Aging of Cu–Be alloys with and without cobalt

Abstract

The effect of prior cold work on the decomposition of the supersaturated solid solution of Cu–Be alloys with and without cobalt addition has been studied by means of electron microscopy and hardness measurements. The results suggest that during aging from 200 to 500°C two temperature regions characterized by different aging products may be distinguished. In the lower temperature region the following continuous precipitation sequence is found: supersaturated solid solution → Guinier–Preston zones →γ′ in the higher temperature region the discontinuous precipitation reaction is: supersaturated solid solution →γD→γ equilibrium phase. An increase in beryllium content raises the limit of the lower temperature region, i.e. the critical temperature separating two aging regions is located between 250 and 350°C for Cu–1·5 Be and between 350 and 400°C for Cu–2Be and Cu–1·8Be–0·2Co (all in wt–%).The addition of cobalt, and the prior cold work, have a retarding effect on aging processes in the lower temperature region, but no effect has been observed in the higher region. The activation energies for formation of Guinier–Preston zones and γ′ phase were calculated and discussed in terms of the reaction of solute atoms with vacancies.MST/242