Formation of nanocrystalline AZ31B Mg alloys via cryogenic rotary swaging

A bulk nanocrystalline AZ31B Mg alloy with extraordinarily high strength was prepared via cryogenic rotary swaging in this study.The obtained alloy shows finer grains,higher strength,and a negligible tension-compression yield asymmetry,compared with that prepared via room-temperature rotary swaging.Transmission electron microscopy investigations showed that at the initial stage,multiple twins,mostly tension twins,were activated and intersected with each other,thereby refining the coarse grains into a fine lamellar structure.Then,two types of nanoscale subgrains were generated with increasing swaging strain.The first type of nanoscale subgrain contained twin boundaries and low-angle grain boundaries.This type of subgrain appeared at the twin-twin intersections and was mainly driven by high local stress.The second type of nanoscale subgrain was formed within the twin lamellae.The boundaries of this type of subgrain did not contain twin boundaries and were transformed from massive dislocation arrays.Finally,randomly oriented nanograins were obtained via dynamic recrystallization,under the combined function of deformation heat and increased stored energy.Compared with room-temperature rotary swaging,cryogenic rotary swaging exhibits a slower grain refinement process but a remarkably enhanced grain refinement effect after the same five-pass swaging.

Experimental investigation of vibration pretreatment-microwave curing process for carbon fiber reinforced resin matrix composites

The vibration pretreatment-microwave curing process is an efficient,low energy consumption,and high-quality out-of-autoclave curing process for carbon fiber resin matrix composites.This study aims to investigate the impact of vibration pretreatment temperature on the fiber weight content,microscopic morphology and mechanical properties of the composite laminates by using optical digital microscopy,universal tensile testing machine and thermo-gravimetric analyzer.Additionally,the combined mode of Bragg fiber grating sensor and temperature measurement fiber was employed to explore the effect of vibration pretreatment on the strain process during microwave curing.The study results revealed that the change in vibration pretreatment temperature had a slight impact on the fiber weight content when the vibration acceleration remained constant.The metallographic and interlaminar strength of the specimen formed at a vibration pretreatment temperature of 80℃ demonstrated a porosity of 0.414% and a 10.69% decrease in interlaminar shear strength compared to autoclave curing.Moreover,the introduction of the vibration energy field during the microwave curing process led to a significant reduction in residual strain in both the 0°and 90°fiber directions,when the laminate was cooled to 60℃.

Effect of thermo-mechanical treatment on microstructure and mechanical properties of wire-arc additively manufactured Al-Cu alloy

Wire-arc additive manufacture(WAAM)has great potential for manufacturing of Al-Cu components.However,inferior mechanical properties of WAAM deposited material restrict its industrial application.Inter-layer cold rolling and thermo-mechanical heat treatment(T8)with pre-stretching deformation between solution and aging treatment were adopted in this study.Their effects on hardness,mechanical properties and microstructure were analyzed and compared to the conventional heat treatment(T6).The results show that cold rolling increases the hardness and strengths,which further increase with T8 treatment.The ultimate tensile strength(UTS)of 513 MPa and yield stress(YS)of 413 MPa can be obtained in the inter-layer cold-rolled sample with T8 treatment,which is much higher than that in the as-deposited samples.The cold-rolled samples show higher elongation than that of as-deposited ones due to significant elimination of porosity in cold rolling;while both the T6 and T8 treatments decrease the elongation.The cold rolling and pre-stretching deformation both contribute to the formation of dense and dispersive precipitatedθ′phases,which inhibits the dislocation movement and enhances the strengths;as a result,T8 treatment shows better strengthening effect than the T6 treatment.The strengthening mechanism was analyzed and it was mainly related to work hardening and precipitation strengthening.

