| Summary: | Sintering provides a way of producing bulk products from materials below their melting temperatures. This creates an avenue to utilize material systems that are difficult to produce from other traditional manufacturing processes. While originally used to process ceramic components, it has become a way to produce metal parts that are conventionally difficult to produce, such as parts made from refractory metals. Various sintering methods such as liquid phase sintering and supersolidus liquid phase sintering have been used to enhance densification and reduce the sintering time of the materials. Nanophase separation sintering (NPSS) was first explored by Dr. Mansoo Park. In this process, a mechanically alloyed powder experiences phase separation on the surface. This second phase promotes neck formation and rapid densification. By being a solid state process, parts made via this method can be used at higher temperatures that they are sintered at and allow for high melting temperature materials such as tungsten to be sintered quickly at low temperature. Nanophase separation sintering has also been shown to allow the production of bulk nanocrystalline products. To fully harness such a technique, it is necessary to fully understand the thermodynamic and kinetic phenomena surrounding it.
First, we explore how to better understand the critical processes occurring during NPSS. Previous techniques such as the Master Sintering Curve (MSC) assume a single mechanism for sintering when the reality of a sintering process in more complicated. For this reason, a Kissinger style analysis for sintering dilatometric data was derived. This allowed us to understand how different temperature dependent mechanisms, represented by densification rate peaks, facilitate the different stages of NPSS. First, however, the Kissinger style analysis was properly derived based on the combined stage sintering model. The error in using the traditional Kissinger analysis for sintering dilatometry data was explored highlighting the necessity for our derivation for accelerated sintering techniques. Finally, the method was evaluated against 3 separate test cases to ensure our obtain activation energies were in line with established methods.
With this new tool, we moved to re-evaluate the previous explored systems of W-Cr, Cr-Ni and Ti-Mg. By analyzing the W-Cr system in more detail, it was possible to ascertain that there were two critical sintering events: neck formation and neck redissolution. These results were verified quantitatively using a Kissinger style analysis and qualitatively through ex-situ SEM imaging. Further confirmation came from examining the Cr-Ni system, which also exhibits NPSS. Finally, when examining the Ti-Mg system which sintered via NPSS but did not achieve the extensive densification of the previous systems. It is show that Ti-Mg does not exhibit redissolution and thus does not fulfill the newly-understood criteria of NPSS.
With this understanding of NPSS, novel alloy systems were designed to exhibit it. First, the binary Mo-Cr systems was explored and shown to fully exhibit NPSS. The Mo-Cr system was used to explore optimizing NPSS in materials including temperature and sintering aid minimization. Beyond binary systems, the Mo-W-Cr ternary was explored. It was shown to ii sinter exceptionally well, similarly to the Mo-Cr binary. The relative impact of the ternary elements is shown and compared to previous NPSS systems. Mechanical testing is performed to understand it usefulness compared to differently processed alloys.
The accumulation of these results verify the thermodynamic requirements and kinetic mechanisms that define nanophase separation sintering. Through this work, novel alloys, including those beyond the binary systems traditionally examined, can be designed to exhibit NPSS and further optimized for improved processing and usefulness.
|
|---|