Materials Science Forum Vol. 782

Abstract: The study presents the results of the identification of the phase composition of creep resistant cast austenitic 30Ni-18Cr steel with the addition of Ti and Nb after the annealing process conducted at 900°C for 10 and 500 hours. The microstructure of the as-cast steel showed mainly the presence of simple carbides. Depending on the alloy chemical composition, those were the TiC and NbC carbides. In the initial phase of annealing process, the nucleation of small secondary precipitates of chromium carbides Cr₂₃C₆ was observed to take place in cast steel microstructure at the matrix / simple carbides interface. After a long-time annealing, a new phase (G phase) was developing; the symbols of the phase were Ni₁₆Ti₆Si₇ and Ni₁₆Nb₆Si₇, respectively. The G phase was enclosing simple carbides forming with them the complexes of primary and secondary precipitates of a very intricate morphology. Chromium carbides Cr₂₃C₆ were also present there.

Abstract: The creep resistance of Elektron 21 magnesium alloy containing Zn, Nd, Gd and Zr has been investigated. Test has been conducted at 200°C, 225°C and 250°C with constant load amounts to 90, 120 and 150 MPa up to 100 hours. Some specimens cracked during the test. Metallographic and fractographic research has been performed in order to identify the microstructural changes occurring during the creep resistance test. Microstructure has been observed with light microscopy and scanning electron microscopy. Chemical composition of microstructural components has been investigated with energy dispersion spectroscopy. Research revealed presence of voids, microcracks and inclusions which can significantly influence creep resistance of material.

Abstract: Combination of centrifugal melt spraying and hot die-forging of a rapidly solidified semi-product was presented as a promising and inexpensive method for processing of aluminium based alloys of unconventional chemical compositions, e.g., those containing high concentrations of thermally stabilizing transition metals. In our study, the use of this processing method is illustrated for the Al–23Si–8Fe–5Mn (wt. %) alloy. Structure was examined by LM, SEM, EDS and XRD. Mechanical properties were determined by hardness and compressive tests. Thermal stability was assessed by measuring the hardness development during long-term annealing, elevated temperature compressive tests and creep tests. The research showed that the investigated alloy exhibits excellent thermal stability as compared with commercial thermally stable aluminium alloys currently used in automotive and aerospace industry.

Abstract: Alloys based on TiAl and FeAl aluminides are modern materials for high-temperature applications in automotive or aerospace industry due to low density combined with good high-temperature mechanical properties and oxidation resistance. Previous works proved that the addition of silicon to these alloys improves the oxidation resistance as well as the thermal stability. In this work, the mechanism of the silicon effect was investigated by observing the microstructure of the oxide layer and the near-surface area of the Ti-Al-Si and Fe-Al-Si alloys prepared by reactive sintering powder metallurgy. It was found that silicon improves the compactness of the oxide layers. The oxide layers on Fe-Al-Si alloys are formed by Al₂O₃ and small amount of iron oxide (Fe₂O₃) while Ti-Al-Si alloys cover by TiO₂ and Al₂O₃ during the oxidation. Due to aluminium depletion of the alloy, a layer of silicides is formed under the oxide layer, thus acting as the additional protection against high-temperature oxidation.

Abstract: The morphology control of intermetallic phases is very important in secondary aluminium cast alloy, because these alloys contain more of additional elements, which forms various intermetallic phases in the structure. Improvement of the mechanical properties is strongly depending upon the morphology, type and distribution of the second phases, which are in turn a function of the alloy composition and cooling rate. The iron intermetallic phase has the greatest influence on mechanical properties. It is necessary to study microstructure of Al-Si alloys, because the metallographic evaluation of aluminium alloys is not simple and these alloys are used for production many mechanical components, especially for cars, aerospace and rail vehicles. The study of iron intermetallic phases was performed using light microscope Neophot 32 and SEM observation with EDX analysis. For study the morphology of these phases were samples deep-etched for 30 s in HCl solution, in order to reveal the three-dimensional morphology of the iron phases.

