Processing of Bulk Nanocrystalline Metals at the US Army Research Laboratory

Summary

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.

Abstract

Given their potential for significant property improvements relative to their large grained counterparts, much work has been devoted to the continued development of nanocrystalline metals. Despite these efforts, the transition of these materials from the lab bench to actual applications has been blocked by the inability to produce large scale parts that retain the desired nanocrystalline microstructures. Following the development of a method proven to stabilize the nanosized grain structure to temperatures approaching that of the melting point for the given metal, the US Army Research Laboratory (ARL) has progressed to the next stage in the development of these materials - namely the production of large scale parts suitable for testing and evaluation in a range of relevant test environments. This report provides a broad overview of the ongoing efforts in the processing, characterization, and consolidation of these materials at ARL. In particular, focus is placed on the methodology used for producing the nanocrystalline metal powders, in both small and large-scale amounts, that are at the center of ongoing research efforts.

Introduction

Nanocrystalline metals prepared by high energy mechanical alloying have been shown to exhibit superior mechanical strength as compared to their coarse-grained counterparts. However, as dictated by thermodynamic principles, nanocrystalline microstructures are subject to grain coarsening at elevated temperatures. As such, processing and applications of these materials is currently limited by the ability to create stabilized microstructures in bulk form. Given the potential of these materials, two primary methods are being pursued in an effort to develop such systems. The first, based on a kinetic approach, utilizes several mechanisms to apply a pinning force on the grain boundaries (GBs) in order to prevent grain growth. Typical mechanisms employed to pin the GBs are secondary phases (Zener pinning)^1,2,3 and/or solute drag effects^4,5. The second method, based on a thermodynamics approach, suppresses grain growth by reducing the GB free energy through solute atoms partitioning to the GBs^{6,7,8,9,10,11,12,13,14,15,16}.

As the first step to developing alloys with a nanograined microstructure, the fundamental understanding into thermodynamic and kinetic principles that govern grain growth and microstructural stability at elevated temperatures was established. Computational materials science was also used to guide alloy development. Using these insights, small scale lots of various alloy powders were produced using high energy milling and evaluated for a broad range of physical and mechanical properties. For the more promising systems, advanced characterization techniques were developed in order to fully link the microstructure of the powder to the observed properties and performance.

Simultaneously, the infrastructure and equipment needed to produce bulk components from the nanocrystalline powders was acquired. Once this equipment was in place, the processing science required to fully consolidate bulk materials from the alloy powders was developed through a series of small scale experiments. Once bulk specimens were available, a series of experiments were performed to understand the mechanical response of these materials under a broad range of conditions (such as fatigue, creep, high strain rate, etc.). The knowledge gained from these experiments has been used to develop possible application spaces that will enable the commercialization of the stabilized bulk nanocrystalline alloys.

Collectively, meeting these tasks has led to the development within the U.S. Army Research Laboratory (ARL) of a nanocrystalline metals research center consisting of 4 main labs. This laboratory complex represents a total investment of 20 million USD and is unique in that it spans aspects of fundamental, applied, and manufacturing science. The primary purpose of these labs is to transition proof-of-concept ideas to the pilot-scale and pre-manufacturing levels. In doing so, it is anticipated that the labs will enable the production of prototype parts, develop the necessary know-how and manufacturing science for scaled-up processing, and allow for linkages internally as well as to external research institutes or industrial partners via the commercialization and transition of this advanced powder technology.

As indicated earlier, the first step is to identify, produce, and rapidly assess new alloy prototypes for both feasibility of synthesis and fabrication into prototype parts. To accomplish this, several unique, custom-designed high energy shaker mills have been constructed with the capability to process powders over a wide range of temperatures from -196 °C to 200 °C. As the name implies, these mills produce approximately 10-20 g of fine powders through the violent shaking action that causes repetitive impacts between powder and grinding media to produce powders in which each particle has a composition in proportion to the starting elemental powder mix. While suitable for the rapid screening of powders, mills of this type are clearly not suitable for powder production on the (near) industrial scale (e.g., kilograms).

