On the Quaternary System Ti-Fe-Ni-Al Xinlin Yan, A. Grytsiv, P. Rogl, V. Pomjakushin, and H. Schmidt (Submitted November 6, 2007; in revised form April 10, 2008) The homogeneity ranges of the Laves phases and phase relations concerning the Laves phases in the quaternary system Ti-Fe-Ni-Al at 900 °C were defined by x-ray powder diffraction (XPD) data and electron probe microanalysis (EPMA). Although at higher temperatures the Laves phase forms a continuous solid solution, two separate homogeneity fields of TiFe 2 -based (denoted by k Fe ) and Ti(TiNiAl) 2 -based (denoted by k Ni ) Laves phases appear at 900 °C. The relative locations of Laves phases, G phase, Heusler phase, and CsCl-type phase as well as the associated tie-tetrahedra were experimentally established in the quaternary for 900 °C and presented in three-dimensional (3D) view. Furthermore, a partial isothermal section TiFe 2 -TiAl 2 -TiNi 2 was constructed, and a connectivity scheme, derived for equilibria involving Laves phases in the Ti-Fe-Ni-Al quaternary system at 900 °C was derived. As a characteristic feature of the quaternary phase diagram, the solid solubility of fourth elements in both the TiFe 2 -based and Ti(NiAl) 2 -based Laves phases is limited at 900 °C and is dependent on the ternary Laves phase composition. A maximum solubility of about 8 at.% Ni is reached for composition Ti 33.3 Fe 33.3 Al 33.4 . Structural details have been evaluated from powder x-ray and neutron diffraction data for (i) the Ti-Fe-Ni ternary and the Ti-Fe-Ni-Al quaternary Laves phases (MgZn 2 -type, space group: P6 3 /mmc) and (ii) the quaternary G phase. Atom site occupation behavior for all phases from the quaternary system corresponds to that of the ternary systems. For the quaternary Laves phase, Ti occupies the 4f site and additional Ti (for compositions higher than 33.3 at.%Ti) preferably enters the 6h site. Aluminum and (Fe,Ni) share the 6h and the 2a sites. The compositional dependence of unit cell dimensions, atomic coordinates, and interatomic distances for the Laves phases from the quaternary system is discussed. For the quaternary cubic G phase, a centrosymmetric as well as a noncentrosym- metric variety was observed depending on the composition: from combined x-ray and neutron powder diffraction measurements Ti 33.33 Fe 13.33 Ni 10.67 Al 42.67 was found to adopt the lower symmetry with space groupF  43m. Keywords crystal chemistry, diffraction (x-ray/neutron powder), intermetallics, phase equilibria, site occupancy, ternary and quaternary titanium aluminides, titanium- iron-nickel-aluminum 1. Introduction Intermetallic alloys based on the Ti-Fe-Ni-Al system are in the focus of aeroengine applications. Most inves- tigations in the system have been dedicated to improving the alloy properties for desired high-temperature applica- tions. It is well known that properties of alloys are controlled by several factors such as composition, heat treatment, phase constitution, and site occupancies. There- fore, a reliable phase diagram and a complete understand- ing of phase equilibria in a certain temperature range are important for modern alloy design based on thermodynamic modeling of the constitutional diagram. Recent reviews for the subsystems of the Ti-Fe-Ni-Al quaternary provide such information. [1-15] However, on the extension from the ternary systems to the quaternary system, there are only few investigations available in literature. [16-24] Past efforts associated with the quaternary system include: (i) the relation between structure and magnetic properties of a magnetically semipermanent alloy with specific composi- tions Fe-(10-25)Ni-(3-5)Al-1Ti (wt.%), [16] Fe-18Ni-3.8 Al-1Ti (wt.%), [17] Fe-13.8Ni-7.0Al-1.7Ti (at.%), [18] and Fe-16.2Ni-3.1Al-1.13Ti (wt.%), [19] (ii) alloying effects on superplastic behavior for Ti-based alloys Ti-5Fe-xAl-yNi (x = 4-8, y = 0, 1, or 2) (wt.%), [22] (iii) phase relation between CsCl-type and Heusler phases within the NiAl- TiNi-TiFe-FeAl section, [21] (iv) Partitioning behavior of alloying elements among the phases of c¢, g, b 1 , b 2 , and H in the Ni-Al-Ti base systems, [23] and (v) the effect of Ti additions on the microstructure of Ni-Al-Fe alloys. [20] In this work, we evaluate the homogeneity regions of the quaternary Laves phase with MgZn 2 -type structure in continuation of our investigation on the ternary Laves phases Ti-Fe-Al [25] and Ti-Ni-Al. [26] Several research- ers [4,9,27-29] have dealt with the phase diagram Ti-Fe-Al, in which a large solid solubility of Al in the binary Laves phase TiFe 2 exists. Almost 33.5 at.% Fe can be replaced by Al at 800 °C, and a value of 47 at.% can be reached at 1000 °C. [28] In a previous work, we showed that Al Xinlin Yan, A. Grytsiv, P. Rogl, and H. Schmidt, Institute of Physical Chemistry, University of Vienna, Wa ¨hringerstrasse 42, A-1090 Wien, Austria; and V. Pomjakushin, Laboratory for Neutron Scattering, ETH Zurich & Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland. Contact e-mails: peter.franz.rogl@univie.ac.at, andriy.grytsiv@univie. ac.at, and yanxinlin13@hotmail.com. Section I: Basic and Applied Research JPEDAV (2008) 29:414–428 DOI: 10.1007/s11669-008-9352-6 1547-7037 ÓASM International 414 Journal of Phase Equilibria and Diffusion Vol. 29 No. 5 2008