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.

Influence of solution treatment on microstructure and properties of in-situ Mg_2Si/AZ91D composites

The influence of solution treatment on the microstructure and properties of Mg2Si/AZ91D composites fabricated from Mg-SiO2 system via in-situ processing method was investigated.The results show that coarse Chinese script shape Mg2Si phases can be formed by adding SiO2 into AZ91D magnesium alloy with Si content up to 1.5% of the alloy melt.During solution treatment,the morphology and distribution of the coarse Chinese script shape Mg2Si phases are modified.Meanwhile,the β-Mg17Al12 phase is dissolved into the magnesium matrix.With increasing holding time,the coarse Mg2Si phases tend to dissolve,break and spheroidize.After solution treatment at 420 ℃ for 16 h,Mg2Si phases become the finest and relatively well-distributed phase.The tensile strength and elongation are increased by 14.9% and 38.9%,respectively.It is believed that the Mg2Si phases continuously dissolve and break,and finally the spheroidized Mg2Si particles are obtained due to the interface tension of Mg2Si/Mg interface.

Microstructure and mechanical properties of AZ31-Mg_2Si in situ composite fabricated by repetitive upsetting

AZ31-4.6% Mg2Si （mass fraction） composite was prepared by conventional casting method. Repetitive upsetting （RU） was applied to severely deforming the as-cast composite at 400 ℃ for 1, 3, and 5 passes. Finite element analysis of the material flow indicates that deformation concentrates in the bottom region of the sample after 1 pass, and much more uniform deformation is obtained after 5 passes. During multi-pass RU process, both dendritic and Chinese script type Mg2Si phases are broken up into smaller particles owing to the shear stress forced by the matrix. With the increasing number of RU passes, finer grain size and more homogeneous distribution of Mg2Si particles are obtained along with significant enhancement in both strength and ductility. AZ31-4.6%Mg2Si composite exhibits tensile strength of 284 MPa and elongation of 9.8%after 5 RU passes at 400 ℃ compared with the initial 128 MPa and 5.4%of original AZ31-4.6%Mg2Si composite.

高能超声对Mg_2Si/AZ91D复合材料的影响

为改善原位颗粒增强镁基复合材料的性能,采用原位合成技术制备了Mg2Si/AZ91D复合材料,通过在熔体中施加高能超声,研究了超声时间和超声功率对复合材料组织性能的影响.结果表明:随着超声时间的延长或超声功率的增大,复合材料中粗大的汉字状Mg2Si相变得细小、分布均匀,同时细小分布均匀的球化状β-Mg17Al12相增多;超声时间为6 min、超声功率为1.2 kW时,组织中呈短棒状的Mg2Si颗粒和球化状β-Mg17Al12相分布均匀,且复合材料的抗拉强度和伸长率达到最大,分别为220.5 MPa和2.6%,较未施加超声的复合材料试样提高了22.3%和38.9%;再延长处理时间或增大输出功率,组织有粗化的趋势,复合材料的抗拉性能及伸长率也呈现先升后降趋势.

浇注方式和冷却速度对AZ91-0.45Si合金中Mg_2Si形态的影响

分别采用底注式和顶注式两种浇注系统,得到不同的AZ91-0.45Si镁合金组织,通过分析其铸态显微组织,比较合金中Mg2Si的形貌,探讨浇注方式和冷却速度对Mg2Si相形貌的影响。结果表明,含0.45Si的AZ91镁合金中存在两种形态的Mg2Si相,底注式试样Mg2Si呈不规则多边形,顶注式试样Mg2Si呈"汉字状"。分析认为,Mg2Si相形成的形态与冷却速度相关,冷却速度越快,过冷度和凝固速度就越大;Mg2Si相有足够的时间继续向四周长大,故顶注式试样中的Mg2Si相呈"汉字状"。

钇对Mg_2Si/AZ91D复合材料中Mg_2Si相变质的影响

研究了不同含量的钇对Mg2Si/AZ91D复合材料中Mg2Si相形貌的影响。结果表明:Y对Mg2Si/AZ91D复合材料中共晶Mg2Si有明显的细化变质效果,合金液经0.8%Y变质后,共晶Mg2Si相由粗大的树枝状或汉字状转变为颗粒状。在1.2%添加范围内,随Y加入量的增加,对共晶Mg2Si相的细化变质效果增强,未出现过变质现象。固溶处理能使Mg2Si/AZ91D复合材料中枝状或汉字状Mg2Si发生部分溶解,并依靠原子扩散自发粒状化,Y的细化变质能促进Mg2Si相固溶处理时的粒状化。

