均匀化处理对高Zn超高强铝合金微观组织演变的影响

采用金相光学显微镜、扫描电子显微镜(SEM)等,检测分析了高Zn超高强度铝合金铸锭均匀化前后的组织形貌及变化。结果表明,合金铸态中存在严重的枝晶偏析,合金中的相主要是由MgZn_(2)相和T(AlZnMgCu)相组成的可溶金属间化合物,以及由Mn、Fe和Ni三种元素形成的难溶金属间化合物。铸锭均匀化热处理后的结果显示,与7050合金和7055合金等Zn含量较低的铝合金不同,在合金中Zn含量高达9.8%的情况下没有出现T相向S(Al_(2)CuMg)相的转化。分析表明,这一现象与Zn含量及Zn的扩散密切相关,目前知道Zn含量越高,转化过程越困难。

高Zn铝合金峰值时效工艺的研究

高Zn铝合金构件由于尺寸大,合金含量高,时效处理时易时效不充分或过时效,本文通过制定一系列单级时效试验方案,对不同单级时效工艺下高Zn铝合金力学性能和显微组织的分析比较,得到一个较适宜单级时效处理制度。

Sc含量对Al-10Zn-2.5Mg-1.6Cu-0.15Zr铝合金板材组织与力学性能的影响

利用金相显微镜(OM)、透射电子显微镜(TEM)结合万能材料试验机研究Sc含量(质量分数)从0.034%增大至0.215%时,Sc含量对T6态Al-10Zn-2.5Mg-1.6Cu-0.15Zr铝合金板材中过剩结晶相、弥散相、析出相、晶粒及力学性能的影响。结果表明:Sc含量从0.034%增加至0.215%,T相和Al_(7)Cu_(2)Fe相过剩结晶相粒子组态无明显变化;纳米级Al_(3)(Sc,Zr)弥散相粒子的尺寸不变但数量呈现出先增多后减少的趋势;析出相粒子尺寸先细化后粗化,数量先增多后减少。当Sc含量为0.189%时,合金板材中出现3~5μm的初生Al_(3)(Sc,Zr)粒子,其数量随Sc含量增加而增多。T6态Al-10Zn-2.5Mg-1.6Cu-0.15Zr-(0.034-0.139)Sc合金板材的晶粒呈纤维状,且随Sc含量增加,纤维状晶粒逐渐细化、长度逐渐缩短;当Sc含量为0.189%时,合金板材发生明显再结晶,其晶粒均为细小类球状等轴晶;当Sc含量从0.034%增加至0.215%,T6态合金板材的强度和伸长率均随Sc含量增加呈现出先升高后降低的趋势;当Sc含量为0.139%时,合金板材的性能最佳,其抗拉强度、屈服强度和伸长率分别为701 MPa、665 MPa和8.0%。

Thermodynamic calculation of high zinc-containing Al-Zn-Mg-Cu alloy

Phase fraction and solidification path of high Zn-containing Al-Zn-Mg-Cu series aluminum alloy were calculated by calculation of phase diagram （CALPHAD） method. Microstructure and phases of Al-9.2Zn-1.7Mg-2.3Cu alloy were studied by X-ray diffraction （XRD）, differential scanning calorimetry （DSC） and scanning electron microscopy （SEM）. The calculation results show that η（MgZn2） phase is influenced by Zn and Mg. Mass fractions of η（MgZn2） in Al-xZn-1.7Mg-2.3Cu are 10.0%, 9.8% and 9.2% for x=9.6, 9.4, 8.8 （mass fraction, %）, respectively. The intervals of Mg composition were achieved for θ（Al2Cu）＋η（MgZn2）, S（Al2CuMg）＋η（MgZn2） and θ（Al2Cu）＋S（Al2CuMg）＋η（MgZn2） phase regions. Al3Zr, α（Al）, Al13Fe4, η（MgZn2）, α-AlFeSi, Al7Cu2Fe, θ（Al2Cu）, Al5Cu2MgsSi6 precipitate in sequence by no-equilibrium calculation. The SEM and XRD analyses reveal that α（Al）, η（MgZn2）, Mg（Al,Cu,Zn）2, θ（Al2Cu） and Al7Cu2Fe phases are discovered in Al-9.2Zn-1.7Mg-2.3Cu alloy. The thermodynamic calculation can be used to predict the major phases present in experiment.

