2+Y2O3//NiO,Ni,Pt $$\begin{array}{l}\mathrm{Re}+\text { kanthal, } \mathrm{Ag}_{x}-\mathrm{In}_{y}-\mathrm{Sb}_{(1-x-y),} \\ \mathrm{In}_{2} \mathrm{O}_{3} / / \mathrm{ZrO}_{2}+\left(\mathrm{Y}_{2} \mathrm{O}_{3}\right) / / \mathrm{NiO}, \mathrm{Ni}, \mathrm{Pt}\end{array}$$ Thermodynamic properties of the liquid phase were described by the Redlich – Kister –Muggianu formula and the phase relations in the ternary system were calculated. The results of the calculations were compared with experimental data available from different literature sources.

The densities of liquid copper, cobalt, and iron, their binary and ternary alloys have been measured over a temperature range including the undercooled regime. A non-contact technique was used, consisting of electromagnetic levitation combined with optical dilatometry. For all samples, the density was a linear function of temperature. The concentration dependence was studied by means of the excess volume which was negligible for Co– Fe and positive for Cu– Fe, Cu– Co, and Cu–Co – Fe. The density of the ternary alloy could be predicted from the excess volumes of the binary phases without the need to introduce any ternary interactions.

Several Co–Cr alloys have been investigated by Differential Scanning Calorimetry (DSC) as well as by Differential Thermal Analysis (DTA) in the temperature range 298 – 1823 K. The compositional range investigated was from 0 to 87 wt.% Cr with special attention given to alloys with low Cr contents, i. e. 0 to 10 wt.% Cr. From the DSC measurements molar heat capacity values, phase transformation temperatures, enthalpies of transformation and Curie transition temperatures were derived. From the DTA measurements, the melting temperatures and melting enthalpies were evaluated. The trends obtained were compared with the results for pure Co as well as the proposed phase diagram reported in literature.

The effects of a low frequency electromagnetic field applied during semi-continuous direct-chill casting on AZ80 ingots were investigated. The experiments were performed both in the absence and in the presence of the electromagnetic field. In contrast to the ingot cast in the conventional process, the microstructure of the ingot was significantly refined by the electromagnetic field and a high degree of grain refinement was realized when the operating frequency was at 22 Hz. The ingot exhibited a relatively increased and uniform distribution of Vickers hardness and decreased segregation of Al when the operating frequency of the electromagnetic field was in the range of 15– 30 Hz.

The formation of a compound, “white” layer on the surface of Fe-7 wt.% Cr alloy, by exposure to a gas mixture of ammonia and hydrogen at 580 °C, was investigated. The microstructure of the surface layer was analysed employing light and scanning electron microscopy, X-ray diffraction and electron probe microanalysis. For the first time a quantitative (phase) analysis of such a white layer was carried out, demonstrating the presence of a dispersed CrN phase within a γ | iron-nitride matrix.

The effect of ageing on the tensile behaviour, mode I fracture toughness and mixed mode I/III fracture toughness of 7010 aluminium alloy plate was investigated. It was found that the 0.2% proof stress in this alloy increased from the under-aged temper to the peak-aged (T6) temper and then subsequently decreased in the over-aged (T73) temper. On the other hand, the ductility exhibited a monotonic decrease from the under-aged temper to the over-aged temper. The Ludwigson equation was found to best describe the flow behaviour in all the three ageing tempers. Under mode I as well as mixed mode I/III loading, the highest fracture toughness was seen in the under-aged temper, whereas the fracture toughness values in the peak-aged and over-aged tempers were comparable.

