Volume 88, Issue 2

Field-Ion Microscopy (FIM), equipped with a time of flight detection facility (Atom Probe APFIM) has been established as a sensitive analytical method for the investigation of the local chemical compositions on nanometer scales. Because of its high in-depth resolution of a few tenths of a nanometer, the magnification of some 107, and the equal detection sensitivity for all elements, this method has helped to solve key problems in surface science, materials research and materials engineering. The newly developed Tomographic Atom Probe (TAP) is equipped with a position sensitive detection system and is able to provide spatially resolved information in real space on the chemical microstructure of materials and compounds with atomic resolution. The sampled volume is about two orders of magnitude larger than that usually analysed with the APFIM. The extended set of data collected by the TAP is stored and may be treated with special visualising tools. Three dimensional images can be created, which include all the information on microstructure and chemical composition of the sampled volume. The paper demonstrates the capability of the TAP by presenting results from two investigations regarding phase separation and characterization of layer structures, respectively. For the first category TAP images of the time evolution of the decomposition of the Cu 44.7 Ni 47.3 Fe 8 alloy during thermal ageing at 773 K are discussed. For the second one, results on the structure of the interfaces of sputter deposited Ni/Zr multilayers are treated. Such layers appear to be amorphous already after room temperature preparation. The thermal stability of Cu/Pd interfaces in sputter deposited multilayers is discussed with regard to the determination of interdiffusion coefficients from these data.

The structure and properties of iron-based Cr15Mo l martensite with about 0.3 wt.% C and N each are studied after tempering at 200 to 650 ° C by means of hardness measurements, dilatometry, transmission electron microscopy (TEM) and Mössbauer spectroscopy. The results are compared with the tempering behaviour of Cr15Mo l (0.6% C) and Cr15Mo l (0.6% N) martensites. After quenching from 1100 ° C the C + N alloy contains a high fraction of retained austenite (RA), a mixture of twin and dislocation martensites and undissolved coarse cementite particles. Tempering at 200 ° C causes precipitation of ε-carbide and ε-nitride with hexagonal close packed (HCP) lattices. A delay of the following carbide/nitride transformations occurs as compared to the pure nitrogen and carbon martensites. At 500 ° C ε-carbide transforms into cementite, while cubic CrN nitride is formed from ε-nitride. An increase of temperature up to 650 ° C is accompanied by CrN → Cr 2 N transformation, while the cementite particles coarsen. Chromium carbonitrides are not observed. An enrichment of cementite in chromium is observed after tempering at 650 ° C while the chromium content in the coarse cementite globules retained after high temperature solution treatment is nearly equal to that in the matrix. An evidence for nitrogen-assisted short range atomic ordering of the FeCrN and, in particular, FeCr(C + N) solid solutions is derived from Mössbauer spectra, which means a more homogeneous distribution of Cr atoms in the nitrogen containing martensites as compared to that in the FeCrC martensite. This peculiarity of chromium distribution is supposed to be a reason for a delayed precipitation and small size of the particles in tempered FeCrN and FeCr(C + N) martensites. RA is stable during tempering up to 600 ° C. A significant splitting of dislocations obviously caused by segregation of the interstitials (C, N) is observed in the RA tempered at 500 to 600 ° C.

A melt-spinning technique was used to produce TE–TL alloy ribbons with TE = Zr and Hf and TL = Fe, Co or Ni for compositions around the TL-rich eutectic (about 90 at.% TL content). In this part of a series of papers on these alloys, experimental results on the low-temperature magnetic properties of the Zr–Fe and Zr–Ni ribbons will be given. The ac susceptibility ( χ ac ) data on the amorphous Zr–Fe alloys indicated the onset of magnetic ordering at T c ≈ 200 K and the occurrence of a second magnetic transition at T f ≈ 70 K. It was also found that hydrogenation changed both the Curie temperature T c and the temperature T f in the same sense as an increase of the Zr content. Furthermore, it could be established from χ ac measurements that there is a considerable spatial redistribution of hydrogen within these samples during a period of up to two weeks after hydrogenation by electrolysis. The Curie points of the amorphous and bcc phase of the Zr 9 Ni 91 alloy were practically the same and much smaller than the previous reported values of amorphous alloys of similar compositions. The critical concentration for the onset of magnetic ordering in the Zr–Ni system was found to be around 89 at.% Ni.

An elemental powder composite Ti-48 at.% Al was prepared by cold extrusion. The kinetics of annealing-induced phase formation at 800 °C was studied using quantitative X-ray diffraction, and the results were analyzed in terms of a one-dimensional multiphase diffusion model. During heating to 800 °C, a reaction takes place which results in a multiphase mixture containing Ti 3 Al, TiAl, Al 2 Ti and Al 3 Ti. Subsequent annealing at 800 °C leads to the growth of the phases TiAl and Al 2 Ti at the expense of Ti 3 Al and Al 3 Ti. In the competitive growth stage, the diffusion through Al 2 Ti is 1.4 times quicker than through TiAl. After consumption of Al 3 Ti, the growth of TiAl at the expense of Al 2 Ti and Ti 3 Al can be described by a classical t -law where t is the annealing time. Due to the statistical nature of the original microstructure, the final stages of TiAl growth are slowed down according to Johnson-Mehl-Avrami growth kinetics with exponent n = 0.61.

