Band 102, Heft 1

The electron channelling contrast imaging technique was applied in a scanning electron microscope equipped with a field emission gun and high beam current mode for imaging stacking faults, in particular in two different variants of cast TRIP steel after tensile and cyclic deformation. Besides well developed deformation bands, individual stacking faults with low dissipation width were observed in the metastable austenitic matrix.

Based on the thermodynamic calculations extrapolated from the corresponding binary sub-systems, four decisive alloys in the Fe–Mg–Mn system and three in the Fe–Mg–Ni system were selected and prepared using a powder metallurgy method to measure the isothermal sections at 500 8C in both systems. The prepared samples were annealed at 5008C, and then subjected to X-ray diffraction, optical microscopy, scanning electron microscopy with energy-dispersive X-ray spectrometry as well as electron probe microanalysis. Taking into account the presently obtained experimental data and the experimental data available in the literature, thermodynamic modeling was performed for the above systems. It was found that a direct extrapolation from the corresponding three binary systems can well reproduce all the experimental data in the Fe–Mg–Mn system, while two thermodynamic parameters are needed in the Fe–Mg–Ni system to fit all the experimental data. The liquidus projections and reaction schemes for the Fe–Mg–Mn and Fe–Mg–Ni systems are also presented.

The nitrogen uptake of high-alloyed steels, especially tool steels, from gaseous atmospheres at elevated temperatures is a well known effect. Besides, for instance, the nitrogenbased case hardening, one application arises in the field of powder metallurgy. Here, introducing nitrogen containing atmospheres during sintering of high-alloyed powder metallurgically produced tool steels could lead to an optimization of the sintering process and an improvement of the materials properties. In this context, several works deal with the application of computational thermodynamics for considering the nitrogen uptake. The scope of this contribution is to investigate in detail the agreement of calculated and experimentally found nitrogen contents. Therefore, one typical gas-atomized powder of a ledeburitic cold work steel was sintered at a temperature of 1230°C, varying the nitrogen partial pressure during sintering between 0.02 MPa and 0.2 MPa. The measured nitrogen contents are compared with values obtained from equilibrium calculations. Additionally, the distribution of nitrogen in the microstructure is investigated by time-resolved optical emission spectrometry and energy dispersive X-ray spectrometry measurements. The results exhibit good agreement between calculated and measured values for low partial pressures of less than 0.1 MPa and an increasing deviation for higher partial pressures. Furthermore, the transformation of vanadium-containing MC carbides to M(C, N) carbonitrides was verified.

At a constant alloy composition, the properties of directionally solidified material depend strongly on the evolution of its microstructure affected by the variation of growth rate V and the temperature gradient G in the vicinity of the solidification front. Several research groups are currently attempting to improve the directional solidification process with mathematical modeling and/or development of a sensor technology for in-situ monitoring of crucial solidification parameters during the process. In this paper the applicability and efficiency of the combined in-situ electrical resistance and temperature measurements for characterization of directional solidification of the eutectic Sn–Pb alloy is presented. Sensor technology shown in this paper exploits the large differences in the electrical resistivity of solid and liquid phases for determination of the current position X(t) and growth rate V(t) of the solidification front.

The microstructural, mechanical and magnetic properties of FeCr30Co8 alloy in the a state were examined after upsetting, followed by torsion. The temperatures (750, 800, 850 and 900°C) and deformation rates corresponded to the superplasticity conditions of the Fe–Cr–Co alloys. Due to the torsional deformation, which was applied only to the bottom of the samples, a gradient microstructure was formed in the sections that were parallel to the upsetting direction. In the temperature range from 750 to 850°C, the deformation led to intermetallic r-phase precipitation. The maximum refinement of the microstructure and the maximum r-phase precipitation were observed in the material that was deformed at 800°C. Measurement of the magnetic properties showed a decreasing coercive force in the deformed alloy, in contrast to the mechanical properties of the samples after the magnetic treatment.

The microstructural evolution of the alloy Ti-6Al-4V was analyzed after hot compression tests in the temperature range of 800–1150°C and the strain rate range of 0.001 s −1 to 1 s −1 . The slope of work hardening and that of flow softening were conspicuously higher in the two-phase microstructure than those obtained in the single-phase β region. It was found that in the two-phase region the peak strain, the strain for highest rate of flow softening and the steady state strain were almost independent of the Zener–Hollomon parameter, Z. While in the single-phase region, characteristic strains were dependent on Z. The microstructural observations indicated that globularization in the α phase contributed to the flow softening. It was also shown that the kinetics of globularization decreased with strain rate. Otherwise, it was concluded that dynamic recrystallization comes into operation during hot deformation of the single-phase β microstructure. In the single-phase region the recrystallization softening and the average grain size diminished with strain rate and increased with temperature.