High density dislocations enhance creep ageing response and mechanical properties in 2195 alloy sheet

The creep strain of conventionally treated 2195 alloy is very low,increasing the difficulty of manufacturing Al-Cu-Li alloy sheet parts by creep age forming.Therefore,finding a solution to improve the creep formability of Al-Cu-Li alloy is vital.A thorough comparison of the effects of cryo-deformation and ambient temperature large pre-deformation(LPD)on the creep ageing response in the 2195 alloy sheet at 160℃with different stresses has been made.The evolution of dislocations and precipitates during creep ageing of LPD alloys are revealed by X-ray diffraction and transmission electron microscopy.High-quality 2195 alloy sheet largely pre-deformed by 80%without edge-cracking is obtained by cryo-rolling at liquid nitrogen temperature,while severe edge-cracking occurs during room temperature rolling.The creep formability and strength of the 2195 alloy are both enhanced by introducing pre-existing dislocations with a density over 1.4×10^(15)m^(−2).At 160℃and 150 MPa,creep strain and creep-aged strength generally increases by 4−6 times and 30−50 MPa in the LPD sample,respectively,compared to conventional T3 alloy counterpart.The elongation of creep-aged LPD sample is low but remains relevant for application.The high-density dislocations,though existing in the form of dislocation tangles,promote the formation of refined T1 precipitates with a uniform dispersion.

Achieving High Strength and Tensile Ductility in Pure Nickel by Cryorolling with Subsequent Low-Temperature Short-Time Annealing

Ultra fine-grained pure metals and their alloys have high strength and low ductility.In this study,cryorolling under different strains followed by low-temperature short-time annealing was used to fabricate pure nickel sheets combining high strength with good ductility.The results show that,for different cryorolling strains,the uniform elongation was greatly increased without sacrificing the strength after annealing.A yield strength of 607 MPa and a uniform elongation of 11.7%were obtained after annealing at a small cryorolling strain(ε=0.22),while annealing at a large cryorolling strain(ε=1.6)resulted in a yield strength of 990 MPa and a uniform elongation of 6.4%.X-ray diffraction(XRD),transmission electron microscopy(TEM),scanning electron microscopy(SEM),and electron backscattered diffraction(EBSD)were used to characterize the microstructure of the specimens and showed that the high strength could be attributed to strain hardening during cryorolling,with an additional contribution from grain refinement and the formation of dislocation walls.The high ductility could be attributed to annealing twins and micro-shear bands during stretching,which improved the strain hardening capacity.The results show that the synergistic effect of strength and ductility can be regulated through low-temperature short-time annealing with different cryorolling strains,which provides a new reference for the design of future thermo-mechanical processes.

不同应力水平对2195-T34 Al-Li合金时效行为的影响机制研究

研究了不同应力水平下2195-T34 Al-Li合金的时效特性变化。当应力低于材料的高温屈服强度(YS)时,低应力时效(LSA)和无应力时效(SFA)试样的屈服强度和延伸率(EL)相近。当应力超过材料的高温屈服强度时,试样的极限拉伸强度(UTS)和伸长率显著下降,表明应力的增加降低了材料的抗破坏性。透射电子显微镜(TEM)和扫描电子显微镜(SEM)的观察结果表明:应力对材料时效性能的影响归因于内部析出物特征和晶界富铜析出物的共同作用。应力的增加促使铜原子在晶界发生偏析形成富铜析出物,同时促进了晶界无沉淀区(PFZ)的形成,而富铜析出物是脆性相,可作为损伤的起始位置,容易导致裂纹萌生和发展,降低材料的延展性。

Effect of three-step homogenization on microstructure and properties of 7N01 aluminum alloys

The effect of different homogenization treatments on the microstructure and properties of the 7N01 aluminum alloy was investigated using hardness measurements, electrical conductivity measurements, tensile and slow strain rate tests, electron probe microanalysis, optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results revealed that three-step homogenization improved the uniformity of Zr distribution by eliminating segregation of the main alloying elements. During the second homogenization step at 350 °C for 10 h, coarse and strip-like equilibrium η phases formed which assisted the nucleation of Al3Zr dispersoids and reduced the width of the precipitate-free zone of A13Zr dispersoids. As a result, coarse recrystallization was greatly reduced after homogenization at 200 °C, 2 h ＋ 350 °C, 10 h ＋ 470 °C, 12 h, which contributed to improving the overall properties of the 7N01 aluminum alloys.