Microstructure evolution and thermal expansion of Cu-Zn alloy after high pressure heat treatment

The thermal expansion coefficients of Cu-Zn alloy before and after high pressure treatment were measured by thermal expansion instrument in the temperature range of 25？700 ℃,and the microstructure and phase transformation of the alloy were examined by optical microscope,X-ray diffractometer（XRD） and differential scanning calorimeter（DSC）.Based on the experimental results,the effects of high pressure treatment on the microstructure and thermal expansion of Cu-Zn alloy were investigated.The results show that the high pressure treatment can refine the grain and increase the thermal expansion coefficient of the Cu-Zn alloy,resulting in that the thermal expansion coefficient exhibits a high peak value on the α-T curve,and the peak value decreases with increasing the pressure.

Precipitation behavior of 14H LPSO structure in single 18R phase Mg-Y-Zn alloy during annealing at 773K

The microstructural evolution of a 18R single phase （S 18） alloy during annealing at 773 K for 100 h was investigated in order to reveal the formation mechanism of 14H phase. The results showed that the as-cast S 18 alloy was composed of 18R phase （its volume fraction exceeds 93%）, W particles and α-Mg phase. The 18R phase in S18 alloy was thermally stable and was not transformed into 14H long period stacking ordered （LPSO） phase during annealing. However, 14H lamellas formed within tiny α-Mg slices, and their average size and volume fraction increased with prolonging annealing time. Moreover, the 14H phase is nucleated within α-Mg independently on the basis of basal stacking faults （SFs）. The broadening growth of 14H lamellas is an interface-controlled process which involves ledges on basal planes, while the lengthening growth is a diffusion-controlled process and is associated with diffusion of solute atoms. The formation mechanism of 14H phase in this alloy could be explained as α-Mg＇→α-Mg＋14H.

Influence of minor Sc additions on grain refinement and microstructure characteristics of a high Zn-containing Al-Zn-Mg-Cu-Zr alloy

In the present work, scandium elements with a series of contents(0.06 wt.%, 0.10 wt.%, 0.14 wt.%,0.17 wt.%, 0.20 wt.% and 0.25 wt.%) were added in a high Zn-containing Al-Zn-Mg-Cu-Zr alloy and the corresponding as-cast microstructure characteristics including grains and phases were thoroughly investigated. The results indicated that fine grain boundaries existed in these alloys and fine MgZn2phases discontinuously distributed on them. Besides,AlZnMgCu eutectic phases and Sc, Zr-containing phases with flocculent morphology were observed. As scandium contents vary from 0.06 wt.% to 0.17 wt.%, the average grain size continuously decreased and its equiaxial characteristics were strengthened. Meanwhile, the content of AlZnMgCu eutectic phase showed a decrease trend. When scandium contents were 0.20 wt.% and 0.25 wt.%, no further enhancement on grain refinement was observed, so as to the reduction of AlZnMgCu eutectic phase content. Besides, Sc, Zr-containing phases with blocky morphology were observed and the alloy with a scandium content of 0.25 wt.% possessed a larger amount of blocky Sc, Zr-containing phase than the alloy with a scandium content of 0.20 wt.%. Grain refinement and reduction of AlZnMgCu eutectic phase content associated with scandium addition were discussed.

