NANOMETRIC GRAIN FORMATION IN DUCTILE POWDERS BY LOW-ENERGY BALL MILLING J. Guerrero-Paz and D. Jaramillo-Vigueras Department of Metallurgical Engineering, ESIQIE-IPN, D.F., Me´xico, CP 07300 (Received August 23, 1999) (Accepted October 25, 1999) Abstract—Based on microstructural observations by TEM and in particle size distribution done by sedimentation- photometry, a new grain size refinement mechanism for ductile powders in mechanical alloying is proposed. A 90 –95% of the particle population was of submicrometric fragmented particles. These were detected from the beginning of the milling process up to 90 ks. It seems that the fragmentation of the original particles occurred under dynamic conditions to generate those submicrometric ones. Under these conditions the original grain size (100 nm to 350 nm) was preserved and a low level of dislocations was observed at these submicrometric particles. Once these submicrometric particles were deformed, grains smaller than 20 nm were observed. It seems from TEM results that the submicrometric fragmented particles were also deformed under dynamic conditions. This could be a new grain size refinement mechanism present in ductile metallic powder systems where the fragmentation is the dominant stage from the beginning of the milling up to some intermediate milling time. In the Cu-20at%Ni, Cu and Ni systems where the particle coalescence process was the dominant stage during all the milling process, a derivation of the mechanism proposed by Hellstern [3] was identified. In our case, powders were mainly deformed by slip and not by shear. It recognizes that the way to refine the grain size in milled powders is influenced at least by the metallic system used as well as by the equipment and the process conditions employed. ©2000 Acta Metallurgica Inc. 1. Introduction Events of deformation, fracture and cold welding occur in mechanical alloying (MA) [1]. The grain size of milled powder is refined up to the nanometric size (smaller than 100 nm). It has been established that a nanostructured material, which has a high density of grain boundaries, exhibits unusual properties over those of coarse-grained polycrystalline materials [2]. However, in spite of the importance that the nanometric grain has as the responsible of the unusual properties of the mechanically alloyed materials, a complete exploration of the variables that influence its refinement has not been made. New grain size refinement mechanisms or derivations from the actual proposed ones are expected. Hellstern et al [3] milled Al and Ru powders to form the intermetallic AlRu by high-energy ball milling. They proposed that the nanometric grain size could be attained due to the deformation by microshear bands, which were approximately 0.5 mm wide. Dislocation networks were formed within the shear bands. With subsequent deformation, dislocation arrays became subgrains. Finally, high-angle nanometric grains were achieved by rotation of its boundaries. By other side, Klassen et al. [4] found nanometric grains of an intermetallic alloy (TiAl 3 ) between pure Ti and Al layers after 36 ks of milling (high-energy equipment). These two previous works suggest that different mechanisms to attain a nanometric grain size by MA seem to exist. From our previous studies of particle size evolution during milling, emerged the idea that the nanometric grain formation could have a strong relation with the submicrometric fragmented particles that appear from the early milling times in Cu-15at%Al powders milled in a horizontal ball mill [5]. It Pergamon NanoStructured Materials, Vol. 11, No. 8, pp. 1123–1132, 1999 Elsevier Science Ltd Copyright © 2000 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$–see front matter PII S0965-9773(99)00403-1 1123