Miroslav Karlík

Electron backscattering diffraction analysis of secondary cleavage cracks in a reactor pressure vessel steel

An analysis of secondary cleavage cracks in the A 508 C1.3 bainitic steel has been carried out using electron backscattering diffraction (EBSD) and scanning electron microscopy (SEM). The crystallographic orientation in the vicinity of the secondary cracks was studied. Secondary cleavage crack propagation planes were identified to be {100}, {110}, {112} and {123}. The secondary cracks propagated mostly in the range of one crystallographic grain (bainitic packet) and were arrested on high-angle twist type boundaries or in the upper bainite carbide colonies. The size of cleavage facets on the fracture surface corresponds to the size of the bainitic packets.

Differences in microstructure and texture of Al-Mg sheets produced by twin-roll continuous casting and by direct-chill casting

Over the last two decades, the use of aluminum sheets in automotive applications has increased. Aluminum sheets are currently produced from direct-chill (DC) cast plates. The need for low-cost aluminum sheets is a challenge for the development of new materials produced by twin-roll continuous (TRC) casting and cold rolling. It is expected that the sheets produced from these different casting procedures can differ in their microstructure. Therefore, they can exhibit different formability behavior. The paper presents the results of the microstructural characterization and texture evaluation of aluminum sheets produced by both technologies. Sheets produced from twin-roll cast materials have much finer and more numerous second-phase particles, the grain structures of both types of materials are similar. Electron backscatter diffraction (EBSD) and X-ray diffraction techniques were used for texture evaluation and both confirmed the presence of stronger cube texture in the strips produced from DC-cast plates.

The influence of Cr and Ce additions on the mechanical properties of Fe3Al based alloys

Abstract The influence of Cr and Ce solutes on the hardness evolution of an Fe–28 at.% Al alloy during isothermal and isochronal annealing in the temperature range 300–1000°C was studied. In the Cr-containing alloys, Cr–Fe–(C) precipitates were found after hot working and cooling. These precipitates were dissolved during annealing at 1000°C for 2 h. Subsequent isothermal and isochronal annealing caused their reprecipitation starting above 300°C. Preferentially they nucleate on dislocations, nearest-neighbor antiphase boundaries and subgrain and grain boundaries. It appears that the precipitate formation decreases the hardness of the alloy. The Ce addition and the Ce-containing particles influence neither the formation of Cr–Fe–(C) precipitates nor the evolution of hardness of the alloy.

Tensile deformation and fracture micromorphology of an Fe–28Al–4Cr–0.1Ce alloy

Abstract Fracture and mechanical properties of a vacuum cast and hot extruded Fe–28Al–4Cr–0.1Ce (at.%) alloy were studied as a function of temperature and strain rate. The tensile deformation tests were carried out at temperatures ranging from 20 to 700°C, at strain rates of ∼10 −4 s −1 and ∼10 −2 s −1 . The effects of strain rate and testing temperature are well recognized in the fractographic features. At room temperature the main fracture mechanism is intergranular decohesion with a certain proportion of transgranular cleavage. With increasing temperature the proportion of transgranular cleavage and ductile dimple fracture gradually increase at the expense of intergranular decohesion. The plastic deformation at each testing temperature is more developed at lower strain rate. The changes in the micromorphology of fracture correlate better with reduction of area than with elongation to fracture. Room temperature ductility of 6.5 and 8.6%, was found for the strain rate of ∼10 −4 s −1 and ∼10 −2 s −1 , respectively.

Microstructure of a Thin Cast Al-Fe-Mn-Si Strip

Abstract The recent trend in Twin Roll Casting has been to reduce the casting gauge to less than 3 mm in order to reduce the processing costs, to take advantage of increasing rates of solidification and to increase productivity. Conventional casters typically cast strips 6 to 10 mm thick and operate at essentially constant production rates in this range. The new generation casters, on the other hand, are able to cast strips down to 1mm and offer a notable increase in the casting speed and thus in the caster productivity. It has been reported that the thin strips show some marked differences with respect to conventional strips owing to the temperature gradients and deformation involved in the thin-gauge casting process and may require different down stream processing cycles. The characterization of the thin strip structure is thus essential. This paper describes the microstructure of a thin-cast (3 mm) Al-Fe-Mn-Si (AA8006) strip in comparison with another strip of the same alloy cast at conventional gauge (6 mm).

Fracture Behaviour of Fe3Al and FeAl Type Iron Aluminides

Fracture behaviour of two intermetallic alloys based on FeAl and Fe3Al was studied. On the alloys Fe-40Al-1C (at%) and Fe-29.5Al-2.3Cr-0.63Zr-0.2C (at%) (FA06Z), a basic characterization, the fracture toughness tests and fractographic analysis were carried out. Tensile tests and fracture toughness tests were performed at 20, 200, 400 and 600°C. The fracture toughness values range from 26 MPa.m1/2 at 20°C to 42 MPa.m1/2 at 400°C. In addition, Jintegral dependence on a obtained by potential method was measured. The fractographic analysis showed that samples fractured at 20, 200 and 400°C in the tensile or fracture toughness tests exhibit transgranular cleavage fracture, while at 600°C the ductile dimple fracture predominates.

