Chemical analysis of alloy phases

Before we discuss some field ion microscope and atom-probe studies of partitioning in alloys, let us touch briefly here upon some of the methods commonly used for analyzing atom-probe data. Obviously if a precipitate or an alloy phase can be distinguished from the matrix from the field ion image, the size can easily be found. For the composition the probe-hole may be aimed at the precipitate, or the alloy phase, and analyze the composition of this volume. If the precipitate is very small and the probe-hole is large, matrix atoms will be detected concurrently regardless of where the probe-hole is aimed. Another problem is aiming 

The method most commonly used is an autocorrelation analysis.46 A set of data collected are divided into n subsets of the same size. The correlation coefficient R(k) is calculated according to 

In compositional analysis of very small precipitates, or in interface segregation studies, using a probe-hole type atom-probe, one is always faced with the fact that the probe-hole may cover both the matrix and the precipitate phases, or the interface as well as the matrix. Thus any abrupt compositional changes will be smeared out by the size of the probe-hole and also by the effect of ion trajectories. A similar uncertainty seems to exist in the compositional analysis of nitride platelets formed in nitrided Fe-3 at.% Mo alloy, aged between 450 and 600°C, where Wagner  

Many other atom-probe analyses of different phases in different types of steels exist as steels are one of the most important materials. It is possible to investigate how the magnetic properties of alloys are correlated to the microstructures of different phases in the alloys.57,58,59 The chemical contents, growth process and structures of metallic carbides in different alloy steels have been studied with the field ion microscope and the atom-probe field ion microscope.60 61 62 63 We refer the reader to some of the original papers published on these subjects. 

GP[2] zones, or two Cu 001 layers separated by a few aluminum layers, have also been observed in the field ion microscope in an aged Al-4 wt% Cu alloy.71 A GP[2] zone formed on the (200) plane and observed on the (022) surface of the [001] oriented tip can be observed as two rows of bright image spots if an odd number of the (200) atomic layers are present between the Cu layers. The imaging condition at the (022) surface is more complicated if an even number of the (022) layers are present 

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In previous sections we have shown how the structures of elemental materials may be rationalized on the basis of microscopic considerations. Our idea was to build up the free energy of a discrete set of competitors and then to make a free energy comparison between these competitors as a function of temperature and perhaps pressure. We next turn to the analysis of alloy phase diagrams. Here we will have to expand the scope of our previous analysis in order to account for the fact that the presence of more than one chemical constituent will at the very least alter the configurational and vibrational entropy. We define alloy in the present context to include any system in which there is more than one... 

Wil] Chemical analysis of carbon content and calculation of carbon activity in (yFe) phase. Pure iron-copper strip and metastable iron carbide as a carbon source placed in the sealed tubes were heated in furnace from 24 h at 1050°C to 7 d at 850°C, then the tubes were quenched to room temperature. Fe-alloys with 0.72, 1.36, 3.13, 5.61, 6.74 mass% Cu and 0.2 to 1.5 mass% C. Measurements at 850, 925 and 1050°C. 

Samples were tested on in a melt of salts (75% Na SO, 25% NaCl) at 950°C in an air atmosphere for 24 hours. Micro X-rays spectrum by the analysis found that the chemical composition of carbides of an alloy of the ZMI-3C and test alloys differs noticeably. In the monocarbide of phase composition of an alloy of the ZMI-3C there increased concentration of titanium and tungsten is observed in comparison with test alloys containing chemical composition tantalum. The concentration of more than 2% of tantalum in test alloys has allowed mostly to deduce tungsten from a mono carbide phase (MC) into solid solution. Thus resistance of test alloys LCD has been increased essentially, as carbide phase is mostly sensitive aggressive environments influence. The critical value of total molybdenum and tungsten concentration in MC should not exceed 15%. 

