Ni-Fe alloys

Low Expansion Alloys. Binary Fe—Ni alloys as well as several alloys of the type Fe—Ni—X, where X = Cr or Co, are utilized for their low thermal expansion coefficients over a limited temperature range. Other elements also may be added to provide altered mechanical or physical properties. Common trade names include Invar (64%Fe—36%Ni), F.linvar (52%Fe—36%Ni—12%Cr) and super Invar (63%Fe—32%Ni—5%Co). These alloys, which have many commercial appHcations, are typically used at low (25—500°C) temperatures. Exceptions are automotive pistons and components of gas turbines. These alloys are useful to about 650°C while retaining low coefficients of thermal expansion. Alloys 903, 907, and 909, based on 42%Fe—38%Ni—13%Co and having varying amounts of niobium, titanium, and aluminum, are examples of such alloys (2). 

Experimentally it is found that the Fe-Co and Fe-Ni alloys undergo a structural transformation from the bee structure to the hep or fee structures, respectively, with increasing number of valence electrons, while the Fe-Cu alloy is unstable at most concentrations. In addition to this some of the alloy phases show a partial ordering of the constituting atoms. One may wonder if this structural behaviour can be simply understood from a filling of essentially common bands or if the alloying implies a modification of the electronic structure and as a consequence also the structural stability. In this paper we try to answer this question and reproduce the observed structural behaviour by means of accurate alloy theory and total energy calcul ions. 

To summarize we have reproduced the intricate structural properties of the Fe-Co, Fe-Ni and the Fe-Cu alloys by means of LMTO-ASA-CPA theory. We conclude that the phase diagram of especially the Fe-Ni alloys is heavily influenced by short range order effects. The general trend of a bcc-fcc phase transition at lower Fe concentrations is in accordance with simple band Ailing effects from canonical band theory. Due to this the structural stability of the Fe-Co alloys may be understood from VGA and canonical band calculations, since the common band model is appropriate below the Fermi energy for this system. However, for the Fe-Ni and the Fe-Cu system this simple picture breaks down. 

Horvath, J. and Uhlig, H., Metallurgical Factors Affecting the Critical Potential for Pitting Corrosion of Cr-Fe-Ni Alloys , J. Electrochem. Soc., 114, 201c (1%7)... 

Cid etal. studied the corrosion resistance of Ni, 5% Fe-Ni and 10% Fe-Ni alloys in the trans-passive region in sulphuric acid. For a given acid concentration the addition of iron reduced the corrosion rate. It was concluded that the addition of small percentages of Fe was doubly beneficial, decreasing both general and intergranular corrosion. 

In addition to nickel alloys, nickel also forms an important alloying element in stainless steels and in cast irons, in both of which it confers additional corrosion resistance and improved mechanical and engineering properties, and in Fe-Ni alloys for obtaining controlled physical and magnetic properties (see Chapter 3). With non-ferrous metals nickel also forms important types of alloys, especially with copper, i.e. cupro-nickels and nickel silvers these are dealt with in Section 4.2. 

Galvanic corrosion reports have emerged from two sources. In the first , the chemical compatibility of uranium carbides and Cr-Fe-Ni alloys was discussed. Evaluation was by thermodynamic modelling and experimental... 

The second stage in the carburisation process, that of carbon ingress through the protective oxide layer, is suppressed by the development of alumina or silica layers as already discussed and in some cases protective chromia scales can also form. Diffusion and solubility of carbon in the matrix has been shown by Schnaas et to be a minimum for binary Fe-Ni alloys with a nickel content of about 80<7o, and Hall has shown that increasing the nickel content for the nickel-iron-2S<7o-chromium system resulted in lower rates of carburisation (Fig. 7.54). 

Fig. 8.38 (Left) The Mossbauer spectrum of the rock called Heat Shield rock, clearly shows with high intensity the mineral Kamacite, an Fe-Ni alloy with about 6-7% Ni (Right) The iron-nickel meteorite Meridiani Planum (originally called Heat Shield Rock ) at Opportunity landing site, close to the crater Endurance. The meteorite is about 30 cm across (Courtesy NASA, JPL, Cornell University)...

In this context, it is interesting to note that it has been claimed (56) that single-crystal Fe-Ni alloy films can be prepared by deposition on heated rock salt substrates in vacua of 10-3-10-4 Torr. Other workers (57) have found that the use of UHV permits single-crystal films of Fe-Ni to be formed (at deposition rates of 14 A/min) without the annealing necessary after deposition at 1(U5 Torr. Single-crystal Au-Pd films have also been prepared (58) and after quenching from 500°C gave an electron dif-... 

Figure 4.12 Secondary neutral and ion mass spectra of a 1 1 Fe-Ni alloy in the mass regions of monomers lop) and dimers (bottom). The dimer distribution indicates that iron and nickel are atomically mixed, as expected in an Fe-Ni alloy. Note the higher sensitivity of SIMS for iron and the manganese impurity (from ter Veen [36]).

Quantitation is usually achieved by comparing the X-ray yields from the sample with yields obtained from standards. The ease with which measurements can be interpreted quantitatively depends on the sample. As illustrated in Fig. 7.7, the volume that is activated by the 10-100 keV electron beam has the shape of a pear with typical dimensions of a few pm. As a consequence, X-rays formed in the interior may be absorbed on their way out, and may stimulate the emission of photoelectrons, Auger electrons and again X-rays. The latter process, secondary fluorescence, can lead to an overestimate of the concentrations. For example, if the specimen is a bulk Fe-Ni alloy, Ni Ka radiation is adsorbed by iron and causes... 

