Verformungsinduzierte Martensitbildung in metastabilen austenitischen Stählen

The volume of strain induced α-martensite in metastable steels depends on the stacking fault energy. If its value makes more than ∼100 ergs/cm², a direct γ → α transformation occurs, this being not or little quantitatively influenced because of the complicated shearing mechanism of lattice change by external stresses or strains. The tendency to form extended stacking faults or hexagonal ϵ-martensite increases to the degree as by adding of certain alloying elements, as e. g. Cr + Ni, Cr + Mn or Mn + C, the stacking fault energy is lowered. The indirect γ → ϵ → α or γ → SF → α-transformation occurs, where ϵ-martensite and stacking faults only gradually differ from eachother. Because of the simple shearing mechanism of the γ/ϵ- and γ/SF transformation external stresses are strongly favouring the indirect transformation. Especially in crossing regions of ϵ-plates or stacking faults, but also in their internal parts, shearings are generated which are prestages or nuclei of α-crystals. If the SFE is below ∼20 ergs/cm² a complete strain induced α-martensite formation becomes possible, if this is in accordance with the thermodynamic stability relations.     

Bildung von α-Martensit im Schnittbereich von ϵ-Martensitplatten

In steels the nucleation of crystals of α martensite from austenite is facilitated in the presence of ϵ martensite. Particularly within the range of crossing platelets of ϵ martensite with 〈110〉_γ direction α martensite is formed easily. That has been demonstrated by metallography in alloys with suitable composition and pretreatment. Also within the range of crossing of other shear products, as stacking faults or twins, the atomic arrangement for nuclei of α martensite is favourable. By the aid of matrices the strain in crossing 2 ϵ martensite plates, ϵ martensite plate and twin, and 2 twins resp., has been calculated and the result compared with the strain during the martensitic γ → α transformation.     

Anomalous Up-and-Down Variation of Positron Lifetime Near Tc in High-temperature Superconductors

The temperature dependence of the positron lifetimes in the polycrystalline high T_c superconducting Y Ba Cu O and Bi Sr Ca Cu O and the non-superconducting Y Ba Cu O have been investigated by using the position lifetime technique. It is found from the experiments that there is an evident temperature dependence for the two kinds of superconducting samples but no temperature dependence for non-superconducting sample. It is important that the anomalous up-and-down variation of positron lifetime near T_c has been observed in the measured superconducting samples. We suggest that this strange temperature dependence of the positron lifetimes near T_C may result from some changes induced by the phase transition.     

The crystallization mechanism in high strength glass-ceramics

The addition of NiO to a MgO Al₂ Al₂O₃ SiO₂ basic glass has an influence on the phase separation and crystallization mechanism. The differentiation of the components already detectable in the amorphous state continues in the crystallization process of the nickel-rich glasses. The formation of Ni Al-spinel in the bulk restricts the precipitation of high quartz crystals to the skin. The result is a high strength glass-ceramic with clearly distinguishable bulk and surface regions.     

Umwandlungsplastizität und Form-Gedächtnis-Effekt in einer Eisenlegierung mit 16% Mangan

During the quenching of wire shaped specimens of steel X8Mn16 fec austenite changes under reduction of length in hcp ϵ-martensite. If the transformation takes place under the influence of an external tensile stress an elongation results. This transformation plasticity consists of a reversible and an irreversible part. The reversibel part rises by the formation of stress-induced ϵ-martensite in preferred orientation and is built back under reduction of length during the following heating over the Af-temperature. By this it is verified that the strain-memory-effect can occur in iron alloys too. A plastic deformation of the austenite below the recristallisation temperature prevents the γ → ϵ-transformation.     

Synthesis and X-ray Crystal Structure Analysis of CU2(CH3CH2COO)4(OP)2

The complex of [CU(CH₃CHCOO)₂(OPph)]₂ has been synthesized and its X-ray crystal structure determined at room temperature, M = 975.96, tetragonal, space group: 14_1/a(#88), Dx = 1.39 g/ cm³, The final R is 0.067 for 2087 independent observed reflections with 1 > 3α(I). The molecule has an inversion center on the middle of the Cu Cu axis. The bond-length of the Cu Cu is 2.61/(2) Å. The coordinate polyhedron of Cu corresponds to a tetragonal bipyramid. The angle of Cu O P is significantly smaller than that of its adducts [Cu(O₂CR)₂L]₂.     

The Nature of Dislocation Damage in Electron Irradiated Al-1% Cu and Stainless Steels

Specimen of Al-1% Cu aged to contain θ′ and stainless steels of various Ni and Cr contents have been irradiated in an HVEM. It has been found that the nature of the dislocation damage in the stainless steels is a complex function of composition and temperature; very irregular faulted loops, regular faulted loops and regular unfaulted loops are formed in varying proportions. In the Al-1% Cu, aged for varying times at 190 °C to produce θ′, the ageing times influences the amount of radiation damage in a complex manner. Virtually all the visible damage in these alloys is found either at the precipitate interface or at the electron entrance surface.     

