Martin Švec

Microstructural evolution and creep of Fe–Al–Ta alloys

Abstract The microstructural evolution in Fe – Al – Ta alloys containing 23 – 31 at.% Al and 1.5 – 2.2 at.% Ta has been studied in the temperature range 650 – 750 °C by annealing for 1, 10, 100 and 1 000 h. The experiments confirm that in this temperature range the precipitation of the stable hexagonal C14 Laves phase is preceded by formation of coherent, metastable L21 Heusler phase precipitates within the Fe – Al matrix. However, precipitates of C14 are observed after much shorter annealing times than previously assumed. Creep strength increases substantially with increasing Al content of the alloys because the solid solubility for Ta in the Fe – Al matrix increases with increasing Al content and solid-solution hardening contributes substantially to the observed high creep strength. It may therefore be that the microstructural changes during creep have no noticeable effect on creep strength.

The Effect of Low Concentrations Nb and C on the Structure and High-Temperature Strength of Fe3Al Aluminide

The Fe3Al iron aluminide alloyed by low concentrations of Nb and C (cNb, cC) is studied. The influence of the cNb/cC ratio on the structure and high-temperature yield strength of iron aluminide was investigated. The structure and phase composition were studied by scanning electron microscope equipped with EDS and EBSD. The strengthening mechanisms are detected as strengthening by incoherent precipitates of NbC and as a solid solution hardening by Nb atoms.

The Effect of Laves Phase (Fe,Al)2Zr on the High-Temperature Strength of Carbon-Alloyed Fe3Al Aluminide

The effects of carbon on the phase structure and on the yield stress σ0.2 in the temperature range from 873 K to 1073 K (600 °C to 800 °C) of the Fe3Al type aluminides alloyed by Zr are analyzed. Four alloys with Zr and C in ranging from 1.0 to 5.0 at. pct of additives were used. The appearing of either Laves phase (Fe,Al)2Zr and/or carbides depend on the difference in concentrations, cZr − cC. This parameter (cZr − cC) has been selected instead of the concentration ratio cZr/cC used in previous works since it exhibits a significantly better correlation with the Laves phase concentration which influences the high-temperature yield stress, σ0.2, of the tested alloys. The presence of Laves phase or eutectic (matrix—Laves phase), respectively, enhances the value of the yield stress σ0.2. The amount of Laves phase is decreased by the presence of C due to the affinity of carbon to Zr.

The Analysis of Causes of Pressure Bottles Heads Destruction

The research deals with the cause of destruction of pressure bottles heads during ordinary operation. The scanning electron microscope (SEM) equipped with an energy dispersive detector (EDX) for local chemical analysis was used for material investigation. The analysis revealed selected oxidation penetrating deep into the material causing the loss of cohesion of phase boundaries leading to brittle fracture.

Fractographical Analysis of Screws Used for Truck Cab Holding

The research deals with determination of the breaking cause of the screws used to attach the truck cabins. Scanning electron and light optical microscopy were used for engineering investigation of the fracture cause. Fractographical and structural analysis revealed fatigue mechanism of the screw fracture with morphology corresponding to the combination of tensile and bending stresses. Based on the acquired results a structural adjustment was recommended to the customer.

The Effect of High-Temperature Deformation on the Structure of Iron Aluminides

The iron aluminides are very perspective materials with many advantages – for example excellent corrosion resistance and/or good strength up to 600 °C. Due to these benefits they appear as suitable materials for use in high temperature applications. But some negative properties (sharp drop in strength above 600 °C or limited ductility at room temperature) are the major obstacles for their use. One way, how to improve mechanical properties of iron aluminides, is doping of binary alloy by alloying elements, which can form strengthening precipitates. Depending on their size the precipitates of secondary phases can block dislocation movement or act as strengthening phase as known in composite materials.One from many properties, which is important to know before a new type of material is used in structural applications, is its deformation behaviour. In present paper there was studied the structure of Fe-25Al-5Zr (in at. %) sample in as cast state and in states after high-temperature deformation at 600 and 800 °C in detail. A high resolution SEM equipped with EDX and EBSD was used for the study of structure. It was measured that huge grains are presented in the structure of as cast state. After deformation at 600 °C there were still observed huge Fe3Al-matrix grains, but strongly deformed. Also formation of sub-grains due to deformation is obvious. Deformation at 800°C led to formation of small grains due to recrystallization. Higher deformation degree in non-eutectic areas led to preferential recrystallization while Fe3Al phase in eutectic regions remained deformed (nonrecrystallized).

Possibilities of Usage of EBSD for Study of DC06 Steel Deformation

EBSD is an analytical technique, which uses diffraction of backscattered electrons on the sample’s crystal lattice to determine the crystal structure and orientation. Point-by-point mapping of crystal orientation and misorientation, calculation of sub-grain boundaries angles and statistical processing of grain data allow identifying and displaying elastically and plastically deformed zones, textures. Orientation mapping and pole-and inverse pole figures allow to identify preferred slip planes and texture, however misorientation reveals areas where higher stress concentration appears, dislocation arrays and internal defects.

The Effect of Heat Treatment on the Structure and Mechanical Properties of Austempered Iron with Vermicular Graphite

Austempered vermicular-graphite iron seem to be a perspective material because of its promising mechanical properties. These properties are strongly affected by parameters of applied heat treatment. Different heat treatment conditions (austenitizing dwell time, isothermal temperature and isothermal dwell) were varied and their effect on the structure and mechanical properties was evaluated and described. Properly selected heat treatment conditions lead to formation of ausferritic matrix with which is responsible for substantial increase in mechanical strength while maintaining sufficient ductility.

The Influence of Microstructure, Heat-Treatment and Type of Crystal Lattice of Iron Aluminides on Coefficient of Thermal Expansion

The iron aluminides seem to be very perspective materials for high temperature structural application. They have many advantages, but unfortunately also some negative properties – e.g. sharp drop in strength above 600°C or limited ductility at room temperature. These disadvantages can be reduced by alloying of binary alloy by other elements.Present work deals with a study of coefficient of thermal expansion (CTE). It was investigated the influence of microstructure and heat-treatment on the values of CTE. Secondary, it was studied the possibilities, how to determine phase transition temperatures from CTE curves. Influence of type of iron aluminides lattice on CTE values was also examined as well as the influence of addition of alloying elements into binary iron aluminides.