Microstructure and strengthening mechanism of Mg-5.88Zn-0.53Cu-0.16Zr alloy solidified under high pressure

Mg-5.88 Zn-0.53 Cu-0.16 Zr(wt.%)alloy was solidified at 2-6 GPa using high-pressure solidification technology.The microstructure,strengthening mechanism and compressive properties at room temperature were studied using SEM and XRD.The results showed that the microstructure was refined and the secondary dendrite spacing changed from 35μm at atmospheric pressure to 10μm at 6 GPa gradually.Also,Mg(Zn,Cu)2 and Mg Zn Cu eutectic phases were distributed in the shape of network,while under high pressures the second phases(Mg(Zn,Cu)2 and Mg7 Zn3)were mainly granular or strip-like.The solid solubility of Zn and Cu in the matrix built up over increasing solidification pressure and reached 4.12%and 0.32%respectively at 6 GPa.The hardness value was HV 90 and the maximum compression resistance was 430 MPa.Therefore,the grain refinement strengthening,the second phase strengthening and the solid solution strengthening are the principal strengthening mechanisms.

Microstructural evolution and mechanical properties of an ultrahighstrength Al-Zn-Mg-Cu alloy via powder metallurgy and hot extrusion

In this work,a novel ultrahigh-strength Al-10Zn-3.5Mg-1.5Cu alloy was fabricated by powder metallurgy followed by hot extrusion.Investigations on microstructural evolution and mechanical properties of the fabricated samples were carried out.The results show that the grain size of sintered samples matches with the powder particles after ball milling.The relative densities of sintered and hot extruded samples reach 99.1%and 100%,respectively.Owing to the comprehensive mechanism of grain refinement,aging and dispersion strengthening,the ultimate tensile strength,yield strength and elongation of the Al-10Zn-3.5Mg-1.5Cu alloy after hot extrusion and subsequent heat treatment achieve 810 MPa,770 MPa and 8%,respectively.

Refinement and strengthening mechanism of Mg−Zn−Cu−Zr−Ca alloy solidified under extremely high pressure

Mg−Zn−Cu−Zr−Ca samples were solidified under high pressures of 2-6 GPa.Scanning electron microscopy and electron backscatter diffraction were used to study the distribution of Ca in the microstructure and its effect on the solidification structure.The mechanical properties of the samples were investigated through compression tests.The results show that Ca is mostly dissolved in the matrix and the Mg_(2)Ca phase is formed under high pressure,but it is mainly segregated among dendrites under atmospheric pressure.The Mg_(2)Ca particles are effective heterogeneous nuclei ofα-Mg crystals,which significantly increases the number of crystal nuclei and refines the solidification structure of the alloy,with the grain size reduced to 22μm at 6 GPa.As no Ca segregating among the dendrites exists,more Zn is dissolved in the matrix.Consequently,the intergranular second phase changes from MgZn with a higher Zn/Mg ratio to Mg7Zn3 with a lower Zn/Mg ratio.The volume fraction of the intergranular second phase also increases to 22%.Owing to the combined strengthening of grain refinement,solid solution,and dispersion,the compression strength of the Mg-Zn-Cu-Zr-Ca alloy solidified under 6 GPa is up to 520 MPa.

High temperature mechanical properties and microstructure of die forged Al-5.87Zn-2.07Mg-2.42Cu alloy

The high temperature mechanical properties(250 ℃) and microstructure of a die-forged Al-5.87 Zn-2.07 Mg-2.42 Cu alloy after T6 heat treatment were investigated. High temperature tensile tests show that as the temperature increases from room temperature to 250 ℃, the ultimate tensile strength of the alloy decreases from 638 to 304 MPa, and the elongation rises from 13.6% to 20.4%. Transmission electron microscopy(TEM) and electron backscattered diffraction(EBSD) were applied for microstructure characterization, which indicates that the increase of tensile temperature can lead to the coarsening of precipitates, drop of dislocation density, and increase of dynamic recovery. After tensile testing at 250 ℃, a sub-grain structure composed of a high fraction of small-angle grain boundary is formed.