Acta mater. 48 (2000) 3833–3845 www.elsevier.com/locate/actamat NANOSCALE INHOMOGENEITIES IN MELT-SPUN Ni–Al P. L. POTAPOV 1 , P. OCHIN 2 , J. PONS 3 and D. SCHRYVERS 1 ² 1 Electron Microscopy for Materials Research (EMAT), University of Antwerp, RUCA, Groenenborgerlaan 171, B-2020 Antwerp, Belgium, 2 Laboratoire de Me ´tallurgie Structurale, ENSCP 11, Rue Pierre et Marie Curie, 75231 Paris Cedex 05, France and 3 Departament de Fı ´sica, Universitat de les Illes Balears, Ctra. de Valldemossa km 7.5, E-07071 Palma de Mallorca, Spain ( Received 7 April 2000; received in revised form 16 June 2000; accepted 21 June 2000 ) Abstract—Ni x Al 100 - x material with x = 62.5 or 65 was rapidly quenched to room temperature by the melt- spinning technique and studied using X-ray diffraction, different transmission electron microscopy (TEM) modes and calorimetry measurements. Similar to bulk material, the initial B2 structure undergoes a martensitic transformation to the L1 0 or 14M structure. However, the transformation proceeds very inhomogeneously and results in a mixed microstructure consisting of transformed and untransformed regions. The structure of the transformed regions varies from faulted L1 0 to faulted 14M and shows a variety of morphologies and features like wave-like interfaces and curvature of twin planes. The potential factors responsible for such an inhomogeneous behaviour, i.e. internal stresses, lattice defects, incomplete atomic ordering and compositional variations, are investigated and discussed. Finally, we conclude that the special structural state of the melt- spun material is explained mainly by solute segregation appearing during the crystallisation process. Thus, contrary to most other melt-quenched materials, in Ni–Al, solute segregation cannot be suppressed by the rapid quenching procedure.  2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Rapid solidification; Transmission electron microscopy (TEM); Intermetallic; Phase transform- ations; Martensite 1. INTRODUCTION The Ni–Al intermetallic compound in the compo- sition range from 42 to 70 at.% Ni (Fig. 1) attracts much attention due to the unique combination of a high melting point, a low density and a high oxidation resistance. Like many other B2 compounds, Ni-rich Ni–Al (with about 62–69 at.% Ni) undergoes a dif- fusionless {110}110 martensitic shear transform- ation on cooling resulting in a multiply twinned tetra- gonal L1 0 structure [1–4]. On heating, the B2 structure is restored by the reverse shear. In compo- sitions with less than 63 at.% Ni, the low temperature structure is more complicated and can be described as L1 0 with periodical microtwins on 111 L1o planes [5,6]. This structure typically has a stacking period of seven layers with a (52 ¯ ) sequence and is denoted as 14M [7] (formerly referred to as 7R or 7M). Occasionally, other stacking sequences, such as a 10- layered stacking, have been observed [8]. Whether or not the 14M structure should be considered as an independent structure or simply a variation of L1 0 ² To whom all correspondence should be addressed. Tel.: + 32-3-2180247; fax: + 32-3-2180257 E-mail address: schryver@ruca.ua.ac.be (D. Schryvers) 1359-6454/00/$20.00  2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(00)00188-9 with fine twins accommodating the transformation strain, is still a matter for discussion [8–10]. Still, as most reports on Ni–Al refer to the (52 ¯ ) stacking, the 14M structure will be used as the reference case for the long period martensite in the present discussions. Annealing Ni-rich Ni–Al at moderate temperatures results in the appearance of other phases such as Ni 2 Al [11–14], Ni 5 Al 3 [14–19] and Ni 3 Al [20] formed by decomposition or by a coupled diffusive–displace- ment mechanism. These transitions proceed relatively quickly and can interfere with the martensitic B2→L1 0 (14M) transformation. As a result, a strong dependence of the structural state of Ni–Al on the quenching rate has been reported several times [14, 21]. These days advanced preparation techniques involving the direct quench from the melt with a coo- ling rate of up to 10 6 K/s are readily available. These techniques allow for the avoidance of unpredictable annealings at moderate temperatures during conven- tional cooling or quenching which hamper a closer look at the low temperature diffusionless B2↔L1 0 (14M) transformation. Surprisingly, the first studies of rapidly quenched Ni–Al revealed an even more complicated structural situation than that in conven- tional (bulk) material. Kennon et al. [22] found the