Band 105, Heft 9

The recently developed Arrhenius formula of a modified Sutherland equation was employed to calculate the self- and impurity diffusivities in liquid Sn, Ag, Bi, Cu and Pb. The reliability of the calculated self- and impurity diffusivities was validated by comparing the calculated diffusivities with the critically reviewed literature data. Based on the reliable tracer, intrinsic and chemical diffusivities available in the literature, the atomic mobility parameters in an Sn–Ag–Bi–Cu–Pb melt were then evaluated by the DICTRA (DIffusion-Controlled TRAnsformations) software package with the aid of the available thermodynamic description for the liquid phase. Comprehensive comparisons show that most of the measured and theoretical diffusivities in Sn–Ag–Bi–Cu–Pb melts can be reasonably reproduced by the currently obtained atomic mobilities. The atomic mobilities were further verified by comparing the model-predicted concentration profiles with the measured ones in various liquid Ag–Pb, Bi–Pb and Sn–Bi–Pb diffusion couples. Finally, the simulation of dissolution of Ag and Cu substrates into liquid solder alloys during the reflow process was performed by using the presently obtained atomic mobilities in the Sn–Ag–Bi–Cu–Pb melts.

The 700°C isothermal section of the Fe–Cr–Si ternary phase diagram has been determined experimentally by means of scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray powder diffraction. Ten three-phase regions exist in the 700°C isothermal section. The binary σ phase contains 0–17.6 at.% Si and 31.4–59.2 at.% Cr; the Fe 5 Si 3 phase is stable at 700°C because of the dissolution of Cr. At this temperature, Fe and Cr cannot be entirely substituted by each other to form the FeSi or CrSi phases: the maximum possible Cr content in FeSi 2 , Fe 5 Si 3 and D0 3 is 3.9, 20.7 and 15.2 at.%, respectively, and the maximum soluble Fe in CrSi 2 , Cr 5 Si 3 and Cr 3 Si is 2.5, 20.4 and 16.8 at.%, respectively.

In order to study the effect of Mo on the microstructure, mechanical properties and corrosion behavior of commercially pure titanium (cp-Ti), Ti- x Mo ( x = 5, 10, 15, and 20 wt.%) alloys were investigated. The phase and microstructures were characterized using X-ray diffraction, optical microscopy, scanning electron microscopy and transmission electron microscopy. The results indicated that the Ti-5Mo alloy was mainly composed of α′ phase with a small fraction of α″ phase. The Ti-10Mo was dominated by orthorhombic α″. The Ti-15Mo alloy was mainly composed of α″ phase with a small fraction of β phase. The volume percentage of the β phase increased with increasing Mo content. The Ti-20Mo alloy was mainly composed of β phase. We also investigated the effect of alloying with Mo on the Vickers hardness and corrosion behavior of Ti- x Mo alloys. The addition of Mo not only caused hardening of cp-Ti but also improved its oxidation protective ability. Electrochemical results showed that the Ti- x Mo alloys exhibited improved corrosion resistance over cp-Ti.

The microstructural evolution of undercooled Ni-1 at.% Fe alloy was investigated by means of fluxing and cyclic superheating. Within the achieved undercoolings, there were two grain refinement processes and a grain coarsening process. The evolution mechanisms of the microstructure were studied applying the calculation of a stress accumulation model and dimensionless superheating. The first grain refinement can be ascribed to the dendrite break-up or ripening induced by remelting. The second one was caused by recrystallization induced by stress accumulation originating from the rapid solidification.

Applying a melt-fluxing method combined with cyclical superheating, rapid solidification of undercooled Co 70 Cu 30 , Co 50 Cu 50 and Co 30 Cu 70 melts was performed to investigate the effects of alloy composition and immiscibility on the resultant non-equilibrium solidification microstructure. As for the samples without liquid separation, grain refinement of primary α-Co dendrite occurs clearly with increases in both copper content and initial undercooling. In contrast, the influence of alloy composition on microstructure evolution is weakened due to the occurrence of liquid separation at larger undercooling, where slight variations in the solute concentration and micro-hardness for Co-rich phase are obtained. After a quantitative thermodynamic calculation, the process of liquid separation is described, which gives an excellent explanation for the observed experimental results.

In this study, the influence of the solidification temperature range of an alloy system on Gasar structures is investigated from the aspect of solidification mode and mushy zone length. The solidification mode and mushy zone length for various Cu–Mn alloys during unidirectional solidification are predicted by theoretical analysis. The calculated data indicate that it is reasonable, possible, but difficult to fabricate porous Cu-34.6 wt.% Mn, Cu-24 wt.% Mn, and Cu-46.6 wt.% Mn alloys by the Gasar process, respectively. Accordingly, lotus-type porous Cu-34.6 wt.% Mn alloy with regularly oriented pores was prepared and solidified with a cellular structure. Porous Cu-24 wt.% Mn alloy with directional pores could be fabricated, solidifying with a columnar dendritic structure. Porous Cu-46.6 wt.% Mn alloy with irregular large pores was obtained, and solidified with an equiaxed dendritic structure.

