Ultrahigh-Strength Nanocrystalline Copper Alloys Improve Performance While Reducing Environmental Impacts
SERDP funded a proof-of-concept research project at the U.S. Army Research Laboratory (ARL) to identify a viable method to produce an environmentally benign and occupationally safe alternative to copper-beryllium alloys. In this project, Army researchers successfully produced a nanostructured copper-tantalum alloy exhibiting properties that meet or exceed those of copper-beryllium alloys. ARL’s process produces bulk, thermodynamically stable alloys that can be readily fabricated in scaled up quantities. This process would drastically reduce the need for beryllium in the near future for applications that require high strength, wear resistance, and superior electrical and heat conductivity.
Beryllium is categorized as a carcinogen and is known to be responsible for Chronic Beryllium Disease, an incurable condition that occurs through occupational exposure. U.S. Department of Defense (DoD) employees are exposed to beryllium dust and fumes as a result of fabrication operations involving these alloys. Because of the uncertainty in determining a “safe” exposure level, the Nuclear Regulatory Commission recommends that DoD eliminate as many job tasks involving exposure to beryllium particles as possible and to minimize the number of workers performing those tasks. Developing alternatives to these alloys would reduce the health risk to these employees and reduce the costs associated with employee monitoring and testing as well as exposure mitigation procedures. Despite these risks, there is a need within the DoD for beryllium alloys because of their unique performance properties. In particular, the addition of beryllium to copper increases its strength and conductivity leading to exceptional performance and manufacturability.
Nanostructured materials exhibit mechanical properties that are superior to conventional materials and have been considered as potential alternatives to beryllium alloys. However, current production processes used to generate nanostructured materials tend to be unstable and may not exhibit the desired properties in extreme environments. Nanostructured metals are inherently unstable because, unlike coarse-grained materials, they are not in a stable energy state. In order to reach a lower energy state, nanosized grains will ultimately grow into grain sizes associated with conventional, non-nanostructured alloys.
Traditionally, materials researchers have attempted to overcome this by incorporating microscopic precipitates or particles into the alloys to act as physical barriers to impede grain growth. However, with an increase in temperature or stress to the alloy, such physical barriers are not strong enough to withstand natural thermodynamic forces and are unable to prevent the grains from growing.
ARL researchers developed an approach that takes advantage of thermodynamic forces instead of trying to overcome them. By forcing a nominally incompatible material, like tantalum, to dissolve into the copper host metal using high energy milling of metal powders, the alloy has no choice but to form nanosized grains to reach its lowest energy state. In turn, the alloy becomes a thermodynamically stable nanostructured material.
Researchers used computational modeling combined with laboratory validation to publish a library of over 1,000 alloy compositions that will form stable nanostructured alloys. These alloys have shown superior mechanical properties, withstanding extreme environments, even at temperatures nearing their melting point. Processing techniques that can be used to produce many different types of intricate bulk materials for a range of applications were identified. Expected benefits from using ultrahigh-strength nanocrystalline copper alloys include the development of materials with superior properties, environmentally safe alternatives that do not include beryllium, and energy saving production processes.
The Final Report for this project (WP-2438) is expected to be posted in early 2015.