Karel Saksl

Crystallization of Cu60Ti20Zr20 metallic glass with and without pressure

Structural stability of a Cu 6 0 Ti 2 0 Zr 2 0 metallic glass under pressure up to 4.5 GPa was investigated by x-ray diffraction. The sample exhibited a supercooled liquid region of 33 K and a ratio of the glass-transition temperature to the liquidus temperature of 0.63. The glass crystallized in two-step transformation processes in the pressure range of 0-4.5 GPa; the first was a primary reaction to form a Cu 5 1 Zr 1 4 -type structure crystalline phase with a spacing group P6/m (175) and lattice parameters a = 11.235 A and c = 8.271 A, and then the residual amorphous phase crystallized into a MgZn 2 -type structure crystalline phase with a spacing group P6 3 /mmc (194) and lattice parameters a = 5.105 A and c = 8.231 A. Both crystallization temperatures increased with pressure having a slope of 19 K/GPa. The increase of the first crystallization temperature with increasing pressure in the glass can be explained by the suppression of atomic mobility. No significant structural change was detected in the Cu 6 0 T1 2 0 Zr 2 0 glass annealed in vacuum at 697 K for 1 h as compared to the as-prepared sample from x-ray diffraction measurements.

Soft x-ray free electron laser microfocus for exploring matter under extreme conditions

We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: lambda = 13.5 nm, pulse length 15 fs, pulse energy 10-40 microJ, 5 Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor controlled six-axis gimbal. Beam imprints on poly(methyl methacrylate) - PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of < or =1 microm. Observations were correlated with simulations of best focus to provide further relevant information.

X-ray laser-induced ablation of lead compounds

The recent commissioning of a X-ray free-electron laser triggered an extensive research in the area of X-ray ablation of high-Z, high-density materials. Such compounds should be used to shorten an effective attenuation length for obtaining clean ablation imprints required for the focused beam analysis. Compounds of lead (Z=82) represent the materials of first choice. In this contribution, single-shot ablation thresholds are reported for PbWO4 and PbI2 exposed to ultra-short pulses of extreme ultraviolet radiation and X-rays at FLASH and LCLS facilities, respectively. Interestingly, the threshold reaches only 0.11 mJ/cm2 at 1.55 nm in lead tungstate although a value of 0.4 J/cm2 is expected according to the wavelength dependence of an attenuation length and the threshold value determined in the XUV spectral region, i.e., 79 mJ/cm2 at a FEL wavelength of 13.5 nm. Mechanisms of ablation processes are discussed to explain this discrepancy. Lead iodide shows at 1.55 nm significantly lower ablation threshold than tungstate although an attenuation length of the radiation is in both materials quite the same. Lower thermal and radiation stability of PbI2 is responsible for this finding.

Origin of the pre-peak in Zr70Cu29Pd1 metallic glass

High-resolution x-ray diffraction experiments were performed for Zr70Cu30?xPdx (x = 0, 1, 5, 10, 20 and 30?at.%) metallic glasses together with reverse Monte Carlo. A pre-peak arising in the low-Q range of S(Q) in Zr70Cu29Pd1 metallic glass was detected whereas no pre-peak was observed in the other samples. The origin of the pre-peak is identified to be the medium-range order around Pd atoms occupying an octahedral-like cluster in the metallic glass. When a pre-peak in S(Q) exists for metallic glasses containing heavy metal elements at a low concentration (such as noble metal or rare earth elements), medium-range order, centered with these heavy atoms, might occur in metallic glasses.

Structural and Magnetic Characterization of Fe–Mn–Al–Ni Pseudo-Heusler Alloy

In this paper, the production and structural, superelastic, thermoelastic, and magnetic properties of melt-spun Fe43.5Mn34Al15Ni7.5 alloy have been investigated with the aim to employ it for contactless strain measurements. Structural properties were observed by scanning electron microscope and X-ray diffraction, which shows the existence of two phases (fcc and bcc). Magnetic properties were studied by vibrating sample magnetometer and LCR-Bridge. Superelasticity was investigated by straining a ribbon. Measured superelastic strain at room temperature was ~0.5%. Within that range we achieved 12% change of permeability that arises from the structural transition between two recognized phases. It proves that Fe43.5Mn34Al15Ni7.5 superelastic alloy is a promising potential contactless sensor of strain.

