回归再时效对超高强铝合金力学性能及组织的影响

在传统的回归再时效(retrogression and re-aging,RRA)工艺(峰时效)基础上降低预时效或再时效温度,对Fe和Si杂质含量高的超高强Al-Zn-Mg-Cu合金挤压棒材进行RRA处理,通过拉伸性能和疲劳性能测试以及扫描电镜和透射电镜观察,研究RRA工艺对合金力学性能与组织的影响。结果表明:降低预时效或再时效温度都可明显提高该合金的塑性和抗疲劳损伤性能,略微降低合金的抗拉强度。采用峰时效温度(120℃)RRA处理后的合金,晶内的主要析出相为尺寸较大的η′相,不能被位错切割,合金强度较高(674 MPa),但塑性和抗疲劳损伤性能差,伸长率为11.1%,最终应力强度因子幅值ΔK=26.8 MPa·m1/2;降低时效温度可增加析出相中GP区粒子的比例,减小η′相的尺寸,从而提高塑性变形能力以及抗疲劳损伤性能。     

温度对Mullite/Al-4.0Cu复合材料及其基体合金时效行为的影响

用挤压铸造方法制备Mullite/Al-4.0Cu复合材料及其基体合金。用硬度测试（HB）、差示扫描量热仪（DSC）和透射电镜（TEM）等手段，研究了温度对复合材料及其基体合金时效行为的影响，结果表明：复合材料和基体合金具有相似的时效硬化曲线及相同的时效析出序列，随时效温度的升高，峰值硬度降低、时析析出过程加快；莫来石纤维除了能明显提高Al-4.0Cu合金的时效硬度外，还能加速其时效析出进程，但对GP区的形成具有明显的抑制作用，而对θ相的析出影响不大。     

高Cu/Mg比Al-Cu-Mg-Ag(2139)合金时效析出行为研究

利用差示扫描量热法(DSC)研究了2139铝合金在时效过程中的相变行为和析出序列,并测试了合金不同状态下的硬度及电导率,利用透射电镜(TEM)研究了合金的微观组织特征.结果表明:2139合金在时效过程中,随着温度的升高,DSC曲线上出现3个放热峰,其峰值温度分别为171,230和276℃,分别对应着合金中的3个析出过程,即Cu原子的偏聚形成GP,Ω相在{111}<,Al>面上的析出和θ/θ相在{001}<,Al>的析出过程.合金的时效析出序列为:α<,sss>→GPzone,Mg-Ag cluster→Ω相→θ'相→θ相.在时效初期,由于固溶原子脱溶而降低了晶格畸变程度,从而使得电导率略有降低,而后随着时效温度的升高,析出相的数量和种类也越来越多,电导率逐渐升高;合金的硬度随着时效温度的升高而升高,但当温度达到330℃时,由于析出相发生粗化和长大而导致硬度降低.     

Al-Cu合金应力时效的动力学研究

对Al-Cu合金分别进行不同的单级和“双级”应力时效处理,结合电阻率测量和透射电子显微镜两种手段,研究了外加应力对于片状析出物析出动力学和择优取向效应的影响。结果表明:在较低温度下时效,相对电阻率在时效初期时出现了较小的上升,说明外加应力对Al-Cu合金的片状GP区有促进作用。在较高温度下时效,相对电阻率没有出现上升阶段,但其下降幅度明显降低,说明外加应力对θ″相的析出动力学也有影响。不同温度下的“双级”时效处理结果表明:外加应力对时效初期择优取向的影响作用要强于对时效后期择优取向的。用扩散理论证明:在共格片状相析出时,扩散系数呈现出明显的各向异性,从而形成了观察到的择优取向效应。     

对铝—锂合金薄板应变时效的研究

本文主要研究了2091合金薄板在预变形8%和12%,150℃和170℃不同时效时间下的组织和性能.随时效时间延长,时效温度升高,预应变增加,强度增加而塑性下降;时效初期合金主要断裂形式为穿晶断裂,随时效时间延长,沿晶断裂比例增大,这也是塑性下降的主要原因;时效时间延长,温度升高和预应变增加,δ’相析出增多长大,时效后期的s’相也析出增多,成为合金强化的主要原因;时效后期的S’相、共面滑移带和无析出区(PFZ)共同作用使塑性曲线下降平缓.     