Correlations among stress corrosion cracking,grain-boundary microchemistry,and Zn content in high Zn-containing Al-Zn-Mg-Cu alloys

The correlations among the corrosion behaviour,grain-boundary microchemistry,and Zn content in Al-Zn-Mg-Cu alloys were studied using stress corrosion cracking(SCC)and intergranular corrosion(IGC)tests,combined with scanning electron microscopy(SEM)and high-angle angular dark field scanning transmission electron microscopy(HAADF-STEM)microstructural examinations.The results showed that the tensile strength enhancement of high Zn-containing Al-Zn-Mg-Cu alloys was mainly attributed to the high density nano-scale matrix precipitates.The SCC plateau velocity for the alloy with 11.0 wt.%Zn was about an order of magnitude greater than that of the alloy with 7.9 wt.%Zn,which was mainly associated with Zn enrichment in grain boundary precipitates and wide precipitates-free zones.The SCC mechanisms of different Zn-containing alloys were discussed based on fracture features,grain-boundary microchemistry,and electrochemical properties.

Creep aging behavior and performance of Al-Zn-Mg-Cu alloys under different parameters in retrogression aging treatment

A study was conducted to better understand how different parameters, namely, regression aging time and regression aging temperature, affect the creep aging properties, i.e., the creep deformation and performance of Al-Zn-MgCu alloy during regressive reaging. The corresponding creep strain and mechanical properties of samples were studied by conducting creep tests and uniaxial tensile tests. The electrical conductivity was measured using an eddy-current conductivity meter. The microstructures were observed by transmission electron microscopy(TEM). With the increase in regression aging time, the steady creep strain first increased and then decreased, and reached the maximum at 45 min.The steady creep strain increased with the increase in regression aging temperature, and reached the maximum at 200 ℃.The level of steady creep strain was determined by precipitation and dislocation recovery. Creep aging strengthens 7B50-RRA treated with regression aging time at 190 ℃ for 10 min, and the difference in the mechanical properties of alloy becomes smaller. The diffusion of solute atoms reduces the scattering of electrons, leading to a significant improvement in electrical conductivity and stress corrosion cracking(SCC) resistance after creep aging. The findings of this study could help in the application of creep aging forming(CAF) technology in Al-Zn-Mg-Cu alloy under RRA treatment.

Preface to the special issue on aluminum alloys for transportation

Aluminum alloys have been used widely as structural materials in the field of aerospace,high speed train and car etc.due to their advantages such as low density,high strength,and good corrosion resistance.Improved properties of these materials can increase the reliability,durability and life of the structural components,and decrease the cost and CO_(2) emission.The special issue focuses on the recent development of advanced aluminum alloys applied in the transportation field,particularly aerospace,train and car.

Research on Residual Stress Measurement of Magnesium Alloy Cabin Castings

Owing to the nonuniform wall thickness and complex internal structure,the measurement of the residual stress on magnesium alloy cabin castings is complex and difficult extremely,and thus seldom research has focused on the residual stress of magnesium alloy castings.In this paper,the blind-hole method,the X-ray diffraction(XRD)method,and the contour method are used to conduct comprehensive and systematic residual stress tests for a magnesium alloy cabin casting.The results show that the residual stress on the surface of the casting obtained by the blind-hole method is between-20.03 MPa and-71.03 MPa,the residual stress obtained by the XRD method is between-26.01 MPa and-87.11 MPa,while the residual stress obtained by the contour method is between-45.89 MPa and 76.87 MPa.The study can lay a basis for the subsequent research of magnesium alloy cabin castings,and provide a reference for the residual stress test of magnesium alloy castings.

Mg-10Gd-3Y-0.4Zr合金高温变形过程的修正本构模型研究

为了研究Mg-10Gd-3Y-0.4Zr镁合金在高温变形过程中的流变行为,在573^773 K温度范围和0.001^1.0 s^(-1)应变速率范围内进行了热压缩实验。实验结果表明,流变应力随着温度的升高和应变速率的降低而降低。流变应力的变化与应变变化有关,建立了考虑应变补偿的改进Arrhenius本构方程。然而,具有应变补偿的Arrhenius模型在低温和高应变速率下显示出与真实应力-应变曲线的明显偏差。误差分析表明,Arrhenius模型中的材料参数不仅与应变有关,还与应变速率和变形温度有关。因此,通过将修正模型中的计算应力与实验应力进行比较,考虑高温压缩过程中材料参数与成形参数之间的关系,建立了修正的Arrhenius本构模型。

