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 grai....

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 per....

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