Quasicrystalline and related crystalline phases in rapidly solidified 2024-2Li alloy

Abstract

The formation and decomposition of quasicrystalline and crystalline phases in as-rapidly solidified and annealed commercial AISI 2024 aluminium alloy containing 2 wt% Li have been investigated by detailed transmission electron microscopy, including a combination of bright field and dark field imaging, selected area diffraction pattern analysis and energy dispersive X-ray microanalysis. The microstructure of as-melt spun 2024-2Li consists of α-Al cell, containing small coherent δ' precipitates, and particles or a continuous network of the icosahedral phase at the cell boundaries. After annealing at 300° C, the intercellular particles of the icosahedral phase coarsen progressively and assume a more faceted shape; after annealing at 400°C, particles of the decagonal and crystalline O phases precipitate heterogeneously on preexisting particles of the icosahedral phase; and after annealing at 500°C, the icosahedral and decagonal phases dissolve completely, and small particles of the crystalline O phase remain together with newly precipitated plates of the T₁ phase. The icosahedral phase in melt spun and melt spun/annealed 2024-2Li belongs to the Al₆CuLi₃ class of icosahedral phases, with a quasilattice constant of 0.51 nm, a stoichiometry of (Al,Si)₆(Cu, Mn, Fe) (Li, Mg)₃ and an average composition of Al-24.1 at.% Cu-64 at.% Mg-1.7 at.% Si-0.3 at.% Mn-0.5 at.% Fe as melt spun and Al-21.9 at.% Cu-6.3 at.% Mg-1.0 at.% Si-0.5 at.% Fe as-heat-treated. The decagonal phase in melt spun/annealed 2024-2Li belongs to the Al₄Mn class of decagonal phases, with a periodicity of 1.23 nm along the 10-fold symmetry axis, a stoichoimetry of Al₃(Cu, Mn, Fe) and an average composition of Al-10.3 at.% Cu-13.8 at.% Mn-2.3 at.% Fe. The crystalline O phase in melt spun/annealed 2024-2Li has an orthorhombic structure with lattice parameters of a = 2.24 nm, b = 2.35 nm and c = 1.23 nm, a stoichiometry of Al₃(Cu, Mn, Fe), and an average composition of Al-11.0 at.% Cu-14.8 at.% Mn-3.9 at.% Fe. Detailed analysis of selected area diffraction patterns shows a close similarity between the icosahedral, decagonal and crystalline O phases in melt spun and melt spun/annealed 2024-2Li. In particular, the decagonal phase and crystalline O phases have a similar composition, and exhibit an orientation relationship which can be expressed as: • [D10]//[001]_o • [D2_a]//[100]_o • [D2_b]//[010]_o suggesting that the orthorhombic O phase is an approximant structure for the decagonal phase.