The microstructure of hydroxyapatite (HAP) ceramic coatings deposited by plasma spraying on TiNi shape memory alloy substrates was investigated. With respect to metal/ ceramic interface strength, excellent coatings of HAP on TiNi substrates were prepared. From the point of view of adherence, these coatings possess higher strength than Epoxy 1200 adhesive (≈ 30 MPa). The observed excellent metal/ceramic interface strength is attributed to the formation of chemical bonding according to the reaction Ca 10 (PO 4 ) 6 (OH) 2 + 2TiO 2 → 3Ca 3 (PO 4 ) 2 + CaTi 2 O 5 + H 2 O and the energy dissipation due to stress-induced martensite formation (SIM) and/or martensite reorientation (RE) during stressing the composite and failure. The applied power parameters of the plasma spraying influence the porosity and fusion between particles of the ceramic coatings.

Tube spinning combined with thermomechanical processing is developed to fabricate thin-walled 2014 aluminum long tubes with high precision tolerance and improved mechanical properties via proper control of the spinning, heat treatment conditions, and material properties as verified by mechanical, stress corrosion, and hydraulic tests. Based on the microstructure investigations using optical, scanning, and transmission electron microscopy, the thermomechanically spun tubes contain distorted grains, strain lines, sub-grains formed by tangled dislocations, and finely dispersed CuAl 2 (θ ″ or θ ′ ) precipitates uniformly distributed in the deformed matrix. The strength, ductility improvement and corrosion resistance accruing from the combination of cold working and precipitation heat treating is a result of a significant dislocation network, refined grains, and uniformly dispersed fine precipitates due to the increased plastic strain.

Large crystals of π-AlMgSiFe have been grown in a molten aluminium alloy of Al-17.6 wt.% Si-7.6 wt.% Mg-1.0 wt.% Fe on the basis of information obtained through thermodynamic calculations, differential thermal analysis and micro-structure investigations. The π-crystals have been separated from the metal matrix by dissolving the alloy in butanol. X-ray diffraction and microprobe analysis of the phase reveal that it is hexagonal with lattice parameters a = 0.6623 nm and c = 0.7928 nm and has the composition 49.1 at.% Al-27.7 at.% Si-17.6 at.% Mg-5.6 at.% Fe. Differential thermal analysis of the above alloy shows that the π-AlMgSiFe is formed during cooling at 641 °C with a heat of formation of about 940 kJ/kg. Then α-Si starts forming at 612 °C and Mg 2 Si at 576 °C. Finally, the metal is solidified as eutectics in two reactions at 558 °C and 549.5 °C. The combined heat of fusion of the two eutectics is ca. 525 kJ/kg, and the phases formed are α-Al, α-Si, Mg 2 Si and π-AlMg-SiFe.

Für zwei Vergütungsstähle wurden dilatometrische sowie umformdilatometrische Untersuchungen dürchgeführt. Ziel der Versuche war die Untersuchung des Einflusses der Austenitisierungs-, Umform- und Abkühlbedingungen auf das Umwandlungsverhalten. Besonderes Augenmerk wurde auf die Besonderheiten der Gefügebildungsprozesse während der Umformung im Zweiphasengebiet gelenkt.

A thermodynamic evaluation of the Ti–Al–O ternary system has been made by using thermodynamic models for the Gibbs energy of individual phases. A consistent model parameter set was determined so that the calculation of isothermal sections and other thermodynamic quantities becomes possible. The agreement between calculation and corresponding experimental data was generally good along large temperature and composition range.

The heats of decomposition of γ-AgMg 4 and ɛ ′ -Ag 17 Mg 54 have been measured as 1800±200 Jmol -1 for the peritectoid reaction γ→(Mg) + ɛ ′ at 469 ° C and 19±1 kJmol –1 for the peritectic reaction ɛ ′ → β ′ + Liq at 491 ° C. A thermodynamic assessment of phase data for the full range of the Ag–Mg binary alloy system reproduces the features of the phase diagram except for details of the ordered α ′ and β ′ phase fields. The single-parameter order –disorder model, chosen for its compatibility with models for higher-order systems, is insufficiently flexible to reproduce the variation in order in these phases. The sequence of four reactions observed on heating γ-AgMg 4 indicates that the liquid composition in the peritectic decomposition of ɛ ′ -Ag 17 Mg 54 is approximately 81 at.% Mg.

Nanosize amorphous Fe – Ni –P–B powders have been first prepared successfully by reducing iron and nickel salts with NaH 2 PO 2 and KBH 4 in aqueous solution. X-ray diffraction patterns showed that the powders were amorphous. Electron microscopy showed that the powders were nearly spherical with diameters in the range of 20 to 200 nm. The thermal properties of the amorphous powders were studied by differential scanning calorimetry (DSC). It was found that exothermic processes were different for powders with different concentrations. The first peak crystallization temperatures and crystallization heats were 622.8 to 708.5 K and 3.83 to 9.42 kJ/mol for the powders studied, respectively.

The phase equilibria in the Sm–Fe –Ti system at 600 ° C were investigated by using diffusion triple technique and electron microprobe analysis (EMPA). A tentative isothermal section of this system at 600 ° C was constructed on the basis of the present experimental results. The isothermal section consists of 10 single-phase regions, 17 two-phase regions, and 9 three-phase regions.