Volume 89, Issue 6

The transition critical temperatures and transition enthalpies of the transition series AuCu I → AuCu II → Disorder in stoichiometric AuCu are determined by Differential Scanning Calorimetry, taking due account of the various structural states before and after the transitions. The results are discussed and compared with those obtained from standard calorimetry.

A method earlier proposed for the calculation of critical data of metals has been applied to aluminium and copper. This procedure should be reliable only for well-known vapor pressures at high temperatures. Otherwise – as for aluminium and copper – a “reasonable” valence at the critical point should rather be used for the calculation. The critical data derived in this way have been used to select vapor pressure curves for the high-temperature range.

Al 1– x Ti x B 2 particles extracted from Al–Ti–B alloys have been investigated by means of a transmission electron microscope equipped with an X-ray energy-dispersive spectroscopic analyser. The solid solubility Al 1– x Ti x B 2 was found to be unlimited (0 ≤ x ≤ 1). The composition of boride particles extracted from an Al–Ti–B alloy with a composition within the solid solubility range was widely spread, whereas it was close to TiB 2 for Ti concentrations in excess of the TiB 2 stoichiometry. The particle size varied from 0.02 to 2 μm. The largest particles were almost pure TiB 2 or AlB2, but the small ones were solid solutions of varying compositions. Those small particles tended to form relatively large agglomerates. Due to impurities in the master alloys, vanadium was present in some boride particles, implying that we actually have an Al 1– x (Ti 1– y V y ) x B 2 solid solution (0 ≤ x + y ≤ 1). The y value was found to vary within the range 0 ≤ y ≤ 0.07. It was found that master alloys containing boride particles with a narrow size distribution and a composition close to TiB 2 gives better grain refinement results than master alloys with wide distributions of particle size and composition.

The Ni–Ti–C ternary phase diagram is constructed via two steps. In the first step, all of the experimental phase diagram data available from the literature were critically reviewed and assessed by using thermodynamic models for the Gibbs energies of individual phases. In the second step, samples were prepared by arc melting of cold-pressed pellets and annealing at 900 °C for 288 h. Water-quenched samples were analyzed by using X-ray diffraction (XRD), optical microscopy, scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) and differential thermal analysis (DTA) techniques. A consistent thermodynamic data set for the Ni–Ti–C system is finally obtained by considering the experimental results from the present work and reliable literature data. Comparisons between the calculated and measured phase diagrams show that most of the experimental information is satisfactorily accounted for by the thermodynamic modeling.

A non-isothermal method for the determination of the gas diffusivity in metals and alloys is proposed. The method allows to determine the temperature dependence of the gas diffusion coefficient in a wide temperature range with a limited number of experiments. The results obtained with this method (for the diffusivity of nitrogen in steels), agree favourably with the diffusion data obtained with other experimental techniques. Hence, the proposed method leads to reliable results and can be successfully used in further studies.

The tensile properties of zinc–aluminium based commercial alloys: No.3, ZA-8 and ZA-27 in pressure-diecast and gravity-cast forms have been investigated over a temperature range from 20 °C to 260 °C. At low temperatures, the maximum stress, proof stress and the modulus of elasticity of the pressure-diecast alloys were superior to those of the gravity- cast alloys, but as the temperature increased they decreased sharply and castings in both forms had virtually the same maximum and proof stresses at about 200 °C, and the same elasticity of modulus at about 90 °C. The elongation results were also temperature dependent, but the rates of increase with temperature were higher for the pressure- diecast alloys. Scanning electron microscopy showed that the proportions of the phases in these castings, and their sizes, differed according to the method of casting used. These differences were considered to be the principal reason for the differences in mechanical property variations with temperature.

In order to clarify the effects of the annealing temperature and the Mg content on the development of the {100} ⟨001⟩ orientation in the annealing textures of Al–Mg alloys, high purity Al and Al–Mg alloys containing Mg up to 5 % were cold rolled 95 % and annealed at temperatures between 300 and 450 °C. Textures developed in these alloys at each annealing temperature were investigated in detail by using the orientation distribution function analysis. lt was found that, in high purity Al, the {100} ⟨001⟩ orientation increased remarkably by annealing at higher temperatures. Such increases were mainly caused by the preferential growth of the {100} ⟨001⟩ grain during grain growth. In the annealing textures of Al-Mg alloys, the {100} ⟨001⟩ orientation was generally weak. Its development was more strongly suppressed with increasing Mg content and with increasing annealing temperature. In these alloys, shear banding which occurs extensively during cold rolling, appears to destroy possible nucleation sites of the {100} ⟨001⟩ recrystallized grains. The {100} ⟨001⟩ recrystallized grains observed after complete recrystallization were thus smaller in number and more widely distributed. During subsequent grain growth, they were therefore readily consumed by grains of other orientations. From these results, it was concluded that the positive introduction of shear bands during cold rolling, combined with annealing at high temperatures, effectively reduces the {100} ⟨001⟩ orientation in the annealing texture in Al alloys, improving their press formabilities.

Deformation processing and microstructural development of an α 2 /O aluminide alloy Ti-25Al-15Nb (at.%) was studied in the temperature range of 950 to 1200 °C and strain rate range of 10 –3 to 100 s –1 . Regions of processing and instability were identified using dynamic materials model. Dynamic recrystallization (DRX) of α 2 / O phase and β phase were seen to occur in the region of 950 to 1050°C/0.001 to 0.05 s –1 and 1125 to 1175 °C/0.001 to 0.1 s –1 , respectively. Unstable flow was seen to occur in the region of 1050 to 1190°C/10 to 100 s –1 . Thermal activation analysis showed that DRX of α 2 /O and β was controlled by cross-slip.

The microhardness of the constituents (transformed β-Ti, Z and eutectic (β + Z)) was measured for Ti-rich Ti–Si–Al alloys. It depends both on the aluminium content in the alloy as well as on the character of mutual substitution of the atoms in the phases at the boundaries of the homogeneity ranges. The results are discussed on the basis of the Ti- rich part of the Ti–Si–Al phase diagram.

Impact toughness of eutectic Al–Si casting alloy is evaluated with the objective of investigating the metallurgical factors affecting this property. The metallurgical factors such as eutectic Si size, aspect ratio, shape factor and Si particles spacing are picked up. Thus, the relationship between the impact toughness and the microstructure is established. Impact toughness is found to be strongly affected by the size and morphology of eutectic Si particles. Other feature which greatly influences the impact toughness is the ratio of the silicon particles spacing normalized to the equivalent diameter, rather than the Si particles spacing. Impact toughness also correlate well with the void growth parameter proposed by the authors.