Spinodal decomposition in age hardening copper-titanium alloys

The structure and properties of age hardened Cu-Ti alloys

The Cu−Ti (Copper-Titanium) system

Discontinuous precipitation results in the formation of a two-phase lamellar structure behind moving grain-boundaries. The reaction is of interest because it generally has a deleterious effect on the mechanical, physical, and chemical properties of many commercial alloys. The combination of heterogeneous boundary precipitation and concurrent migration is complex and has resulted in a diverse range of proposed reaction mechanisms, several models of the growth kinetics, and many empirical observations of the effects of ternary additions and lattice strain. This review summarizes existing theories and attempts to reconcile them with the latest experimental data for reactions which take place from supersaturated solid solutions (eutectoid decomposition is specifically excluded). It is considered that fuller understanding requires increased application of the latest electron -optical techniques such as high-resolution microanalysis and in situ experiments in the high-voltage microscope. Furthermore, to...

Grain boundary discontinuous precipitation reactions

A survey of the properties of copper alloys for use as fusion reactor materials

Electron transmission microscopy and diffraction have revealed directly the fine-scale precipitation processes and the mechanism of formation of the periodic microstructures which characterize the decomposition of Cu-Ti “sideband” alloys. The initial stages of decomposition involve the development of «100» compositional waves giving rise to the well-known sideband or satellite reflections along the «100» directions. This «wave-like» clustering producing a triaxially modulated structure involves the formation of two disordered phases; ordering does not accompany the initial clustering. Prolonged aging produces a periodic array of interpenetrating rods along the «100» directions of the matrix. During aging the periodic structures coarsen according to a t1/3 law. The experimental activation energy for the coarsening of the modulated structures is approximately 48 kcal/ mole. Maximum strength is associated with the formation of the transition phase β’ which forms byin situ transformation of the titanium-rich regions; this transformation is not accompanied by a loss of coherency in the modulated structures at the aging temperatures studied in this investigation. It is suggested that these periodic structures arise from spinodal decomposition.

W. A. Soffa

Papers

An electron microscopy study of precipitation in Cu-Ti sideband alloys