Precipitation in CuTi and CuTiAl alloys; discontinuous and localised precipitation

Abstract

Discontinuous precipitation in a Cu-4% Ti binary alloy is studied through transmission electron microscopy. Different types of contrast conditions are used and moire fringes and interfacial dislocations are revealed. The grain refinement that occurs during discontinuous precipitate growth is due to translational and rotational subgrain boundaries formed on account of the different habit planes of precipitates from the different nucleation sites. It is also shown that discontinuous precipitation is absent in a Cu-2.1% Ti-2.4% Al and a Cu-2.1% Ti-5% Al alloys over a wide range of aging temperatures. However, a pronounced tendency for localised precipitation is observed in these alloys. It is found that the ternary CuTiAl alloys have lower coherent solvus temperatures. Solute supersaturation is therefore relieved through precipitation of the equilibrium phase by heterogenous nucleation at grain boundaries, twin boundaries, dislocations and other structural imperfections where a lower activation energy barrier is present.