Given the need to produce powder in large quantities and in as continuous a process as possible, a search was undertaken to identify potentially viable methods and equipment. Planetary ball mills use a support disk which rotates in the opposite direction from the vertically oriented vials, resulting in particle size reduction due to both grinding and collisions caused by centrifugal forces. Lot sizes for most planetary mills range up to approximately 2 kg. Unlike conventional mills, attritor mills consists of a series of impellers inside a vertical drum. The rotation of the impellers cause the motion of the grinding media, resulting in particle size reduction through collisions between powder, balls, and the impellers. Larger attritor mills are capable of producing over 200 kg per run. Although both of these mills offer significant increases in lot sizes relative to shaker mills, they are not capable of running in a continuous fashion but must rather be loaded and unloaded manually for each run.

Due to these shortcomings, attention shifted to a series of high energy, horizontal rotary ball mills. Capable of processing as much as 200 kg per batch, these mills are also capable of operating under inert atmospheres as well as vacuum. Finally, the milling chamber has been designed with an airlock that allows for the rapid and automated removal of powder once the milling process has been completed. Combined with an automatic powder injection system, this means that the ball mill is capable of running in a fairly continuous manner, thereby making it a highly viable system for industrial settings. Due to these combination of features, ARL has recently purchased and installed two mills and is now engaged in upscaling internal powder processing efforts.

While the powder processing efforts represent a central aspect of on-going efforts, the characterization and consolidation of the most promising alloy powders are also areas of focused research. Indeed, as detailed below, ARL has made notable investments in the requisite analytical and test equipment needed to fully evaluate key features of the new powders. Moreover, successful consolidation of samples now allows for conventional full scale mechanical testing and characterization (e.g., tension, fatigue, creep, shock and ballistic evaluation) of these materials which has typically not been feasible for this class of material. This article reports the protocols utilized at ARL for initial synthesis, scale-up, consolidation and characterization of bulk nanocrystalline metals and alloys.

The two main labs for powder synthesis can be seen in Figure 1. Figure 1A shows the small-scale powder processing lab which enables the rapid development of concepts and alloy design. This lab contains several custom-designed high energy mills with the capability to process powders over a range of temperatures (room temperature to 400 °C and 10 to -196 °C). The lab also contains a custom horizontal tube furnace designed for the rapid assessment of the thermal and microstructural stability (e.g., grain growth studies) of new metal alloys. Finally, the lab also houses several unique small-scale mechanical test setups including tension, shear punch, and impression creep testing devices, as well as a state-of-the-art instrumented nano-indenter. Once thoroughly tested and of shown promise, selected alloys are moved to the large scale processing lab (Figure 1B), where the engineering and manufacturing protocols are developed to allow large scale (e.g., kilogram) production of the specific powder. In total, the labs represent a total investment on the order of 2 million USD and covers the transition of novel metal powders from the lab bench to the pilot-scale manufacturing levels, thereby enabling the production of prototype parts.

High energy ball milling/mechanical alloying is a versatile process for producing nanocrystalline metals and alloys in powder form¹⁷. Starting with coarse grained powders (typically mean grain size ~5-10 µm), it is possible to obtain nanocrystalline powders with mean grain size < 100 nm after milling. This milling is routinely performed in a vibratory/shaker mill. The milling vial is filled with the desired amount of powder as well as milling balls, typically stainless steel. This mill shakes the vials in a motion that involves back and forth oscillations with short lateral movements at a rate of approximately 1080 cycles min^-1. With each complex motion the balls collide with one another, impact against the inside of the vial and the lid, and simultaneously reduce the powder to finer size. The kinetic energy imparted into the powder is equal to half the mass times the square of the average velocity (19 m s^-1) of the bearings. The mill power, e.g. the energy delivered per unit time, increases with the frequency of the mill (15-26 Hz). Taking the typical number of balls and the lowest frequency for a given 20 h period, the total number of impacts exceeds 1.5 billon. During these impacts the powder undergoes repeated fracturing and cold-welding until the point where the constituents are mixed at the atomic level. Microscopically this mixing and refinement of the microstructure is facilitated by localized deformation in the form of shear bands as well as a high density of dislocations and point defects which breaks down the microstructure. Eventually, as the heat of collision raises the local temperature, recombination and annihilation of these defects occurs at a steady state with their generation. The defect structures eventually, though reorganization, result in the formation of smaller and smaller high angle equiaxed grains. Thus, ball milling is a process that induces severe plastic deformation manifested by the presence of a high density of defects. This process allows for increased diffusivity of solute elements and the refinement and dispersion of secondary phases and the overall nanostructuring of the microstructure.

