Design of Low-Melting Point Compositions Suitable for Transient Liquid Phase Sintering of PM Steels Based on a Thermodyn
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LIQUID phase sintering is commonly employed for the consolidation of a variety of PM alloyed systems. Its widespread use is related to the activation of the sintering kinetics through faster mass transport rates in the liquid, which favors higher degrees of homogenization in comparison to the solid-state process and, in some cases, enhanced densification.[1–7] One possible source of liquid is the presence of a low-melting point component in the starting powder mix specifically selected to melt during the sintering cycle. For this kind of liquid-forming system, the amount, duration, and spreading capacity of the liquid phase determine the extent of the densification as well as the microstructure evolution. A controlled quantity of liquid phase homogenously distributed and able to exert an attractive ELENA BERNARDO, Ph.D. Student and Teaching Assistant, and MO´NICA CAMPOS, Associate Professor, are with the Department of Materials Science and Engineering, IAAB, Universidad Carlos III de Madrid, Avda. Universidad 30, 28911 Legane´s (Madrid), Spain. Contact e-mail: [email protected] RAQUEL DE ORO, Postdoctoral Researcher, is with the Department of Materials and Manufacturing Technology, Chalmers University of Technology, Ra¨nnva¨gen 2A, 412 96, Go¨teborg, Sweden. JOSE´ MANUEL TORRALBA, Full Professor, is with the Department of Materials Science and Engineering, IAAB, Universidad Carlos III de Madrid, and also Deputy Director with the IMDEA Materials Institute, Eric Kandel 2, 28906 Getafe, Madrid, Spain. Manuscript submitted September 2, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
force between the base particles induces rearrangement of the solid and leads to fast densification. However, this latter scenario is only possible if the interaction phenomena between the liquid and the solid at high temperatures are favorable. Such interaction can be defined by the contact angle evolution between the liquid and the solid, the solubility between the phases, and the distribution capacity of the liquid.[4,8,9] For instance, the solubility ratio between the coexisting phases that can be deduced from the phase diagram plays a main role, since it determines the permanent or transient nature of the liquid; in other words, it sets the time interval during which activation of sintering is possible. Accordingly, the prediction of the liquid/solid interaction-related phenomena provides the means to improve sinterability and to control dimensional behavior. In the case of conventional systems processed by liquid phase sintering (such as cemented carbides or heavy alloys), the solubility condition is determinant. In these systems, intense densification is linked to a high unipolar solubility: a high solubility of the solid particles in the liquid, but a limited reverse solubility. A schematic representation of the phase diagram of a hypothetical system with a limited intersolubility is presented in Figure 1. This solubility feature favors the formation of a permanent liquid phase which, under favorable wetting conditions, ena
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