Enhanced age-hardening response and creep resistance of an Al-0.5Mn-0.3Si (at.%) alloy by Sn inoculation

Precipitation-strengthening at ambient and high temperatures is examined in Al-0.5Mn-0.3Si (at.%) alloys with and without 0.02 at.% Sn micro-additions. Isochronal aging experiments reveal that Sn inoculation results in a pronounced age-hardening response: a hardening increment of 125 MPa is achieved at peak-aging (475 °C), which is five times greater than that of a Sn-free alloy. Scanning electron microscopy and synchrotron x-ray diffraction analyses demonstrate that, while the structure of the α-Al(Mn,Fe)Si precipitates formed in the peak-aged alloys is identical, their mean radius is smaller (R ~ 25 vs. 100-500 nm) and their number density is greater (~10²¹ vs. ~10^19-20 m ^-3) in the Sn-modified alloy. Atom-probe tomography analyses reveal that the enhanced dispersion of the α-precipitates is related primarily to the formation of Sn-rich nanoprecipitates at intermediate temperatures, which act as nucleation sites for Mn-Si-rich nanoprecipitates. High-resolution transmission electron microscopy analyses demonstrate that these Mn-Si-rich nanoprecipitates exhibit icosahedral quasicrystal ordering (I-phase), which transform into the cubic-approximant α-phase upon peak aging. Significant Sn segregation at the semi-coherent interfaces of the α-precipitates in the peak-aged Sn-modified alloy is observed via APT, which promotes homogeneous nucleation of the I/α-precipitates at aging temperatures > 400 °C. At 300 °C, creep threshold stresses are observed in both alloys in the peak-aged state, which increases from ~30 MPa in the Sn-free alloy to ~52 MPa in the Sn-modified alloy. This boost in creep resistance is consistent with the enhanced aging response (higher Orowan stress).