Dynamic consolidation of metastable nanocrystalline powders
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INTRODUCTION
NANOSTRUCTURED materials (polycrystalline with grain sizes of < 100 nm) have been investigated in numerous studies in recent years. These materials, which have a significant percentage of the total atoms located at the grain boundaries, often possess quite different properties than their coarse-grained counterparts and, thus, become candidates for advanced materials with novel properties. Several recent reviews of nanostructured materials are available. I1'2'3J Although there are several methods of synthesizing nanostructured materials, mechanical processing of powders with high-energy ball milling[4,5,6~is one of the more promising methods capable of being scaled up to produce bulk quantities. Mechanically processed micronsized powders have been shown to possess nanosized substructures or even to become amorphous with appropriate milling parameters, tTJ In most systems, the nanograins or amorphous structure represents a "far from equilibrium" condition that is generally quite metastable in nature. For most commercial applications, the nanostructured powders must be consolidated and, oftentimes, further processed to create a final shape. Conventional consolidation and forming processes such as hot isostatic pressing (HIP) or hot extrusion require elevated temperatures for extended times, which can considerably coarsen the metastable structure and greatly compromise the novel properties of the as-milled powder. Dynamic or shock wave consolidation is a means of consolidating particulates without high bulk temperatures. Passage of a large-amplitude compressive stress wave resulting from plate impact or detonation of an explosive can cause densification by plastic flow in the particulate. Under appropriate conditions, bonding occurs by interparticle melting and rapid resolidification. Melting is generally G.E. KORTH, Principal Scientist, Metals and Ceramics, and R.L. WILLIAMSON, Senior Engineering Specialist, are with the Idaho National Engineering Laboratory, Lockhead Idaho Technologies Co., Idaho Falls, ID 83415-2218. This article is based on a presentation made in the symposium "Dynamic Behavior of Materials," presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
restricted to near-surface regions, and the bulk of the powder particles remains relatively cool. A review article by Gourdin I8] assesses the dynamic consolidation of metal powder materials; both experimental and analytical techniques are addressed in detail. Recently, shock consolidation from plate impacts (in a gas gun) was shown to largely retain the nanostructure of a ball-milled Fe-28A1-2Cr intermetallic, t9~ The purpose of this study is to investigate the retention of nanosized grain structures in titanium aluminide and iron powders during conventionaI and dynamic consolidation processes. Hot isostatic ~pressing and a more rapid pseudo-HIP techniq
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