© 2006 The Authors Journal compilation © 2006 The Royal Microscopical Society Journal of Microscopy, Vol. 224, Pt 1 October 2006, pp. 58–61 Blackwell Publishing Ltd Scanning and transmission electron microscopy microstructure characterization of mechanically alloyed Nb –Ti–Al alloys MAGDALENA ROZMUS, MAREK BLICHARSKI & STANIS L AW DYMEK AGH University of Science and Technology, Faculty of Metallurgy and Materials Science, Mickiewicza 30, 30–059 Kraków, Poland Key words. Mechanical alloying, niobium aluminides, titanium aluminides. Summary Results are presented of an investigation of the microstructure development during mechanical alloying and following con- solidation of an Nb15Ti15Al alloy. The alloy was synthesized from elemental as well as pre-alloyed powders. The micro- structure of this material was examined by transmission electron microscopy, scanning electron microscopy and X-ray diffraction. The use of pre-alloyed TiAl powder for synthesis of the Nb15Ti15Al alloy meant that a much shorter time was required to complete the mechanical alloying process compared with the synthesis of elemental powders. The investigation indicates that three phases were present in the consolidated materials: the Nb solid solution, the Nb 3 Al intermetallic phase and the dispersoid. Introduction Niobium aluminides have been considered as potential materials for high-temperature applications because of their specific properties, such as high melting point, high strength- to-weight ratio, and excellent elevated-temperature properties (Shyue et al., 1993). However, these intermetallics generally have very low ductility at room temperature, which has ren- dered their widespread use impossible. One of the methods of improving the ambient temperature ductility of any brittle material is to reduce its grain size. The other method is the incorporation of a ductile phase into the alloy microstructure. Mechanical alloying (MA) takes advantage of both possibilities (Suryanarayana, 2001). Also, as a solid state process, it enables the synthesis of elements with a large difference in melting point, like Nb (2300 °C) and Al (660 °C). The main aim of this work was to employ MA for synthesis of the alloy Nb15Ti15Al, which we supposed would exhibit better mechanical properties than the same alloy produced by other methods. Also, the differences in microstructure of alloys with the same compositions but produced from elemental or pre-alloyed powders were examined. This work is an extension of the research on binary Nb–Al (Dymek et al., 1997; Dollar & Dymek, 2003) and ternary Nb–Al-V (Dymek et al., 2000, 2001) alloys. Experimental procedure Two materials produced by MA were investigated. The first one was produced from pure elemental powders of Nb, Ti and Al (the alloy designated as A), and the other was produced from pure Nb and intermetallic TiAl powders (the alloy desig- nated as B). The target compositions of both materials were the same: 70% niobium, 15% titanium and 15% aluminium (atomic percentage). The composition was selected on the basis of isothermal sections of the Nb–Ti–Al ternary phase dia- gram in order to get a ductile Nb solid solution phase (Nb ss ) and a hard Nb 3 Al-based one. MA of both alloys was conducted in a Szegvari laboratory attritor mill (Akron, OH) in an argon atmosphere with the controlled oxygen level reduced to less than 10 p.p.m. In both cases the milling for the first 4 h was carried out at cryogenic temperature, using liquid nitrogen as a coolant, whereas the milling for the rest of the time was carried out at room temperature, using water as a coolant. Powder samples for analysis were taken after selected periods of milling. The total milling time for the first blend was 100 h, and for the second one it was 50 h. The changes of powder morphology upon milling were examined by scanning electron microscopy (SEM) using secondary electron images. The phase analysis was performed by X-ray diffraction with CoKα radiation. The X-ray diffraction patterns were used for the evaluation of lat- tice parameters, lattice strains and X-ray crystallite size. This allowed us to monitor the progress of the MA process of both blends. Also, the microstructure of powder particles was examined using SEM on pre-polished particle sections; the Z- contrast formed by backscattered electrons was used in this case. The powders collected at the end of the milling process were sieved through a 45 μm mesh and consolidated by hot Correspondence to: Magdalena Rozmus. Fax: (+4812) 617 3190; e-mail: rozmus@galaxy.uci.agh.edu.pl