Microstructural design for advanced light metals
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troduction Advanced lightweight alloys for structural applications share a common feature—they are all strengthened by second-phase particles. The crystal structure (i.e., ordered intermetallic or solid-solution phases), volume fraction, size and shape, orientation relationship with the matrix phase and habit plane inclination, coherency state and spatial distribution of these second-phase particles, collectively called precipitate microstructure, determine the deformation behavior and mechanical properties of these alloys. While the crystal structure and volume fraction are controlled mainly by alloy composition (i.e., phase equilibria), the remaining microstructural attributes depend on specific solid-state phase transformation pathways (TPs) leading to the precipitation of these secondphase particles during a specific thermal or thermomechanical treatment. Thus, the major strategy in advanced lightweight alloy design has been to control the specific TPs for a given alloy to optimize the previously mentioned microstructure attributes, regulate microstructure–dislocation interactions and tailor the microstructure–property relationship.1 This article is an overview of the current understanding of mechanisms of phase transformation, strengthening, and the role of microalloying elements in Al, Mg, and Ti alloys. Emphasis is on nucleation and growth of precipitates, precipitate–dislocation interactions, solute segregation at precipitate–matrix interfaces and planar defects, and the
development of strengthening models that account for the real (as opposed to an assumed spherical) particle shape in light alloys. We will show that emerging modeling and advanced characterization tools are not only adding a new understanding, but opening up new pathways for alloy design.
Aluminum alloys Roles of microalloying elements The key strengthening precipitate phase in binary Al-Cu alloys is θ′, which forms as a {001}α plate, with (001)θ’ //{001}α as its broad surface. The (001)θ,//{001}α interface was traditionally thought to have a mixture of Cu and Al, but recent Z-contrast scanning transmission electron microscopy (STEM) imaging2 indicated that it actually contains 100% Cu. First-principles density functional theory (DFT) calculations revealed that the presence of excessive Cu (i.e., Cu segregation) in this interface reduced the interfacial energy.2 Additions of 0.3 wt% Mg and 0.4 wt% Ag to Al-4 wt% Cu alloy fully replace θ′ by the Ω phase that forms as thin plates on {111}α with an aspect ratio larger than θ′. The resultant Al-Cu-Mg-Ag alloys exhibit a tensile yield strength above 500 MPa. While it is known that Mg or Ag alone cannot facilitate Ω precipitation, their precise roles in Ω nucleation and growth are not fully understood. Atom probe tomography and Z-contrast STEM indicated that Ω evolved from preexisting Mg-Ag clusters that formed in the very early stage of
Jian-Feng Nie, Department of Materials Science and Engineering, Monash University, Australia; [email protected] Yunzhi Wang, Department of Materials Scien
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