Cluster Structure Model For BCC Substitutional Solid Solutions And The Application For Alloy Composition Design

Common industrial alloys are usually based on solid solutions, partly or completely. However, existing theories and methods for alloy composition design are not available due to the lack of suitable structural model for solid solution alloys. It is necessary to further investigate the structure feather of solid solutions and to explore a new way for alloy composition design. In this thesis, the cluster-plus-glue-atom model for BCC substitutional solid solutions will be developed from the viewpoint of chemical short range orders. A relevant method for alloys composition design is then proposed and applied in composition interpretation and optimization of industrial alloys.The cluster-plus-glue-atom model for BCC substitutional solid solutions is first established in fine details. In this model, local structure is described by a basic structural unit consisting of a CN14 rhombic-dodecahedral cluster and some glue atoms situated outside the clusters. The cluster part represents strong interactions between solvent and solute atoms, and glue atoms represent relative weak interactions. The basic structure unit can be described as cluster formulas [(center atom)-(shell atoms)14](glue atoms)1-8. Especially, for the cluster formula with x=1, the cluster packing reaches the maximum density and the glue atoms perfectly match the interstitial sites between the clusters. Such a local structural unit should possess special structural stability.Next, the cluster model is applied in the composition interpretations of BCC industrial alloy specifications in Ti-, Nb-, Ta-, Mo-, W-, and U-based systems. It is pointed out that all the industrial alloys conform to the cluster formulas [(center atom)-(shell atoms) 14](glue atoms)x, and in particular to that with x= 1, thus unveiling universal composition rules for BCC metals in support of the model prediction. The general procedures of alloy composition interpretation and design are consequently defined, which provide an effective and practical way for the development of new alloys.Finally, the cluster formula method is applied in the composition optimization for biomedical Zr-based alloys, irradiation resistance Zr-based nuclear alloys, and high temperature corrosion resistance V-based alloys.Zr-based alloys with low Young's modulus and low magnetic susceptibility are considered as new implant materials. The key issue for their alloy design is to coordinate the BCC structural stability and with the addition of BCC destabilizing elements. The produced alloys should possess a relatively stable BCC structure for the desired low Young's modulus; at the same time, minor secondary precipitates ?" and ? in the BCC matrix is also desirable for high strength and low magnetic susceptibility. According to cluster-based structure model, it can be deducted that the universal cluster formula should be [(Mo,Sn)-(Zr,Ti)14]Nb, with glue atom x=1. Alloying elements are all properly configured within the framework of this cluster formula. The new alloys thus designed [(Mo0.5Sn0.5)-Zr14]Nb (Zr87.5Nb6.25Mo3.13Sn3.13 at.%, Zr86.45Nb6.29Mo3.25Sn4.02 wt.%) and [(Mo0.5Sn0.5)-(Zr13Ti1)]Nb (Zr81.25Nb6.25Ti6.25Mo3.13Sn3.13 at.%, Zr82.7Nb6.48Ti3.34Mo3.35Sn4.14 wt.%) possess even lower magnetic susceptibilities than that of pure Zr metal. The alloys also possess low Young's modulus of E= 77?79 GPa and high hardness with HV= 288?311 kgf·mm-2, which approach those of Zr-6Nb alloy.Zirconium alloys are widely used as cladding materials owing to the good irradiation stability. To meet the increasing burn-up of nuclear reactors, their corrosion resistance should be further improved. In light of the basic cluster formula [M-Zr14]M with x=1,a super cluster formula is proposed{[M-Zr14]M-([Zr-Zr14]Zr)12}([M-Zr14]M)3, which is the composition formula for dilute BCC solid solutions. According to this super cluster formula, the maximum content of solute solubility is 8/256= 3.13 at.%. Composition design of new Zr-based alloys is then practiced. The alloy structure is characterized by an a-Zr matrix with secondary C14 Laves phases Zr(Nb,Fe,Cr)2 precipitates that are distributed uniformly inside grains and on grain boundaries. The corrosion resistance is enhanced obviously and the strength demand is also satisfied. The effects of new elements Cu and V in corrosion behavior are also discussed. The weight gain in autoclave of new Zr-Sn-Nb-Fe-Cr alloys Zr-l/256Nb (No.1) and Zr-1.5/256Nb (No.2) is only 70% of the presently used N36 alloy. The alloys possess tensile strength with ?UTS=458?461 MPa, which approach those of N36 alloy.V-based alloys with BCC structure, such as V-4Ti-4Cr, are candidate materials in nuclear fusion field for their corrosion resistance at high temperature. The cluster formula for this kind alloys is proposed as [M-V14]M, where M=V1/3Cri/3Ti1/3 is the equal-molar mixing of the three constituent elements. Composition optimizations are further realized based on cluster formulas via adding elements Ta, Zr and via adjusting their respective proportions. The alloys should possess single BCC structure without second precipitates and satisfy low irradiation activation demand. The strength and corrosion resistance of the alloys are simultaneously improved. The corrosion rate in KC1 solution of the new alloy with M=V1/6Ta1/6Cr1/3Ti1/3 (V89.58Ta2.08Cr4.17Ti4.i7 at.%, V85.2Ta7.04Cr4.04T13.72 wt.%) is 30% of V-4Ti-4Cr. The alloys also possess a fairly high hardness of HV= 176 kgf·mm-2, which is 15% more than that of the reference alloy.