铋含量对原位自生Mg_2Si/AZ91D复合材料组织与性能的影响

制备原位自生Mg_2Si/AZ91D复合材料,研究铋变质剂含量对复合材料组织与力学性能的影响。结果表明,加入铋变质剂后,原位自生Mg_2Si/AZ91D复合材料中初生相Mg_2Si由粗大的树枝状变成细小的多边形状和颗粒状,共晶相Mg_2Si由汉字状变成短棒状和蠕虫状,Mg17Al12相由连续网状分布变成弥散板条状分布。铋含量为0.8%时,初生相Mg_2Si最细小,平均晶粒尺寸为13μm,比未变质时减小了81%。铋含量为0.8%时Mg_2Si/AZ91D复合材料力学性能最佳,抗拉强度和延伸率分别为290 MPa和6.7%,比未变质时分别提高25.5%和109%。

Te变质处理对原位自生Mg_2Si/AZ91D复合材料的影响

研究了Te变质处理对原位自生Mg_2Si/AZ91D复合材料组织与力学性能的影响,探索了变质机理。结果表明:Te变质使复合材料中初生Mg_2Si基本消失,共晶Mg_2Si数量增多,尺寸减小,并且部分共晶Mg_2Si由汉字状变成了蠕虫状和短杆状,β-Mg_(17)Al_(12)由断续的网状变成了细小的板条状、颗粒状。α-Mg基体的晶粒尺寸随Te含量的增加而减小。Te含量为0.7%时,复合材料的室温和高温(150℃)力学性能最佳。

Si含量对Mg_2Si增强AZ91D复合材料组织与性能的影响

熔炼并浇注了Mg2Si增强AZ91D复合材料铸锭,并研究了Si含量对其铸态组织和力学性能的影响规律。结果表明:Mg2Si增强AZ91D复合材料中初生Mg2Si数量随着Si含量的增加而增多,其形状由细小的树枝状、块状、颗粒状变成粗大的树枝状。Si含量为2%时,初生Mg2Si最细小,平均晶粒尺寸达到15μm。随着Si含量的增加,Mg2Si增强AZ91D复合材料室温力学性能呈下降的变化趋势,Si含量为2%时,室温抗拉强度、条件屈服强度、伸长率分别达到281 MPa、132.3 MPa和6.4%。Si含量为2%时,Mg2Si增强AZ91D复合材料拉伸断口中出现了许多韧窝,撕裂棱非常明显,说明材料的塑性较好。

半固态等温热处理对原位Mg_(2)Si/AZ91复合材料组织及性能的影响

为解决高体积分数Mg_(2)Si增强镁基复合材料中初生Mg_(2)Si相尺寸粗大导致的复合材料力学性能不佳的问题,提出了等温热处理细化Mg_(2)Si形貌的方法。研究了等温热处理对体积分数为10%的Mg_(2)Si/AZ91复合材料中Mg_(2)Si形貌、基体组织及硬度的影响规律,并揭示其演化机理。结果表明:在等温热处理过程中,初生Mg_(2)Si相曲率较小处优先溶解,导致初生Mg_(2)Si相在枝晶壁处熔断形成独立的颗粒,随后转变为具有最低表面能的球状,随着保温时间的延长,发生合并长大。对复合材料进行不同参数的等温热处理后发现,在510℃保温60min后,初生Mg_(2)Si相的细化效果最好,颗粒尺寸达到30.6μm,较原始试样减小了58.6%。在热处理过程中,α-Mg枝晶与沿晶界分布的Mg_(17)Al_(12)相发生成分均匀化,α-Mg晶界圆整化并且晶粒尺寸减小;复合材料硬度在等温热处理后显著降低,但随着初生Mg_(2)Si相和的α-Mg晶粒的细化,硬度有所回升,其硬度变化的主要机制是固溶强化、硬质相消失、细晶强化的共同作用。该研究为Mg_(2)Si/AZ91复合材料形貌改善以及性能提升提供了实验基础和理论依据。

Mechanical properties of Al-15Mg_(2)Si composites prepared under different solidification cooling rates

The effect of different cooling rates(2.7,5.5,17.1,and 57.5℃/s)on the solidification parameters,microstructure,and mechanical properties of Al-15Mg_(2)Si composites was studied.The results showed that a high cooling rate refined the Mg_(2)Si particles and changed their morphology to more compacted forms with less microcracking tendency.The average radius and fraction of primary Mg_(2)Si particles decreased from 20μm and 13.5%to about 10μm and 7.3%,respectively,as the cooling rate increased from 2.7 to 57.5℃/s.Increasing the cooling rate also improved the distribution of microconstituents and decreased the grain size and volume fraction of micropores.The mechanical properties results revealed that augmenting the cooling rate from 2.7 to about 57.5℃/s increased the hardness and quality index by 25%and245%,respectively.The high cooling rate also changed the fracture mechanism from a brittle-dominated mode to a high-energy ductile mode comprising extensive dimpled zones.