High energy cryomilling is a milling process similar to high energy ball milling except for the fact that the milling vial is maintained at cryogenic temperature during the milling process. In order to achieve a uniform temperature in the vial, the mill has been modified as follows. The milling vial is first placed inside a Teflon sleeve which is then sealed with a Teflon cap. The sleeve is connected to a dewar containing the appropriate cryogen (liquid nitrogen (LN₂) or liquid argon (LAr)) through stainless steel and plastic tubing. The cryogen flows through the sleeve throughout the milling process to cool the milling vial and maintain the milling vial at the boiling temperature of the cryogen, such as -196 °C for LN₂ and -186 °C for LAr. The low temperatures of cryogenic processing lead to the increased fragmentation of more ductile metals which otherwise cannot be milled at room temperature. Additionally, the cryogenic temperatures reduce thermally activated diffusional processes such as grain growth and phase separation thereby allowing increased refinement of the microstructure and solubility of insoluble elemental species.

The high energy horizontal rotary ball mill is a high energy milling system that consists of a horizontal stainless-steel milling jar with a high-speed rotor with several blades fixed on a drive shaft. The powder to be milled is transferred inside the jar along with the milling balls. Movement of the balls and powder is achieved through the rotation of the shaft inside the jar. The shaft rotates at high speed and the milling steel balls collide, accelerate, and transfer their kinetic energy to the powders. The range of rpm is 100 - 1000 and the average velocity of the balls is 14 m s^-1. In particular, mills are equipped to operate over a range of milling temperature (-30 °C to 200 °C high) and can be run under vacuum (mTorr) or in over pressure mode (1500 Torr) (utilizing various types of cover gas). In addition to the base unit, the mill is equipped with a carrier gas discharge unit as well as connection assemblies which allows the loading and unloading of powder under inert gas cover. This apparatus can be seen in Figure 2A along with a typical 8 L steel milling jar (Figure 2B). In addition to the larger mill, ARL has purchased a smaller mill which has been converted to run under liquid nitrogen (Figure 2C). This mill can produce between 100-400 g of processed powder per running cycle.

Protocol

1. Small Scale Synthesis of Nanocrystalline Powders under Ambient Conditions

1. In a controlled argon atmosphere glove box, place 10 g of the primary element (e.g., Fe in FeNiZr alloy) and 100 g of stainless steel/tool steel milling balls in the desired milling jar.
   NOTE: Loading of powder into milling jar inside a glove box is required to ensure minimal uptake in oxygen and/or moisture content ^18,19.
2. After loading, seal the jar and remove from the glove box. After removal, ensure that jar is fully sealed and load into the appropriate milling machine.
3. After performing a 1 h milling cycle, remove the vial and transfer it back into the argon-filled glove box.
   NOTE: This short run serves to coat all surfaces with the primary element, thereby helping to reduce the transfer of contaminants from milling jar and media to the alloy being produced.
4. To synthesize the alloy powders, add a total of 10 g of elemental powders in the desired ratios to the just coated milling jar inside the glove box. Add the required amount of just coated milling balls to the jar such that there is a 10:1 ratio of the mass of the balls to mass of powder. The lid should be placed and tightened on the milling jar before removal from the glove box. After removal, further tightening of the lid should be performed using a wrench and a vice.
5. Place the vial in the high energy shaker mill and initiate milling operation (typically on order of 20 h). After the milling is completed, remove the vial and transfer it to the glove box. Carefully remove the lid and transfer the milled powder to the desired sample vial for storage.
   NOTE: A typical high energy shaker mill used in mechanical alloying is shown in Figure 3A. A schematic showing how high energy milling results in nanocrystalline materials is shown in Figure 3B, with an image showing an average final particle size between 10 and 500 µm shown in Figure 3C.

