Microstructural evolution and mechanical properties of AZ31 Mg alloy fabricated by a novel bifurcation-equal channel angular pressing

In this work,a novel type of short-process deformation technology of Mg alloys,bifurcation-equal channel angular pressing(B-ECAP),was proposed to refine grain and improve the basal texture.The cylindrical billets were first compressed into the die cavity,then sequentially flowed downward through a 90°corner and two 120°shear steps.The total strain of B-ECAP process could reach 3.924 in a single pass.The results of microstructure observation showed that DRX occurred at upsetting process in the die cavity and completed at position D.The grains were refined to 6.3μm at being extruded at 300℃ and grew obviously with the extrusion temperature increase.The shear tress induced by 900 corner and two 120°shear steps resulted in the basal poles of most grains tilted to extrusion direction(ED)by±25°.Compared with the original billets,the extruded sheets exhibited higher yield strengths(YS),which was mainly attributed to the grain refinement.The higher Schmid factor caused by ED-tilt texture resulted in a fracture elongation(FE)more than that of the original bar in ED,while was equivalent to that in transverse direction(TD).As the extrusion temperature increased,the variation of UTS and YS in ED and TD decreased gradually without ductility obviously decrease.

Microstructure homogeneity and mechanical properties of laser-arc hybrid welded AZ31B magnesium alloy

Laser-arc hybrid welding of AZ31B magnesium alloy was carried out,the effects of welding parameters on weld formation,microstructure homogeneity and mechanical properties were investigated.The results showed that laser-arc hybrid welding was beneficial to improve the weld formation of magnesium alloy by inhibiting the defect of undercut and pores.The weld microstructure was mainly columnar grains neighboring the fusion line and equiaxed grains at the weld center.It was interesting that the grain size at the upper arc zone was smaller than that at the lower laser zone,with the difference mainly affected by laser power rather than welding current and welding speed.The welding parameters were optimized as laser power of 3.5 kW,welding current of 100 A and welding speed of 1.5 m/min.In this case,the weld was free of undercut and pores,and the tensile strength and elongation rate reached 252 MPa and 11.2%,respectively.Finally,the microstructure homogeneity was illustrated according to the heat distribution,and the evolution law of tensile properties was discussed basing on the weld formation and microstructure characteristics.

Microstructure evolution,mechanical properties and fracture behavior of Al-xSi/AZ91D bimetallic composites prepared by a compound casting

In this paper,the effect of the Si content on microstructure evolution,mechanical properties,and fracture behavior of the Al-xSi/AZ91D bimetallic composites prepared by compound casting was investigated systematically.The obtained results showed that all the Al-xSi/AZ91D bimetallic composites had a metallurgical reaction layer(MRL),whose thickness increased with increasing Si content for the hypoeutectic Al-Si/AZ91D composites,while the hypereutectic Al-Si/AZ91D composites were opposite.The MRL included eutectic layer(E layer),intermetallic compound layer(IMC layer)and transition region layer(T layer).In the IMC layer,the hypereutectic Al-Si/AZ91D composites contained some Si solid solution and flocculent Mg_(2)Si+Al-Mg IMCs phases not presented in the hypoeutectic Al-Si/AZ91D composites.Besides,increasing Si content,the thickness proportion of the T layer increased,forming an inconsistent preferred orientation of the MRL.The shear strengths of the Al-xSi/AZ91D bimetallic composites enhanced with increasing Si content,and the Al-15Si/AZ91D composite obtained a maximum shear strength of 58.6 MPa,which was 73.4% higher than the Al-6Si/AZ91D composite.The fractures of the Al-xSi/AZ91D bimetallic composites transformed from the T layer into the E layer with the increase of the Si content.The improvement of the shear strength of the Al-xSi/AZ91D bimetallic composites was attributed to the synergistic action of the Mg_(2)Si particle reinforcement,the reduction of oxidizing inclusions and the ratio of Al-Mg IMCs as well as the orientation change of the MRL.