Aluminium/alumina/graphite hybrid metal matrix composites prepared by using the stir casting technique were heat treated to enhance mechanical properties such as strength, hardness and wear resistance. The present study evaluates the effect of ageing time on the hardness, tensile strength and tribological behaviour of both unreinforced Al-Si10Mg alloy and aluminium hybrid metal matrix composites. Results for composite samples were compared with those of as-cast samples. Results showed that heat treated samples of alloy and composite showed higher hardness, tensile strength and tribological properties compared to both as-cast alloy and composite. Among the different ageing times employed, specimens aged for 4 hours showed maximum mechanical and tribological properties. Investigations of the worn surfaces were carried out using scanning electron microscopy to understand the wear mechanism.

The present paper describes correlations between microstructure and tensile properties of a friction stir (FS) welded Al–Mg–Si alloy (6061–T6 Al). Mechanical tests were performed to determine the hardness and tensile stress–strain properties for the base material and for the nugget (or stirred) zone in the FS weld. The microstructure in the nugget zone was characterized using a weak-beam technique, high-resolution electron microscopy, and scanning transmission electron microscopy. Reduction of flow stress in the nugget zone was caused by thermal softening of the base material during the friction stir welding (FSW) process. Fine needle-like precipitates, dislocation segments, and fine lattice defects were observed in the base material, while only isolated long dislocations were found in the nugget zone. These results suggest that dynamic recrystallization occurs during FSW. Deterioration of tensile properties in the nugget zone was caused by the annihilation of lattice defects formed during the thermal refining process. The strain-hardening rate of the nugget zone was higher than that of the base material, and this was caused by a simple dislocation multiplication due to the dynamic recrystallization of the base material.

The aim was to determine the effect of extrusion process parameters of the KoBo method (initial billet temperature, extrusion rate and die oscillation frequency) on the Lüders phenomenon occurrence in aluminum wires. The material used in the experiment was 99.7% pure Al billet. The extrusion was carried out with the following parameters: extrusion ratio λ = 100, initial billet temperature in the range of 293–493 K, extrusion rate of 0.1–0.5 mm · s −1 and frequencies of 3, 5 and 8 Hz (the die oscillation angle was a constant ± 8°). Output materials were mechanically tested and examined microscopically. Based on the results it was found that: the Lüders phenomenon in aluminum wires extruded by the KoBo method appears only i) in a specific range of the extrusion force, namely 450–650 kN, ii) a specific range of, yielding strength (100–140 MPa), iii) dependent on the combination of process parameters its extent and position can vary over a product's length, iv) tests have also shown that this phenomenon has a continuous propagation nature only at the initial stage of the tensile test.

TiO 2 -reduced graphene oxide nanocomposites with different diameters and crystallinity of titania nanoparticles were synthesized via the sol-gel method followed by calcination in air or treatment under vacuum. The materials were characterized with transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, photoluminescence spectroscopy and diffuse-reflectance UV–vis spectroscopy. The photocatalytic activity of the resulting materials was examined in the process of phenol decomposition under UV–vis light irradiation. The influences of TiO 2 loading and calcination treatment on photocatalytic activity of the composites were investigated. It was found that higher TiO 2 concentrations resulted in higher photocatalytic activity. This is in agreement with the band gap energy values, as lower visible light absorption and higher E g values were obtained for the samples prepared with higher TiO 2 loading. Furthermore, photoactivity was affected by the calcination treatment. Higher activity under UV–vis was shown by the samples calcinated under vacuum, which was attributed to the better crystallinity compared to the samples treated with air.

A modelled constitutive equation derived from hot working tests to predict hot rolling mill loads is proposed and validated against industrial hot rolling data for AISI 321 stainless steel. Good correlation is found between the predicted mean flow stress, the Zener–Hollomon Z parameter and actual industrial mill load values from mill logs if allowances are made for differences in von Mises plane strain conversion, friction and front or back end tension. The multipass hot working behaviour of this steel was simulated through Gleeble thermomechanical compression testing with the deformation temperature varying between 1200°C down to 800°C and the strain rate between 0.001 s −1 and 5 s −1 . At strain rates greater than 0.05 s −1 , dynamic recovery as a softening mechanism was dominant, increasing the dynamic recrystallisation to dynamic recovery transition temperature to higher temperatures. This implies that through extrapolation to typical industrial strain rates of about 60 s −1 , most likely no dynamic recrystallisation in plant hot rolling occurs in this steel but only dynamic recovery. Grain refinement by dynamic recrystallisation is, therefore, unlikely in this steel under plant hot rolling conditions. Finally, mill load modelling using the hot working constitutive constants of the near-equivalent AISI 304 instead of those specifically determined for AISI 321, introduces measurable differences in the predicted mill loads. The use of alloy-specific hot working constants even for near-equivalent steels is, therefore, recommended.

A two-dimensional X-ray diffraction technique for stress measurement is reported. The proposed technique determines the stress from the Fourier series of a Debye–Scherrer ring. This technique does not require sample inclination to several different angles to determine the stress as the conventional sin 2 ψ technique does. Although the cos α technique has the same advantage, the proposed technique can also determine stress from an imperfect Debye–Scherrer ring. To measure the effectiveness of the proposed technique, a four-point bending test was performed on a steel specimen to determine the Fourier series of Debye–Scherrer rings. The stresses determined from the Fourier series were consistent with those determined using the cos α technique. The proposed technique is expected to extend the application range of current X-ray stress measurement.