Evaluating Microstructure, Wear Resistance and Tensile Properties of Al-Bi(-Cu, -Zn) Alloys for Lightweight Sliding Bearings

One of the most important routes for obtaining Al-Bi-x monotectic alloys is directional solidification. The control of the thermal solidification parameters under transient heat flow conditions can provide an optimized distribution of the Bismuth (Bi) soft minority phase embedded into an Al-rich matrix. In the present contribution, Al-Bi, Al-Bi-Zn, and Al-Bi-Cu alloys were manufactured through this route with their microstructures characterized and dimensioned based on the solidification cooling rates. The main purpose is to evaluate the influence of typical hardening elements in Al alloys (zinc and copper) in the microstructure, tensile properties, and wear of the monotectic Al-Bi alloy. These additions are welcome in the development of light and more resistant alloys due to the growing demands in new sliding bearing designs. It is demonstrated that the addition of 3.0 wt.% Cu promotes microstructural refining, doubles the wear resistance, and triples the tensile strength with some minor decrease in ductility in relation to the binary Al-3.2 wt.% Bi alloy. With the addition of 3.0 wt.% Zn, although there is some microstructural refining, little contribution can be seen in the application properties.

Effect of TiC on coarsening and macrosegregation of Al–Bi alloys

The microstructures of an Al-Bi immiscible alloy and the corresponding composites containing TiC (1 wt.% and 2 wt.%) were explored for melt temperatures of 800 °C, 850 °C, and 900°C. It was demonstrated that serious coarsening and macrosegregation of Bi-rich minority phase particles occurred, which was slightly alleviated by increasing the melt temperature from 800 °C to 900 °C. By adding TiC particles, the coarsening and macrosegregation of Bi-rich minority phase particles were significantly impeded. Scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed that TiC particles were located inside and on the surface of Bi-rich minority phase particles, exhibiting heterogeneous nucleation and self-assembly behaviour. By properly increasing the holding time of the melt, finer and more uniform Bi-rich minority phase particles were obtained.

Effect of microgravity on the solidification of aluminum–bismuth–tin immiscible alloys

Directional solidification experiment was carried out with Al-Bi-Sn immiscible alloy under microgravity environment onboard the Tiangong 2 space laboratory of China. Sample with a well-dispersed microstructure was obtained by properly designing the experimental scheme, the matrix shows equiaxed morphology, and there is no visible gas cavity or pinhole in the sample. In contrast, the reference samples solidified on earth show phase-segregated structure and contain some gas cavities or pinholes. The grain morphology of the terrestrial sample depends on the solidification direction, it is equiaxed when the sample ampoule was withdrawn against the gravity direction, while it is columnar when the sample ampoule was withdrawn along the gravity direction. The solidification process and affecting mechanisms of microgravity on the microstructure formation are discussed. The results indicate that the microgravity conditions can effectively diminish the convective flow of the melt and the Stokes motions of the minority phase droplets and gas bubbles, which are helpful for suppressing the occurrence of macro-segregation and preventing the formation of porosity. The results also demonstrate that the microgravity conditions favor the detachment between the melt and the wall of crucible, thus increasing the nucleation undercooling of α-Al nuclei and promoting the formation of equiaxed grain.

Tsallis Entropy Index q and the Complexity Measure of Seismicity in Natural Time under Time Reversal before the M9 Tohoku Earthquake in 2011

The observed earthquake scaling laws indicate the existence of phenomena closely associated with the proximity of the system to a critical point. Taking this view that earthquakes are critical phenomena (dynamic phase transitions), here we investigate whether in this case the Lifshitz–Slyozov–Wagner (LSW) theory for phase transitions showing that the characteristic size of the minority phase droplets grows with time as t 1 / 3 is applicable. To achieve this goal, we analyzed the Japanese seismic data in a new time domain termed natural time and find that an LSW behavior is actually obeyed by a precursory change of seismicity and in particular by the fluctuations of the entropy change of seismicity under time reversal before the Tohoku earthquake of magnitude 9.0 that occurred on 11 March 2011 in Japan. Furthermore, the Tsallis entropic index q is found to exhibit a precursory increase.

The Behavior of Liquid Alloys Visualized by X-Rays

Developments in synchrotron and home laboratory X-ray sources and fast low noise X-ray imaging detectors over the last 15-20 years has enabled real time X-radiography of alloy solidification from the melt. These investigations have been an important tool for in–situ investigations of dendrite-, eutectic and monotectic growth, dendrite fragmentation etc. At the same time, the techniques have allowed studies of phenomena in the melt such as convection, formation of solute boundary layers and minority phase droplet interactions. The article will review the X-radiography techniques and some of the results with emphasis on studies of phenomena in alloy melts.