Unveiling the tailorable electrochemical properties of zeolitic imidazolate framework-derived Ni-doped LiCoO_(2) for lithium-ion batteries in half/full cells

As a prevailing cathode material of lithium-ion batteries(LIBs),LiCoO_(2)(LCO)still encounters the tricky problems of structural collapse,whose morphological engineering and cation doping are crucial for surmounting the mechanical strains and alleviating phase degradation upon cycling.Hereinafter,we propose a strategy using a zeolitic imidazolate framework(ZIF)as the self-sacrificing template to directionally prepare a series of LiNi_(0.1)Co_(0.9)O_(2)(LNCO)with tailorable electrochemical properties.The rational selection of sintering temperature imparts the superiority of the resultant products in lithium storage,during which the sample prepared at 700℃(LNCO-700)outperforms its counterparts in cyclability(156.8 mA h g^(-1)at 1 C for 200 cycles in half cells,1 C=275 mA g^(-1))and rate capability due to the expedited ion/electron transport and the strengthen mechanical robustness.The feasibility of proper Ni doping is also divulged by half/full cell tests and theoretical study,during which LNCO-700(167 mA h g^(-1)at 1 C for 100 cycles in full cells)surpasses LCO-700 in battery performance due to the mitigated phase deterioration,stabilized layered structu re,ameliorated electro nic co nductivity,a nd exalted lithium sto rage activity.This work systematically unveils tailorable electrochemical behaviors of LNCO to better direct their practical application.

具有细晶/混晶交替分布组织特征的AZ31镁合金轧板拉伸变形行为仿真

通过中等应变速率轧制获取细晶区和混晶区交替分布的AZ31镁合金轧制板材,构建不同组织特征的黏塑性自洽(VPSC)模型,预测板材在室温拉伸下的变形机制、织构演变和力学响应,研究细晶区与混晶区对塑性变形的贡献。结果表明:经410℃、平均应变速率1.9 s^(-1)、4.8 s^(-1)和7.8 s^(-1)制备的轧制板材中细晶区与混晶区呈现板织构特征,细晶区织构组分相较于混晶区更接近ND方向;随着轧制平均应变速率的增加,板材的细晶区与混晶区的织构组分差异逐渐减小。在室温拉伸塑性变形初期阶段,轧制板材的主要变形机制是基面滑移,次要变形机制是棱柱面滑移和锥面滑移,而拉伸孪生几乎不开启。压缩孪生启动时,部分晶粒向TD方向偏转56°,形成TD方向的织构组分。棱柱面滑移活性的降低,有利于增强锥面滑移的分离效应,促使ND方向织构极密度降低和双峰织构形成。轧制板材在拉伸过程中,由于细晶区的织构组分相较于混晶区更接近ND方向,使得在拉伸过程中细晶区相较于混晶区的基面滑移开启较少,非基面滑移开启较多,促使拉伸变形的过程中细晶区起强化作用,混晶区起弱化作用。织构组分差异逐渐减小,使得细晶区与混晶区的变形机制活性趋于一致,促使其在拉伸过程中对抗拉强度贡献也逐渐趋于一致。

基于Deform 3D的AZ31镁合金不对称挤压变形工艺模拟

通过Deform 3D软件模拟了AZ31镁合金不对称挤压变形工艺过程。结果表明,设置不同水平长度流动距离促使材料不对称地流入挤压通道,进而形成不对称剪切变形,出现应变及应力集中分布的不对称性,促使材料晶粒沿着剪切面偏转,获得细晶、弱织构的高性能镁合金坯料。

中高应变速率轧制AZ31镁合金板的抗拉强度预测

本文采集了不同轧制温度、应变速率以及压下量等三个工艺参数下中高应变速率轧制的AZ31镁合金抗拉强度的27组样本,通过惯性权重、学习因子的改进和引入变异操作对PSO-BP神经网络进行改进,并与BP、PSO-BP神经网络对比,进行抗拉强度的预测。结果表明:BP神经网络不能预测AZ31镁合金抗拉强度的非线性变化,PSO-BP神经网络和改进的PSO-BP(IPSO-BP)神经网络均能较好地预测AZ31镁合金抗拉强度的非线性变化;这三个模型中IPSO-BP神经网络预测最为准确,相较于PSO-BP神经网络,其平均绝对误差从15.3764降低至3.4288,平均相对误差从5.94%降低至1.32%,均方误差从251.3662降低至20.7199,相关系数从0.7753提高至0.8937;通过Pearson相关性计算判断出应变速率、压下量对抗拉强度的影响均大于轧制温度,而应变速率与抗拉强度呈负相关关系,压下量与抗拉强度呈正相关关系。

Ce变质铸造汽车用AZ61镁合金的组织与力学性能

对AZ61镁合金进行了不同Ce添加量的变质铸造试验,通过OM、硬度测试、拉伸试验等手段研究了Ce变质处理对AZ61镁合金组织与力学性能的影响。结果表明:未变质的AZ61镁合金铸态组织由α-Mg相和粗大网状β-Mg17Al12相组成。随着Ce加入量的增加,AZ61镁合金中β-Mg17Al12相析出量逐渐减少,稀土相逐渐增多,β-Mg17Al12相由连续粗大网状逐渐转变为颗粒状,Ce添加量1.0wt%的AZ61镁合金中β-Mg17Al12相最为细小。与未变质的AZ61镁合金相比,经Ce变质的AZ61镁合金的硬度、抗拉强度和伸长率均有了明显提高。随着Ce加入量的增加,AZ61镁合金的硬度、抗拉强度和伸长率均先升高后降低,Ce添加量1.0wt%的AZ61镁合金的力学性能最佳。