Effects of in-situ ZrB_2 particle on grain refinement of ZrB_2/AZ91D magnesium matrix composite

In-situ ZrB2/AZ91D magnesium matrix composite was successfully synthesized with AI/K2ZrF6＋NH4BF4 by means of Direct Melt Reaction. The fabricated ZrB2/AZ91D magnesium matrix composite through direct melt mixing method was investigated. Results from X-ray diffraction （XRD） and energy dispersive X-ray spectroscopy （EDS） confirmed the existence of ZrB2 particles in the AZ91D alloy, and most ZrB2 particles were in the size range of just several microns, some even to 100 nm. The cast specimens were studied through corrosion testing and heat treatment. The average grain size of AZ91D decreased markedly from about 250 pm to 50 IJm. In addition, the shape and size of the ,β-MglTAI12 phase as well as the morphologies of primary a-Mg in the magnesium matrix composite were greatly changed. The network structure of the β-MglTAI12 phase was broken into small blocks and the size of a-Mg decreased significantly.

含金属Mg相的Mg_(2)(Sn,Si)薄膜的微观结构与热电性能

利用高真空磁控溅射设备,采用Mg-Sn-Si合金靶材和纯Mg靶进行顺序沉积,在表面含有500 nm厚氧化硅的单晶Si(100)衬底上制备了不同Mg含量的Mg-Sn-Si薄膜。沉积Mg-Sn-Si薄膜的相组成、化学成分、表面和横截面形貌和热电性能被系统的测量、观察和分析。结果表明,制备的Mg-Sn-Si薄膜由Mg_(2)(Sn,Si)固溶体和金属Mg相组成。随着金属Mg相含量的增多,沉积薄膜载流子浓度升高,迁移率降低。沉积薄膜的电导率随着金属Mg相含量的增多而增大,而Seebeck系数降低。沉积薄膜的金属Mg相含量增加,虽然电导率增加,但因Seebeck系数急剧下降,最终导致功率因子下降。在所研究的金属Mg含量范围内,Mg_(2)(Sn,Si)固溶体薄膜含有部分金属Mg相,对提高复合薄膜的功率因子是不利的。

Sb变质对原位Mg2Si/AZ91复合材料组织的影响

Mg2Si颗粒增强镁基复合材料中Mg2Si是粗大的长条状,严重降低其力学性能。加入微量Sb元素可细化Mg2Si增强相,但是Sb在Mg2Si/AZ91复合材料中的细化机理尚不明确。为此,采用原位铸造的方法研究了不同Sb含量对Mg2Si/AZ91复合材料微观组织的影响。通过光学显微镜、扫描电子显微镜和电子探针显微分析仪观测了不同Sb含量的Mg2Si/AZ91复合材料中Mg2Si的尺寸和形貌,并探究Sb元素细化Mg2Si的机理。采用Image-Pro Plus软件统计了Mg2Si颗粒的平均尺寸以及形状因子的变化。实验结果表明:添加Sb后基体中汉字状的共晶Mg2Si消失,且初生Mg2Si的平均尺寸不断降低,当Sb的质量分数为2%时,初生Mg2Si颗粒尺寸细化至13.8μm(降低80.1%),分布更均匀;随着Sb含量增加,Mg2Si形状因子越来越趋向于1,形貌由粗大的树突状、长条状转变成细小的多边形;在凝固过程中,Sb富集在Mg2Si固液生长界面前沿,降低了初生Mg2Si的实际析出温度和Mg2Si与熔体的界面张力,提高了过冷度,增大了Mg2Si的形核率。同时,富集在Mg2Si表面的Sb毒化了Mg2Si的生长步骤,Mg2Si各向同性生长,形貌得到改善。

P变质对Si合金化AZ91镁合金显微组织和力学性能的影响

采用X射线衍射、高分辨透射电镜等现代分析测试手段研究了Al-P中间合金变质处理对AZ91＋0．7％Si合金的显微组织和力学性能的影响。研究结果表明：AZ91＋0．7％Si合金中加入一定量的Al-P中间合金变质处理后，Mg2Si的形貌发生了显著变化，由汉字状变为细小的颗粒状，其颗粒的平均尺寸为6～10μm，且基体组织的平均晶粒尺寸由原来的100μm减小到45μm左右，β-Mg17Al12相出现不同程度的断网，从而使AZ91＋0．7％Si合金的力学性能得到显著提高。说明Al-P中间合金对Si合金化的AZ91镁合金具有很好的变质强化功效，是一种Mg2Si化合物的强化粒化剂。

