Processing of Bulk Nanocrystalline Metals at the US Army Research Laboratory
March 7th, 2018
This paper provides a brief overview of the ongoing efforts at the Army Research Laboratory on the processing of bulk nanocrystalline metals with an emphasis on the methodologies used for the production of the novel metal powders.
The overall goal of this procedure is to produce mechanically alloyed nanocrystalline powders that can be processed into bulk stabilized nanocrystalline metals and alloys. To meet ever increasing material performance demands, the U.S.Army has undertaken the development of stabilized bulk nanocrystalline alloys for use across a wide range of operating conditions. Ball milling is one of the few synthesis techniques conducive to producing large volumes of nanocrystalline powders, thereby facilitating industrial scale production of this universal structural applications.
Advanced characterization techniques such as transmission electron microscopy and atom protomography coupled with sight specific materials extraction we have focused on the milling, enable the determination of structure, property, processing relationships, and the bulk alloys. In addition to additive manufacturing, ARL has a variety of consolidation techniques including equal channel angular excrusion, or ECAE, field assisted sintering technology, or FAST, and hot isostatic pressing, or HIP. To begin the small scale synthesis, in an argon filled glove box, place 10 grams of the primary element for the alloy and 100 grams of stainless steel or tool steel milling balls in a stainless steel milling jar.
Close the milling jar and remove it from the glove box. Clamp the jar in a vice and tighten the lid with a wrench. Then secure the fully sealed jar in the clamp of a high energy shaker mill.
Perform a one hour milling cycle to coat both the interior of the jar and the milling balls with the primary element. Then remove the jar from the mill and return it to the argon filled glove box. Open the jar and remove loose powder from the milling balls and the jar.
Add to the coated jar a total of 10 grams of elemental powders in the required ratios for the alloy. Add the coated milling balls to obtain a 10 to 1 mass ratio of milling balls to powder. Close the jar and remove it from the glove box.
Tighten the lid of the milling jar with the vice and wrench before loading the jar into the high energy shaker mill. Once the milling jar has been secured in the mill, initiate a milling cycle of a duration appropriate for the alloy being produced. When the milling cycle is finished, return the jar to the argon filled glove box.
Carefully remove the lid and transfer the nanocrystalline powder to a sample vial for storage. To begin the cryogenic small scale synthesis, coat the inside of a milling jar in the appropriate milling media with the primary element of the desired alloy as previously described. Then in an argon filled glove box, place the required amounts of elemental alloying powders in the coated jar with the milling media.
Tightly close the jar and remove the jar from the glove box. Use a vice and wrench to tighten the jar lid completely. Fit the jar into a PTFE sleeve connected to a dewar of liquid nitrogen or liquid argon.
Place a PTFE cap on the jar and sleeve then place the enclosed jar in the clamp of the high energy shaker mill. Ensure that the milling jar is secure and properly sealed. Then open the flow of the cryogen to the milling jar.
Allow the cryogen to flow for about 30 minutes or until the cryogen has been visibly flowing from the outlet for two minutes to ensure that the jar reaches the desired temperature. Initiate a milling cycle of the desired duration. Upon completion, stop the flow of the cryogen and carefully remove the milling jar from the PTFE sleeve.
Place the jar in front of a hot air drier and allow the jar to warm to room temperature. Then transfer the jar into the argon filled glove box. Carefully open the jar and transfer the nanocrystalline powder to a storage vial.
To begin the large scale synthesis, in an argon filled glove box load the primary elemental powder for the alloy into a glass jar. Load up to one kilogram of the elemental alloying powders into a second glass jar. Seal both jars with caps designed to transfer powder to the milling vessel without exposure to air.
Remove the sealed jars from the glove box. Next, connect a stainless steel eight liter milling vessel with a cooling jacket to a high energy horizontal rotary ball mill, then load approximately one kilogram of 440C stainless steel ball bearings into the vessel. Connect an argon gas line and ethylene glycol cooling lines to the vessel.
