Alloys, interstitial, 147 --- physical properties

Although the distinction is not always clear, ternary nitrides (and nitrides in general) often are classified into two groups (1) intermetallic-type and (2) ionic/covalent-type. Intermetallic nitrides are those in which metal-metal (M-M) interactions are dominant and where the nitrogen atoms are interstitial within the metal array.3 Because these phases are stabilized by M-M interactions, the structure and physical properties are similar to those of many other metallic systems, such as alloys, metals, and... 

Physical Properties. An overview of the metallurgy (qv) and solid-state physics of the rare earths is available (6). The rare earths form alloys with most metals. They can be present interstitially, in solid solutions, or as intermetallic compounds in a second phase. Alloying with other elements can make the rare earths either pyrophoric or corrosion resistant. It is extremely important, when determining physical constants, that the materials are very pure and well characterized. All impurity levels in the sample should be known. Some properties of the lanthanides are listed in Table 3. 

Solubility is one of the important properties of an alloy. Interstitial impurities (for example, hydrogen, oxygen, carbon, boron, and other chemical elements) in metals and alloys change considerably their physical properties [1-18],... 

Thus far, we have focused primarily on Fe-C alloys, with carbon atoms positioned within vacant interstitial sites within the iron lattice. As you may expect, a variety of other elements may also be present in steel that will alter its overall physical properties. For example, all steels contain manganese that assists in hardening mechanisms, as well as removing sulfur and oxygen atoms from the matrix. This prevents FeS formation and removes bubbles in the molten state of steels, both of which would greatly contribute to brittleness of the final product. 

Substitutional solid solutions can have any composition within the range of miscibility of the metals concerned, and there is random arrangement of the atoms over the sites of the structure of the solvent metal. At particular ratios of the numbers of atoms superstructures may be formed, and an alloy with either of the two extreme structures, the ordered and disordered, but with the same composition in each case, can possess markedly different physical properties. Composition therefore does not completely specify such an alloy. Interstitial solid solutions also have compositions variable within certain ranges. The upper limit to the number of interstitial atoms is set by the number of holes of suitable size, but this limit is not necessarily reached, as we shall see later. When a symmetrical arrangement is possible for a particular ratio of interstitial to parent lattice atoms this is adopted. In intermediate cases the arrangement of the interstitial atoms is random. 

Interstitial alloys An interstitial alloy is formed when the small holes (interstices) in a metallic crystal are filled with smaller atoms. The best-known interstitial alloy is carbon steel. Holes in the iron crystal are filled with carbon atoms, and the physical properties of iron are changed. Iron is relatively soft and malleable. However, the presence of carbon makes the solid harder, stronger, and less ductile than pure iron. 

An alloy is a mixture of two or more materials, at least one of which is a metal. Alloys can have a microstructure consisting of solid solutions, where secondary atoms are introduced as substitutionals or interstitials (discussed further in the next chapter and Module 5, Plant Materials) in a crystal lattice. An alloy might also be a crystal with a metallic compound at each lattice point. In addition, alloys may be composed of secondary crystals imbedded in a primary polycrystalline matrix. This type of alloy is called a composite (although the term "composite" does not necessarily imply that the component materials are metals). Module 2, Properties of Metals, discusses how different elements change the physical properties of a metal. 

A review is presented of some recent applications of neutron vibrational spectroscopy to the study of disordered metal-hydrogen systems. The examples discussed cover a range of systems from simple dilute solutions in bcc or fee metals to amorphous alloy hydrides. It is shown that neutron inelastic scattering studies of the vibrational density of states provide a powerful and sensitive probe of the local potentials and bonding sites of hydrogen in metals and often reveal critical information on the novel microscopic physical properties and behavior of disordered metals-hydrogen systems, including those influenced by interstitial or substitutional defects. 

The same situation is met in R-M-N ternary nitrides in which the nature of the M element determines the dominating type of bond involved in the material. This is illustrated by the fact that with lithium (or barium) as a cationic element, the R-M-N corresponding nitride is essentially ionic in character, whereas with silicon, more covalent nitrido-silicates are formed. In addition, metallic nitrided alloys exist, with nitrogen located as an interstitial element in octahedral voids of the metal atom lattice. The presence of insertion nitrogen (as well as carbon) in such compounds is sometimes necessary for their existence, and can strongly modify the physical properties. 

Carbon forms binary carbides in which it is the more electronegative element. SiC is a hard abrasive substance with the same atomic arrangement as diamond. The salt-like metal carbides, such as Bc2C and CaC2, form methane and acetylene, respectively, on hydrolysis. Interstitial carbon atoms in metals form alloys such as steel the carbon has a profound effect on the physical properties of the metal. 

Introduction of symmetry in otherwise asymmetrical structure is also found in super lattices discovered in 1923 in AuCus aUoj and foimd later to exist in a number of alloys below a temperature known as critical temperature and they are PtCus, FeNis, MnNi3, and (MnFe) Nis alloys. Ordinarily an alloy of say A and B elements exists in solid solutions wherein the atoms of A and B are arranged randomly in the interstitials. This is the state of affairs in the alloys other than those mentioned above. In these alloys, the random structures are available at an elevated temperature, and when they are cooled down below a particular temperature called critical temperature, an ordered state happens wherein a particular set of lattice sites are occupied periodically by say A atoms and the other particular sites by B atoms. The solution is then said to be ordered and the lattice thus constituted is known by super lattice. This is a sort of disorder-order transformation and is manifested by an extra reflection in X-ray diffraction pattern. This is an important phenomena not only because of the fact that this ordered state exhibits different physical and chemical properties, but it is also an example of asymmetry to symmetry transformation. The long range order that exist in the super lattice of AuCus alloys can be explained as follows ... 

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