| Industrial alloys all have specific chemical compositions, which indicates that there must be particular structure units on which the compositions reply. However, so far the specified compositions have been derived only through empirical routes. In order to understand alloy compositions, properties and alloy design methods, necessarily the basic solid solution structures should be examined. In the present paper, the structure model and relevant structural units of substitutional-type FCC solid solutions are established for the first time, which are derived from the short-range ordered structures via the "cluster plus glue atom" approach. Such local units satisfy ideal atomic interactions between solute and solvent atoms and can be described as [cluster] (glued atoms)1～6, where the number of glued atoms corresponds to different local configurations. At the formulated compositions the solid solutions have stable local structures. These cluster formulas quantitatively explain the specification compositions of Cu-(Zn,Ni,Au) and Fe-Ni industrial alloys, thus providing a new practical method for analysis and design of engineering alloys.This paper first reviews the structural characteristics of industrial alloys. It has been pointed out long time ago that that the properties of alloys were related to short-range ordered structures in solid solutions. Until the 1990s, short-range ordered structures were experimentally unveiled by diffraction experiments describing, confirming that the compositions of alloys with good properties are related to the special short-range ordered structures. However, the description of short-ranged structures is only limited in a few parameters that cannot reflect the whole structural feature of the solid solutions and the composition units. Therefore, the composition selection rules for the engineering alloys rest unsolved. In the previous works of our group, from the viewpoint of short-range ordering, composition rules and structural characteristics of complex alloys such as quasicrystals, amorphous and solid solution alloys have been analyzed and a new structural approach, the "cluster plus glue atom" model, has been proposed to address the short-range ordered structures. This new method provides a new route towards solving the problem of the composition selection of industrial alloys.Then, this paper describes the main innovative point, i.e., the structural modeling of FCC substitutional solid solutions. Considering the short-range ordering between solute and solvent atoms to meet the ideal chemical interaction among atoms, and in terms of the cluster plus glue atom approach, an ideal local structural unit consists of a first-nearest-neighbor cubic octahedral cluster plus a few glue atoms outside the cluster. The corresponding formula is expressed as [cluster](glue atoms) i～6.The number of glue atoms is confined by the stacking of clusters in isolated manners in order to avoid the cluster-type short-range ordering from the extension to longer range. Also at most clusters should be separated by a single layer of glue atoms so that the glue atoms do not overweight the cluster part. These restrictions restrict the range of inter-cluster vectors that lead to one to six glue atoms configurations. As a counter example, in the cluster stacking configuration with seven glue atoms, the nearest neighbor distances between clusters fall beyond the aforementioned vector range.In the end, some typical specifications of binary FCC solid solution alloys are analyzed. The short-range ordering reflects strong inter-atomic interaction within local units in accordance with the enthalpy of mixing and Cowley short-range-order parameter. Two elements with a negative mixing enthalpy tend to nearest-neighbored, forming a cuboctahedral cluster [A-B12], i.e., a solute atom A is shelled by twelve solvent atoms. The relevant cluster formula is [A-B12]A]-6. The glue atom positions can be occupied by both Al and B in case of a weak A-B interaction, and the cluster formula becomes [A-B12](A,B)1～6. The common specifications of Cu-Zn brasses, typical alloys of negative enthalpy of mixing, are interpreted by cluster formulas [Zn-Cui2]Zn1～6 and [Zn-Cu12](Cu,Zn)6, with the latter one applicable to the Zn-lean alloys. For binary alloys with positive enthalpy of mixing, the atoms of the same kind tend to be nearest-neighbored, so that the cluster is [A-A]2] and the glue sites are occupied by either six A and B or by one to six B, i.e., [A-A12]B1～6 and [A-A12](A,B)1-6, which respectively correspond to strong and weak enthalpy of mixing. The latter formula well explains the Cu-Ni alloys with weak positive mixing enthalpy, including Ni-rich Monel alloy [Ni-Ni12]Cu6 and Cu-rich cupronickel alloys [Cu-Cui2](Cu,Ni)6. Similarly, it has been confirmed that a number of other FCC single-phase solid solution alloys also satisfy the same types of cluster formulas, such as iron nickel alloys presented as [Ni-Fe12]Nix or [Fe-Ni12]Fex, gold and copper alloys as [Au-Cu12]Aux, [Cu-Au8Cu4](Au,Cu)x and [Au-Cu8Au4](Au,Cu)x, with the cluster [Cu-Au8C4] derived from the ordered phases AuCu and AuCu3. The above analyses show that the compositions of mature industrial specifications reflect the short-range ordered nature of solid solutions. It is anticipated that the clusters plus glue atom approach can be used as a practical alloy design method, thus opening up a brand new research field for further investigation. |
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