X-ray scattering studies at a renewed pc-Ag/electrolyte interface366,823 provide evidence for assuming that fast relaxation and diffu-sional processes are probable at a renewed Sn + Pb alloy surface. Investigations by secondary-ion mass spectroscopy (SIMS) of the Pb concentration profile in a thin Sn + Pb alloy surface layer show that the concentration penetration depth in the solid phase is on the order of 0.2 pm, which leads to an estimate of a surface diffusion coefficient for Pb atoms in the Sn + Pb alloy surface layer on the order of 10"13 to lCT12 cm2 s i 820 ( p,emicai analysis by electron spectroscopy for chemical analysis (ESCA) and Auger ofjust-renewed Sn + Pb alloy surfaces in a vacuum confirms that enrichment with Pb of the surface layer is probable.810... 

Chapter 10 provides an exhaustive description of how these techniques can be applied to a large number of industrial alloys and other materials. This includes a discussion of solution and substance databases and step-by-step examples of multi-component calculations. Validation of calculated equilibria in multi-component alloys is given by a detailed comparison with experimental results for a variety of steels, titanium- and nickel-base alloys. Further selected examples include the formation of deleterious phases, complex precipitation sequences, sensitivity factor analysis, intermetallic alloys, alloy design, slag, slag-metal and other complex chemical equilibria and nuclear applications. 

The ultimate analysis of a mixture of organic compounds, however important it may be, is not decisive for its structural identification likewise, knowledge of the elementary composition of an alloy is not sufficient to describe its physical and mechanical behaviour, because the nature and phase composition of the alloy are left undetermined. The properties of silicates, e.g. glass, may be greatly influenced by thermal treatment while the chemical composition of the products remains unaltered. 

The Rietveld plots of both powder diffraction data sets are shown in Figure 7.10 and Figure 7.11. Visual analysis of both figures indicates a good fit, which was expected from the low residuals Table 7.7). The model of the crystal structure Table 7.8) appears to be complete and makes both physical (reasonable atomic displacement parameters) and chemical sense [no overlapping atoms, the 3(g) sites are occupied simultaneously by atoms that have close atomic volumes (Ni and Sn), the chemical composition of the major phase established from x-ray data is nearly identical to the known composition of the alloy]. Therefore, the outcome of this crystal structure determination may be accepted as satisfactory. 

G.19 A. Taylor. X-Ray Metallography (New York Wiley, 1961). X-ray diffraction, radiography, and microradiography. Structures of metals and alloys determination of phase diagrams, texture, grain size, and residual stress chemical analysis and studies of ceramics. 

The peculiar temperature and composition dependence of the valence of chemically collapsed phases of SmS was first observed in the system Sm1 xGdxS (55). Recently a phase diagram in the (x, T)-plane has been proposed for this system (41) as well as for CeTh alloys (44). When cooled below about 200 K the chemically collapsed phases of SmS show a dramatic lattice expansion, in some cases with explosive character (55) and disintegration of the crystal into a black powder. An example of this transition towards a more divalent state of Sm on cooling in SmAs 18 S 82 and Sm 81 Y19S is shown in Fig. 22 and 23, where we show the data of Poliak et al (63). The temperature dependent configurational mixing of Sm ions which is directly visualized in the XPS data is the source of the anomalous temperature dependence of the lattice constant. No detailed analysis of the dependence of the Sm 4/lineshape on temperature... 

The structural properties of the metal systems described above naturally challenge consideration from a purely chemical point of view, for occasionally the results of structure analysis cannot readily be reconciled with accepted chemical principles and demand a re-orientation of the chemical picture of the metallic state. It has often been the practice of the metallurgist to seek to represent the several phases in an alloy system by simple chemical formulae corresponding to idealized stoichiometric compositions, departures from which were interpreted as solid solution in the ideal phase of excess of one or other of the components. Such formulae inevitably tend to convey the impression, implicitly if not explicitly, that these phases are to be regarded as definite chemical compounds, and it is necessary to consider carefully to what extent such a viewpoint is justifiable, and even whether there are valid grounds for attributing to the phases any formulae at all. 

The distribution of impurities between the main phases of the alloys was studied, since it affects the susceptibility of material to hot cracking during welding and the mechanical properties of welded joints. Metallographic analysis and chemical analysis were carried out with the help of JSM-U3 electron microscope, which had an attachment for energy-dispersive x-ray analysis. For this investigation, test samples were taken from the argon arc welds on 5-mm-thick Al-6Cu-Mn plates. The alloys contained various amounts of Si and Fe the concentration differences of... 

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