Materials Chemistry and Physics 81 (2003) 538–541 Substructure of titanium alloys after cyclic heat treatment W. Szkliniarz ∗ , J. Chrapo ´ nski, A. Ko´ scielna, B. Serek Department of Materials Science, Silesian University of Technology, Krasi´ nskiego 8, 40-019 Katowice, Poland Abstract The fundamental structure parameter determining the properties of titanium alloys is the grain size. The grain size can be controlled by means of the lattice strain and the internal work-hardening which accompany the phase transitions occurring during repeated heating ↔ cooling cycles. During cyclic heat treatments, there are conditions facilitating the cumulation of internal work-hardening. The structural research carried out has shown that in the pre-treatment cycles the structural defects refer mainly to the  phase and manifest itself by an increase in the microtwin density. Larger numbers of cycles increase the dislocation density mainly in the  phase and then in the  phase. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Substructure; Titanium alloys; Cyclic heat treatment; Internal work-hardening 1. Introduction A coarse grain structure has a particularly unfavorable im- pact on the mechanical properties of titanium and its alloys. The effective methods of grain refining have fundamental importance for this group of alloys. There are possibilities of grain refining in the processes of melting and crystalliza- tion, plastic working and heat treatment. Heat treatment of titanium alloys is based on the phase transitions which take place during heating and cooling due to the occurrence of two allotropic forms of titanium. Through heat treatment one can produce desirable struc- ture changes and consequently, changes of various proper- ties. During slow continuous heating, when the transition  +  →  takes place, instead of the expected structure refining, only a considerable coarsening is observed [1]. Austenite grain refining during heating of steel is con- nected with the internal work-hardening accompanying the  →  transformation [2]. The effect of internal work-hardening depends on the temperature, the time and the mechanisms of internal stress relaxation. Owing to a small difference in the specific volumes of the  and  phases, the stresses produced during the transformation do not lead to grain refining even at high heating rates. The high temperature of phase transitions and the tendency of the body-centered cubic  phase to polygonization rather than to recrystallization have an influence on the lack of grain refining during heating of titanium alloys. ∗ Corresponding author. Tel.: +48-32-2563197; fax: +48-32-2563197. E-mail address: szkliniarz@polsl.katowice.pl (W. Szkliniarz). The volume change occurring in titanium during the al- lotropic transformation was investigated by Sadovski et al. already in 1960 [3]. They carried out investigations aiming at an evaluation of the possibilities of grain refining in titanium through recrystallization caused by internal work-hardening and temperature. Specimens of unalloyed titanium were sub- mitted to quenching in water from temperatures at which the  phase occurs and subsequently annealed at a temperature close to the transformation temperature. However, no signs of recrystallization were found either below or above the transformation temperature, up to the melting temperature. The recrystallization annealing carried out for a comparison on cold-deformed specimens led to producing a new struc- ture consisting of polyhedral grains. Both Sadovski et al. [3] and other authors [4,5] saw the reason for this in an insuf- ficient size of the internal work-hardening which accompa- nies the allotropic transformation. The work-hardening is so small that heating above the temperature of allotropic trans- formation, regardless of the heating and cooling conditions, in principle never allows to obtain the same grain size such as it was before the treatment. The main factor determin- ing the work-hardening is the volume change of the phase transformation. Hitherto, it has been impossible to achieve significant grain refining effects in titanium and its alloys through heat treatment methods analogous to those applied for steel. Much attention has been devoted to the issues of cyclic heat treatment for a long time. This kind of treatment has proved to be one of the most effective ways of forming the structure and mechanical properties of titanium and its alloys, particularly in case of alloys of a coarse grain structure. The basic reason for microstructure changes in 0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00069-5