Interstitial-free steels are widely used as thin sheet for automobile external panels. For this type of application, essential properties of the steel are deep-drawability and stretch-formability. However, low-strength interstitial-free steel is nearly pure iron and work on fatigue fracture behavior of this steel might be adopted as a basic study to understand the behavior of high-strength steels used for structural components. In this study, the fatigue fracture behavior of low-strength interstitial-free steel has been studied at room temperature in air and in an argon atmosphere. Fractographic observations on fatigue fracture surfaces reveal a mixed type of fracture mode (intergranular and transgranular cracking) for all specimens. The major portion of intergranular fracture has been found at the regions of low and medium crack lengths. Further increase in the crack length causes a decrease in the proportion of intergranular cracking and, finally, the fracture mode becomes completely transgranular. At the same test temperature, the proportion of intergranular fracture drastically decreased when the test was carried out in the argon atmosphere.

Fracture-mechanics experiments were carried out on ultrafine-grained (UFG) samples of copper, titanium and an Al alloy to obtain the fracture behaviour under static and cyclic loading. The UFG-materials investigated show crack resistance behaviour under static loading, which was confirmed by ductile fracture surfaces. Under cyclic load the crack growth rate was described well by the ESACRACK model.

This paper summarizes the latest results in the development of a combined pressure-swirl-gas atomizer. The main goals of this investigation are to characterize the generated metal powders in terms of spray pattern and particle size distribution. With the combined pressure-swirl-gas atomizer it is possible to create 150 kg h – 1 of tin powder with mass medians between 30 and 110 μm (tin alloys) and geometric standard deviations below 1.9 with a nitrogen consumption of 100 kg h – 1 . Gas flow and melt mass flow can be varied independently of each other, while their characteristic ratio of mass flows can be held below 0.7, which opens new potential for low gas consumption in gas atomization. The experiments were conducted with water and tin melts as model fluids. The comparison of the dimensionless numbers shows that the model experiments are comparable and basically extendable to other melts as long as the material characteristics allow this.

Oxide dispersion strengthened (ODS) and dual scale particle strengthened (DSPS) copper composites were successfully manufactured by a powder metallurgical processing route, containing Al 2 O 3 particles with a mean diameter of 17 nm as dispersoids and, in the case of the DSPS material, 15% tungsten fibres about 3 mm long. Their mechanical behaviour was investigated at ambient and elevated temperatures. The ODS material displayed mechanical properties comparable to those of commercial GlidCop ® alloys. Addition of tungsten fibres in case of the DSPS material increased the elevated temperature flow strength relative to the ODS counterpart by about 12 %. This result is rationalised by finite element calculations, taking fibre orientation, plastic deformation of the fibres as well as the role of the fibre coating into account.

The electrochemical behaviour of magnesium and different wrought alloys, AZ31, AE42, LAE442, and ZEK100, with and without heat treatment, was examined with the Mini-cell System by linear sweep voltammetry in 0.5 wt.% and 3.5 wt.% NaCl solution. The corrosion damage was characterised by scanning electron microscopy. Magnesium and the different alloys exhibited strong differences in their electrochemical behaviour. Alloying with aluminium increases the corrosion resistance. However, the magnitude of this improvement depends on uniform sizes and distributions of the Al-rich phases. Similarly, the influence of the heat treatment on the corrosion resistance depends on the produced distribution of precipitates. Magnesium and its alloys corrode quite rapidly in comparison with other metals, and there is pronounced hydrogen development. The latter increased with increasing NaCl concentration, although no great differences in the corrosion in both concentrations were observed, except on pure magnesium and ZEK100.

Maraging steels based on iron –nickel martensite constitute a very important family of high-strength steels, which distinguishes itself by demonstrating an unparalleled combination of excellent fabricability and high strength and fracture toughness after heat treatment. Heat treatment of these steels has now been perfected to ensure consistently high levels of strength, ductility, and toughness in a variety of product shapes and sizes. Cobalt-free variants have been commercialized as part of efforts to save production costs. Further knowledge has been generated on 18% nickel maraging steels regarding phases precipitating during aging, thermal embrittlement, thermal cycling and austenite reversion/retention and their effect on mechanical properties. The paper reviews the progress made over the last one and half decades.