The aim of this paper is to present an overview of the tensile behavior of ultrafine grained Mg-3%Al alloy. The bulk ultrafine grained Mg alloy was synthesized via consolidation of mechanical alloying. The grain size of the sample was calculated from X-ray diffraction line profile analysis. Tensile properties of ultrafine grained samples tested at temperatures of 25°C to 250°C are compared with that of a coarse grain microcrystalline sample. Plastic deformation of the ultrafine grained sample exhibits a combination of work hardening (15–48% plastic strain) and perfect plastic behavior characterized by a constant flow stress. At room temperature, the sample showed a gain in strength of about two times and enhanced ductility compared to the microcrystalline sample. The strain rate sensitivity was observed to be three times higher in the present ultrafine grained sample. A stress drop with increasing test temperatures was observed in both the samples. The effect of temperature on flow behavior in terms of work hardening rate, temperature sensitivity, ductility, yield strength, and tensile strength are also examined precisely. The present results show important evidence of the temperature dependence of the mechanical properties of the ultrafine grained Mg-3%Al alloy in comparison to the microcrystalline sample.

Cu-10Cr-0.4Zr alloy and in-situ composites based on the alloy were prepared. Microstructural evolution and mechanical properties of the Cu-10Cr-0.4Zr filamentary insitu composites were investigated. It was found that with the increasing of drawing strains from 2.1 to 6.2, the thickness of Cr filaments decreased from about 5 100 nm to 300 nm, and the ultimate tensile strength increased from 510 MPa to 1089 MPa. With increasing drawing strain up to 4.2, the Cr filaments were constrained to fold or twist (even overlapped together) on cross-sections, this could be attributed to deformation compatibility between the Cu matrix and Cr filaments, as well as the deformation mechanisms of bcc-Cr. Meanwhile, with the increasing of drawing strains from 2.1 to 6.2, the ultimate tensile strength values calculated by a modified strengthening model were in good agreement with the experimental values.

Aluminium was incorporated into a polycrystalline iron electrode surface by underpotential deposition from an equimolar AlCl 3 + NaCl melt at 200°C, 250°C and 300°C. The process was studied using linear sweep voltammetry and potentiostatic deposition/galvanostatic stripping. The deposits were characterised by electron microprobe analysis, glancing incidence X-ray diffraction and Auger electron spectroscopy. The electrochemical measurements showed clear evidence of formation of three intermetallic compounds. This was confirmed by the physical analysis, which showed three layers of successive bulk intermetallic compounds. The constant-potential regions measured during the low-current stripping corresponded to the coexistence of pairs of the three intermetallic phases. The kinetics of growth of the identified compounds are described.

Die casting moulds are exposed to high cyclic temperatures, mechanical loads and severe chemical conditions. Thermal and mechanical loads cause high local stresses and consequently surface cracks. This paper introduces a new approach to establishing thermal fatigue cracks in die casting moulds by measuring the resulting defect-fins on aluminium alloy castings in the actual die casting process. The investigation showed that cracks occurred sooner and were bigger if the mould material had a lower hardness. The maximum depth of the defect-fin observed on the casting due to thermal fatigue crack on the mould with the lowest hardness (42 HRc) at 10000 cycles was 0.37 mm, the maximum width of the defect-fin was 0.76 mm and maximum length was 9.6 mm. The observed cracks were bigger closer to the entrance of melt flow, especially at the places with stress concentrators, due to higher melt temperature and higher melt flow.

At ambient temperature, hexagonal close packed magnesium has limited ductility due to low availability of active slip systems. In this work, 3 methods of enhancing the ductility of wrought magnesium are discussed, namely: (1) addition of microparticles such as Ti, Mo and SiC (Method 1), (2) addition of various oxide nanoparticles as well as carbon nanotubes (Method 2) and (3) use of aluminium based core as continuous reinforcement (Method 3). In Method 1, the magnesium matrix is softened (Ti microparticle case), delocalized in deformation (Mo microparticle case) or in a reduced state of stress around the microparticle after heat treatment (SiC microparticle case). In Methods 2 and 3, the basal plane in magnesium is sometimes tilted from 0° to 45° (to the force axis), enabling basal slip to occur more easily in the composite. In the cases where the basal plane is oriented parallel to the force axis, non-basal slip at room temperature occurs more easily between the aligned basal planes. In Method 2, the nanoparticles present also provide sites where cleavage cracks are opened ahead of the advancing crack front. An attempt is made to correlate the ductility enhancement of wrought magnesium as a function of the way it is reinforced.

