STRUCTURE, STRENGTH AND TOUGHNESS OF NANOCRYSTALLINE FeAl M.A. Morris-Mun ˜oz, A. Dodge and D.G. Morris Department of Physical Metallurgy, CENIM, CSIC, Avenida Gregorio del Amo 8, 28040 Madrid, Spain (Received August 4, 1999) (Accepted August 15, 1999) Abstract—Refining grain size to the nanocrystalline level has been suggested as a way of improving strength while enhancing ductility and toughness. In the present study, nanocrystalline bulk FeAl has been prepared by mechanical alloying and hot forging. Powders quickly reach a state of partial order during milling, and low temperature annealing is sufficient to chemically homogenise and give full order. Contamination during milling leads to the formation of carbide and oxide particles, which stabilise fine grains during heating. Bulk materials show grain sizes of 20nm to 100nm depending on the consolidation temperature. Hardness and compression strength show little change over this grain size range. Fracture toughness stays high down to moderately small grain sizes, falling only for consolidation at the lowest temperatures. There appears to be a reasonable range of fine grain sizes (40 –100nm) where good interparticle bonding and high densities can be achieved leading to good strength and toughness. ©1999 Acta Metallurgica Inc. Introduction The behaviour of FeAl powders during milling has been studied on many occasions (e.g. 1– 6) and it is well established that a partially-ordered nanocrystalline state is obtained after a long period of milling, independent of whether the starting state is fully ordered alloy or elemental powders. The powders re-order at low temperatures during heating, and this is followed by the loss of more stable structural defects and eventually by recrystallization and grain growth. Similarly high temperatures are required for good consolidation of powders to bulk material, that is to remove essentially all porosity and to achieve good interparticle bonding (e.g. 7–12), and indeed one of the biggest challenges of such consolidation procedures is to retain a fine grain size by limiting the time and temperature required for consolidation. Finer grain sizes may be retained during such heating when solid solution alloy powders instead of elemental powders are examined, since the presence of solute may lower diffusivity, and when fine particles are present to pin grain boundaries (13). Obtaining bulk materials with nanocrystalline grain sizes is of interest because of the improved hardness and strength found, but also because of expectations of better ductility and toughness (7–12). However, while increased hardness and strength in compression has often been reported, at least down to grain sizes of the order of 20 –50nm (7–12), only few studies have examined mechanical behaviour under some form of tensile loading (such as tension or bend testing) (7,9,11,12,14 –16). In most, if not all, of the cases reported brittle behaviour is found at very small grain sizes and, even when some ductility is observed, there is little evidence of improvements in ductility or toughness over that typical of materials of conventional grain sizes. Pergamon NanoStructured Materials, Vol. 11, No. 7, pp. 873– 885, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$–see front matter PII S0965-9773(99)00385-2 873