Co-Al-W基高温合金的团簇成分式

Cluster formulas of Co-Al-W-base superalloys

Co-Al-W基高温合金具有类似于Ni基高温合金的γ+γ'相组织结构.根据面心立方固溶体的团簇加连接原子结构模型,Ni基高温合金的成分式即最稳定的化学近程序结构单元可以描述为第一近邻配位多面体团簇加上次近邻的三个连接原子.本文应用类似方法,首次给出了Co-Al-W基高温合金的团簇成分式.利用原子半径和团簇共振模型,可计算出Co-Al-W三元合金的团簇成分通式,为[Al-Co_(12)](Co,Al,W)_3,即以Al为中心原子、Co为壳层原子的[Al-Co_(12)]团簇加上三个连接原子.对于多元合金,需要先将元素进行分类:溶剂元素——类Co元素Co (Co, Cr, Fe, Re, Ni,Ir,Ru)和溶质元素——类Al元素Al (Al,W,Mo, Ta,Ti,Nb,V等);进而根据合金元素的配分行为,将类Co元素分为Co~γ(Cr, Fe, Re)和Co^(γ')(Ni, Ir, Ru);根据混合焓,将类Al元素分为Al, W (W, Mo)和Ta (Ta, Ti, Nb, V等).由此,任何多元Co-Al-W基高温合金均可简化为Co-Al伪二元体系或者Co-Al-(W,Ta)伪三元体系,其团簇加连接原子成分式为[Al-Co_(12)](Co_(1.0)Al_(2.0))(或[Al-Co_(12)] Co_(1.0)Al_(0.5)(W,Ta)_(1.5)=Co_(81.250)Al_(9.375)(W,Ta)_(9.375) at.%).其中,γ与γ'相的团簇成分式分别为[Al-Co_(12)](Co_(1.5)Al_(1.5))(或[Al-Co_(12)] Co_(1.5)Al_(0.5)(W,Ta)_(1.0)=Co_(84.375)Al_(9.375)(W,Ta)_(6.250) at.%)和[Al-Co_(12)](Co_(0.5)Al_(2.5))(或[Al-Co_(12)] Co_(0.5)Al_(0.5)(W, Ta)_(2.0)=Co_(78.125)Al_(9.375)(W,Ta)_(12.500)at.%).例如,Co_(82)Al_9W_9合金的团簇成分式为[Al-Co_(12)]Co_(1.1)Al_(0.4)W_(1.4)(～[Al-Co_(12)]Co_(1.0)Al_(0.5)W_(1.5)),其中γ相的团簇成分式为[Al-Co_(12)]Co_(1.6)Al_(0.4)W_(1.0)(～[Al-Co_(12)]Co_(1.5)Al_(0.5)W_(1.0)),γ'相的团簇成分式为[Al-Co_(12)]Co_(0.3)Al_(0.5)W_(2.2)(～[AlCo_(12)]Co_(0.5)Al_(0.5)W_(2.0)).

Having a γ/γ’ microstructure similar to Ni-base superalloys and also including various alloying elements such as A1 and W, new Co-base superalloy, namely Co-Al-W-base alloy, has been widely studied as a kind of potential alternative of Ni-base superalloy, which is the most important high-temperature structural material in industrial applications. Besides, Co-Al-W-base alloy has also excellent mechanical properties, for example, creep properties comparable to those of the first-generation Ni-base single crystal superalloys. In our previous work,the ideal composition formula of Ni-base superalloy has been obtained by applying the cluster-plus-glue-atom structure model of faced centered cubic solid solution, which shows that the most stable chemical short-rangeorder unit is composed of a nearest-neighbor cluster and three next-neighbor glue atoms. In this paper, the ideal cluster formula of Co-Al-W-base superalloy is addressed by using the same approach. Based on cluster-plusglue-atom model theory, according to lattice constants and atom radii, calculations are carried out. The results show that the atom radius of A1 is equal to Covalent radius(0.126 nm) and for γ’ phase the atom radius of W changes obviously(0.1316 nm). After analyzing atomic radii, the chemical formula for Co-Al-W ternary alloy is calculated to be [Al-Co12](Co,A1,W)3, which signifies an A1 centered atom and twelve Co nearest-neighbored cluster atoms plus three glue atoms, which is in good consistence with that for Ni-base single crystal superalloy.For multi-element alloy, the alloying elements are classified, according to the heat of mixing between the alloying elements and Co as well as partition behavior of alloying elements, as solvent elements-Co-like elements Co(Co, Ni, Ir, Ru, Cr, Fe, and Re) and solute elements-Al-like elements Al(Al,W,Mo,Ta,Ti,Nb,V,etc.).The solvent elements can be divided into two kinds according to partition behaves: Coγ(Cr, Fe, and Re) and Coγ’(Ni, Ir, and Ru). The latter is further grouped into Al, W(W and Mo, which have weaker heat of mixing than Al-Co) and Ta(Ta, Ti, Nb, V, etc., which have stronger heat of mixing than Al-Co). Then all chemically complex Co-Al-W-base superalloys are simplified into Co-Al pseudo-binary or Co-Al-(W, Ta) pseudo-ternary system. Within the framework of the cluster-plus-glue-atom formulism and by analyzing the compositions of alloy, it is shown that the Co-Al-W-base superalloy satisfies the ideal formula [Al-Co12](Co1.0Al2.0)(or[Al-Co12] Co1.0Al0.5(W, Ta)1.5=Co81.250Al9.375(W,Ta)9.375 at.%). In the same way, those of γ and γ’ phases are respectively [Al-Co12](Co1.5Al1.5)(or [Al-Co12] Co1.5Al0.5(W, Ta)1.0=Co84.375Al9.375(W, Ta)6.250 at.%)and [Al-Co12](Co0.5Al2.5)(or [Al-Co12] Co0.5Al0.5(W,Ta)2.0 = Co78.125Al9.375(W,Ta)12.500 at.%). For example, alloy Co82Al9 W9 and its γ and γ’ phases are formulated respectively as [Al-Co12]Co1.1Al0.4W1.4(~[Al-Co12]Co1.0Al0.5W1.5),[Al-Co12]Co1.6Al0.4W1.0(~[Al-Co12]Co1.5Al0.5W1.0),and[Al-Co12]Co0.3Al0.5W2.2(~[AlCo12]Co0.5Al0.5W2.0).