Composition Design Methodology And Properties Of Zr-Co-Al Amorphous Alloys Derived From Their Cluster Formulae

Amorphous alloys are readily formed in a variety of binary and multi-component alloy systems. In particular in some multi-component systems, the amorphous formation ability has been found to be exceedingly large within a limited composition range. The composition rules that govern the ease of amorphous alloy formation have long been a fundamental question in solid state physics and materials science. Useful amorphous alloys as structural materials are frequently related to the problems of a large amorphous formation ability, a good combination of properties and low material cost.Structurally, amorphous alloys are known to exhibit short-range order, which is embodied in the specific topology and chemistry of the nearest neighbor coordination polyhedron around a given atom(or vacancy). Such a atomic polyhedron is usually called an atomic cluster. The atomic cluster idea has been used to describe the principal structural features of amorphous alloys, and to understand why an amorphous state is very easily reached at some specific compositions. In our previous work, Dong and colleagues have developed an atomic-cluster-plus-glue-atom model for an ideal amorphous structure. The model says that an ideal amorphous structure is composed of atomic clusters and the glue atoms located in the interstitial sites of the disordered atomic cluster lattice. Accordingly, the composition formula of an ideal amorphous alloy can be expressed as [cluster](glue)x, where x is the number ratio between the atomic cluster and the glue atoms. Composition design of amorphous alloys therewith can be done in terms of the framework of the formula expression. The basic ingredients of the alloy design are the identification of the concrete form of the atomic cluster and the nature and number of glue atoms.Zr-Co-Al is known to be a fundamental amorphous system. We recall the amorphous alloy composition design using the atomic cluster model. In particular, Zr-Co is determined to be the basic system and Al is regarded as the alloying element for the constitution of a ternary amorphous system. Zr-Co eutectic compositions are first transformed into an atomic-cluster-plus-glue-atom form using the atomic cluster identified from the eutectic reaction related intermetallic phases. The atomic cluster is assumed to play the role of the basic cluster in the cluster formula of ternary Zr-Co-Al. A series of ternary Zr-Co-Al compositions are derived from the composition design. Bulk amorphous alloys are prepared at at these compositions, and the thermal glass stability and the mechanical property of these alloys are studied.The results of the experimental study are summarized as follows:(1) The crystalline structures of tI12-Zr2Co and cP2-ZrCo eutectic intermetallic phases are deciphered, and a Co-centered Co3Zr8and a Zr-centered Zr7Co8cluster are determined. The eutectic Zr-Co compositions Zr46.6Co53.4, Zr65Co35and Zr78.5Co21.5are expressed [Zr7Co8](CoZr)=Zr47Co53,[Co3Zr8](Co2Zr)=Zr64.3Co35.7and [Co3Zr8]Zr3=Zr78.6Co21.4.(2) Since the present study puts an emphasis on the Zr-based alloys, Co3Zr8can be the favorable cluster of Zr-based Zr-Co amorphous alloys, and Co/Zr are taken as the glue atoms. Taking the atomic size and the electron concentration factors into consideration, the possible composition formulae of Zr-Co amorphous alloys are obtained:[Co3Zr8]Co,[Co3Zr8]Co2,[Co3Zr8]Co3and [Co3Zr8]Zr. And since eutectic compositions are good amorphous alloy formers, the eutectic formulae [Co3Zr8](Co2Zr) and [Co3Zr8]Zr3are also considered to be the basic binary amorphous compositions.(3) Third-element Al is then used to do substitution alloying in the six composition formulae. Notice that the atomic size relationship, Rco+Zr=RAl, one Co plus one Zr shell atoms in the Co3Zr8atomic cluster are replaced by two Al atoms. The alloying strategy delivers six Zr-Co-Al compositions:[Co2Zr7A12]Co,[Co2Zr7Al2]Co2and [Co2Zr7Al2]Co3;[Co2Zr7Al2]Zr,[Co2Zr7Al2](Co2Zr) and [Co2Zr7Al2]Zr3. The corresponding alloy compositions are Zr58.3Co25Al16.7, Zr53.8Co30.8Al15.4, Zr50Co35.7Al14.3, Zr66.6Co16.7Al16.7, Zr57.1Co28.6Al14.3and Zr71.4Co14.3Al14.3. The water-chilled copper mould casting experiment revealed different amorphous formation abilities at these compositions: except for Zr71.4Co14.3Al14.3and Zr66.6Co16.7Al16.7, all the other alloys can be made into2mm diameter amorphous rods. The critical diameters for amorphous formation are larger than8mm at the compositions Zr58.3Co25Al16.7, Zr53.8Co30.8Al15.4and Zr57.1Co28.6Al14.3. Among them, the Zr53.8Co30.8Al15.4(cluster formula:[Co2Zr7Al2]Co2) showed the largest critical diameter of12mm. For comparison, the known best amorphous former Zr56Co28Al16showed a critical diameter of10mm under the same casting conditions.(4) To check the validity of the present composition design, a number of alloy compositions around Zr53.8Co30.8Al15.4deriverd form the optimized cluster formula [Co2Zr7Al2]Co2have also been studied. Copper mould casting experiment has been carried out at compositions Zr85-xCoxAl15(x=24～34), Zr54Co46-yAly (y=14～18), Zr84-x’Cox’Al16(x’=28～32) and Zr53Co47-y’Aly’(y’=15～17). The detailed experimental investigation showed that the best amorphous formation abilities peak at Zr54Co31Al15and Zr54Co30Al16, which are very close to the [Co2Zr7Al2]Co2formula composition (i.e., Zr53.8Co30.8Al15.4). The amorphous formation ability of Zr-Co-Al alloys are discussed with the experimental evidence. And the room-temperature mechanical properties of these amorphous alloys are finally studied.