Early Diagnostics of Alloys For Spinodal Decomposition: A Case of Preventive Prediction of Phase Delamination in an Irregular Fe-Cr-C Solid Solution (The Design Stage)

The article presents an improved approach to thermodynamic modelling and early, preventive prediction of spinodal decomposition processes with phase delamination of irregular three-component α - solid solutions into separate equilibrium, immiscible phases. Using the obtained model, it is possible to analytically predict the critical concentration-temperature conditions under which the noted phase segregation can be induced, contributing to the premature aging of metallic materials and reducing the operational reliability of machine parts made from them. Consequently, the proposed approach will allow in advance, even at the stage of development of the alloy, to eliminate the risk associated with its structural-phase decay during the operation of the product made from it. The results of calculations obtained on a widely used model alloy of the Fe-Cr-C system are presented. It has been established that the spinodal decomposition of the noted three-component stainless heat-resistant solid solution can lead to concentration stratification into the following three equilibrium phases, with the content of elements in molar parts: 1) Fe=0.14, Cr=0.29, C=0.57; 2) Fe=0.53, Cr=0.29, C=0.18; 3) Fe=0.14, Cr=0.68, C=0.18. Such phase segregation can be initiated in a solid solution with an initial content of these elements of 0.72, 0.25, and 0.03 mol, in the case of its rapid forced cooling to a critical temperature of 342 K for this system, since this leads to the maximization of free energy and transfers it in a thermodynamically non-equilibrium state. As a preventive measure to avoid the process of spinodal decomposition, seeking to zeroing the free energy of the system by its concentrative enriched or depleted delamination and, consequently, microstructural embrittlement, it is recommended to technologically exclude the probability of producing and operating an alloy with a predetermined non-equilibrium chemical composition and critical extent of forced cooling.