Effect of off-stoichiometry on the mobility of point-like defects and damping in binary Cu–Al martensites

An attempt is undertaken to define a factor, apart from temperature, which allows for the mobility of quenched-in defects in a Cu–Al system of martensitic alloys. Experiments were performed with single-and polycrystalline alloys with nominal Al content ranging from 10 to 13.5 wt.%. An acoustic technique, operating for temperatures ranging from ambient down to 10 K, was applied to detect the existence of mobile quenched-in defects. No mobility of quenched-in defects was detected for either single-or polycrystalline alloys with Al contents close to and below the Cu3Al stoichiometric one (Cu–12.4 wt.% Al), corresponding to perfect DO3 ordering. For the alloy Cu–13 wt.% Al, mobility of quenched-in defects was detected starting from temperature of around 70 K. It is suggested that the off-stoichiometry of the alloys is accommodated through anti-site defect formation on the Al-deficient side and through vacancy formation on the Al-rich side of the phase diagram. The composition dependence of damping of Cu–Al martensites exhibits a sharp maximum around the stoichiometric composition Cu3Al (Cu–12.4 wt.% Al), which is presumably formed by variations of ordered domain size and point defect concentration on both sides of the stoichiometry.