Acta mater. 49 (2001) 1725–1736 www.elsevier.com/locate/actamat INFLUENCE OF Fe SUBSTITUTIONS ON THE DEFORMATION BEHAVIOR AND FAULT ENERGIES OF Ni 3 Ge–Fe 3 Ge L1 2 INTERMETALLIC ALLOYS T. J. BALK†, MUKUL KUMAR‡ and K. J. HEMKER§ Departments of Mechanical Engineering and Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218-2686, USA ( Received 28 December 2000; accepted 14 February 2001 ) Abstract—Ni 3 Ge exhibits a yield strength anomaly, whereas the yield strength of Fe 3 Ge shows a normal decline with temperature, and there is a gradual transition from anomalous to normal behavior as Fe content increases. A dramatic strengthening for 77 K deformation has also been noted to occur in these alloys as a result of increasing Fe content. The combined use of transmission electron microscopy (TEM) and image simulations has facilitated identification of the operative deformation mechanisms and allowed for a quantitat- ive measure of superdislocation dissociations. A transition from octahedral glide and Kear–Wilsdorf locking to cube glide of superdislocations has been observed to coincide with an increase in either deformation temperature or Fe content. The low-temperature strengthening has been correlated with enhanced cross-slip, which is aided by a significant lowering of the cube-plane antiphase boundary energy with increasing Fe content. It is proposed that the strengthening and the transition to cube glide are promoted by an increase in the complex stacking fault energy, which enhances both cross-slip and cube-plane mobility.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Transmission electron microscopy (TEM); Dislocations (mobility); Intermetallic compounds; Mechanical properties (plastic); Image simulation 1. INTRODUCTION The anomalous increase of flow strength with increas- ing temperature exhibited by many L1 2 intermetallic compounds has attracted considerable attention in the literature. The deformation mechanism responsible for the yield strength anomaly is well documented, see for example [1]. The anomaly is associated with the thermally activated formation of Kear–Wilsdorf (KW) locks [2]; the process involves cross-slip of screw-oriented superdislocations from a {111} octa- hedral plane, where they are mobile, to a {010} cube cross-slip plane, where they are subjected to high fric- tion stresses. The dissociation of a superdislocation in the L1 2 -ordered crystal structure results in the for- mation of an antiphase boundary (APB) bounded by two similar 1/2110 superpartials, and further sub- dissociation of the superpartial results in a complex † Present address: Max-Planck-Institut fu ¨r Metallfor- schung, Stuttgart, Germany. ‡ Present address: Lawrence Livermore National Labora- tory, University of California, 7000 East Avenue, L-356, Livermore, CA 94550 USA. § To whom all correspondence should be addressed. Tel.: +1-410-516-4489; Fax: +1-410-516-7254 E-mail address: hemker@jhu.edu (K. J. Hemker) 1359-6454/01/$20.00  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(01)00097-0 stacking fault (CSF) bounded by dissimilar Shockley partials. The planarity of the dissociated core geometry, and, in particular, the widths of the APB and the CSF that are governed by the respective fault energies, have a profound effect on the dislocation mobility and, consequently, the gross deformation behavior [1]. There are several L1 2 alloys that do not exhibit the anomalous yield stress behavior. In order to study the transition from anomalous to normal mechanical behavior of L1 2 intermetallics, the model system (Ni x Fe 1-x ) 3 Ge was chosen. This system is pseudobi- nary and has been reported to exhibit complete solid solubility if the Ge content is held constant at 25 at% and Fe is substituted for Ni as the composition varies from Ni 3 Ge to Fe 3 Ge [3]. The L1 2 compounds Ni 3 Ge and Fe 3 Ge show dramatically different mechanical behavior. Ni 3 Ge has been reported to have a strong anomaly [3], whereas Fe 3 Ge shows a more normal decrease of yield strength with increasing temperature [3, 4]. Moreover, Suzuki et al. [3] have reported that the temperature dependence of flow strength changes from anomalous to normal as the Fe content is increased, and this has also been corroborated by the current authors [5, 6]. The dislocation structures have been described for binary Ni 3 Ge [7] and Fe 3 Ge [8],