浇铸温度对AZ31镁合金铸轧凝固焊合线的影响

基于φ880 mm×400 mm水平双辊铸轧机的实际工况,通过有限元模拟分析了浇铸温度对铸轧区AZ31镁合金凝固焊合线的影响规律。结果表明,浇铸温度对凝固焊合线位置影响较为明显,但对其形状影响较小。随着浇铸温度提升,凝固焊合线逐渐向出口靠近,边部Kiss点(K')和Kiss线上的凸点(K')连线与过K'点平行于铸轧方向的直线的夹角α先减小后增大。当浇铸温度为965 K时,铸轧过程中极易出现漏液,合适的浇铸温度范围为915~959 K。对比不同浇铸温度下的仿真结果,浇铸温度为945 K时,∠α最小,有利于缓解镁合金铸轧板带边裂。

添加0.5%Si_(3)N_(4)对车用AZ61镁合金微观组织和性能的影响

研究了添加0.5%Si_(3)N_(4)(质量分数)对车用AZ61镁合金微观组织、力学性能以及耐腐蚀性能的影响。结果表明添加0.5%Si_(3)N_(4)(质量分数)后,铸态和挤压态AZ61镁合金的晶粒均细化。添加0.5%Si_(3)N_(4)(质量分数)后,部分Si_(3)N_(4)与Mg,Al发生反应生成了AlN与Mg2Si,另一部分Si_(3)N_(4)以第二相的形式存在于镁合金中,铸态AZ61镁合金屈服强度由66.7 MPa提升到了73.2 MPa,抗拉强度由105.4 MPa提升到了113.3 MPa,延伸率由1.5%提升到了1.9%,与AZ61镁合金相比,分别提升了9.7%,7.4%和26.7%。添加0.5%Si_(3)N_(4)(质量分数)后挤压态AZ61镁合金屈服强度由145.0 MPa提升到了162.7 MPa,抗拉强度由274.6 MPa提升到了284.2 MPa,延伸率由16.0%提升到了18.1%,与AZ61镁合金相比,分别提升了12.2%,3.5%和13.1%。添加0.5%Si_(3)N_(4)(质量分数)后挤压态AZ61的耐腐蚀性能也得到了提升,腐蚀电流密度由142.9μA/cm^(2)降低到了56.7μA/cm^(2)。添加0.5%Si_(3)N_(4)(质量分数)可以有效提升车用AZ61镁合金的力学性能和耐腐蚀性能。

AZ31镁合金和7050铝合金ECAP过程的数值模拟研究

利用Abaqus有限元软件分别对AZ31镁合金和7050铝合金进行等通道转角挤压(ECAP)数值模拟研究。利用有限元(FE)模拟得到应力场、应变场及载荷-位移曲线,分析了在不同挤压温度、挤压速度、摩擦条件和不同截面下等通道转角挤压的变形规律。结果表明,挤压温度升高可以降低挤压载荷,这有利于AZ31镁合金挤压,在摩擦较小情况下进行变形有利于降低挤压载荷,提高变形均匀性;在不考虑变形过程中温升效应的情况下,挤压速度对挤压载荷影响较小,当挤压速度提高时,最大等效应变值有所提高;圆形截面比方形截面更适合进行等通道转角挤压。

预制孪晶对AZ31镁合金绝热剪切敏感性的影响

为了分析不同孪晶体积分数对材料绝热剪切敏感性的影响,预制具有不同孪晶体积分数的AZ31镁合金。采用分离式霍普金森压杆在200℃、944 s^(-1)条件下对AZ31镁合金帽状试样进行高速冲击试验,采用电子背散射衍射仪和光学显微镜对试样在高应变速率变形前后的微观组织进行观察,计算了试样发生绝热剪切时的吸收能量,比较了绝热剪切带内外的显微硬度。结果表明:不同孪晶体积分数试样内均产生了绝热剪切带,随着孪晶体积分数的增加,绝热剪切带宽度和吸收能量减小,而绝热剪切敏感性增大。

Acceleration of aging behavior and improvement of mechanical properties of extruded AZ80 alloy through(10–12) twinning