Backfill and purge the vessel with argon gas to remove air then connect the glass jar to the primary elemental powder to the powder loading assembly of the ball mill. Use a double ball valve to transfer the primary elemental powder from the glass jar to the milling vessel under an argon atmosphere. Close the valve to seal the chamber then connect the powder extraction system to the milling vessel.
Backfill and purge the extraction system with argon gas to remove air. Perform a short coding run with the primary elemental powder. Remove loose powder under an argon atmosphere when finished, then transfer the elemental alloying powders from the glass jar to the milling vessel under an argon atmosphere as previously described.
Start flowing ethylene glycol at minus 25 degrees celsius through the outer jacket of the vessel. Once the vessel is cooled, run the ball mill for 12 to 30 hours at a rotational speed of 400 to 800 rpm. Finally, transfer the alloy powders to the storage jar under an argon atmosphere using the powder extractor.
Store the jar in an argon filled glove box. Small scale powder synthesis generally resulted in nanocrystalline microstructures with an average powder particle size between 10 and 500 micrometers. Powders were processed with ECAE which allowed significant refinement of the microstructure and texture.
Bright-field STEM of an ECAE processed copper 10 atomic percent tantalum sample allowed grain sizes to be measured from the clearly resolved grains. High-angle Annular Dark Field STEM showed the high number density of tantalum particles and the wide range of particle sizes. For example, this 40 nanometer wide tantalum particle was surrounded by tantalum particles with diameters of 5 to 20 nanometers.
The large tantalum particle featured a partial shell around it's lower half. The high number density of tantalum particles was quantified with atom probe tomography. APT was also used to locate and quantify tungsten dioxide and sodium particles in an electroplated nickel tungsten alloy.
Along with STEM, the 3D atom maps and mass spectra from APT were essential for a full understanding of the microstructure and mechanisms of mechanically alloyed nanocrystalline powders. Bulk sample consolidation of iron nickel zirconium powder by hot isostatic pressing resulted in a maximum density of about 96%Conventional excrusion at 1000 degrees celsius successfully consolidated bulk iron nickel zirconium samples from powders. The powder processing methods developed at the Army Research Lab have resulted in a variety of nanocrystalline stabilized metal alloy powders.
The excellent thermal stability of these powders enables the consolidation and evaluation of bulk components in ways that were previously not possible.
View the full transcript and gain access to thousands of scientific videos
Overview
This paper provides an overview of the Army Research Laboratory's efforts in processing bulk nanocrystalline metals, focusing on methodologies for producing novel metal powders.
Key Study Components
Area of Science
- Materials Science
- Metallurgy
- Nanotechnology
Background
- The U.S. Army is developing stabilized bulk nanocrystalline alloys.
- These materials are intended for use in diverse operating conditions.
- Ball milling is a key technique for synthesizing large volumes of nanocrystalline powders.
- Advanced characterization techniques are employed to analyze structure-property relationships.
Purpose of Study
- To produce mechanically alloyed nanocrystalline powders.
- To facilitate industrial-scale production of nanocrystalline materials.
- To explore various consolidation techniques for these materials.
Methods Used
- Ball milling for powder synthesis.
- Transmission electron microscopy for characterization.
- Atom tomography for materials analysis.
- Consolidation techniques including ECAE, FAST, and HIP.
Main Results
- Successful production of nanocrystalline powders.
- Characterization techniques provided insights into material properties.
- Demonstrated feasibility of industrial-scale production.
- Established processing relationships for bulk alloys.
Conclusions
- The study advances the understanding of nanocrystalline metal processing.
- It highlights the importance of synthesis techniques in material performance.
- Future work will focus on optimizing production methods.
