Oral Presentation 24th Australian Conference on Microscopy and Microanalysis 2016

Precipitate phases and phase transformations in Al-Ag alloys  (#95)

Zezhong Zhang 1 , Laure Bourgeois 1 2 , Julian M. Rosalie 3 , Nikhil Medhekar 1
  1. Department of Materials Science and Engineering, Monash University , Melbourne, Victoria, Australia
  2. Monash Centre for Electron Microscopy, Monash University , Melbourne, Victoria, Australia
  3. Erich Schmid Institute of Materials Science, Leoben, Austria

The mechanisms of phase transformations are one of the fundamental problems in solid-state physics. They directly relate to the mechanical properties of engineering alloys.  Al-Ag alloys, having been studied extensively since last century, have served as a model system for understanding phase transformations and precipitation in the solid-state. Although binary Al-Ag alloys have limited applications, Ag is a common addition to aluminium alloys as it can dramatically accelerate or even modify precipitation and thus improve the mechanical properties of high-strength aluminum alloys.1  Recent studies have shown that in this role, Ag segregates at the interfaces of a wide range of precipitates. 234 

To understand the common role Ag plays during phase transformations in various aluminium alloys, we revisited the Al-Ag system and characterised the atomic structures of the different precipitate phases using high angle angular dark field scanning transmission electron microscopy (HAADF-STEM). The instruments used were a dual-aberration-corrected FEI Titan3 at 300 kV, a JEOL JEM 2100F and a FEI Tecnai G2 F20 at 200 kV within the Monash Centre of Electron Microscopy. 

In particular we examined the porosity of Guinier-Preston (GP) zones and the structure of the γ' (Ag2Al) precipitate phase. Surprisingly, we found a new intermediate precipitate phase AgAl with a layered structure. The structures deduced from the HAADF-STEM observations are shown to be in excellent agreement with density functional theory calculations of the energetics for corresponding phases.5 This new layered structure can be regarded as analogous to the layered segregation of Ag at precipitate interfaces in aluminium alloys.

  1. Polmear, I.J. "The effects of small additions of silver on the ageing of some aluminium alloys." Trans. Met. Soc. AIME, 230 (1964).
  2. Rosalie, Julian M., and Laure Bourgeois. "Silver segregation to θ′(Al 2 Cu)–Al interfaces in Al–Cu–Ag alloys." Acta Materialia 60.17 (2012): 6033-6041.
  3. Kang, Sung Jin, et al. "Determination of interfacial atomic structure, misfits and energetics of Ω phase in Al–Cu–Mg–Ag alloy." Acta Materialia 81 (2014): 501-511.
  4. Kang, Sung Jin, et al. "Atomic structure and growth mechanism of T 1 precipitate in Al–Cu–Li–Mg–Ag alloy." Scripta Materialia 109 (2015): 68-71.
  5. Zhang, Z., Bourgeois, L., Rosaile, J. M., and Medhekar, N. V. "The metastable bilayer precipitate phase AgAl in the Al-Ag system". (to be submitted)