There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

The evolution of the Ω-phase dispersion during aging in an Al-5.6Cu-0.72Mg-0.5Ag-0.32Mn-0.17Sc-0.12Zr-0.1Ge-0.08Ti-0.02Fe-0.01Si (wt%) alloy was examined at the temperatures, T , of 200 and 250 °C with dwelt times, τ , of 1.8–86.4 and 0.6–25.2 ks, respectively. The precipitation sequence during aging can be written as SSSS→Ag-Mg clusters→Ω-phase+θ′-phase→θ-phase+Ag,Mg,Cu-enriched phases. Only a minor portion of Cu is consumed for the precipitation of the Ω-phase. Approximately 3.3 wt% of Cu is retained in the solid solution after peak-aging. Therefore, the Ω-phase is a transition phase with a high free energy. The Ω-phase plates exhibit superior coarsening resistance. The kinetics of the coarsening is successfully predicted by the equation in a form of C tt × τ 0.2 , where C tt is a coefficient depending on aging temperature. Effect of aging on yield stress (YS) was analyzed in terms of additive sum of the contribution arising from the grain boundary strengthening, dislocation strengthening, solid solution strengthening and precipitation strengthening mechanisms. The contribution of the precipitate strengthening can be predicted satisfactorily by Nie and Muddle’s model of dislocation shearing of the {111} α plates. It was shown that the Ω-phase is a most efficient strengthening agent in the Al-Cu-Mg(-Ag) alloys and gives the major contribution to the overall YS. It is attributed to high efficiency of interfacial strengthening and high number density of the Ω-phase plates.

Precipitation structure and strengthening mechanisms in an Al-Cu-Mg-Ag alloy

A high performance Cu-Cr-Zr alloy with an ultimate tensile strength of 612 MPa, a uniform elongation of 5% and an electrical conductivity of 84.7% IACS was achieved by the severe rotary swaging and two-step peak-aging treatment. A perfect ultrafine grained microstructure with uniformly distributed nano-precipitates was identified to be responsible for the excellent properties of the studied alloy. The proposed method of rotary swaging in combination with the two-step aging treatment shows good potential for producing the high performance Cu-Cr-Zr alloy, which can not only fulfill the industrial requirement of strength and electrical conductivity, but also is suitable for mass production.

Optimizing the strength, ductility and electrical conductivity of a Cu-Cr-Zr alloy by rotary swaging and aging treatment

Effects of natural aging on the formation and strengthening effect of G.P. zones in a retrogression and re-aged Al–Zn–Mg–Cu alloy

The close association between the Ω precipitation and various Ag additions is systematically investigated by quantitative transmission electron microscopy and atom probe tomography analysis. Our results suggest that the precipitation of Ω phase is strongly dependent on Ag variations. Increasing the bulk Ag content favors a denser Ω precipitation and hence leads to a greater age-hardening response of Al-Cu-Mg-Ag alloy. This phenomenon, as proposed by proximity histograms, is directly related to the greater abundance of Ag solutes within Ω precursors. This feature lowers its nucleation barrier and increases the nucleation rate of Ω phase, finally contributes to the enhanced Ω precipitation. Also, it is noted that increasing Ag remarkably restricts the precipitation of θ' phase.

Quantitative transmission electron microscopy and atom probe tomography study of Ag-dependent precipitation of Ω phase in Al-Cu-Mg alloys

In this work, the microstructures of the electron-beam welded joints of cast Al–Cu–Mg–Ag alloy plate were investigated in terms of the element segregation, grain structure and precipitation behavior by scanning electron microscopy, transmission electron microscopy and atom probe tomography observation as well as differential scanning calorimetry and electron probe micro analysis. The hardness distribution of the weld cross-section was also described using microhardness mapping. A model of strengthening mechanisms was proposed to analysis the microstructure-strength relationship of the welded joint. It is found that the occurrence of softening behavior in fusion zone after welding is closely related to the dissolution of Ω and θ′ strengthening phases. The precipitation of a large number of fine Ω, θ′ plates and some small dispersed precipitates (Ag-rich particles, GP zones and Mg–Ag phases) may be the main reason for the increase in the strength of the welded joint after post-welding heat treatment. It is because the supersaturated solid solution produced after welding due to the rapid cooling rate as well as the strong binding energy between Ag atoms and vacancies/Mg atoms promotes the precipitation of small dispersoids.

Microstructure evolution and mechanical properties of the electron-beam welded joints of cast Al–Cu–Mg–Ag alloy

The influence of preaging on the strength and precipitation behavior of a deformed Al-Cu-Mg-Ag alloy

After extended aging at 200 °C for various periods, the microstructures of Al–Cu–Mg–Ag alloys with and without small Er addition is intentionally investigated by optical metallographic examination, scanning electron microscopy (SEM) and quantitative transmission electron microscopy (TEM) characterization to explore the potential effect of Er on the tensile properties. The present results reveal that the Ω precipitation is obviously suppressed by Er, thereby degrading the strength of Al–Cu–Mg–Ag alloy. Meanwhile, the S precipitates, which can be found in Er-free alloy during the whole aging at 200 °C, is rarely observed in Er-added alloy after extended aging for 100 h, suggesting the precipitation of S phase is significantly retarded by Er. Inversely, the precipitation of θ′ phase is accelerated by Er and it coarsens at a much lower rate. Compared to Er-free alloy, the coarser recrystallized grain size in Er-added alloy eliminates the grain refinement effect by Er. Besides, small Er addition is capable of reducing the amount and size of the residual particles in Al–Cu–Mg–Ag alloy.

Jian Wang

Papers

Effects of natural aging on the formation and strengthening effect of G.P. zones in a retrogression and re-aged Al–Zn–Mg–Cu alloy

Quantitative transmission electron microscopy and atom probe tomography study of Ag-dependent precipitation of Ω phase in Al-Cu-Mg alloys

Microstructure evolution and mechanical properties of the electron-beam welded joints of cast Al–Cu–Mg–Ag alloy

The influence of preaging on the strength and precipitation behavior of a deformed Al-Cu-Mg-Ag alloy

Effects of small Er addition on the microstructural evolution and strength properties of an Al–Cu–Mg–Ag alloy aged at 200°C