Solid State Phenomena Vol. 211

Abstract: Magnesium alloys are one of the lightest structural metallic materials. Their specific strength and stiffness is comparable to this, characterizing aluminum alloys and even some groups of steel and titanium alloys. Their main disadvantage is low maximum working temperature (about 120°C for Mg-Al-Zn alloys). This led to development of Mg-RE-Zr alloys, which can work up to 250°C. The paper presents results of the investigations of influence of subsequent melting operations on the Elektron 21 and WE43 magnesium alloys. Elektron 21 alloy had been prepared from the pure ingots, while WE43 alloy from the scrap material. Average area of the grain flat section and eutectics volume fraction had been evaluated quantitatively. The results of the evaluation have been verified by means of Mann-Whitney U-Test and Kolmogorov-Smirnov statistical tests. The liquid metal treatment led to refinement of the grain only in Elektron 21 alloy (from Ᾱ=3559μm² to Ᾱ=1849 μm²). Multiple modification of the WE43 alloy does not lead to further decrease of the average area of grain flat section (from Ᾱ=1638μm² to Ᾱ=1871 μm²).

Abstract: The casting defects, microstructure and hardness of Elektron 21 Mg sand casting alloy was investigated. Visual and radiographic examination showed the presence of non-metallic inclusions and porosity. Quantitative analysis of microstructure was performed, no correlation between section thickness, grain size, volume fraction of eutectic areas and hardness was observed. Casting defects were responsible for cracking at magnesium casting/steel tube interface.

Abstract: The Elektron 21 is a commercial magnesium alloy containing Zn, Nd, Gd and Zr. The addition of rare earth elements improve its mechanical properties at elevated temperatures, what contribute to widen the range of applications. Constructions working at elevated temperatures can be exposed to its rapid changes. The purpose of present paper is to present results of study on reaction of Elektron 21 on applied thermal shock. As-cast material was heat treated according to four different variations. Afterwards, five cycles of thermal shocks, consisted of heating to 200°C (service temperature) and rapid cooling in water, have been applied. After heat treatment and each cycle of thermal shock the hardness and microstructure have been studied. The investigated material revealed satisfactory resistant to thermal shock, regardless of applied various heat treatment.

Abstract: Usefulness of the magnesium alloys for construction of structural components is determined, apart from their low density, by a number of favourable mechanical properties and in the case of their use for components of transport means additionally by good fatigue strength. In this study, 12 mm diameter extruded rods of AZ31 and AZ61 magnesium alloys were used as test material. After extrusion the rods were annealed at a temperature of 400°C, with a 60 min soaking period and subsequent cooling in air. Cylindrical specimens with a diameter of d₀ = 8 mm were made for the fatigue test under high-cycle rotary bending conditions with the cycle asymmetry coefficient R = -1. The tests were carried out for a limited fatigue strength range. Examination of microstructure of tested alloys and fatigue fractography were also performed. During the high-cycle fatigue tests it was found that the AZ61 alloy has a longer fatigue life. Based on the obtained results, fatigue life characteristics of the tested materials were drawn up.

Abstract: The paper organises the current state of knowledge concerning the effect of hydrogen on stress corrosion of magnesium alloys. This review describes phenomena and mechanisms connected with stress-corrosion cracking (SCC) in commonly used magnesium alloys from Mg-Al-Zn system. In addition, some information about SCC in alloys from Mg-Y-RE-Zr and Mg-Al-RE systems is described. It seems that microstructural factors (e.g., matrix α-Mg and intermetallic phases) related to the presence of Y, Zr and rare earth elements (RE) plays an essential role in hydrogen-induced cracking (HIC).

Abstract: The purpose of the study is to assess electrochemical corrosion resistance of magnesium alloy AZ31 with additives of 4.5, 7.5 and 15 % lithium in NaCl solutions. Corrosion tests were performed in solutions with concentration 0.01 2 M NaCl with application of electrochemical testing system VoltaLab®PGP201. Resistance to electrochemical corrosion was evaluated on the ground of registered anodic polarisation curves by means of potentiodynamic method. Results of performed tests show unequivocally deterioration of corrosion characteristics of the alloy together with increase of molar concentration of NaCl solution. As chloride ions concentration increases, decrease of corrosion potential and polarisation resistance, as well as increase of corrosion current density are observed. Deterioration of corrosion characteristics of AZ31 alloy was shown with the increase of lithium content. It must be highlighted that irrespective of molar concentration of NaCl solution, there is also presence of pitting corrosion in the tested alloy. It proves that magnesium alloy AZ31-Li is not resistant to that type of corrosion. Test results prove that it is necessary to apply protective films on elements made of magnesium alloy with lithium additive.

Abstract: Magnesium alloys containing yttrium and neodymium are known to have high specific strength, good creep and corrosion resistance up to 523 K. The addition of ceramic particles strengthens the metal matrix composite resulting in better wear and creep resistance while maintaining good machinability. In the present study, WE43 magnesium matrix composite reinforced with SiC and carbon particulates were fabricated by stir casting. The microstructure of the composite was investigated by optical microscopy, quantitative metallography, scanning electron microscope and XRD analysis. Microstructure characterization of WE43 MMC showed inhomogeneous reinforcement distribution and presence of shrinkage porosity. Reinforcing particles are well bonded with the matrix, however, in some cases thin reaction layers was detected. The presence of SiC particles assisted in improving hardness.

Abstract: The magnesium alloy ZRE1 (2.7%Zn, 0.53%Zr, 3,14% rare earth elements) was applied as a matrix of composite reinforced with glassy carbon particles obtained by hot pressing of powder mixture. An influence of matrix chemical composition on interaction processes occurring at the interface was analyzed. The interface microstructure was characterized by scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). It was revealed the continuous bonding between components and a presence of zone enriched with zirconium, rare earth elements and oxygen in comparison to the matrix composition.

Abstract: In the paper, results of the study on aluminium evaporation from the Ti-Al alloy (6.5%mass) during smelting in a vacuum induction furnace (VIM) are presented as well as the kinetics of the process is discussed. The experiments were performed at 51000 Pa for 1972 K and 2023 K. Based on the determined values of aluminium overall mass transfer coefficient, the mass transfer coefficient in the liquid phase and the evaporation rate constant, the stages that determined the investigated process were defined. It was demonstrated that Al evaporation from the aluminium-titanium alloy is kinetically controlled.

Abstract: In this work the influence of the particular stages of a multi-stage heat treatment on microstructure and properties of the Ti-47Al-2W-0.5Si alloy was presented. The alloy was melted in a vacuum induction furnace at a special graphite crucible. The received alloy in form of cylindrical ingot with a diameter of 16 mm and a length of 120 mm was homogenized at 1400°C for 1 h with furnace cooling (Stage 1). The proposed further treatment consists of a cyclic heat treatment (Stage 2), under-annealing carried out in a two-phase α+γ area (Stage 3) and a short full annealing conducted in α-phase area (Stage 4). Selection of particular stage parameters including the upper cycle temperature, the soaking time at the upper cycle temperature, the cooling rate from the upper cycle temperature and number of cycles for the second stage and the temperature and time of annealing for the third and forth stage was presented. The purpose of the proposed treatment is microstructure refining. This refinement is associated with the massive transformation that occurs in the alloy after heating to a temperature of 1405°C, short soaking time and cooling in air. The effect of heat treatment parameters on the microstructure and hardness of the investigated alloy was characterized.