The effects of pre-existing {10–12} extension twins on the precipitation behavior of an extruded AZ80 material during aging and on its mechanical properties after peak aging are investigated. The material containing {10–12} twins-which are formed by compression before aging(twinned material)-has a finer grain size and higher dislocation density than the extruded material. Although the peak hardnesses of the twinned and extruded materials are almost the same, the time to reach the peak hardness is considerably shorter in the former material than in the latter(4 h and 24 h, respectively). In the twinned material, the high dislocation density of the {10–12} twins promotes continuous precipitation, which results in the formation of numerous fine Mg17Al12precipitates within the twins in the early stage of aging.The formation of these continuous precipitates reduces the driving force for discontinuous precipitation, which consequently suppresses the formation and growth of coarse Mg17Al12precipitates at the grain boundaries. Despite its shorter peak-aging time, the 4 h-peak-aged twinned material shows higher tensile strength and elongation than the 24 h-peak-aged extruded material. These higher mechanical properties of the former material are attributed primarily to the presence of more abundant fine continuous precipitates, which are effective in strengthening the material, and less abundant coarse discontinuous precipitates, which can act as crack initiation sites. These results demonstrate that the introduction of {10–12} twins into wrought Mg–Al-based alloys can accelerate the Mg17Al12precipitation kinetics considerably and improve the strength and ductility of the peak-aged alloys simultaneously.

AZ91镁合金中变形孪晶与析出相交互作用结构的电子显微学研究

本文综合利用透射电子显微镜(transmission electron microscopy,TEM)及高分辨透射电子显微镜(high resolution transmission electron microscope,HRTEM)电子显微表征技术,对冲击变形时效AZ91镁合金中变形孪晶与β⁃Mg17Al12析出相交互作用引发的变形结构进行了观察和表征。结果表明:(1)β析出相阻碍{1012}孪晶生长使其界面形成台阶结构导致孪晶板条呈现粗细不均的形貌,更严重的交互作用可导致孪晶界发生大角度弯折,同时诱发基体中扭折带的形成以协调孪晶变形;(2){1011}孪晶与β析出相的交互作用会产生局域应力集中,导致连锁诱发{1012}孪晶形核,表明孪晶与析出相的交互作用可以促进孪晶形核。

Difference in extrusion temperature dependences of microstructure and mechanical properties between extruded AZ61 and AZ91 alloys

The effects of extrusion temperature on the microstructure and tensile properties of extruded AZ61 and AZ91 alloys are investigated by subjecting them to hot extrusion at 300 and 400℃.Although the average grain size of the extruded AZ61 alloy slightly increases from 9.5 to 12.6μm with increasing extrusion temperature,its resultant microstructural variation is insignificant.In contrast,the average grain size of the extruded AZ91 alloy significantly increases from 5.7 to 22.5μm with increasing extrusion temperature,and the type of Mg17Al12 precipitates formed in it changes from fine dynamic precipitates with a spherical shape to coarse static precipitates with a lamellar structure.As the extrusion temperature increases,the tensile yield strength of the extruded AZ61 alloy increases from 183 to 197 MPa while that of the extruded AZ91 alloy decreases from 232 to 224 MPa.The tensile elongations of the extruded AZ61 and AZ91 alloys decrease with increasing extrusion temperature,but the degree of decrease is significant in the latter alloy.These different extrusion temperature dependences of the tensile properties of the extruded AZ61 and AZ91 alloys are discussed in terms of their microstructural characteristics,strengthening mechanisms,and crack initiation sites.

Microstructural characteristics and low-cycle fatigue properties of AZ91 and AZ91-Ca-Y alloys extruded at different temperatures

The commercial AZ91 alloy and nonflammable SEN9(AZ91-0.3Ca-0.2Y,wt%)alloy are extruded at 300°C and 400°C.Their microstructure,tensile and compressive properties,and low-cycle fatigue(LCF)properties are investigated,with particular focus on the influence of the extrusion temperature.In the AZ91 and SEN9 materials extruded at 300°C(300-materials),numerous fine Mg_(17)Al_(12)particles are inhomogeneously distributed owing to localized dynamic precipitation during extrusion,unlike those extruded at 400°C(400-materials).These fine particles suppress the coarsening of recrystallized grains,decreasing the average grain size of 300-materials.Although the four extruded materials have considerably different microstructures,the difference in their tensile yield strengths is insignificant because strong grain-boundary hardening and precipitation hardening effects in 300-materials are offset almost completely by a strong texture hardening effect in 400-materials.However,owing to their finer grains and weaker texture,300-materials have higher compressive yield strengths than400-materials.During the LCF tests,{10-12}twinning is activated at lower stresses in 400-materials than in 300-materials.Because the fatigue damage accumulated per cycle is smaller in 400-materials,they have longer fatigue lives than those of 300-materials.A fatigue life prediction model for the investigated materials is established on the basis of the relationship between the total strain energy density(ΔW_(t))and the number of cycles to fatigue failure(N_(f)),and it is expressed through a simple equation(ΔW_(t)=10·N_(f)-0.59).This model enables fatigue life prediction of both the investigated alloys regardless of the extrusion temperature and strain amplitude.