含Li的Al-Zn-Mg-Cu合金时效析出过程

采用小角度X射线散射技术和硬度测试,对含0.65wt.%Li的Al-Zn-Mg-Cu合金定量研究了在120℃单级时效过程中析出相尺寸和体积分数的演变规律,探讨了Li元素在该时效前中期对析出相析出行为的影响,分析了时效过程中显微硬度与析出相之间的关系。结果表明,随着时效时间的延长,析出相半径分成了高速、中速和低速三个增长阶段,而体积分数则分成了快速增加和渐缓增加两个阶段;Li在时效前中期作用显著,不仅促进了合金固溶-淬火后GP(I)区的迅速形成,还缩短了析出相体积分数达到最大值的时间;显微硬度与析出相半径和体积分数在时效前中期均呈现出良好的正相关关系,析出相类型转变过程的缓慢导致合金硬度维持峰值时间较长。     

高温预析出对Al-Zn-Mg系铝合金时效硬化和应力腐蚀的影响

研究了高温固溶后降温处理工艺对中强可使预析出LC52和7039铝合金的组织、时效硬化和应力腐蚀的影响。金相观察发现,高温预析出可优先在晶界处产生,并提高随后时效状态下晶界析出相的不连续分布程度,温度降低到一定程度晶内和晶界产生大量析出。合金拉伸性能和应力腐蚀结果表明,预析出在保持强度和塑性的同时,可提高抗应力腐蚀性能。而预析出温度降低,合金强度呈下降趋势。     

时效温度对1460铝锂合金薄板力学性能与微观组织的影响

采用拉伸性能测试及透射电镜(TEM)分析手段研究了1460铝锂合金薄板在145和175℃时效时的力学性能和微观组织的演化。研究结果表明:相比于145℃时效,1460铝锂合金175℃时效响应速率明显加快。在120 h时效时间范围内,175℃时效时合金的延伸率、最高抗拉强度以及屈服强度与抗拉强度差均低于145℃时效时,特别是175℃近峰时效时合金屈服强度与抗拉强度差明显降低。1460铝锂合金145,175℃两个温度时效时的主要脱溶产物均为δ'相(Al_3Li),合金时效初期均快速析出均匀弥散分布的δ'相;175℃时效时δ'相的粗化速度明显高于145℃时效,导致其尺寸随时效时间延长显著粗化,密度明显降低。145℃时效时随时效时间延长,合金在析出δ'相后形成大量的G.P.区;时效后期G.P.区长大,没有T1相(Al_2CuLi)的析出。175℃时效过程中合金在析出δ'相后快速析出T1相;时效后期T1相数量明显减少而尺寸显著粗化。     

Al-Li合金断裂韧性的研究

研究了时效工艺对一种高强高模Al-Li合金断裂韧性的影响.研究结果表明,时效前的预时效或预变形可促进T_1相析出,从而改善合金的断裂韧性,但在过时效状态,预时效或预变形的有利影响减弱.为获得高的断裂韧性.时效时应避免粗大晶界平衡相的析出和晶界无析出带变宽.依据组织观察结果,讨论了时效制度、微观组织和断裂韧性之间的关系.     

时效工艺对7003铝合金组织与性能的影响

采用透射电镜、维氏硬度仪和拉伸试验机,研究了时效温度和时间对7003铝合金显微组织与力学性能的影响。结果表明,随着时效时间的延长,铝合金基体中依次沉淀析出GP区、η'相(亚稳定相MgZn_2)和η相(MgZn_2),其中GP区和η'相都能阻碍位错运动,对铝合金起到很好的强化作用。当GP区数量最多时,铝合金的强度达到第一个峰值;当GP区转变为过渡相η'时,铝合金的强度达到第二峰值;当η'转变为η相时,铝合金的强度开始下降。随着时效温度的提高,铝合金强度达到峰值的时间缩短,并且两个峰值之间的时间间隔也缩短。时效温度为120℃时,铝合金的第二峰值强度高于第一峰值强度;时效温度高于120℃时,铝合金的第一峰值强度高于第二峰值强度。     

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Abstract

The ageing in SAP-type (Al–Mg–Si)–Al₂O₃ composites is investigated using resistivity measurements, hardness measurements, mechanical tests, and electron microscopy observations. An oxide content of about 3% completely inhibits GP zone formation during natural ageing because of the absence of quenched-in vacancies. In contrast, GP zone formation occurs during artificial ageing, the process being maintained by thermal vacancies and by diffusion along grain boundaries. To take advantage of both dispersion and precipitation hardening, it appears advisable to keep the oxide content below 1·5%. A considerable improvement in mechanical properties of the present composites over the conventional alloy is observed.     