Morris et al. 2006). The data concerning the ternary system Al-Fe-Ti have been reviewed and discussed by Palm and Lacaze (2006) the assessments of the limiting binary systems (especially of Ti-Al and Fe-Al) have also been reported and commented. The Fe-Ni and Ti-Fe systems have been examined and discussed in papers dedicated to the assessment of Ti-Fe-Ni alloys (Cacciamani et al. 2006, Riani et al. 2006). 

Menstruum process (fluxprocess) The menstruum process was described for the preparation of carbides which are distinguished by good wettability and a very low content of impurities (such as N, O). As a most suitable auxiliary metal bath for the reaction between transition metals and carbon, a Fe-Ni alloy was suggested. The amount of this alloy (70 Fe-30 Ni mass%) was about four times the volume of the transition metal (Ti, Zr, V). After the reaction the product was crushed and put in a warm HC1 solution to dissolve the menstruum alloy. 

Purpose of this work was to study the structure, phase composition and mechanical properties of surface diffusion layers formed in the preliminary cold deformed a-Fe and Fe-Ni alloys after nitriding. 

The nitrided layers formed in Fe-Ni alloys some differ from those in a-Fe. These differences consist in the presence of additional layer of 4 -phase on the surface (fig. 2). Due to < -phase high hardness and brittleness this layer can be destroyed during mechanical treatment. 

The preliminary plastic deformation considerably effects on the phase formation, structure, microhardness and thickness of nitrided layers in -Fe and Fe-Ni alloys. The high microhardness of the diffusion layers results from the formation of the s- and y- nitrides. Iron doping with Ni leads to changing of the s-, f-phases composition. The existence of narrow intervals of deformations of 3-8 % and 20-30 %, in which the considerable (about 2 times) rise of microhardness of surface nitrided layers due to accelerated formation of s- and f-phases, was found. 

Five main types of CNFs, platelet (P-CNF), tubular (T-CNF), thick herringbone (thick FI-CNF), thin herringbone (thin H-CNF) and very thin herringbone (very thin FI-CNF) vere selectively prepared and examined as supports of anode catalysts for DMFCs. P-CNF was synthesized from carbon monoxide over a pure iron catalyst at 600 °C, whereas thick H-CNF was obtained from ethylene over a copper-nickel catalyst [Cu-Ni (2 8 w/w)]. An Fe-Ni alloy (6 4 w/w) was used for the selective synthesis of T-CNFs from carbon monoxide gas at 650 °C [15, 16]. 

Buchwald, U.F. Clarke, R.S.Jr. (1989) Corrosion of Fe-Ni alloys by Cl-containing akaga-neite (P-FeOOH) The Antarctic meteorite case. Am. Min. 74 656-667 Buckland, A.D. Rochester, C.H. Topham,... 

Magnetically soft Fe-Ni alloys can have their properties altered by heat treatment. The compound NisFe undergoes an order-disorder transformation at about 500°C. Since the susceptibility of the ordered phase is only about half that of the disordered phase, a higher susceptibility is realized when the alloy is quenched from 600°C, a process that retains the high-temperature, disordered structure. Heat treatment of Fe-Ni alloys in a magnetic field further enhances their magnetic characteristics (see Figure 6.61), and the square hysteresis loop of 65 Permalloy so processed is desirable in many applications. A related alloy called Supermalloy (see Table 6.19) can have an initial susceptibility of approximately one million. 

The homogeneous nucleation of martensite in typical solids is too slow by many orders of magnitude to account for observed results. Calculations of typical values of AQc using the classical nucleation model of Section 19.1.4 (see Exercise 19.3) yield values greatly exceeding 76 kT. Furthermore, nearly all martensitic transformations commence at very sparsely distributed sites. Small-particle experiments [14] have yielded typical nucleation densities on the order of one nucleation event per 50 pm diameter Fe-Ni alloy powder particle.3 Thus, nucleation of martensite is believed to occur at a small number of especially potent heterogeneous nucleation sites. 

The crystallography of the f.c.c.— b.c.t. martensitic transformation in the Fe-Ni-C system (with 22 wt. %Ni and 0.8 wt. %C) has been described in Section 24.2. In this system, the high-temperature f.c.c. solid-solution parent phase transforms upon cooling to a b.c.t. martensite rather than a b.c.c. martensite as in the Fe-Ni system. Furthermore, this transformation is achieved only if the f.c.c. parent phase is rapidly quenched. The difference in behavior is due to the presence of the carbon in the Fe-Ni-C alloy. In the Fe-Ni alloy, the b.c.c. martensite that forms as the temperature is lowered is the equilibrium state of the system. However, in the Fe-Ni-C alloy, the equilibrium state of the system in the low-temperature range is a two-phase mixture of a b.c.c. Fe-Ni-C solid solution and a C-rich carbide phase.5 This difference in behavior is due to a much lower solubility of C in the low-temperature b.c.c. Fe-Ni-C phase than in the high-temperature f.c.c. Fe-Ni-C phase. If the high-temperature... 

The thermal conductivity of diamond at 300 K is higher than that of any other material, and its thermal expansion coefficient at 300 K is 0.8 x 10". lower than that of Invar (an Fe-Ni alloy). Diamond is a very widc-band gap semiconductor Eg = 5.5 eV), has a high breakdown voltage (I07V cm-1), and its saturation velocity of 2.7 x I01 cm s-1 is considerably greater than that of silicon, gallium arsenide, or indium phosphide. 

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