Progression of creep deformation from grain boundaries to grain interior in Al-Cu-Mn-Zr alloys

• Published in: Acta materialia 2023-05, Vol.250 (15 May), p.118886, Article 118886
• Main Authors: Rakhmonov, J.U., Milligan, B., Bahl, S., Ma, D., Shyam, A., Dunand, D.C.
• Format: Article
• Language: English
• Subjects: Al-Cu alloys; Creep; Grain-boundary precipitates; In-situ neutron diffraction; Load transfer; MATERIALS SCIENCE; θ′-precipitates
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2023.118886

Creep mechanisms are studied in θ′-Al2Cu-strengthened Al-Cu-Mn-Zr alloys at 300 and 350°C for (i) ACMZ, a base alloy without further alloying elements and (ii) RR350, a commercial alloy with additions of Ni and Co forming distinct grain-boundary precipitates. At high stresses, creep is dominated by dislocations bypassing θ′ precipitates within grains via the Orowan mechanism, as evidenced by (ⅰ) very high stress exponent (n∼20-25) and (ⅱ) α-Al and θ′ lattice strains (measured via in-situ neutron diffraction) evolving during creep in a manner consistent with load transfer from the plastically-deforming α-Al matrix to elastically-deforming θ′ precipitates. At intermediate stresses, both alloys exhibit a n∼3 regime, where α-Al and θ′ lattice strains scale near-linearly with applied stress while remaining largely unaffected by strain accumulation, indicating that Orowan looping or dislocation pile-up around θ′ is now inactive within the grains. Rather, dislocation motion occurs solely in θ′-precipitate-free zones (θ′-PFZ) where high dislocation densities are observed via TEM after creep deformation. Plastic flow at θ′-PFZ and/or localized pipe diffusion are expected to enable grain-boundary sliding (GBS), which is proposed as the rate-limiting mechanism in the n∼3 regime. Ni/Co-rich precipitates at RR350 grain-boundaries, with negligible θ′-PFZ around them, share load (as determined via neutron diffraction) with the α-Al matrix more effectively than θ-Al2Cu precipitates at ACMZ grain-boundaries, with wide surrounding θ′-PFZ. Thus, high creep resistance in the n∼3 GBS regime of RR350 is enabled by coarsening-resistant grain-boundary precipitates, forming without concomitant development of weak θ′-PFZ, which effectively share load with the grains. [Display omitted]