原位Mg_2Si颗粒对镁合金基体应力分布影响的模拟研究

利用大型商业有限元软件Ansys模拟AZ91D镁合金和原位Mg2Si颗粒增强AZ91D基复合材料在相同单向外载荷条件下的应力分布情况,并获得原位Mg2Si增强颗粒对镁基复合材料力学性能的影响。结果表明:材料远离受力面而靠近约束面上的尖角处易应力集中;Mg2Si颗粒的产生提高了材料抵抗弹性变形的能力,从而使该复合材料得到强化;在拉伸应力场超过屈服极限的情况下,基体开裂或界面脱落是失效的主要形式。

Sb变质对Mg_2Si/Mg-Al基复合材料中Mg_2Si相的影响

采用金相显微镜、扫描电镜、X射线衍射仪和同步热分析仪研究了Sb变质对Mg_2Si/AZ91D复合材料中Mg_2Si相的影响,探索了变质机理。结果表明:0.2%Sb变质使Mg_2Si/AZ91D复合材料中初生Mg_2Si由粗大的枝晶状变成细小的多边形状,共晶Mg_2Si的形貌由粗大的汉字状变成针状、团絮状、短杆状、蠕虫状和颗粒状。继续增加Sb含量,对初生Mg_2Si的形状和尺寸影响不大。但是增加Sb含量使得颗粒状的共晶Mg_2Si数量增多,尺寸减小,分布更均匀。加入Sb变质剂后,Mg_3Sb_2可作为初生Mg_2Si的形核核心,细化晶粒。共晶Mg_2Si的凝固开始温度降低,其形核过冷度增大,形核率增加。

Ce变质对Mg_2Si增强镁基复合材料组织与力学性能的影响

研究了Ce含量对原位自生Mg_2Si/AZ91D复合材料组织与常温、高温力学性能的影响。结果表明:Ce对初生Mg_2Si的形状影响不大,但是可细化初生Mg_2Si,使共晶Mg_2Si由树枝状、汉字状变成短杆状、颗粒状,并细化β-Mg_(17)Al_(12)。Mg_2Si/AZ91D复合材料的室温和高温力学性能均随Ce含量的增加呈先增大后减小的变化趋势。加入0.5%Ce时,材料的室温和高温力学性能最佳。室温抗拉强度和伸长率分别比未变质时提高了18.4%和74.1%;高温下抗拉强度比室温下降低了22.6%,伸长率比室温下提高了52.7%。

Si对喷射成形AZ91镁合金微观组织的影响

在保护气氛中采用喷射成形技术制备AZ91和AZ91+2.0%Si镁合金,采用OM、SEM、XRD及TEM等技术分别对合金的微观组织进行观察和分析。结果表明:与相同成分的铸造镁合金相比,喷射成形技术显著细化合金组织;喷射成形沉积态AZ91镁合金具有均匀、细小的等轴晶组织,平均晶粒尺寸约为17μm,离异共晶β-Mg17Al12相在晶界的偏析被有效改善;对于喷射成形AZ91+2.0%Si镁合金,喷射成形过程中高的冷却速度抑制了Mg2Si相的长大,细小的Mg2Si相呈现为近球形或多边形态,与基体结合良好且弥散分布,并可以作为α-Mg异质形核核心,使合金组织得到充分细化。

挤压温度对Al-25%Mg_(2)Si复合材料组织和性能的影响

用熔铸法制备高增强相质量分数的Al-25%Mg2Si复合材料。在300~400℃挤压温度、2.64∶1挤压比和0.001 m/s挤压速度下,获得热挤压材料。利用OM、SEM和XRD分析材料的相和显微组织以及对应的抗弯性能。结果表明:铸态组织由枝晶状或漏斗状初生Mg2Si(Mg2Sip)、片状共晶Mg2Si(Mg2Sie)和α-Al组成;挤压变形后,Mg2Sip钝化破碎,Mg2Sie片或Mg2Sie颗粒间距增大,部分Mg2Sie被破碎为点状或短棒状;由于增强相的破碎和孔隙率降低,复合材料的强度和塑性大幅提高,随挤压温度的降低,增强相Mg2Si的破碎效果更显著,复合材料的抗弯强度不断升高,最大为183 MPa,而塑性先升后降,在360℃达到最大值,为1.9%。