Field ion microscopy and 3-D atom probe analysis of Al3Zr particles in 7050 Al alloy

Full text: For high strength 7xxx series Al alloys, Zr is an important trace alloy element which is often added to optimise properties, having effects such as refining grain size, inhibiting recrystallization, and improving stress corrosion cracking resistance and quench sensitivity. In addition, it has been reported recently that Zr addition also has a significant influence on early stage ageing behaviour of a 7xxx series Al alloy. Zr equilibrium solubility in solid Al is extremely low. After solution or ageing treatment, most Zr is present as small spherical Ai3Zr dispersoids approximately 20 nm in diameter, distributed at grain boundaries as well as within the Al matrix. The crystallographic nature of intermetallic phase Al3Zr has been well studied in the literatures. So far, no direct measurement of the chemistry of the Al3Zr particles in 7xxx series Al alloys has been published. It is unclear if there is significant Zn, Mg or Cu included in the particles. In this research, 3DAP has been employed for the first time to investigate ionisation behaviour of Al3Zr particles and determine the chemistry of the particles in 7050 Al alloy. Using field ion microscopy, the local evaporation radius of the Al3Zr particle has been measured to be equivalent to 36 nm for a 10 kV tip, less than the equivalent tip radius for the Al matrix of ∼68 nm. Using the matrix Al evaporation field (19 V/nm) as a reference, this allows the evaporation field of Al3Zr to be calculated as 35 V/nm, the same as the field calculated for evaporation of Al as Al2+ (35 V/nm), and that of Zr as Zr3+ (35 V/nm). This result is consistent with Al2+ and Zr3+ being the main species observed in the mass spectrum during analysis of Al3Zr particles. Using 3DAP, the chemical compositions of Al3Zr particles are determined to be 64.8∼67.7 at% Al, 23.6∼24.8 at% Zr, 6.9∼9.1 at% Zn, 0.4∼0.7 at% Cu, 0.5∼1.2 at% Mg, with a (Al+Zn)/Zr ratio close to 3. Choice of specimen temperature of 25 K or 80 K shows little effect on the measured chemical compositions. Use of the calculated evaporation field of Al3Zr has allowed accurate reconstruction of the morphology of the particles. The difference in the charge state of Al ions evaporated from particle and matrix has allowed the trajectory overlaps between Al matrix and Zr particles to be estimated as ∼ 1.7 nm. (author)