An investigation of the Ni 5 Al 3 phase
- PDF / 667,818 Bytes
- 5 Pages / 595.44 x 841.68 pts Page_size
- 30 Downloads / 257 Views
I. INTRODUCTION The Ni 5 Al 3 phase, having the Pt 5 Ga 3 -type structure, was first identified by Enami and Nenno1 in tempered nickel-rich NiAl martensite by transmission electron microscopy. They determined its space group to be orthorhombic D \9h (Cmmm) with a unit cell as shown in Fig. 1. The positions of nickel atoms in this unit cell are 0 0 0, 1/2 1/2 0, 1/4 1/4 0, 3/4 3/4 0, 1/4 3/4 0, 3/4 1/4 0, 0 1/4 1/2, 0 3/4 1/2, 1/2 3/4 1/2, 1/2 1/4 1/2. The positions of aluminum atoms are 1/2 0 0, 0 1/2 0, 1/4 0 1/2, 3/4 1/2 1/2, 3/4 0 1/2, 1/4 1/2 1/2. Later Robertson and Wayman2 studied this phase in some detail and reported it to be a stable phase up to approximately 700 °C. Robertson and Wayman2 employed x-ray diffraction in addition to transmission electron microscopy in their study. However, x-ray diffraction intensities in their study were reportedly influenced by preferred orientation effects arising from the use of bulk specimens. These authors did not report any x-ray diffraction data. Singleton, Murray, and Nash 3 incorporated this phase in the revised Ni-Al phase diagram based upon the above two studies as well as that of Litvinov and ArkhangeFskaya.4 A portion of their phase diagram is reproduced in Fig. 2. The Ni 5 Al3 phase is of particular significance because it occurs in an important region between NiAl and Ni3Al. The Ni 5 Al 3 phase results from a transformation of Ni-rich B2 NiAl that is stable at high temperatures. The Ni-rich B2 NiAl, with excess nickel atoms on aluminum sites, undergoes Bain distortion and transforms to L l 0 martensite upon quenching. A simple ordering of the nickel atoms on the aluminum sublattice (which can take place at low temperatures) is then required to transform the martensitic structure to Ni 5 Al 3 . 5 This sequence of transformations is depicted in Fig. 3. Here B2 NiAl is represented in its tetragonal orientation with a J. Mater. Res. 2 (2), Mar/Apr 1987
http://journals.cambridge.org
c/a ratio of 0.707, which changes to the L l 0 martensite with a larger c/a ratio, which then orders itself further to the Ni 5 Al 3 orthorhombic structure. The available information on the lattice constants of the Ni 5 Al 3 phase and the changes occurring in the structure during the above sequence of transformations has been obtained primarily from electron diffraction studies. It is the purpose of this paper to provide more accurate information based on a detailed x-ray diffraction study of an alloy that goes through these transformations. This information is particularly critical in view of the renewed interest in the nickel aluminides, both NiAl and Ni3Al.6 II. EXPERIMENTAL The alloy studied was obtained by hot extrusion of a blend of Ni3Al (24 at. % Al) and NiAl (44 at. % Al)
FIG. 1. Unit cell of Ni5Al3 [orthorhombicD\l (Cmmm)}.
0003-6951 /87/020163-05S01.75
Downloaded: 13 Mar 2015
© 1987 Materials Research Society
IP address: 129.21.35.191
163
P. S. Khadkikar and K. Vedula: Investigation of the Ni5AI3 phase
The mild steel can was removed from the extrusion by centerless
Data Loading...
