Modelling of defects and amorphisation by ball milling of g-TiAl P.T. Jochym a, * , K. Parlinski a , W. Maziarz b , J. Dutkiewicz b a Department of Computational Materials Research, Institute of Nuclear Physics, PAN, ul.Radzikowskiego 152, 31-342 Cracow, Poland b Institute of Metallurgy and Materials Science, PAN, ul.Reymonta 25, 30-059 Cracow, Poland Received 20 December 2005; accepted 9 January 2006 Available online 20 March 2006 Abstract The formation and site preference energies and volume changes of single and pair of defects of ternary alloying elements in g-TiAl inter- metallic compound were studied by the density functional theory. Slight tendency to clusterization of antisite defects has been found. This may lead to disorder in the system. The V and Cr atoms prefer to reside in the Ti sublattice. The formation energy for CreCr, CreV and VeV nearest neighbour pairs are in the 1.3e2 eV range. The Al antisite in Ti sublattice requires much less energy than the Ti antisite in Al sublattice. The amorphisation process of TiAl alloy was studied by means of high energy ball milling of Ti and Al elemental powders, which produces amor- phous structure after 40 h. The amorphous states were studied by the DFT calculations of many random atomic configurations and the results were compared with the NiAl compound. Possible explanation for the amorphisation of the TiAl compound is presented. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl; D. Defects: point defects; E. Ab-initio calculations; B. Glasses, metallic 1. Introduction Several ordered intermetallic alloys, such as NiAl and TiAl with other alloying additions, are currently being studied as light weight materials for high temperature applications [1e9]. g-TiAl (tetragonal L1 0 structure) is one of the most promising compounds because of a good balance between density, strength and oxidation resistance, but it suffers from brittleness at low temperatures. One of the possible methods for improving low temperature ductility is decrease of grain size by powder metallurgy start- ing with ball milling [1e3]. Ball milled samples show substan- tial grain refinement leading even to the amorphous structure after prolonged milling times [1e5,10e12]. Small additions of other elements like V or Nb catalyse formation of the amor- phous structure at compositions near TiAl [5,6]. Contrary to TiAl, ball milling of powders of compositions near NiAl never results in the amorphous structure [7e9]. It results in the nanocrystalline phase of B2 structure. The additions of up to 6 at.% of Fe, Ga, Mo, Co or Ti [8,9] do not change final B2 nanocrystalline structure. These materials were also studied by various theoretical methods[14e19]. In this work we aim to extend these previous studies and to investigate the issue of the amorphisation of the TiAl compound. We will first present an experimental structural study of TiAl and NiAl after ball milling and demonstrate experimental evidence for much better amorphisation of the TiAl alloy, as compared to NiAl. Then we will report on ab initio calcula- tions of various aspects of this system having possible influ- ence on the process of formation of the TiAl alloy. We will show the results for the formation and site preference energies, volume changes caused by Cr, V atomic impurities and antisite defects in TiAl. We will close the paper with the ab initio study of amorphisation process in this system. 2. Experimental procedure Powders of titanium (110 mm size and purity >99.5%), aluminium (150 mm size and purity >99.5%) and nickel * Corresponding author. Tel.: þ48 12 662 8269; fax: þ48 12 662 8458. E-mail address: pawel.jochym@ifj.edu.pl (P.T. Jochym). 0966-9795/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2006.01.053 Intermetallics 14 (2006) 1397e1402 www.elsevier.com/locate/intermet