热处理状态对2195铝锂合金热变形行为及组织演变的影响

2195铝锂合金在工业生产中应用广泛,不同的初始热处理状态影响其热变形行为。因此,本文旨在研究热处理状态对2195铝锂合金的组织演变和热变形行为的影响。实验在Gleeble-3800热模拟机上进行,温度范围为350~500℃,应变速率为0.01~10 s^(-1)。将热变形后各区域的微观组织进行对比,并计算出合金的本构模型和加工图。在较高的变形温度(400~500℃)下,T3和T8试样的峰值应力基本相同,同时发现热变形后合金中存在低角度晶界(LABs)和动态再结晶。由于固溶强化和形变强化作用,2195-T8铝锂合金表现出比2195-T3铝锂合金更大的形变激活能。本文确定2195-T3和2195-T8铝锂合金的最佳热加工条件均为475~500℃和0.01~0.1 s^(-1)。本文可为铝锂合金在后续热加工过程中设置热处理参数提供有益的帮助。

预处理对喷射沉积Al-Cu-Li合金中Al_(3)Zr粒子析出及再结晶行为的影响

本文为探究如何调控喷射沉积Al-Cu-Li合金中Al_(3)Zr粒子的析出以及其析出机制,通过扫描电镜、透射电镜和电子背散射衍射研究了预处理对喷射沉积Al-Cu-Li合金中Al_(3)Zr粒子的析出行为及其对再结晶的影响。研究结果表明,喷射沉积Al-Cu-Li合金中Al_(3)Zr粒子存在分布不均匀,其粒子数量密度在Zr原子过饱和度高的晶内更高,在过饱和度低的晶界附近更低;晶界附近和晶内均存在局部无沉淀析出带,且晶界附近存在少量粒子线性团簇。在均匀化过程中Zr原子与位错攀移之间发生交互作用使得Al3Zr粒子析出。在均匀化前热变形预处理增加了基体中位错数量进而促进了Al_(3)Zr粒子析出,改善了粒子分布,抑制了局部无沉淀析出带。相比于(573 K/12 h)退火预处理和673 K下压缩10%预处理,673 K下压缩30%预处理试样中Al3Zr粒子密度更高,分布更均匀,尺寸更小。经过热压缩和固溶处理后,3种热压缩试样大角度晶界占比分别为30.3%、21.6%和17.3%。本研究将为喷射沉积Al-Cu-Li合金的研发提供依据。

微波能场均匀性调控及其对复合材料温度场分布的影响研究

微波加热技术凭借选择性加热、节能、高效等一系列优点成为目前复合材料成型领域的研究热点之一。然而,谐振腔内电场的不均匀分布将会引起复合材料构件的加热不同步,导致固化后构件的严重变形和质量缺陷,是制约该工艺广泛工程化应用的主要瓶颈。本文联合非线性电磁本构和功率流密度函数,查明了复合材料构件微波加热过程中电场均匀性与温度场分布之间的关联关系;结合有限元仿真技术,揭示了多种调控方式,例如微波激励源的设置、模式搅拌器的引入、载物平台的相对移动等对电场分布均匀性的作用机制;基于自主研发的微波加热均匀性调控设备,开展系列性实验探究了不同调控模式下复合材料构件温度场分布的演变规律,为仿真分析提供了数据支撑;最后,深入分析了固化过程中温度场分布均匀性对复合材料性能的影响。

剪切应力下温度对纳米晶Al-Mg-Si合金近表面微观结构演变的影响

采用分子动力学模拟方法研究了不同压力和速度下纳米晶Al-Mg-Si的微观结构演变中的温度效应。通过微观结构的定量表征提出了变形机制图,给出了不同温度和载荷下弹性-塑性转变、堆垛层错、晶粒细化和剪切层形成的“相图”。温度的变化会改变堆垛层错(SFs)的运动和微观结构的完整性。在300 K时,SFs以单条迹线的形式出现,而在77 K时则以多条平行线的形式出现。与300 K相比,77 K下样品晶粒旋转和滑移运动受限,微观结构难以重排。在77 K时,在外部载荷作用下晶粒细化程度更高(最高可细化13.2%,而300 K下最高仅为8.3%),从而产生更多晶界面,而且特殊晶界(以Σ3为代表)变化幅度明显,且总体含量较高。在105倍大气压的载荷下,样品的结构完整性突然下降。在最快速度下的变形恢复到与最慢速度几乎相同的低水平。

