Development of a High Temperature Maximun Bubble Pressure Apparatus for Measuring Surface Tension of Molten Metals

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Computer modeling of metallurgical processes can be used to enhance process control and product quality. Full exploitation of the high-level of currently available computational tools requires improved accuracy of the physical property input data. In particular, surface tension of molten metals critically influences the characteristics of metals processing. For high-quality measurements of surface tension, it is mandatory to establish the concentration of surface-active elements present in the melt. The most ubiquitous species is oxygen; it is present under all practical conditions involving molten metals and is a major factor effecting surface tension, even at parts per million levels. Since it is practically impossible to totally eliminate the presence of oxygen in reactors and vessels, the oxygen potential needs to be closely monitored. This award from the Instrumentation for Materials Research program support a program to build a high temperature maximum bubble pressure (MBP) device equipped with a solid electrolyte based sensor/control to monitor the oxygen potential in the probe. This combination is uniquely suited for measuring surface tensions of reactive alloys since: (1) the MBP probes beneath the surface of the melt and the probed surface area is small compared to the volume of the melt; and (2) the oxygen potential in the gas stream responsible for creating the probed surface is monitored through an oxygen sensing/controlling device. The instrumentation developed under this program will be made available to students at Carnegie Mellon during laboratory session designed to study the structure and properties of metals in their molten state. Furthermore, it is expected that the ability to establish the effect of the oxygen potential on the surface tension and the effects of the presence of 2D surface phases, will significantly contribute to ongoing and future research projects at Carnegie Mellon that study the surface thermodynamics of alloys. In order to fully appreciate the tools offered by computer models in predicting metallurgical processes for process control and product quality, it is necessary to parallel the advances in process modeling with improved accuracy of the physical data that are used as input. Surface tension of molten metals is a key property since it influences (1) the conversion of liquid droplets to solids during the production of metal-alloy powders; (2) the formation of gas pores during casting which diminish the final metal strength; and (3) the formation and shape of impurity inclusions during the processing of steels. However, it is experimentally extremely difficult to perform accurate surface tension measurements of molten reactive alloys such as stainless steels and superalloys. This is due to the fact that oxygen is always present in trace amounts, which will react with the metals. Since even low levels of dissolved oxygen in the metal can change the surface tension significantly, it is critical to control its concentration accurately. The award from the Instrumentation for Materials Research program instrumentation supports the development of a novel combination of a probe that measures the surface tension under the surface of metals and an oxygen pump/sensor that controls and monitors the oxygen level. The instrument will benefit the metals processing industry and increase the understanding of the physical chemistry of surfaces in the molten state and thereby positively impacting research as well as education in materials chemistry. The instrument built under this program will be implemented in laboratory classes at Carnegie Mellon changing the fact that the important molten state of materials has so far been little studied by students.