Influence of zirconium and copper on the early stages of aging in Al-Zn-Mg alloys

The effects of zirconium and copper on the early stages of the precipitation processes in an Al-5.5 wt pct Zn-1.2 wt pct Mg alloy have been studied by differential scanning calorimetry (DSC) thermal analysis. Electron diffraction has been used as a complementary technique to aid in the interpretation of the thermal effects observed in the DSC thermograms. The results show that the initial stages of Guinier-Preston zone I (GP(I)) formation at room temperature are not affected by the presence of zirconium, but the rate of Guinier-Preston zone II (GP(II)) precipitation is slowed down significantly. For aging at 100 °C, the stability of GP zones is reduced by the addition of zirconium, and this leads to a reduction in the amount of η′ produced during aging. The addition of copper to an Al-5.4 wt pct Zn-1.2 wt pct Mg-0.2 wt pct Zr alloy intensifies the electron diffraction spots from GP(I), suggesting that the strong electron-scatterer copper may be incorporated into GP zones. The rate of growth of GP(I) at room temperature is unaffected by the presence of copper, but the rate of formation of GP(II) at room temperature is retarded. For artificial aging at 100 °C, the development of GP(I) and GP(II) is not affected significantly by the presence of copper, but the formation of η′ is stimulated, producing a high number density of very fine η′ precipitates. Preaging at room temperature results in accelerated η′ formation during subsequent aging at 100 °C in the zirconium-containing alloy. However, this acceleration of η′ formation is absent when copper is present in the alloy.     

The Temperature Limit for GP Zone Formation in an Al-Zn-Mg Alloy

The temperature limit for GP zone formation, T_limit was determined for an Al-5 pct Zn-2 pct Mg alloy by examining the formation process of the intermediate phase. Studies of changes in hardness and electrical resistivity, together with electron metallography permitted a conclusion that the T_limit for this alloy is 125 °C. The nucleation mechanism of the intermediate phase is also discussed. Two types of precipitation mode were found the partly coherent phase;i.e. heterogeneous nucleation on GP zones and homogeneous nucleation respectively. It is concluded that the dominant nucleation mode is determined by the quenching conditions. formerly with Graduate School of Tokyo Institute of Technology, is now with Kobe Steel Ltd., Nagoya formerly with Tokyo Institute of Technology, is now Associate Professor, The Technological University of Nagaoka, Nagaoka.     

时效对超高强含钪铝合金组织和性能的影响

采用维氏硬度测量、室温拉伸性能测试和显微组织结构分析,研究了不同时效制度下Al-Zn-Cu-Mg-Sc-Zr合金的力学性能、腐蚀性能和显微组织。结果表明,合金具有显著的时效硬化效应,随时效温度的升高,合金达到时效硬度峰值的时间缩短。合金适宜的时效制度为120℃/24 h。此时,合金的抗拉强度、屈服强度、伸长率和维氏硬度分别为696 MPa、654 MPa、11.1%和211.2 HV。合金中主要强化相为GP区和η′相,主要强化作用为沉淀强化及弥散强化。时效过程中Al3Sc和Al3(Sc,Zr)质点表现出较强的热稳定性;合金抗晶间腐蚀能力随时效时间的延长而增强。     

Effect of δ alumina fibers on the aging characteristics of 2024-based metal-matrix composites

The age-hardening precipitation reaction in aluminum matrix composites reinforced with discontinuous alumina fibers was studied using the differential scanning calorimetry (DSC) technique, microhardness tests, and transmission electron microscopy (TEM) observation. Composites fabricated with the 2024 alloy matrix were infiltrated through a ceramic preform using a squeeze-casting process. The alumina fibers had a considerable effect on the aging response of the matrix alloy in composites. Alumina fibers caused suppression of Guinier—Preston (GP) zone formation in composite that reduced the peak hardening during artificial aging. The suppression of GP zone formation in composites is believed to be due to the fiber-matrix interface, which acts as a sink for vacancies during quenching. Moreover, the presence of reinforcement does not alter the kinetics of the subsequent artificial aging of these Al₂O₃/2024Al composites.     

