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.