2. Small Scale Synthesis of Nanocrystalline Powders under Cryogenic Conditions

1. Perform coating run for milling jar and balls as described in Steps 1.1-1.3.
2. In controlled atmosphere glove box, fill coated milling jar with desired amount of elemental powders and milling media. After tightening the jar, remove from the glove box.
3. Place the milling jar inside a Teflon sleeve and cap, which is then placed in the clamp of the high energy shaker mill.
4. Open the dewar containing the cryogen and allow it flow for about 30 min to ensure the milling jar has reached the desired temperature (-196 °C for liquid nitrogen and -186 °C for liquid argon).
5. Upon reaching equilibrium, initiate the milling operation until the desired duration has been reached. Upon completion, close the dewar, carefully remove the milling jar from the sleeve and place it in front of a dryer to bring it to room temperature.
6. Once the milling jar reaches room temperature, transfer it back inside the controlled atmosphere glove box. Carefully open the milling jar and transfer the powders to desired storage vial.
   NOTE: A picture of the high energy shaker mill adapted for use at cryogenic temperatures is shown in Figure 4A. Shown in Figure 4B is a milling vial immediately after it has been removed from a cryomilling operation. Figure 4C provides an idea of the number of milling balls typically used in a processing operation.

3. Large Scale Synthesis of Nanocrystalline Powders

1. Load the required elemental alloying powders into a glass jar inside an argon glove box, seal, and remove.
2. After attaching the vessel to the high-energy horizontal rotary ball mill, load approximately 1 kg of 440C stainless steel ball bearings into a stainless steel 8 L vessel contained within a cooling jacket.
   Note: Images of the various parts of the high energy horizontal rotary ball mill are shown in Figure 5.
3. Connect the argon gas line and coolant lines to the vessel. Back-fill and purge the vessel with argon gas to remove air.
4. Using a double ball valve, transfer the alloying elemental powders into the milling vessel and then close the valve to seal the chamber.
5. Connect the powder extraction system to the milling vessel and then back-fill and purge the extraction system with argon gas to remove air.  
6. Start flowing ethylene glycol at -25 °C through the outer jacket of the vessel.
7. Begin the milling process for up to 1 kg of elemental powders for the desired amount of time (typically 12-30 h) using rotational energy of 400-800 rpm. Once the milling is completed, transfer the powders to a jar under argon atmosphere. Store the jar in an argon filled glove box.

Results

Approximately 10 g of powder are produced per each run in the high energy shaker mill. After successful synthesis of novel nanocrystalline metals and alloys in high energy shaker mill, scale-up is conducted in a high energy horizontal rotary ball mill.

Typically, nanostructured powders are generated using high energy milling processes, wherein the grain size of a small amount of powder is refined, approximately 10 g per batch. T...

Discussion

Compared to other synthesis techniques, mechanical alloying is an extremely versatile method for producing metal and alloyed powders with grain sizes <<100 nm. Indeed, mechanical alloying is one of the few ways in which large volumes of nanostructured materials can be produced in a cost effective and easily scalable manner. Furthermore, high-energy ball milling has been shown to vastly increase the limit of solid solubility in many metallic systems in which equilibrium room temperature solubility does not otherwis...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Copper powder	Alfa Aesar	42623	Spherical, -100+325 mesh, 99.9%
Tantalum powder	Alfa Aesar	10345	99.97%, -325 mesh
Iron powder	Alfa Aesar	00170	Spherical, <10 micron, 99.9+%
Nickel powder	Alfa Aesar	43214	-325 mesh, 99.8%
Zirconium powder	American Elements	ZR-M-03-P	99.90%
SPEX mills (high energy shaker mills)	SPEX SamplePrep	8000M
Zoz mills (high energy horizontal rotary ball mill)	Zoz GmbH	CM01 (small mill) CM08 (large mill)
Focused Ion Beam	FEI	Nova600i Nanolab dual beam FIB/SEM
Scanning Electron Microscope	FEI	Nova600i Nanolab dual beam FIB/SEM
Precision Ion Polishing System	Gatan	Model 695
Transmission Electron Microscope	JEOL	2100F	multipurpose field emission TEM
Atom Probe Tomography	CAMECA	LEAP 5000XR
Equal Channel Angular Extrusion	ShearForm	custom built
Hot Isostatic Press	Matsys