Solid State Phenomena Vol. 229

Abstract: The microstructure of Mg-5Si alloy consists of the primary coarse Mg₂Si phase, α-Mg solid solution and eutectic α-Mg + Mg₂Si, in which eutectic Mg₂Si phase solidifies in the form of Chinese script particles. When 0.2 wt.% of Ca was added to the Mg-5Si alloy the size of primary Mg₂Si phase remained unchanged. The modification effect of calcium on the primary Mg₂Si phase was effective only in the Mg-5Si-0.5Ca alloy. The morphology of the primary Mg₂Si phase is changed from the coarse dendrite shape to polyhedral shape and the size of primary crystals is significantly reduced. The addition of 0.6 wt.% Ca to Mg-7Si alloy did not cause the modification of primary Mg₂Si phase.

Abstract: Majority structural elements from magnesium alloys are produced by means of casting. The big and complicated elements are mainly sand cast. The aim of the research was to investigate the influence of wall thickness on the castability and microstructure of modern magnesium alloy with RE and Ag additions – QE22. Eight 145x98mm plates have been cast for the research. Their designed thickness was equal: 0.5mm, 1mm, 1.5mm, 2mm, 3mm, 4mm, 5mm as well as 6mm. The parameters of microstructure constituents in each cast has been estimated quantitatively. The critical wall thickness, at which liquid alloy fill the mould cavity completely, is equal 4mm. With decreasing wall thickness, percentage of filled cavity is decreasing linearly. Mean area of grain flat section increased 7 times in the thickest plate (from 501μm² to 3773μm²). The volume fraction of eutectics as well as alloy hardness decreased in the thickest plate.

Abstract: The microstructure and hardness of as-cast Mg-5Si-7Sn-5Mn alloy after solution and ageing treatments is presented in this paper. It was found that the microstructure of as-cast alloy. is composed of primary dendrites crystals of Mg₂Si phase, α-Mg matrix, long needle-like precipitates of Mn₅Si₃, Chinese script particles of Mg₂Si phase and irregular Mg₂Sn phase. The solution treatment at 500°C causes the dissolution of the Mg₂Sn phase in the α-Mg magnesium solid solution, whereas the remaining intermetallic compounds are stable in this temperature. The hardness of alloy increases from 73 HV2 to 96 HV2 at 250°C. The increase in hardness is a result of the formation of the lath-like precipitates of Mg₂Sn phase within the α-Mg matrix.

Abstract: Following article summarizes results of the researches concerning influence of heating and cooling rate on the phase transformations temperatures in EV31A magnesium alloy. Particular attention has been paid to calibration aspects of the temperature measurement as well as cooling and heating rates in protective atmosphere. The researches were conducted on Multi HTC calorimeter, provided by Setaram company. The samples were heated up to liquidus temperature with controlled rates of: 2.5°C/min, 5°C/min, 10°C/min, 15°C/min and 20°C/min. Just after melting, the specimens were cooled with the same rates. Equilibrium temperatures of phase transformations were calculated by extrapolation to zero heating/cooling rate. Considerable difference between melting point (during heating) and solidification temperature (during cooling) were noticed on cooling/heating curves. The alloy specific heat has also been calculated.

Abstract: Magnesium and its alloys may be applied as cathodic protection for steel constructions. As the protectors work in water environment, one of the main degradation factors is cavitation erosion. The paper presents results of research on the cavitation erosion of magnesium casting alloy – Elektron 21. The cavitation erosion tests were performed using vibratory apparatus. After the cavitation tests eroded surface and the cross-sections of the specimens were observed by means of scanning electron microscopy. Elektron 21 did not exhibit distinct incubation period of cavitation erosion. The cavitation pits are observed mainly in the α-Mg solid solution grains and on the interfaces between intermetallic phases and the α-Mg. On the other hand, the Mg₃(Nd, Gd) eutectic phases are more resistant to cavitation erosion than α-Mg. They are protecting the solid solution from degradation. However, due to their high hardness they undergo cracking and detachment of from the surface. After longer times of cavitation tests, cracks begin to propagate into the material, which leads to crushing of bigger parts of the material.

Abstract: The WE43 cast magnesium alloy is characterized by high development prospects due to its strength properties and maximum operating temperature of up to 250°C. It is used in the automotive and aircraft industries for engine, hull and gear parts. These are mainly individual or low-volume large-size casts, which in the casting process may display some defects in the form of misruns, shrinkage porosities and cracks. These defects are repaired with the application of welding technologies, which are also used to joint castings into structures and to repair castings after operation. The weld joint is an integral part of the structure. Therefore, it decides on the properties of the entire structure. It is important to get to know the microstructure and properties of magnesium alloy weld joints, and in particular those of the fusion weld. In this paper stability of the structure of WE43 cast magnesium alloy welded joint was researched.

Abstract: The application of short carbon fibers in magnesium alloy AZ31 matrix composite fabrication by cold chamber pressure die casting was presented. A technological procedure of small-sized and complex-shaped composite casts manufacturing was shown. The microstructure of the composite was characterized as well as its mechanical properties, friction coefficient and wear resistance. The application of mechanical stirring of melted AZ31 alloy with short fibers and then AZ31-Cf suspension pressure die casting ensured obtaining casts with the reinforcing phase correctly distributed and well bonded with the matrix. Comparision of the AZ31-Cf composite with the AZ31 alloy properties, cast in the same conditions, revealed a considerable increase in bending strength and hardness, and some improvement of ductility and sliding friction parameters as a result of short carbon fibers application.

Abstract: The paper characterized the phase composition, microstructure and selected mechanical properties at room temperature and at temperature corresponding to the expected operating conditions of a new generation of TiAl based alloys melted in a vacuum induction furnace in a special graphite crucibles.

Abstract: The paper concerns a new group of titanium alloys with improved properties caused by presence of carbon. The effect of heat treatment processes like annealing, solution treatment and precipitation hardening on the microstructure of Ti-8Al-1Mo-1V alloy with carbon content not exceeding the limit of solubility of carbon in the alpha phase (0.2 wt. %) were presented. The result of hardness test, static tensile test and creep test of research alloy in both annealing and hardening state was showed. The obtained results were referred to the same alloy without carbon. Investigated alloy is characterized by improved tensile strength with comparable plastic properties. The investigated alloy was characterized better creep resistance after duplex annealing treatment than after hardening.