Evolution of precipitate and precipitate/matrix interface in Al-Zn-Mg-Cu(-Ag)alloys

Evolution of precipitate and precipitate/matrix interface in artificially aged Al-Zn-Mg-Cu(-Ag)alloys has been systematically studied.In the early stage of ageing,Ag,as a fast diffuser,can promote the formation of solute pairs and small clusters.Solute clusters are further demonstrated to be able to act as precursors forη’precipitates by in-situ STEM heating.With prolonged ageing time,the precipitate/matrix interface evolves from the Zn-dominated interface between early-stageη’and Al matrix to the Zn and Mg co-segregatedη’/Al andη_(2)/Al interfaces.Theη’/Al interfacial layers are shown to precede the formation ofη’,while theη_(2)/Al interfaces are found to be closely related to the thickening process ofη_(2)and the involved particular atomic movements are specified.Experimental observations and DFT calculations re-veal that forη’andη_(2),Ag can dissolve into the precipitate as well as locate at the precipitate/matrix interface without showing preference.For Cu,its dissolution in the precipitate and segregation on the interface mainly occur forη_(2)rather thanη’.The incorporation of Ag and Cu does not change the defined precipitate structure.

Effects of Sc on the vacancy and solute behaviours in aluminium

Element Sc is a promising candidate for optimizing the high-temperature mechanical properties of Al alloys.In this study,the Sc-solute,Al_(3) Sc-vacancy and Al_(3) Sc-solute interactions in aluminium are inves-tigated extensively by using first-principles calculations.The correlation between the various interaction energies and the solute atomic size,and the Sc-solute compound formation energy has been evaluated.A negative correlation between the first nearest neighbour Sc-solute binding energies and the lowest Sc-solute compound formation energies has been identified,while the second nearest neighbour Sc-solute binding energies increase monotonically with the solute atomic size.Al_(3) Sc precipitates can bind vacancy strongly at the specific atomic site,but their relatively low number density limits their influence on va-cancy behaviours during the ageing period shortly after quenching.Compared to the atomic size,the trend for solute segregating at the interface between Al_(3) Sc precipitate and Al bulk is more strongly re-lated to the Sc-solute binding energy.The calculated results can clarify the available experimental obser-vations for Al-Sc,Al-Cu,Al-Mg-Si and Al-Zn-Mg-Cu alloys,and it is hoped to guide the design of high-performance Al alloys.

Atomistic Evolution of Hf_(2)S/γ′Interfaces in a Hf‑containing Ni‑based Single‑Crystal Superalloy

The evolution of interfacial structures of needle-like sulfides in a Hf-containing Ni-based single-crystal superalloy has been studied with a combination of the state-of-the-art spherical aberration-corrected scanning transmission electron microscope and three-dimensional electron diffraction methods.The Hf_(2)S precipitates possess an ABACBC stacking sequence with layered structure of Hf-S-Hf-Hf-S-Hf.The Hf_(2)S/γ′interfaces exhibit different types of metastable interfacial structures,including the sharp interface with segregated Hf atom columns,the one with indistinct transition region and the one with ordered transition region.These metastable structures represent the different stages of Hf_(2)S growth during the aging process,based on which a sequential layer-by-layer growth mechanism of Hf_(2)S is proposed.

Atomic-scale study on the precipitation behavior of an Al-Zn-Mg-Cu alloy during isochronal aging

The precipitation behavior of a 7075 Al alloy during isochronal heat treatments at three different heating rates has been studied using differential scanning calorimetry,high-angle-annular-dark-field scanningtransmission-electron microscope and density functional theory calculation.In the early stage of aging,GPI and GPII zones form sequentially and cause two characteristic DSC peaks.Subsequently,the formation ofη_(1) precipitates takes place concurrently withη’.A novel type of metastable phaseη_(1)’is identified as the precursor ofη_(1),which can lower the lattice misfit betweenη_(1) and Al matrix along the direction of[1010]_(η_(1))//[001]_(Al).Accordingly,a pathway for the formation ofη_(1) viaη_(1)’is demonstrated.Precipitatesη’together withη_(1) andη_(1)’contribute to the third DSC peak.With the further increase of temperature,ηprecipitates become prevailing.Based on the quantitative analyses,the influence of the heating rate and ending temperature on the cross section and number density of phases formed is discussed.