An electron microscopy study of modes of intermetallic precipitation in Ti-Cu alloys

Abstract

The precipitation processes that accompany the aging of supersaturatedα-titanium solid solutions containing up to 6 wt pct Cu have been studied by thin foil electron microscopy and X-ray diffraction techniques. Two mechanisms of decomposition have been identified: i) the heterogeneous nucleation and growth of Ti₂Cu at interfaces and internal substructure such as dislocations, and ii) the uniform nucleation of thin, coherent, disk-shaped precipitates which lie on$\{ 10\overline 1 1\}$. The coherent precipitates can form with densities up to 10¹⁷ per cu cm; this and many morphological features of the precipitation process in Ti-Cu are analogous to the well-known behavior of Al-Cu alloys. The coherent precipitates first lose coherency along the edge of the disk and then along the flat faces. The mechanisms by which these two processes occur are considered in detail.