Abstract
The influence of minor Ag on the precipitation evolution of the Al-4.2Zn-2.8Mg-1.0Cu(wt.％)alloy from early stages to over-aged stages at 150 ℃ was investigated.Surprisingly,co-precipitation of strengthening phases T'and η'are found in both Ag-free and Ag-added alloys.With Ag addition,precipitation of both T'and η'is refined and increased,such that the age-hardening capabilities and peak-aged tensile strength are improved.In addition,the quantitative proportion of η'precipitates increases with the increase of Ag content due to the increase in the(Zn+Cu)/Mg ratio of nucleating particles.The narrowed precipitate-free zones(PFZs)are considered responsible for the undiminished fracture elongation in Ag-added alloys.Essentially,these effects of Ag are closely related to the strong Ag-vacancy and Ag-solute interactions.In over-aged stages,the Ag-added alloys still possess higher hardness values compared to the Ag-free al-loy,which is related to precipitate coarsening mechanisms.The Ag-free alloy follows classical coarsening behavior by solid solution mediated diffusion,while the Ag-added alloy follows two possible coarsen-ing mechanisms,coalescence of aggregates and diffusion of atoms.The smaller average size and higher residual number density of precipitates benefited from the slow diffusion-controlled coarsening behavior depending on the precipitate composition characteristics of the two-stage differentiation and the precip-itate distribution characteristics of high-density dispersion in early-aged stages could explain why the hardness of Ag-added alloy keeps at a higher level than that of Ag-free alloy even after 1000 h ageing.Meanwhile,the transformation of metastable phases to stable phases is inhibited due to the addition of Ag,such that GP zones,T',η',η and T phases coexist even after 14 d of ageing.In terms of phase com-position,the addition of Ag decreases the ratio of Mg/(Al+Zn)in T-type phase.For the Ag-added alloy,the sum concentration of Zn+Mg in η'phase is about 10 at.％higher compared to T'phase,and η phase continues to have a high sum concentration of Zn+Mg,besides,the Zn/Mg ratio and Cu concentration exhibit obvious differences from T phase.
基金项目
Chongqing Key Project for Technological Innovation and Application(CSTB2022TIAD-KPX0073)
Natural Science Foundation of Chongqing(CSTB2022NSCQ-LZX0002)
National Natural Science Foundation of China(51871033)
Opening Project of State Key Laboratory for Advanced Metals and Materials(2022-Z03)
Opening Project of State Key Laboratory for Advanced Metals and Materials(2020-ZD02)
NETL
NSTL
万方数据