平底无铆连接的材料流动行为及接头失效机理研究

平底无铆连接在轻量化结构中具有广阔应用前景。本研究旨在分析平底无铆连接过程中的材料流动行为和接头失效机理。在不同的成形载荷下,使用平底无铆连接工艺连接Al5052-H32铝合金板材。建立了平底无铆连接有限元模型,分析其材料流动规律。通过接头的金相观察以及硬度测试得到连接过程中材料的实际流动行为。采用扫描电子显微镜对材料力学性能测试后的失效接头断口进行观察,从而研究其失效机理。结果表明,在平底无铆连接过程中,材料流动最活跃的区域集中在冲头圆角处的上板区域。在剪切试验中,接头断裂的微观形貌表现为脆性-韧性混合断裂。在拉伸试验中,观察到接头断裂的微观形貌具有韧性断裂特征。

Evaluation of elastic-viscoplastic self-consistent models for a rolled AZ31B magnesium alloy under monotonic loading along five different material orientations and free-end torsion

Comprehensive experiments including monotonic tension and monotonic compression of specimens taken from a rolled AZ31B Mg thick plate along five different material orientations with respect to the rolled direction(RD),and free-end torsion of a tubular specimen machined along the thickness direction(ND)were conducted.The experimental results were used to evaluate an elastic-viscoplastic self-consistent model with the consideration of twinning and detwinning(EVPSC-TDT)on magnesium(Mg)alloys.The EVPSC-TDT model provides stress-strain curves and the hardening rates in close agreement with the experimental results of all the 11 loading cases.The model adequately predicts the textures after fracture of all the 11 loading cases and the evolutions of tension twins with increasing strains for tension in the ND,compression in the RD,and torsion along the ND.The Swift effect was observed in the experiment and was properly simulated by the model.

Inhibiting segregation enabled outstanding combination of mechanical and corrosion properties in precipitation-strengthened aluminum alloys

Segregation of solutes to grain boundary(GB)is the dominant restriction on enhancement of mechanical properties and corrosion resistance during aging in precipitation-strengthened aluminum alloys.Here,we innovatively introduce the cyclic deformation by the vibration during aging to eliminate the GB segre-gation,resulting in the formation of the narrow precipitate-free zone(PFZ)widths near the GB,as well as the fine and discontinuous grain boundary precipitations(GBPs).Compared with the traditional peak-aging,the 2014 aluminum alloy treated by thermal cooperative vibration aging(TCVA)exhibits a superb combination of impact toughness and corrosion resistance,and retains the strength and ductility.In ad-dition,the atomic simulations show that TCVA generates numerous vacancies near the GB,but does not change the dislocation density with the increase of cycle time.These results indicate that the vacancy significantly promotes the nucleation and growth of precipitates in the vicinity of GB,resulting in the narrow PFZs and fine discontinuous GBPs.The present work provides the fundamental knowledge and method to inhibit the equilibrium segregation of solutes to GB during aging,and further realizes the precipitation-strengthened aluminum alloys with excellent mechanical and corrosion properties.

Modification of constitutive model and evolution of activation energy on 2219 aluminum alloy during warm deformation process

To investigate the flow behavior of 2219 Al alloy during warm deformation, the thermal compression test was conducted in the temperature range of 483-573 K and the strain rate range of 0.001-5 s^-1 on a Gleeble-3500 thermomechanical simulation unit. The true stress-true strain curves obtained showed that the flow stress increased with the decrease in temperature and/or the increase in strain rate and the softening mechanism primarily proceeded via dynamic recovery. The modification on the conventional Arrhenius-type constitutive model approach was made, the material variables and activation energy were determined to be dependent on the deformation parameters. The modified flow stresses were found to be in close agreement with the experimental values. Furthermore, the activation energy obtained under different deformation conditions showed that it decreased with the rise in temperature and/or strain rate, and was also affected by the coupled effect of strain and strain rate.