Abstract: The present study was performed on primary AlSi10MgMn cast alloy to analyze the morphology and composition of complex microstructure of the intermetallic phases. AlSi10MgMn cast alloy is a typical casting alloy used for parts with thin walls and complex geometry. It offers good strength, hardness and dynamic properties and is therefore also used for parts subject to high loading. In this study, several methods were used such as: optical light microscopy (LM) and scanning (SEM) electron microscopy in combination with EDX analysis using standard etched or deep etched sample to identify intermetallic. Alloy was analyzed in as-cast state (rapidly cooled right after casting) and after T6 heat treatment. T6 treatment (solution annealing, quenching and age hardening) improves mechanical properties. The results show that the microstructure of AlSi10MgMn alloy consisted of several phases: α-matrix, eutectic, Fe-rich intermetallic phases (Al₁₅(FeMn)₃Si₂, Al₅FeSi), Mg₂Si, Al₈FeMg₃Si₆ and of other phases in formation. Iron-rich intermetallic phases are well known to be strongly influential on mechanical properties in Al-Si alloys. The most common morphology was the long platelets of Al₅FeSi phase. After heat treatment were observed spheroidisation of eutectic Si, dissolution and fragmentation of Fe-phases.

Abstract: The laser surface hardening is method which provides increased mechanical properties of secondary (recycled) Al-Si cast alloys for automotive industry. Improvement of mechanical properties and structure of secondary aluminium alloys can often significantly increase the lifetime of casting and reduce costs for fuel and reduction of environmental loading. For study was used a laser beam Nd: YAG lasers, BLS 720, on the test samples of secondary cast alloy AlZn10Si8Mg. AlZn10Si8Mg cast alloy are used for engine and vehicle constructions, hydraulic unit and mould making without the need of heat treatment because this alloy is self-hardened. The effect of laser beam was evaluated with the laser power 50 W and 80 W on the surface of samples. The final microstructure of Al-alloys depend on the laser process parameters. The changes of microstructure after laser surface hardening was observed by using classical techniques of etching (standard black-white contrast - etching by 0.5 % HF, 10 % H₃PO₄ and colour contrast - etching by Weck-Al) and deep etching with HCl. Due to the action of laser beam on the surface of the secondary alloy AlZn10Si8Mg there have been changes in the microstructure of the material. Melting area is alpha-phase with much fine columnar dendrites morphology without the presence of Si-particles and intermetallic phases. In the transition area were observed grain refinement of eutectic Si (finer and rounder Si particles) as the modify action of the laser. By increasing the laser power the microhardness of surface layers decreases. In the surface layer (80 W) were observed cracks due to uneven heat transfer of the material.

Abstract: The determination of ductile fracture criteria as well as friction coefficient, stress-strain curves, constants for Hollomon's equation and a material workability based on analytical methods as a forming limit diagram, a normalized Cockcroft-Latham criteria (nCL)) ring and compression tests for two materials based on aluminum and copper alloys were carried out. A calculation of nCL criteria on the basis of a compression test and numerical simulations was made. The critical values nCL criteria resulting from compression test were determined. Prediction of nCL criteria by numerical simulations were confirmed by laboratory compression tests. The values obtained from numerical simulations and compression tests for both materials show a good coincidence in results.

Abstract: The influence of thermo-plastic processes through methods of severe plastic deformations (SPD) and rolling carried out at ambient and cryogenic temperatures on recovery of two materials was investigated. The aim of this study was to insert strains to materials having middle and high stacking fault energy (SFE) in ambient and cryogenic temperature conditions, respectively and subsequently, through DSC method, to observe an influence of the storage energy on structural recovery of materials. As experimental materials were used oxygen free high conductivity copper (OFHC Cu) and C-Si steel which represent materials with middle and high stacking fault energy (SFE), respectively. The OFHC Cu was subjected to equal channel angular rolling (ECAR) by seven passes. ECAR is a method belonging to a SPD group. It was shown, five ECAR passes have a significant effect on material properties. The rolling performed at cryogenic temperatures using a laboratory duo rolling mill was carried out only once. This study implies that a recovery process (characterized by the mobility of structural defects) starts as follows: for OFHC Cu without ECAR and processed by 5^th ECAR passes: 0.31·T_melt and 0.19·T_melt, respectively, for C-Si steel processed by cryorolling: 0.095·T_melt.

Abstract: Microstructure, mechanical properties, cyclic plastic deformation behavior and fatigue strength of ultrafine-grained (UFG) magnesium alloy AZ91 processed by equal channel angular pressing (ECAP) were investigated. ECAP of originally cast alloy results in development of bimodal structure, improved yield stress, tensile strength and ductility when compared to the as-cast state. Endurance limit based on 10⁷ cycles is also improved, however exhibits large scatter. Initiation of fatigue cracks takes place in regions of large grains in the bimodal structure, where the content of Mg₁₇Al₁₂ particles is low.