| The present work addresses the composition origin of metallic glasses and solid solution alloys.These two types of alloys always exhibit special properties at specific compositions,which have been obtained through tedious and extensive practices.It is highly demanded that the structural origin behind the specific compositions should be understood,so that composition design theory can be established to effectively guide the alloy development.Based on the consideration that these alloys are characterized by short-range ordering,our group has previously utilized a new structural approach,the cluster-plus-glue-atom model,to describe the local orders.Such an endeavor also led to the proposition of composition formulas for alloys in terms of a nearest-neighbor cluster plus a few glue atoms in an empirical manner.However,this formulism still requires further improvements,as its physical basis is obscure and too many speculations are involved,such as the accurate definition of nearest neighbors,the selection of the appropriate clusters,and the quantitative determination of the glue atom part.The research work in the present thesis will be conducted focusing on the key problems mentioned above.After introducing Friedel oscillation and spherical periodicity theory,the cluster-plus-glue-atom structural model is refined and a new concept of chemical units in alloys is proposed that mimics molecules in chemical substances.It is now possible to calculate quantitatively the cluster-plus-glue-atom chemical units for any alloys.The composition origin is then unveiled in terms of short-range-order chemical units,common for both metallic glasses and solid solution alloys and the developed theoretical approach can be used for alloy composition design.Specifically,this thesis covers the following four aspects:(1)First,Friedel oscillation is introduced to accurately define the nearest-neighbor polyhedral clusters.The definition of clusters has always been difficult.Especially,when the nearest-neighbor atoms of clusters are distributed on multi-shells,the cut-off distances of clusters cannot be precisely determined.In this thesis,the cut-off distance is defined by introducing the Friedel oscillation mechanism and the atomic dense packing.The nearest neighbors of a cluster are regarded to be confined within the first effective Friedel minimum in the pair potential,with the radial distance ratio of the outermost shell to the innermost shell being rL/rS=1.5.This ratio is further correlated to atomic radius ratio,facet capping configuration,and cluster coordination number after considering hard-sphere atomic dense packing.The cluster cut-off distance is obtained by multiplying the rL/rS ratio with the measured innermost-shell distance,as are well validated in common cluster types.(2)Second,the selection rule of the proper clusters,that enter into glass-forming composition formulas,is specified.There are always multiple non-equivalent atomic sites in the alloy hases in relevant to metallic glasses.Centered by any one a cluster can be defines.Selecting the most representative one from all these clusters is solved difficult task.In this paper,two important properties of the principal clusters are emphasized,i.e.,spherical periodicity and cluster isolation,both being structural features of metallic glasses.According to these two criteria,the principal clusters are rigorously identified in devitrification phases and are used to construct cluster formulas to explain binary bulk-metallic glasses in binary Cu-Zr,Cu-Hf,Ni-Nb,Ni-Ta,Al-Ca,and Pd-Si systems.(3)A new concept of chemical unit is proposed to describe short-range ordering in solid solutions and its quantitative calculation is realized.After the key assumption that a similar short-range order is inherited between the near-melting-point liquid and its solid solution descendence,a cluster-resonance model is introduced into the structure description of solid solutions in the medium-range order.It is pointed out that the cluster formulas that ideally satisfy the atomic interactions are analogues of molecules of chemical substances,termed chemical units.This is the essential structural homologue inherited between the liquid and the solid states.Equations for calculating the chemical unit formulas are finally established,which are simply relevant to atomic density ?a and cluster radiusrl,with the number of atoms per chemical unit being calculated from Z = c · ?a.rl3.Based on this equation,chemical units in typical FCC commercial alloys are achieved,including Cu-based binary alloys,Ni-based superalloys,and Co-free maraging stainless steels.The calculated chemical units agree well with the most commonly used alloy specifications,thus unveiling the composition origin of commercial industrial alloys as rooted in the specific chemical units.(4)The dual-cluster formula model for Sn-based eutectics is established.Dual-cluster formula model was proposed in our previous work indicating that eutectic liquids can be regarded as being composed of two stable strucrual units,which are respectively from corresponding eutectic phases and are represented by the cluster formulas of ideal metallic glasses.Such a dual-cluster formula model is not valid for Sn-based eutectics for their being very closed to the ?-Sn side.Since the high content of Sn in the eutectic compositions,it can be assumed that two structural units are both based on that of p-Sn phase.So that the cluster formulas can be calculated out via the equation for the solid solutions.Then the stable structural unit in p-Sn phase is determined as[Sn-Sn10]Sn5,i.e.,CN10 cluster plus 5 glue atoms.The other element being the solute atoms,the specific cluster formulas are obtained according to the equation Z = c·?a·rl3.Binary Sn-based eutectics Sn-(Ag,Au,Mg,Pb,Zn,Bi)are interpreted accurately using this model.Then Sn-based multi-element commercial solder alloys are explained,emphasizing that the minor alloying actually corresponds to integer substitutions in the basic formulas.The present approach provides a new composition design tool for Sn-based eutectic solders. |
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