Vladimír Šíma

FeAl-based alloys cast in an ultrasound field

Abstract An ultrasonic device was designed to fabricate relatively small (about 0.7 kg) vacuum chill castings of FeAl-based alloys with improved microstructure. The influence of ultrasound on the solidification microstructure of the Fe-40 at.% Al based castings was investigated. The subsequent hot-rolling of such an as-cast alloy was made using a special stainless steel bandage of the casting. This effectively prevents fraying and cracking caused by the thermal shock arising at the surface of the hot casting, when it comes in contact with a cold roller. The efficiency of ultrasonic vacuum casting, based on the influence of the acoustic cavitation on the solidification of the melt, is manifested by a refined microstructure of the prepared iron aluminides Fe-40 at.% Al with addition of C or Zr and B. First results of improved mechanical properties of these alloys are presented and discussed.

Buquoy's problem

We analyse the one-dimensional motion of a uniform thin fibre which is pulled upwards from a horizontal plane by a constant vertical force exerted against the homogeneous gravitational field. The solution of the equation of motion which describes this variable mass problem is discussed and the character of the resulting damped oscillations is described.

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.

Two models of solid-solid transformation kinetics in the CuAu intermetallic compound

Abstract A simple model of the kinetics of order–disorder and order–order transformations in the stoichiometric CuAu compound is presented. The model describes the evolution of the Gibbs energy of stable and metastable states of two ordered (CuAu(I), CuAu(II)) and one disordered (CuAu(D)) phase. The simultaneous kinetics of all three phases is interpreted as a result of six competitive transformation processes. The model is employed for a semi-quantitative description of the TTT curves, for differential scanning calorimetry modelling and also for an interpretation of the so-called retro-effect in CuAu. The proposed phenomenological description of kinetic processes in CuAu is compared with the classical concept of the theory of nucleation and growth kinetics. Both theories are discussed in terms of real processes, accompanying the transformations in CuAu (elastic effects, anti-phase boundaries, twinning, hysteresis effects, spinodal ordering).

Microstructure and mechanical properties of hot rolled Fe–40 at-%Al intermetallic alloys with Zr and B addition

Abstract The microstructure and mechanical properties of hot rolled Fe–40 at-%Al based intermetallic alloys, with 0·1 at-%Zr and different additions of B (varying from 0·01 to 0·1 at-%), are characterised. The additions of Zr and B improve tensile properties at room and elevated temperatures. Increasing B content is also associated with a number of other effects. First, the fracture mode changes from intergranular decohesion to cleavage, which correlates with significant increases in the fracture toughness. Second, there is a certain stabilisation of dislocations arranged in parallel systems of slip bands, as shown by transmission electron microscopy. Numerous complex stacking faults on {100} planes are also observed in the alloy with the highest B content. Third, B is found to modify the formation of second phase particles; such particles (coarse and fine) are analysed by energy dispersive X-ray spectroscopy and electron energy loss spectrometry to obtain compositional information.

Hysteresis of Long-Range Ordering in CuAu

The effect of heating and cooling on the long-range order transformation in stoichiometric CuAu is investigated by several complementary measuring methods. Measurements of heat flow, resistometry and acoustic emission are done dynamically by linear heating/cooling. It is shown that measuring dynamically yields the expected effect of undercooling, which decreases with decreasing cooling rate. The dependence of undercooling on cooling rate is compared with the concept of continuous cooling for glass forming. A small influence of heating rate on disordering temperature is reported (retro-effect).

Defects and Particles in Laboratory Hot Rolled Steel of C-Mn-Cr-Nb Type

Susceptibility to cracking of the as-cast C-Mn-Cr-Nb steel was studied by laboratory rolling. The variable parameters were the heating temperature (1150 - 1340 °C) as well as the rolling temperature (950 - 1150 °C). Final microstructure of the free-cooled samples was constituted by bainite, pearlite and ferrite with different morphology and various contribution. Deformation temperature below 1000 °C yielded in the incomplete recrystallization of austenite. Surface cracks originated preferentially on the austenite grains boundaries. Size of the present particles (inclusions and precipitates) varied from 101 nm to 101 μm. SEM and EDS analysis revealed that the inclusions ware mostly of the MnS type. TEM analysis confirmed that the grain boundaries were not enriched by any particles. In addition to the Fe3C particles, the discoid niobium carbide particles with approximately 40 nm diameter and 10 nm thickness were detected. These small particles were not connected by any notable pinning of dislocations.

Fully Dense Fine Grained FeAl-Based Intermetallics Prepared by Spark Plasma Sintering Method

Fine grained fully dense material has been prepared by spark plasma sintering method (SPS), the feedstock powder has been prepared by ball milling of the as cast alloy with nominal composition of Fe-40Al-0.1Zr-0.1B (at.%). The sintered material has been tested and analyzed using a variety of experimental methods in order to study its mechanical properties at temperatures up to 800°C. The obtained results show better mechanical characteristics than that for the commercial FeAl40 Grade 3 material. The results are discussed in terms of the phase composition, microstructure and results of chemical analysis of the compacted material.