Accumulative roll bonding of AA8006, AA8011 and AA5754 sheets

Accumulative Roll Bonding (ARB) is a technique of grain refinement by severe plastic deformation, which involves multiple repetitions of surface treatment, stacking, rolling, and cutting. The rolling with 50% reduction in thickness bonds the sheets. After several cycles, ultrafine-grained (UFG) materials are produced. Since ARB enables the production of large amounts of UFG materials, its adoption into industrial practice is favoured. ARB has been successfully used for preparation of UFG sheets from different ingot cast aluminium alloys. Twin-roll casting (TRC) is a cost and energy effective method for manufacturing aluminium sheets. Fine particles and small grain size are intrinsic for TRC sheets making them good starting materials for ARB. The paper presents the results of a research aimed at investigating the feasibility of ARB processing of three TRC alloys, AA8006, AA8011 and AA5754, at ambient temperature. The microstructure and properties of the ARB were investigated by means of light and transmission electron microscopy and hardness measurements. AA8006 specimens were ARB processed without any problems. Sound sheets of AA8011 alloy were also obtained even after 8 cycles of ARB. The AA5754 alloy suffered from severe edge and notch cracking since the first cycle. The work hardening of AA8006 alloy saturated after the 3rd cycle, whereas the hardness of AA5754 alloy increased steadily up to the 5th cycle. Monotonous increase in strength up to 280 MPa was observed in the ARB processed AA8011 alloy.

Necking and softening as a consequence of latent continuum damage

Abstract Necking and softening in ductile polycrystalline metals are modeled as consequences of continuum damage (CD) that consists in braking through of barriers resisting plastic deformation. Our model describes these materials as two-phase continua consisting of microdomains with easy glide and barriers with decreasing continuity in the course of deformation. Macroscopic homogeneity and increase of macroscopic stress with increasing deformation are conditioned by sufficient continuity of these barriers. Our model describes the whole course of engineering stress–strain diagrams up to rupture and the differences in the diagrams for test specimens of different length. The model is compared to experimental data measured on two materials − sorbitic steel and an AlMg3 alloy.

TEM phase analysis of NiTi shape memory alloy prepared by self-propagating high-temperature synthesis

Abstract The effect of annealing a Ni–48at.%Ti alloy prepared by self-propagation high-temperature synthesis (SHS) by means of transmission electron microscopy (TEM) was studied. The alloy in the as-sintered condition shows the presence of B2 NiTi matrix, relatively large particles (2–100 μm) of cubic NiTi2 phase and very fine densely distributed particles of rhombohedral Ni4Ti3 phase. The phase fractions were determined by the Rietveld refinement of neutron diffraction. Twelve-hour annealing at 1000 °C leads to coarsening of Ni4Ti3 particles. Slow furnace cooling from the annealing temperature results in a transformation of the rhombohedral Ni4Ti3 phase to a metastable Ni3Ti2 phase with cubic symmetry. Its particles, coherent with the B2 matrix, have the form of thin oval platelets with lenticular cross-section. The orientation relation is either cube-to-cube or [115]B2 || [111]P and (552)B2 || (121)P. An additional isothermal annealing of the water cooled samples at 720 °C for 3 and 10 h was carried out. According to the scanning and transmission electron microscopy observation of the sample annealed for 3 h, the cubic Ni3Ti2 phase transforms to the high-temperature tetragonal Ni3Ti2–H variant (space group I4/mmm).

High-temperature mechanical properties of Fe-40 at.% Al based intermetallic alloys with C or Ti addition

Abstract Tensile and creep properties of vacuum-induction-melted and cast Fe-40Al-1C and Fe-40Al-1Ti (at.%) were studied. The ingots were hot-rolled at 1200 °C to plates of 12.5 mm thickness using a stainless steel sheath and cooled in air. The alloys showed microstructures with coarse elongated grains having diameters up to 500 and 300 m in the direction of rolling, respectively. Tensile tests were carried out at temperatures 20, 400, 600, 700, and 800 °C. The creep tests were performed under constant load of 100 MPa and 150 MPa at temperatures 550, 600, 650, and 670 °C. Fracture surfaces of tensile specimens tested up to 700 °C exhibited mainly intergranular decohesion. With increasing temperature, the proportion of ductile dimpled fracture increased, and at 800 °C the fracture surfaces of both alloys were practically completely covered by ductile dimples. In comparison to tensile test specimens, fracture surfaces of creep specimens showed an increased fraction of intergranular ductile fracture.