La/Nd比对AZ91-RE合金显微组织和拉伸性能的影响

研究La/Nd比对AZ91-RE合金显微组织和拉伸性能的影响。采用经典表征方法分析第二相的体积分数、平均最近邻距离(NND)和数量密度。利用第一性原理计算确定第二相的形成焓、力学性能和热力学性质。结果表明,La与Nd的质量分数比(w_(La)/w_(Nd))为3/2的合金具有最小的平均晶粒尺寸、最小的平均最近邻距离和最高的第二相数量密度。计算结果表明,Al_(11)La_(3)和Al2Nd的形成在能量上有利。此外,Al_(11)La_(3)和Al_(2)La在抗变形能力、抗剪切变形能力、固体刚度和热稳定性方面都明显优于其他第二相。因此,提高Al_(11)La_(3)和Al_(2)La相的含量有利于改善AZ91-RE合金的力学性能。

AZ31B镁合金轧板差温剪切旋压成形组织和性能的研究

在100~300℃成形温度开展了AZ31B镁合金薄板差温剪切旋压实验,并分析研究了温度对成形工件微观组织和力学性能的影响规律。结果表明:AZ31B板材在150~300℃具有良好的差温剪切旋压成形性;150℃成形时,工件微观组织中以变形孪晶为主,晶界分布着亚微米级再结晶晶粒,随着温度上升,再结晶体积分数增加,在250、300℃成形时,获得完全动态再结晶组织。工件在200℃成形后获得最佳综合力学性能,在150℃旋压后工件强度最高。

注射成形增强半固态AZ91D镁合金的组织及性能

为了研究半固态注射成形技术对AZ91D镁合金显微组织、力学性能以及腐蚀行为的影响,通过对半固态注射成形镁合金进行热处理及电化学腐蚀分析,研究了镁合金成形件在注射成形中与热处理后的组织及性能变化。结果表明,相对于压铸镁合金,半固态注射成形镁合金的组织均匀、力学性能优良、晶粒细小且平均晶粒尺寸为20~30μm,能获得质量良好的显微组织。镁合金的热处理对晶界第二相的数量和分布有较大影响,能作用于材料性能。经时效处理后,镁合金硬度从63.47 HV提高到74.05 HV,抗拉强度从125.56 MPa提高到150.91 MPa,抗拉强度提高了20.2%。在质量分数为3.5%的NaCl溶液的电化学腐蚀中,合金的自腐蚀电位为-959.56 mV,经固溶+时效处理后,自腐蚀电位先升高到-927.55 mV后下降至-988.94 mV,耐腐蚀性能先增高后降低。

Comparative study of extrudability, microstructure, and mechanical properties of AZ80 and BA53 alloys

The extrudability,microstructural characteristics,and tensile properties of the Mg–5Bi–3Al(BA53)alloy are investigated herein by comparing them with those of a commercial Mg–8Al–0.5 Zn(AZ80)alloy.When AZ80 is extruded at 400℃,severe hot cracking occurs at exit speeds of 4.5 m/min or more.In contrast,BA53 is successfully extruded without any surface cracking at 400℃ and at high exit speeds of 21–40 m/min.When extruded at 3 m/min(AZ80–3)and 40 m/min(BA53–40),both AZ80 and BA53 exhibited completely recrystallized microstructures with a<10–10>basal texture.However,BA53–40 has a coarser grain structure owing to grain growth promoted by the high temperature in the deformation zone.AZ80–3 contains a continuous network of Mg_(17)Al_(12) particles along the grain boundaries,which form via static precipitation during natural air-cooling after the material exits the extrusion die.BA53–40 contains coarse Mg_(3)Bi_(2) particles aligned parallel to the extrusion direction along with numerous uniformly distributed fine Mg_(3)Bi_(2) particles.AZ80–3 has higher tensile strength than BA53–40 because the relatively finer grains and larger number of solute atoms in AZ80–3 result in stronger grain-boundary and solid-solution hardening effects,respectively.Although BA53 is extruded at a high temperature and extrusion speed of 400℃ and 40 m/min,respectively,the extruded material has a high tensile yield strength of 188 MPa.This can be primarily attributed to the large particle hardening effect resulting from the numerous fine Mg_(3)Bi_(2) particles.