Frequently Asked Questions
Executive Industry Relevance
The development of stabilized bulk nanocrystalline metals addresses critical material performance gaps in defense and structural applications where conventional alloys fail under extreme conditions. By enabling industrial-scale production of nanocrystalline powders through ball milling and advanced consolidation techniques, this work supports predictive material selection and de-risks early-stage alloy development for high-reliability systems. The methodology provides a scalable pathway from discovery to testing, reducing reliance on trial-and-error in high-stakes material qualification.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of structure-property relationships in mechanically alloyed systems to validate functional hypotheses about grain boundary strengthening and thermal stability.
- Operational Value: Provides reproducible nanocrystalline powder synthesis under controlled inert atmospheres, ensuring batch-to-batch consistency for mechanistic studies.
Screening & Assay Development
- Scientific Value: Produces quantitatively characterized powders with defined particle size distributions (10–500 µm) and nanostructure, enabling reliable input for downstream processing evaluations.
- Operational Value: Supports high-throughput screening of alloy compositions via scalable ball milling protocols that maintain argon-purged conditions to prevent oxidation.
Translational & Preclinical Research
- Scientific Value: Facilitates microstructural refinement through techniques like ECAE and HIP, allowing assessment of processing-structure-property relationships critical for predicting in-service performance.
- Operational Value: Enables generation of bulk test specimens with measurable density (up to 96%) and texture, supporting mechanical evaluation in simulated operating environments.
Pipeline & Workflow Integration
The described process integrates powder synthesis, characterization, and consolidation to form a discovery-to-validation workflow for structural alloys, where mechanical alloying enables hypothesis testing, characterization informs lead selection, and consolidation supports preclinical-like evaluation.
- Discovery Biology: Ball milling under inert conditions allows systematic variation of elemental ratios to test hypotheses about solute drag and grain boundary stabilization in nanocrystalline systems.
- Screening: The use of standardized milling media, mass ratios, and cryogenic controls ensures reproducible powder generation, a prerequisite for comparative alloy screening.
- Analytics: Transmission electron microscopy and atom probe tomography deliver quantitative nanoscale and chemical data essential for comparing processing outcomes across alloy systems.
- Translational Research: Consolidation via HIP and ECAE bridges powder synthesis to bulk property evaluation, enabling risk-adjusted decisions before full-scale prototyping.
- Enterprise Reuse: The argon-purged, scalable ball milling platform can be adapted across multiple alloy systems, serving as a reusable capability for nanocrystalline material discovery.
Operational & Enterprise Impact
- Scientific Value: Predictive confidence in thermal stability and mechanical behavior through direct correlation of processing parameters with nanostructure and chemistry.
- Operational Value: Standardized, inert-atmosphere powder synthesis and consolidation techniques enhance reproducibility and reduce variability in early material evaluation.
- Strategic Value: Early identification of thermally stable nanocrystalline alloys reduces late-stage failure risk in structural component development.
- Portfolio Impact: Enables risk-based prioritization of alloy candidates based on achievable density, grain refinement, and elemental distribution from pilot-scale processing.
Implementation Considerations
- Expertise in mechanical alloying, inert atmosphere handling, and advanced electron microscopy is required for powder synthesis and characterization.
- Instrumentation includes high-energy shaker mills, horizontal rotary ball mills with cooling jackets, and consolidation tools such as ECAE dies and HIP units.
- Cross-team standardization depends on shared protocols for argon purging, milling ball-to-powder ratios, and thermal cycling during cryogenic milling.
- Adaptation across model systems requires adjustment of milling duration, ball media composition, and consolidation temperature based on alloy-specific reactivity and thermal stability.
- Practical limitations include challenges in achieving full density during consolidation and potential contamination from milling media, as noted in the requirement for post-milling powder cleaning steps.
Biopharma FAQs
Chapters in this video
0:04
Title
1:17
Small-scale Synthesis of Nanocrystalline Powders under Ambient Conditions
2:58
Small-scale Synthesis of Nanocrystalline Powders under Cryogenic Conditions
4:32
Large-scale Synthesis of Nanocrystalline Powders
6:39
Results: Characterization and Processing of Nanocrystalline Powders
8:29
Conclusion