Diamagnetism and some peculiarities of interatomic interactions in Cu2Sb

Results are reported for magnetic susceptibility measurements of high purity Cu₂Sb which characterize it as a diamagnetic. The observed anomalies of magnetic susceptibility, electrical resistivity and linear expansion coefficient in the neighbourhood of 370 K verify the existence of a phase transition of nonmagnetic origin in this temperature range. The analysis of the peculiarities of interatomic interaction in Cu₂Sb indicates the covalent character of the bonding. The value of the strength of the Cu Cu and Cu Sb bond is evaluated. It is shown that the Cu Cu bond in the Cu₂Sb compound is the most strong one.     

Hydrogen in Cu?Ti and Ni?Ti Metallic Glasses

The hydrogen solubility and its effect on the crystallization of Cu Ti and Ni Ti glasses were studied by differential scanning calorimetry, thermogravimetry, and X-ray diffraction. Dependence of the crystallization products of the hydrogenated Ti-based alloys on the hydrogen content was found. Whereas in Cu-Ti alloys hydrogenation leads to drastic decreasing in the thermal stability due to phase separation in the amorphous state and to formation of microcrystalline structure during crystallization, in Ni Ti system hydrogen produces hydrides with Ni as well with Ti, which after heat treatment decompose, and finally the same crystalline phases as in unhydrogenated alloy are formed. The isothermal crystallization kinetics of the maximum hydrogenated Cu₅₀Ti₅₀ amorphous alloy was also investigated to obtain additional information about this transformation leading to nano-crystalline material.     

Structural Studies on Ni75(SnSi)25 and Ni75(SnBi)25 Alloys

Ternary alloys on the quasi-binary system Ni₃Sn(r) in equilibrium condition. It has been observed that these phases undergo transformations at high temperatures but upto 500 °C room temperature modifications are stable. The two phases do not have any appreciable solid solubility in either of them. The phase Ni₃Si(r) crystallizes in to Cu₃Au (Li₂) structure where as Ni₃Sn(r) is based on c.p.h. structure with a = 5.305 A°, c = 4.254 A°. No new ternary phase has been detected in Ni₃Sn Ni₃Si section. The investigated alloys of the Ni Sn Bi system contain 75 at.% Ni. All the ternary alloys show simultaneous occurrence of three phases, namely Ni₃Sn(r), NiBi and Ni(Sn) in equilibrium state. The phase NiBi has NiAs(B8) type of structure. Due to non-existence of isostructural phases in the two binary systems (Ni Sn and Ni Bi), single solid solution phases are not formed. Widely differing atomic sizes of nickel and bismuth atoms restrict the formation of solid solution of bismuth in nickel in contrast to Ni(Sn) where atomic size factor is favourable.     

Melting and crystallization of silicon layers on insulator with millisecond lamp heating

Melting and crystallization of silicon layers in a SOI structure (Si SiO₂ Si) at millisecond lamp heating have been studied by model calculations using the solution of conduction equation. Pulse heating conditions that do not lead to silicon substrate melting under SiO₂ have been determined. For pulses of 1 and 4.4 ms duration the silicon melt lifetime on the SiO₂ surface has been estimated. The lengths of the crystal oriented growth from windows in the SiO₂ layer that open the single-crystalline silicon substrate have been measured (25 and 64 μm).     

Positron annihilation in Co alloys containing FCC and HCP phases

Positron annihilation is studied in Co Cu and Co Mn alloys subjected to the heat treatments which result in the structural states with various lattices, phase contents and defects concentrations. The positrons are shown to be insensitive to the hcp layers in fcc lattice which form the stacking faults. Multiple transitions through the interval of martensitic transformation initiate the formation of defects of the higher concentration the larger of the volume effect of transformation.     

Thermal expansion in CuAlTe2 compound

The lattice parameters a and c as well as the axial thermal expansion coefficients α ⟂ and α ∥ in the CuAlTe₂ chalcopyrite-type compound are determined as a function of temperature in the range from 80 to 650 K by a X-ray diffractometry technique. The data obtained are used to evaluate the axial ratio c/a, the tetragonal distortion δ = 2 — c/a, the interatomic distances for Cu Te and Al Te bonds and their temperature coefficients. It is found that the thermal expansion behaviour of CuAlTe₂ is similar to that of other CuB^IIIC^VI₂ compounds in having a relatively small expansivity along the c-axis and a large one in the perpendicular direction. When comparing the results for a series of the CuB^IIIC^VI₂ compounds (B Al, Ga, In; C S, Se, Te) it is shown that the correlations between the thermal expansion coefficients α ⟂, α ∥, α_m, dδ/dT and the tetragonal distortion δ, as well as the molar mass of the compound take place.     