Influence of a Direct Current on the Solidification of Immiscible Alloys

Continuous solidification experiments were carried out with immiscible alloys under the effect of a direct current. The experimental results demonstrate that a direct current shows a significant effect on the migration of minority phase droplets (MPDs) in continuously solidified immiscible alloys. It can promote the formation of a well dispersed microstructure or a core/shell microstructure. A model describing the kinetics of the microstructure evolution in a continuously solidified immiscible alloy was developed. The microstructure formation in the alloys was calculated. The numerical results are in favorable agreement with the experimental ones. They demonstrate that a direct current may affect the microstructure development through changing the spatial motions of MPDs. The alloys show a well dispersed microstructure when they are solidified with such a direct current density that the direct current causing motion of the MPDs is almost equivalent to the radial component of the Marangoni migration velocity of the MPDs. The alloys show a core/shell microstructure when they are solidified with such a direct current density that the direct current causing motion of the MPDs dominates the migration of the MPDs along the radial direction of the sample. A wire or rod with well dispersed microstructure or a core/shell microstructure can be prepared by solidifying immiscible alloys under the effect of a direct current properly chosen.

Fabrication Of Zn4Sb3 Alloys By A Combination Of Gas-Atomization And Spark Plasma Sintering Processes

In this study, single phase polycrystalline Zn4Sb3 as well as 11 at.% Zn-rich Zn4Sb3 alloy having ε-Zn4Sb3 (majority phase) and Zn (minority phase) phases bulk samples produced by gas-atomization and subsequently consolidated by spark plasma sintering (SPS) process. The crystal structures were analyzed by X-ray diffraction (XRD) and cross-sectional microstructure were observed by the scanning electron microscopy (SEM). The internal grain microstructure of 11at.% Zn-rich Zn4Sb3 powders shows lamellar structure. Relative density, Vickers hardness and crack lengths were measured to investigate the effect of sintering temperature of Zn4Sb3 samples which are sintered at 653, 673 and 693 K. Relative density of the single phase bulk Zn4Sb3 sample reached to 99.2% of its theoretical density. The micro Vickers hardness of three different sintering temperatures were found around 2.17 – 2.236 GPa.

On the symmetry peculiarities of Bi2WO6single crystals

Bi2WO6single crystals (a=5.452(1), b=5.433(1), c=16.435(1) Å) were studied by X-ray diffraction (MoKα radiation, diffractometer Xcalibur S, CCD-detector) and electron diffraction techniques. Bi2WO6is an archetypal x=1 member of the Aurivillius family of layered perovskites of general formula Bi2O2Ax+1BxO3x-1. Its high piezoelectric performance and nonlinear optical properties have attracted considerable attention. In addition, these crystals offer high ionic conductivity due to the fast oxygen ion transport. In recent years, this compound has been the subject of intense research in the context of catalytic applications. In this work, the Bi2WO6single crystals were grown from solution in melt of Na2WO4–NaF. There were reflections with indexes 0kl, k=2n+1, in the diffraction pattern, contradicting the sp.gr. P21ab. The structure was solved by direct methods and refined in the sp.gr. R1 (R=3.60%, Rw=3.52%). The group P21ab was found to describe the arrangement of heavy atoms Bi and W only (R=17.5%, Rw=18.68%). The structure can be described by three local groups of symmetry – each atomic layer has inherent symmetry: W atoms and O atoms in equatorial vertices of WO6-octahedra have R11b sp.gr., Bi atoms – Bm11, the rest of O atoms – B11b. The oxygen atoms between two Bi sheets can be also described by B11m sp.gr. Preliminary electron diffraction investigation of the Bi2WO6crystals indicated a presence of small amount of a minority phase B1a1 together with the main P21ab phase. The presence of B1a1 phase can be probably explained by Na content in the crystal originating from the flux. Bi2WO6single crystals were studied earlier [1]. TEM showed coherent intergrowths of two distinct modulated variants having different symmetry. This result couldn't be explained by impurity presence because of investigation of pure crystals grown from melt. The work was done with the partial support of the grant for Leading Scientific Schools NSh-1130.2014.5 and RFBR (proj.14-02-00531a).

Solidification of Immiscible Alloys and Convective Effect

A model describing the microstructure formation in a directionally solidified immiscible alloy under the convective effect is presented. The microstructure evolution in a directionally solidified Al-Pb alloy is investigated. It is demonstrated that convective flows have great effects on the solidification of immiscible alloys. A convective flow against the solidification direction causes an increase in the nucleation rate while a convective flow along the solidification direction causes a decrease in the nucleation rate. The convective flows lead to a more uneven distribution of the minority phase droplets in the melt. It causes an increase in the size of the largest minority phase droplets and is against the obtaining of the immiscible alloys with a well dispersed microstructure.