Pergamon Acta mater. Vol. 45, No. 2, pp. 675-682. 1997 Copyright 0 1997 Acta Metallurgica Inc. Published by Elsevier Science Ltd PII S1359-6454(96)00181-4 Printed in Great Britain. All rights reserved 1359-6454197 $17.00 + 0.00 SUPERDISLOCATIONS AND ANTIPHASE BOUNDARY ENERGIES IN DEFORMED Fe3Al SINGLE CRYSTALS WITH CHROMIUM F. KRAL, P. SCHWANDER and G. KOSTORZ Institut fur Angewandte Physik, ETH Hiinggerberg, 8093 Zurich. Switzerland (Received 23 February 1996; accepted 9 Mav 1996) Abstract-The influence of Cr additions on the positive temperature dependence of the flow stress and on the antiphase boundary (APB) energies of Fe3Al was investigated. Single crystals of binary Fee28 at.% Al and ternary Fe-28 at.% Al-6 at.% Cr were deformed in uniaxial compression between room temperature and 1273 K. The dislocation arrangement and the dissociation of superdislocations were studied by transmission electron microscopy using the weak-beam technique. The operative slip systems are discussed on the basis of direct measurements of the APB energies. Three types of fault energies of superdislocations were analyzed as a function of temperature (up to 1003 K) and chromium concentration: the nearest-neighbor APB energy in the slip plane, the next-nearest-neighbor APB energy in the slip plane and the nearest-neighbor APB energy in the cross-slip plane. A procedure for calculating APB energies from effective pair potentials is given. The calculated results are in good agreement with the experimental data. Copyright 0 1997 Acta Metallurgica Inc INTRODUCTION Fe.?Al with Cr Recently iron aluminides such as Fe,Al with DO, structure and FeAl with BP structure have undergone an extensive study of their microstructure and mechanical properties as high temperature structural materials. Studies of the positive temperature dependence of the yield stress (first reported in Ref. [l]) and the critical resolved shear stress (CRSS) are of special interest. As for many other intermetal- lies, the poor ductility at ambient temperature has been an obstacle for further applications. The ductility of Fe3Al can be improved by adding chromium [2, 31.As a possible explanation McKamey and Liu [4] proposed the suppression of the environmental embrittlement (first studied by Liu and coworkers [S]) by the formation of a CrzO, oxide layer. Therefore Fe,Al with Cr seems to be one of the most promising aluminides for high temperature applications. The main superdislocation structures of the DO, and B2 iron aluminides were first described in Ref. [6] and are shown in Fig. 1. The difference in the antiphase boundary (APB) energies in different slip systems is considered to be the possible driving force for the increase in the CRSS at higher temperatures [7, 81. For the positive temperature dependence of yield stress the transformations between the ordered DO3 and B? structures or between Bz and the unordered b.c.c. (or c() structure are important [9]. The results of X-ray diffraction and electrical resistivity studies of these phase transformations are 4 r c- + Yl Y2 Yl R Fig. 1. Schematic representation of fourfold and twofold dislocations of (a) 4 x (111)/Z and (b) 2 x (111)/2 type. summarized in Table 1. Recently mechanical properties and microstructural features of polycrys- talline Fe,Al with Cr and related alloys have also been studied by transmission electron microscopy (TEM) [13-151. However, studies of the mechanical properties of single crystals are still missing. The use of single crystals eliminates many of the side effects in the movement of dislocations. Mechanical properties of single crystals, combined with a (hitherto unavailable) systematic determination of the APB energies, are then expected to further our understanding of the positive temperature dependence of yield stress on a microscopical level. Table I. Phase transformation temperatures of Fe-28 at.% Al and Fe-28 at.% AI-6 at.% Cr Alloy (at.%) DO1 F’ Bz (K) Bi * a (K) Fe-28 Al 813 [lo] 1170 [ll] Fe-2XAlL6Cr 826 [12] 1203 [12] 675