The decomposition process in high-purity Al-1.7 at.% Cu alloys with trace elements: preservation of quenched-in vacancies by In, Sn and Pb influencing the $$\theta \, '$$ formation

Abstract

Aluminium-copper alloys of the 2xxx type receive their excellent mechanical properties by the formation of copper-rich precipitates during hardening. Size, distribution and crystal structure of the formed precipitates determine the final strength of those alloys. Adding traces of certain elements, which bind to vacancies, significantly influences the decomposition behaviour, i.e. the diffusion of the copper atoms. For high-purity ternary alloys (Al-1.7 at.% Cu-X), we investigate the interaction of copper and trace element atoms (X=In, Sn, and Pb) with quenched-in vacancies by Positron Annihilation Lifetime Spectroscopy (PALS). By employing Vickers microhardness, Differential Scanning Calorimetry (DSC) and Small Angle X-Ray Scattering (SAXS) we obtain a comprehensive picture of the decomposition process: opposite to predicted binding energies to vacancies by ab-initio calculations we find during ageing at room and elevated temperature a more retarded clustering of copper in the presence of In rather than for Sn additions, while Pb, having the highest predicted binding to vacancies, shows nearly no retarding effect compared to pure Al-Cu. If the latter would be due to a limited solubility of lead, it had to be below 2 ppm. Transmission Electron Microscopy (TEM) as imaging method complements our findings. Annealing the quenched Al-1.7 at.% Cu-X-alloys containing 100 ppm In or Sn at $$150\,^\circ {\text {C}}$$ 150 ∘ C leads to finely distributed $$\theta \, '$$ θ ′ -precipitates on the nanoscale, since due to the trace additions the formation temperature of $$\theta \, '$$ θ ′ is lowered by more than $$100\,^\circ {\text {C}}$$ 100 ∘ C . According to TEM small agglomerates of trace elements (In, Sn) may support the early nucleation for the $$\theta \, '$$ θ ′ -precipitates.

In Situ Heating Positron Annihilation Lifetime Spectroscopy Experiments on an Al–Mg Alloy

“…Another way to study such interactions is to conduct isochronal annealing experiments, such as the ones that have been applied to Al–Cu, [ 9 ] Al–Sn, and Al–In alloys. [ 10 ] A preliminary study on Al–Mg alloys was carried out in our group using conventional 22 Na‐based spectroscopy.…”

Identification of Point Defects in Multielement Compounds and Alloys with Positron Annihilation Spectroscopy: Challenges and Opportunities

“…can be highlighted, as well as the direct observation of monovacancy migration at low temperatures in both materials. [18,19] Positron annihilation methods have revealed important vacancy-driven phenomena in the ageing of dilute metal alloys, shedding light on, e.g., Cu precipitates in Fe, [20] interactions of quenched-in vacancies and solutes in Al alloys, [21][22][23][24] or plastic deformation in various steel grades. [25,26] Binary compound semiconductors have provided perhaps the most fruitful field for using positron annihilation spectroscopy for defect studies.…”

“…[ 14 ] In elemental semiconductors, the determination of migration and formation mechanisms of vacancy–multidonor complexes in Si [ 15,16 ] and Ge [ 17 ] can be highlighted, as well as the direct observation of monovacancy migration at low temperatures in both materials. [ 18,19 ] Positron annihilation methods have revealed important vacancy‐driven phenomena in the ageing of dilute metal alloys, shedding light on, e.g., Cu precipitates in Fe, [ 20 ] interactions of quenched‐in vacancies and solutes in Al alloys, [ 21–24 ] or plastic deformation in various steel grades. [ 25,26 ]…”

Directed Energy Deposition‐Arc Repair Al6.5Cu2Ni0.3Ti0.5Zr0.25V Alloy: Microstructure Evolution and Strengthening Mechanism

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Lightweight Al-Cu alloys are widely utilized in aerospace applications due to their high specific strength, production flexibility, and low density. [1][2][3][4][5] Nevertheless, with the industrial demand for high-temperature heat-resistant aluminum alloys, traditional Al-Cu alloys have been difficult to meet the requirements of use. [6] Above 250 °C, the main strengthening phase θ 0 in the traditional Al-Cu alloy coarses rapidly, and the grain boundary pinning effect weakens, resulting in a sharp decline in alloy properties.

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