This work describes the development of texture during hot rolling of three binary alloys, namely Ti-8Nb, Ti-12Nb and Ti-16Nb. The alloys have been unidirectionally hot rolled at 800°C and air cooled. The alloy Ti-8Nb consists of the α (cph) and small volume fraction of β (bcc) phases while the alloys Ti-12Nb and Ti-16Nb show the presence of α n (orthorhombic) and small volume fractions of β phases in as-cast and hot rolled conditions. The Ti-8Nb alloy exhibits nearly c-type texture in terms of basal pole figure in 40% hot rolled condition with a strong and inhomogeneous [0001]⊩ND fibre. Subsequent rolling reductions change texture and weaken the fibre present in 40% hot rolled specimen. The Ti-12Nb alloy exhibits c-type texture in 60% hot rolled specimen with a strong [001]⊩ND fibre. The overall texture of 80% hot rolled specimen is weak and different in terms of basal pole figure. The development of texture during hot rolling of the alloy Ti-16Nb is different from that of Ti-12Nb alloy even though the texture of 80% hot rolled specimens of both the alloys are nearly the same. The high overall intensity of texture in alloy Ti- 16Nb in the 80% hot rolled specimen can be attributed to the presence of a large volume fraction of β phase in comparison to that of the alloy Ti-12Nb.

Phosphorus vapor pressures were measured over tin–phosphorus samples using two independent experimental techniques. Using an isopiestic method, data points were obtained between approximately 0 and 71 at.% P in the temperature interval from 618 to 998 K. Vapor pressures were also measured at lower temperatures over two twophase fields, i. e. in (L + Sn 4 P 3 ) between 559 and 677 K and in (Sn4P3 + Sn3P4) between 552 and 621 K, by a Knudsen cell based mass loss technique. Based on these measurements partial thermodynamic properties and integral Gibbs energies were derived. The composition dependence of the phosphorus activity was also derived for three selected temperatures, 650, 700, and 750 K. Standard molar Gibbs energies of formation of Sn 4 P 3 , Sn 3 P 4 and SnP 3 were determined and are given by: ΔfGm°(Sn4P3)(s)±1.1/kJ⋅mol−1=−166.5+0.2007T/K(T=559−794K)ΔfGm°(Sn3P4)(s)±1.8/kJ⋅mol−1=−234.4+0.3053T/K(T=552−705K)ΔfGm°(SnP3)(s)/kJ⋅mol−1=−113.0+0.1523T/K(T=632−659K)

Some Tb 3+ -, Dy 3+ -, Gd 3+ -doped, and Tb 3+ /Dy 3+ - and Tb 3+ / Gd 3+ -codoped silicate glasses were synthesized by meltquenching. Their luminescence properties were investigated by photoluminescence, X-ray excited luminescence spectra and their corresponding decay curves. It is shown that the enhanced photoluminescence intensity of Tb 3+ emission can be achieved by efficient energy transfer from Dy 3+ (or Gd 3+ ) to Tb 3+ under UV light excitation. However, the X-ray excited luminescence intensity of terbium ions in Tb 3+ /Gd 3+ -codoped glass contrasts remarkably to that of the Tb 3+ /Dy 3+ -codoped one, where the X-ray excited luminescence intensity of terbium ions is obviously lower than that of the Tb 3+ -doped one.

The effect of O on the phase equilibria in Nb–Fe–Cr alloys, more specifically on the formation of a Ti 2 Ni type phase, has been studied. Nb–xFe–yCr alloys with × = 37.4 – 41.4 at.%, y = 7.6 – 8.6 at.% were subjected to long term annealing experiments at 950 8C after oxygenation. The phases were identified and characterized using scanning electron microscopy, X-ray diffraction and wavelength dispersive spectroscopy. In the pure Nb-37Fe-8Cr and Nb-41Fe-9Cr alloys respectively a two phase equilibrium between (Nb) and l FeNb and a three phase equilibrium between (Nb), l FeNb and Fe 2 Nb are established. In the oxygenated alloys a Ti 2 Ni type phase with composition (Fe, Cr) 6 Nb 6 O x appears mainly at the expense of the l FeNb phase. With respect to composition and crystallographic parameters the (Fe, Cr) 6 Nb 6 O x phase has large similarities with the Fe 2.4 Nb 3.6 O and Fe 6 Nb 6 O phase. From wavelength dispersive spectroscopy measurements, it is concluded that × equals 0.75 when (Fe, Cr) 6 Nb 6 O x is in equilibrium with (Nb), l FeNb and Fe 2 Nb. × is higher when this phase is in equilibrium with NbO. The structure of (Fe, Cr) 6 Nb 6 O x is also analyzed with Rietveld refinement. This analysis reveals that Cr atoms partially replace Fe atoms only on the 16d site and not on the 32e site.