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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 further explore novel alloy development via powder processing synergistically combined with innovative consolidation methodologies to fabricate nanocrystalline copper (Cu)-based alloys with thermal stability and properties suitable for substitution of copper-beryllium (Cu-Be) and its applications. Previously, it was demonstrated that both the mechanical properties as well as the electrical properties of the copper-tantalum (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; however, the preliminary study did not provide a sufficiently thorough investigation of the chemistry/properties relationship. Therefore, the specific objectives of the current project were to elucidate the processing-nanostructure-property-performance interrelationships in an array of nanocrystalline Cu-Ta alloys through a more systematic investigation of thermo-mechanical processing conditions and alloy composition effects, to establish and define the property tradeoffs available in this system and to understand the adjustment range of both physical and mechanical properties.
A series of characterization techniques were utilized to study and develop a new family of nanocrystalline alloys based on the immiscible Cu-Ta binary system. Small- and large-scale mechanical alloying was used to identify, assess, and map out how the physical and mechanical properties of this material scale from a proof-of-concept stage into a higher manufacturing environment. Experiments were conducted by tuning the composition from 1 to 10 % Ta. Small-scale testing was mainly accomplished at the Army Research Laboratory’s Center for Advanced Metals Powder Synthesis (CAMPS), Aberdeen Proving Ground, under the original SERDP SEED project WP-2139. In the large-scale sample testing, Cu-Ta alloy powders were produced using a CM08 mill purchased from Zoz GmbH, installed and operated at the Armament Research, Development and Engineering Center’s Powder Metallurgy and Nanotechnology Center (PMNTC) at Picatinny Arsenal. A comprehensive experimental milling program was designed and conducted to investigate the issues associated with scale up and milling kinetics. These issues would occur when utilizing the large capacity mill for the production of significantly larger quantities of nanocrystalline Cu-Ta powder with 1, 5, and 10 % Ta concentrations as compared to those produced in the much smaller SPEX mill used for the earlier seminal studies. Alloy characterization at CAMPS consisted of scanning transmission electron microscopy (STEM), scanning electron miscroscopy (SEM), and X-ray diffraction (XRD) analyses.
Microstructural evaluation and mechanical testing of the resultant alloys revealed that a diverse array of physical properties can be attained. These properties are intimately linked to the as-milled Cu grain size and the distribution of Ta particle dispersiods, both of which can be altered by manipulating the composition and consolidation methodology. To investigate the increasing susceptibility and the degree of thermal softening, the Equal Channel Angular Extrusion (ECAE) processed Cu-Ta samples were subjected to quasi-static compression tests at high temperatures. When subjected to extreme temperatures nearing the melting point of Cu, the high hardness values were still retained, demonstrating a smooth and continuous plastic flow with no signs of failure. Pressure and shear deformation during consolidation had only a moderate effect on decreasing the hardness relative to the isothermal annealing studies. The exhibited flow stress is a strong function of the testing temperature, and there is significant contract to the behavior of coarse-grained Cu.
In combination, the results of the earlier SERDP SEED project and this current project reveal that Cu-Ta alloys are highly capable of meeting and exceeding many of the properties exhibited by Cu-Be alloys. However, further research is needed to improve, optimize, and elucidate the existing alloy formulations towards a tangible application. The successful development of the instrumented Hot Isostatic Pressing (HIP) processing protocols demonstrates that parts fabrication is possible. Transitioning from small-scale processing in a SPEX mill (few grams/day) to larger scale processing (Zoz mill, kilograms/day) has revealed the need for a more critically designed process engineering study. With such an additional effort, the level and role of impurities such as iron, chromium, and oxygen on properties can be mitigated or greatly reduced. Additionally, evaluation of the physical and mechanical properties attained within the larger scale dimensioned parts must be correlated to the properties achieved using smaller scale powder production equipment.