Ongoing work in the development and characterization of sensory materials requires the development of shape memory alloy (SMA) powder or particles. These are embedded in structural material so that the progression of localized damage that occurs during fatigue crack growth will produce an audible acoustic emission (AE) as the SMA transforms from an austenite phase to a martensite phase. In order to set the shape memory effect in these particles, the SMA must be solution-treated (ST) to produce the austenite phase, and rapidly quenched to or below room temperature to preserve the austenite phase at room temperature.
A furnace has been developed for solution treatment, followed by a rapid quench, of metallic powders in an inert environment (e.g., vacuum, partial pressure of argon, or some other inert gas). The furnace permits rapid quenching of material from an elevated temperature (1,300 °C) to cryogenic temperatures, all within an inert environment. The system is able to process particles and/or powder, preserving inert environmental conditions throughout, and faster and more uniform quench rates than existing systems.
Having the ability to ST and quench material without “bagging” material (a process where material is sealed within a vacuum bag) makes this system unique. This is a significant advantage for processing powder, where bagging is less than ideal because it creates a gradient in cooling rates (i.e., powder next to the bag wall will cool faster than powder in the interior of the bag) such that a variation in properties of the powder would be expected.
Processing of SMA powder requires (1) exposing the powder to solution treatment temperature for an extended period of time; (2) maintaining an inert environment, preferably a partial pressure of an inert gas like argon to prevent evaporation of material, throughout the solution treat and quenching phases; and (3) quenching the material in a pool of condensed inert fluid (e.g., argon), bringing it from ST temperature to cryogenic temperature rapidly (less than 1 second) while maintaining inert conditions.
The system consists of a ceramic tube/furnace mounted around the ceramic tube, and the material is heated inside the tube. The ceramic tube and magnetic transporter are mounted onto opposite sides of a chamber. Material is loaded into the crucible through the top of the chamber. The bottom of the chamber is submerged in a dewar of liquid nitrogen. A magnetic transporter uses magnets on the outside of a tube to manipulate a shaft inside the tube through magnetic couplers. A ceramic crucible, which holds the material to be processed, is attached to the end of the transporter shaft. The shaft is moved down the length of the tube to transport material into and out of the furnace hot zone, and position material for quenching.
This work was done by John A. Newman, Terryl A. Wallace, and Stephen W. Smith of Langley Research Center; Michael R. Home of the National Institute of Aerospace; Joel A. Alexa and Harold D. Claytor of Lockheed Martin Corp.; Peter L. Messick of ATK Space Systems; and William P. “Paul” Leser and Patrick E. Leser of North Carolina State University. LAR-18074-1