Modeling for the structure evolution of alloys of the Al-Cu-Mg system during natural ageing

Natural ageing in alloys of the Al-Cu and Al-Cu-Mg systems is investigated and modeled. To determine the kinetic parameters, a method for measuring the electrical resistance is applied as the basic method. With the use of the Avrami equation in the differential form, a model for the structure evolution (changes in the volume fraction of the Guinier-Preston zones (GPZs) and the depletion of the solid solution) and the change in the yield strength for the alloys during natural ageing is constructed. In the case of the Al-Cu-Mg ternary system, two types of zones are present (the GPZ and the Guinier-Preston-Bagaryatskii zone (GPBZ)). In this case, the dissociation of the oversaturated solid solution at room temperature is described by a set of differential equations. The accuracy of the constructed model of the dependence of the yield strength on the alloy composition and the natural ageing time is 6%, which is within the error of experimental determination for this parameter.     

A calorimetric study of precipitation in aluminum alloy 2219

Precipitate microstructures in aluminum alloy 2219 were characterized using transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The DSC signatures of individual precipitate phases were established by comparing the DSC and TEM results from samples that had been aged such that only one precipitate phase was present. These signatures were then used to analyze the commercial tempers. It was found that DSC could readily distinguish between the T3, T4, T6, T8 and O tempers but could not distinguish amongst T81, T851 and T87. Small amounts of plastic deformation between solution treatment and aging had a significant effect on the thermograms. Aging experiments at 130 and 190 °C showed that the aging sequence and DSC response of this alloy were similar to those of pure Al-Cu when the increased copper content is taken into account. Further aging experiments at temperatures between room temperature and 130 °C showed pronounced changes of the GP zone dissolution peak as a function of aging conditions. These changes were found to be related to the effect of GP zone size on the metastable phase boundary and on the GP zone dissolution kinetics.     

工业化制备7050铝合金厚板显微组织与力学性能

采用OM、TEM观察、室温拉伸试验,研究工业化制备大截面7050铝合金厚板微观显微组织和力学性能.结果表明:板材表层、1/4厚度层和芯部处均存在部分粗大晶粒组织以及晶粒尺寸分布不均匀现象,其中芯部粗大晶粒的尺寸、数量以及晶粒尺寸不均匀程度均高于1/4厚度层和表层;合金时效后晶内的析出相主要为η’相、η相以及少量GPⅡ区,表层晶粒内的析出相密度大于1/4层和芯部,且1/4层和芯部粗棒状的η相含量较高;合金的强度、硬度以及延伸率在厚度方向上呈梯度分布,表层硬度、抗拉强度（σ_b）、屈服强度（σ_0.2）和延伸率（δ）均最高,从表层到芯部,硬度、σ_b、σ_0.2以及δ均逐渐减小.      

Microstructure study on the effect of titanium in the aging treatment of an Al-Zn-Mg alloy

A study has been made of the effect of Ti additions on the microstructure in two stage and single stage aging of an Al-5 Zn-2 Mg alloy. The influence of aging treatment was examined for alloys with and without Ti addition. The alloys were solution heat treated at 510°C, air cooled then aged at various temperatures below the GP zone solvus for short times before aging above the GP zone solvus at 200°C. The Ti addition alloy yields a small precipitate free zone (PFZ ∼ 0.3 μ) and a higher density of precipitate near the edge of the PFZ than in the midgrain region. The ternary alloy without Ti addition produces a wide range of PFZ sizes (0.4 to 4 μ) depending upon the aging temperature below the GP zone solvus. Significant changes in the morphology of the matrix precipitate, the amount of grain boundary precipitate, and the width of the PFZ, were observed in single stage aging at 200°C as a result of the Ti addition. These results could be explained in terms of Ti interacting with vacancies or solute atoms, causing the changes in the vacancy and/or solute concentration profiles. A model based upon solute and vacancy concentrations coupled with a time at temperature effect has been developed to allow the interpretation of the observed two-stage aging results in an Al-Zn-Mg alloy. Formerly Graduate Research Assistant, Materials Engineering, Rensselaer Polytechnic Institute, Troy, New York.