P HILOSOPHICAL MAGAZINE A, 1999, VOL. 79, NO. 6, 1279±1294 Electron microscopy investigation of ternary c -brass-type precipitation in a Ni 39. 6 Mn 47. 5 Ti 12. 9 alloy J.- W. S eo ² , D. Schryvers,W.Vermeulen, O. Richard Rijksuniversitair Centrum te Antwerpen, Groenenborgerlaan 171, B-2020 Antwerp, Belgium and P. Potapov Institute for Physical Metallurgy, 9/23, 2nd Baumanskaya, 107005 Moscow, Russia [ Received 13 February 1998 and accepted in revised form 28 July 1998 ] Abstract Homogenized Ni 39.6 Mn 47.5 T 12.9 material was investigated by dierent electron microscopy techniques. Apart from the martensite precursor distortions typical for B2 phase alloys undergoing a thermoelastic martensitic transformation upon cooling, coherent dodecahedron-shaped precipitates with sizes between 20 and 100 nm and faceted by lozenge shapes of {110}-type planes are observed. Selected- area and microdiraction patterns reveal an overall unit cell with a size of 3 3 3 units of the bcc lattice of the matrix and a body-centred symmetry without screw axes. Finally a ternary g -brass-type atomic structure of space group I4 3m is suggested for these precipitates in accordance with the obtained symmetry constraints, the energy-dispersive X-ray measurements and high- resolution transmission electron microscopy images. This is the ®rst time this type of structure is found in an alloy completely consisting of transition-metal elements. § 1. Introduction It has been known for a while that the cubic-to-tetragonal martensitic transfor- mation as well as the shape memory eect in Ni±Mn intermetallic alloys are depen- dent on composition (Adachi and Wayman 1984, Baele et al. 1987) and can also be in¯uenced by introducing ternary alloying elements such as Al (Henninger et al. 1986, Morito et al. 1993, Inoue et al. 1994, Potapov et al. 1996) or Ti (Vintaykin et al. 1990, Potapov 1993, 1994, Udovenko et al. 1995, Potapov et al. 1996). In particular, the martensitic transformation start temperature M s of the ternary alloys in which Ni and/or Mn is replaced by Ti decreases with increasing Ti content (Udovenko et al. 1995; Potapov et al. 1996). Potapov and co-workers also discussed the eect of alloying on the lattice parameters, microstructure and shape memory behaviour using X-ray and neutron diraction and transmission electron microscopy (TEM). From their neutron diraction results they concluded that in alloys with a [Mn]/[Ni]ratio smaller than one the Ti atoms occupy Mn sites and the excess of Mn + Ti with respect to the equiatomic ratio is accommodated by the introduction 0141±8610/99 $12. 00 Ñ 1999 Taylor & Francis Ltd. ² Present addresses: Institut de Physique, Universite  de Neuchatel, CH-2000 Neuchatel, Switzerland, and the IBM Research Division, Zu È rich Research Laboratory, CH-8803 Ruschlikon, Switzerland.