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Demonstration of Ultrahigh-Strength Nanocrystalline Copper Alloys for Military Applications
Dr. Kristopher Darling | U.S. Army Research Laboratory
The objective of this research was to explore novel alloy development via powder processing combined with innovative consolidation methodologies to fabricate laboratory-scale parts made from nanocrystalline Cu-Ta based alloys with thermal stability, ultrahigh strength and hardness, and wear properties suitable for substitution for Cu-Be applications.
High-energy ball milling was used to fabricate non-toxic nanostructured Cu-Ta based alloy powders with a unique ability to retain their ultrahigh strength properties to temperatures nearing the melting point of the Cu alloy. The as-milled powder particles had a mean diameter of approximately 100µm, but with an internal nanocrystalline microstructure, thereby removing the potential hazards associated with nanoscale particulate operations and handling.
The outcome of this project was fully successful. A viable material was generated which can replace and compete with Cu-Be alloys.
The successful implementation of fabricating the Cu-Ta alloy powders into bulk parts was based on a novel approach for alloy design. The conventional methodology to prevent grain growth in nanocrystalline systems is to fight the equilibrium tendencies of grain growth by kinetically pinning the grain boundaries in place. However, by selectively choosing interfacially segregating solutes, the driving force for grain growth can be attenuated. That is, the magnitude of the collective grain boundary free energy can be reduced or potentially even negated, thereby placing the system into a deep, metastable state, preventing the growth of grains at any temperature. This thermodynamic concept, as employed in the Cu-Ta system, is elucidated by extensive experimentation and atomistic simulations.
The proof-of-concept for thermodynamic stabilization was validated, as powder processing was transferred from a laboratory scale to a larger pilot scale production. Within 8 months, powder processing was taken from a SPEX mill producing a few grams a day to a Zoz mill producing 1000g a day. This allowed the fabrication of parts to go from 3mm to over 70mm in dimensions. Two types of consolidation techniques were attempted: Field Assisted Sintering Technology (FAST) and Equal Channel Angular Extrusion (ECAE) both of which resulted in fully dense bulk parts.
It was also demonstrated that the mechanical properties as well as the electrical properties of the Cu-Ta alloys can be tailored by changing the Ta content or by changing the temperature used during consolidation to match or exceed the properties of Cu-Be alloys. The researchers are currently able to provide much larger samples. However, this capability is outside the scope of the given effort. The consolidation techniques were primarily chosen to ensure successful production of bulk parts using methods which are scalable to larger parts while being representative of lower cost, high volume manufacturing.