Fe-based Multi-component Alloys Designed By A Cluster-based Stable Solid Solution Structural Model

Fe-based alloys are the most important engineering materials, where a host of alloying elements need to be added to meet different behaviors in service. However, the kinds and the contents of alloying elements are commonly determined by lots of try-and-error experiments, which results in that it will experience a long time to develop a new alloy. Starting from the local structure of stable solid solution alloys, this work applies the cluster-plus-glue-atom model to design Fe-based multi-element alloys, and amounts of experimental results confirm the validity of this cluster model. More importantly, a series of new Fe-based multi-component solid solution alloys are obtained with guidance of the model. This work possesses significances both in theory research and in engineering applications.The cluster-plus-glue-atom model is originated from the structural analysis and composition design of multi-component metallic glasses and quasicrystals. It dissociates an alloy structure into two parts:the cluster part and the glue atom part, where the cluster is the nearest neighbor coordination polyhedron and generally formed in solute elements with negative enthalpies of mixing (△H<0) with the base element, and the glue atoms are those elements having weak△H. The alloy composition is then expressed with [cluster](glue)x, x denoting the number of glue atoms matching one cluster. Two types of alloy systems, Fe-Ni-Cu and Fe-Cr-Mo-Al-Cu, representing FCC and BCC strucuture respectively, are selected, where the relationships among composition, microstuctrue and properties have been established through structure and property characterizations.Fe-Ni Invar alloys have been extensively used as sealing alloys in electronic industry. Minor amount of Cu addition can improve their corrosion resistance, but excess Cu could induce phase precipitation and thus deteriorates the corrosion resistance. In order to determine the optimum content of Cu in Fe-Ni alloys, the composition of Fe-Ni-Cu Invar alloys are analized with the cluster model and series of cluster formula alloys are designed and experimented. Generally, the Cu contents in conventional Fe-Ni Invar alloys, such as4j42,4j46et al., are less than4mass%, and these alloys possess poor corrosion resistance in high Cl--containing environment. In schematic Fe-Ni-Cu ternary phase diagrams from20℃to700℃, Cu solubility limit maintains constant, from2.5at.％to3.5at.%. The double cluster model [CuNi12][NiFe12]m (expressed in atomic number in cluster model in this paper) for Cu-containing Fe-Ni ternary stable solid solutions is proposed based on the enthalpies between the element pairs (△HFe-Ni=-2kJ/mol,△HNi-cu=4kJ/mol,△Hcu-Fe=13kJ/mol) and the cluster-structural model for stable solid solution. This model assumes that Fe-Ni and Cu-Ni nearest neighbors are allowed but the Cu-Fe ones should be avoided and Cu is dissloved in the Fe matrix in the form of CuNi12. Experimental results affirm that series of alloys formulated by this model maintain monolithic FCC structure and possess excellent corrosion-resistance, superior to those commercial Fe-Ni Invar alloys in3.5wt.%NaCl. In addition, these new alloys have comparable expansion coefficients with those comercial Invar alloys of approximate compisitions.As good corrosion-resistant materials, Cr and Mo elements are generally added into ferrite stainless steels. But excess Mo will raise the cost of materials. And too much Al and Cu could induce the second phase precipitation although they are benifical for corrosion resistantance. In order to determine the optimum contents of these alloying elements, the composition analysis and experiments are carried on with the cluster-plus-glue-atom model. The cluster model is first established for Fe-Al-Cu stable BCC solid solution. In the schematic Fe-Al-Cu ternary phase diagrams, the atomic ratio of Al/Cu is almost maintains constant from20℃to600℃and its value is Al/Cu=8. So the cluster model [CuAl8]Fex is established according to the enthalpies between the elements (△Hfe-Ai=-11kJ/mol,△HAl-cu=-1kJ/mol,△Hcu-Fe=13kJ/mol). And then, Cr and Mo substitute for some Fe due to their similar essence, the Cr content is according to Tammann's n/8law. The addition of Mo is supposed to be the second neighbour of Cu and the fisr neighbour of Al, and the Cu-Al8-Mo6cluster is then established. Finally, the cluster formula [(CuAl8)Mo6](Fe,Cr)x is proposed for Fe-Cr-Mo-Al-Cu quinary solid solution alloys. The experimental results show that the cluster formula alloys maintain single BCC structure. The obtained Cr24Mo7A13Cu and Cr27Mo6A13Cu alloys (mass%) posses better corrosion resistance than commercial ferrite stainless steels with high contents of Cr and Mo in high Cr-contained and strong oxidized environments. Moreover, they have much better oxidation resistance than conventional Fe-23Cr-5Al alloys at1100℃, their mechanical properties are comparalble with that of the Fe-Cr-Mo ferrite stainless steel.Based on the alloying principle of insoluble elements in above two typical systems, a lot of similar ternary phase diagrams are analyzed, and the corresponding stable cluster structure models are then established to forecast new solid solution alloys.