Role of heat accumulation in the multi-shot damage of silicon irradiated with femtosecond XUV pulses at a 1 Mhz repetition rate

The role played by heat accumulation in multi-shot damage of silicon was studied. Bulk silicon samples were exposed to intense XUV monochromatic radiation of a 13.5 nm wavelength in a series of 400 femtosecond pulses, repeated with a 1 MHz rate (pulse trains) at the FLASH facility in Hamburg. The observed surface morphological and structural modifications are formed as a result of sample surface melting. Modifications are threshold dependent on the mean fluence of the incident pulse train, with all threshold values in the range of approximately 36-40 mJ/cm2. Experimental data is supported by a theoretical model described by the heat diffusion equation. The threshold for reaching the melting temperature (45 mJ/cm2) and liquid state (54 mJ/cm2), estimated from this model, is in accordance with experimental values within measurement error. The model indicates a significant role of heat accumulation in surface modification processes.

Development of Advanced and Free-Machining Titanium Alloys by Micrometer-Size Particle Distribution

The chip formation process of four different titanium alloys has been studied in several cutting experiments. Alloys containing more than 50% of a-phase at room temperature and aged metastable b-alloys form segmented chips independent of the cutting conditions. Solution treated metastable b-alloys show a cutting parameter dependent change from continuous to segmented chip formation. Lanthanum has been added to all four alloys. The microstructure of these alloys consists of a titanium matrix and micrometer-size particles. The presence of grain boundary particles leads to enhanced grain stability at elevated temperatures. In addition, short chips are observed during metal cutting only in case pure metallic rare-earth metal particles are present.

Chip Formation and Machinability of Nickel-Base Superalloys

Nickel-base superalloys like Alloy 625 are widely used in power generation applications and in the oil and gas industry due to their unique properties especially at elevated temperatures. The chip formation process of Alloy 625 is not yet well understood. Therefore, the cutting process of this alloy has been studied in detail by means of orthogonal cutting experiments at conventional cutting speeds and in the high-speed cutting regime. Alloy 625 shows a cutting parameter dependent change in the chip formation process from continuous to segmented chips. Silver has been added to Alloy 625 to improve the machinability. During machining of these modified alloys short breaking chips develop so that cutting processes are eased and can be automated.

Structure and thermal behavior of lead-free solders prepared by rapid solidification of their melt

Purpose The paper aims to investigate the structure and thermal stability of newly developed lead-free Sn-based alloys which can be used as novel materials in the soldering of electronic components. Design/methodology/approach Rapid solidification was used to prepare the alloys. Findings The results showed that the microstructure of these solders exhibited uniform distribution and small-sized intermetallic compounds. Also, smaller crystalline size can be expected compared to commercially available counterparts. The analyses revealed a uniform and homogenous distribution of the small intermetallic particles of Cu6Sn5 and Ag4Sn in the microstructure of solders. The practical implications mean an improvement in mechanical properties and thermal stability of such solder joints, which is a precondition of low mechanical, thermo-mechanical stresses in their structure. Originality/value The originality lies in the production of these alloys by the melt spinning technique which was not previously used in the electronics industry.

Terbium-induced phase transitions and weak ferromagnetism in multiferroic bismuth ferrite ceramics

Partial substitution of isovalent rare-earth ions for bismuth is one of the most effective ways to develop room temperature BiFeO3-based multiferroic materials with high resistivity and strong magnetoelectric coupling. However, their structures and properties are composition and processing sensitive, with the underlying mechanisms still far from being completely understood. Here we report on the structural, thermal and magnetic properties of polycrystalline Bi1−xTbxFeO3 (0 ≤ x ≤ 0.30) dense ceramics prepared by spark plasma sintering (SPS). The X-ray diffraction study reveals that increasing terbium content induces a structural transformation from the parental rhombohedral (R3c) polar phase to an orthorhombic (Pnma) non-polar phase at x ≈ 0.20–0.25. Complementary Raman and energy-loss near-edge structure (ELNES) spectroscopy studies indicate that the transition proceeds by the progressive loss of Bi–O hybridization. Suppression of the long-range ferroelectric ordering upon Tb substitution and loss of ferroelectricity at x ≥ 0.25 was also confirmed by differential scanning calorimetry. High-sensitivity magnetic measurements show that the introduction of a small amount of Tb3+ ions at the A-sites of the perovskite structure gives rise to the occurrence of spontaneous magnetization at room temperature. The reduced degree of Fe 3d–4p orbital mixing and the weaker Fe 3d–O 2p hybridization, revealed by ELNES and X-ray near-edge absorption fine structure (NEXAFS) analyses, suggest that the substitution-induced changes in the electronic structure are responsible for the enhanced magnetization in Tb-doped BiFeO3. Among the biphasic (R3c + Pnma) compositions with ferroelectric order, the Bi0.8Tb0.2FeO3 compound shows the highest value of remanent magnetization (Mr ≈ 0.26 emu g−1), which makes this material a potential candidate for magnetoelectric applications.