Anthony J. Roberts

Processing of Bulk Nanocrystalline Metals at the US Army Research Laboratory

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.

Investigation of the Plastic Flow Field in Magnesium Alloy AZ31B in Three Orientations for Empirical Penetration Models

Experimental evidence to the extent of the plastic flow field in magnesium targets after ballistic impact has been lacking. The work described in this paper will include the ballistic characterization of the Magnesium alloy AZ31B-H24 in three orientations: normal direction; transverse direction; and rolling direction. Semi-infinite impacts from penetrators in each of the three directions will be shown. The targets were sectioned and machined using electrical discharge machining in preparation for polishing and etching to determine the extent of plastic flow that can be seen. The authors will investigate the applicability of Effective Flow Stress in the similitude-based empirical model and the Walker-Anderson Penetration Model for small arms penetration prediction. The results will be included in this paper.

Effect of Processing Parameters on the Microstructure of Mechanically Alloyed Nanostructured Al‐Mn Alloys

High-energy mechanical alloying was utilized to study the alloy formation between Al and Mn. A modified SPEX mill was designed to investigate the role of milling energy and milling power on enhancing the solubility of Mn in Al. Despite having a large influence on alloying kinetics (an increased rate of 3.25×) no further grain refinement or degree of alloying was observed with increasing milling energy. It is hypothesized that alloying in this particular binary system takes place and is controlled by the increasing interfacial surface area created between the Mn particles and the Al matrix. However, extensive comminution eventually likely leads to an autocatalytic exothermic reaction between the Mn particles and Al matrix, leading to the formation of the equilibrium phase Al6Mn. This reaction reduces the degree of and, in turn, frustrates further alloying, highlighting the practical limits of creating solid solutions in this system through mechanical alloying.