Infrared reflection spectra of selected modifications of SiO2 and Al2O3

Infrared reflection spectra were measured within the wave number range of 1400 – −400 cm⁻¹ for following SiO₂ and Al₂O₃ modifications: quartz, opal, quartz glass, cristobalite, tridymite, corundum, sapphire, ruby, gibbsite, boehmite, and diaspore. The spectral bands were attributed, according to literature quotations, to individual vibrations of bonds Si O, O Si O, Si O Si, Al O, O Al O, and Al O Al. The evaluation of infrared reflection spectra has shown that they are very sensitive to internal vibrations of Si O, Al O, which are affected by the symmetry of the surrounding crystal field. They are therefore selective for individual minerals and thus suitable for quick identifying purposes.     

Reflectivity measurements of thin SiO2 layers in the soft-X-ray region

Energy and angular dependent reflectivity measurements of differently prepared SiO₂ layers were made in the soft X-ray region in order to study differences in their optical and electronic behaviour caused by the technological process. The optical constants of the layers were determined by fitting the angular dependent reflectivity data with a multilayer model which takes into account the roughness of all interfaces. Detailed knowledge of the polarization level of the primary beam from the synchrotron radiation source is needed for reliable results. The energy dependent reflectivity measurements indicate that high-pressure dry-oxidized layers have a more perfect structure than normal dry-oxidized layers. Independent on the deposition process the Si SiO₂ interface roughness amounts to ± 1 nm. The Si_2p-XANES of Si and SiO₂ were detected.     

In-situ Straining Experiments on Glass Foils in the HVEM

Crack propagation in Na₂O CaO SiO₂ glass foils with thicknesses between 0.4 μm and 4 μm was studied by HVEM in-situ straining experiments. The propagation of cracks could be achieved by a stepwise loading of the glass foils in a quantitative HVEM tensile tilting stage. The contrasts observed in front of the crack tips indicate a plastic deformation of the glass foils. The influence of the imaging electrons on the mechanical properties of the foil material is discussed.     

RHEED investigations of unusual crystalline states in amorphous SiO2 films

Microcrystalline regions of silicon dioxide with structures unobserved in the macro-state are studied by Reflection High Energy Electron Diffraction (RHEED). Their formation can be understood by considering the existence of several possible relatively stable crystalline states of SiO₂, with free energies higher than those of the known silicon oxides. SiO₂ with structures and lattices identical to the structures and the lattices of the CaF₂-, of the rutile-type GeO₂, and of α-PbO₂ have been identified. Considerations for the state of silicon in some of these structures are also included.     

Positron annihilation study of defects in martensite transformation of Fe?Mn alloy

It has been observed that a small number of repetitions of the transformation by thermal cycling enhanced the formation of ϵ-Martensite whereas a large number of repetitions of transformation decreased the amount of ϵ-Martensite formed. PAT analysis and electron microscope show that a few repetitions of the transformation produced less defects, including vacancies and dislocations, whereas a large number of repetitions of transformation produced progressively more defects. The PAT analysis also shows that these accumulated defetcs are mainly dislocations, which form a network of dislocations or a Lomer-Cottrell dislocation net. A small number of dislocations enhanced the nucleation of ϵ-Martensite whereas a large number of dislocations prevented the nucleation. So the dislocations-produced by cycling transformation thermally — has a great effect on the ϵ-formation.     

Glass formation and properties of chalcogenide systems (XXII) the phase diagram of the system PbSe?GeSe2 and on the compound Pb2GeSe4

The phase diagram of the system PbSe GeSe₂ is studied by X-ray analysis and DTA. Pb₂GeSe₄ is the only compound which is formed in the system. It melts incongruently at 590 ± 4°C. The formation of Pb₂GeSe₄ as a definite compound is supported by studying the phase relations in the corresponding area of the ternary system PbSe GeSe GeSe₂.     

Formation and structural characteristics of Pd–Ag/SiO2 and Pd–Cu/SiO2 catalysts synthesized by cogelation

Pd–Ag/SiO₂ and Pd–Cu/SiO₂ xerogel catalysts have been synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of either Pd and Ag or Pd and Cu with 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS). After an extensive study of the influences of synthesis operating variables over structural characteristics of gels, highly dispersed bimetallic Pd–Ag/SiO₂ and Pd–Cu/SiO₂ xerogel catalysts were obtained. These samples are then composed of completely accessible Pd–Ag and Pd–Cu alloy crystallites with sizes of 2–3.5 nm located inside silica particles exhibiting a monodisperse microporous distribution. It appears also that the metal complex acts as a nucleation agent in the formation of silica particles.