Nanostructured Composite Materials with Improved Deformation Behavior

The recent progress in the development of nanostructured composites is described for Zr‐base multicomponent alloys as a typical example for such materials. These advanced composite materials are attractive candidates for structural as well as functional applications. The combination of high strength with high elastic strain of fully nanocrystalline and glassy alloys renders them quite unique in comparison to conventional (micro‐)crystalline materials. However, one major drawback for their use in engineering applications is the often limited macroscopic plastic deformability, despite the fact that some of these alloys show perfectly elastic‐plastic deformation behavior. To improve the room temperature ductility of either fully nanocrystalline or amorphous alloys, the concept of developing a heterogeneous microstructure combining a glassy or nanostructured matrix with second‐phase particles with a different length‐scale, has recently been employed. This review describes the composition dependent metastable phase formation in the Zr‐(Ti/Nb)‐Cu‐Ni‐Al alloy system, which in turn alters the mechanical properties of the alloys. We emphasize the possibilities to manipulate such composite microstructures in favor of either strength or ductility, or a combination of both, and also discuss the acquired ability to synthesize such in‐situ high‐strength composite microstructures in bulk form through inexpensive processing routes.     

Ti-10Mo-XNb合金铸态组织和机械性能的研究

为了研究Nb元素对Ti-10Mo合金组织和性能的影响,采用钨电极熔化、离心浇注工艺制备了4种钛合金(Ti-10Mo-XNb,X=0,3,7,10),分析并测试了Nb元素对Ti-10Mo合金铸态组织和力学性能的影响.研究结果表明:随着Nb含量的增加,3种Ti-Mo-Nb合金的铸态组织和相组成发生了改变,Ti-10Mo-3Nb合金由等轴的α+β晶粒组成,Ti-10Mo-7Nb合金由等轴的β晶粒组成,Ti-10Mo-10Nb合金由少量等轴和大量枝状的β晶粒组成.另外,随着Nb含量的增加,3种Ti-Mo-Nb合金的维氏硬度、压缩强度、弹性模量降低,压缩率和抗弯强度升高,压缩断口和弯曲断口由脆性断裂向韧性断裂转变.Ti-Mo-Nb合金有望成为新型的生物医用材料.     

Effect of Niobium on Microstructure and Properties of the CoCrFeNb_xNi High Entropy Alloys

A series of CoCrFeNb_xNi(x values in molar ratio, x = 0, 0.25, 0.45, 0.5, 0.75, 1.0 and 1.2) high entropy alloys(HEAs) was prepared to investigate the alloying effect of Nb on the microstructures and mechanical properties. The results indicate that the prepared CoCrFeNb_xNi(x 0) HEAs consist of a simple FCC solid solution phase and a Laves phase. The microstructures of the alloys change from an initial single-phase FCC solid solution structure(x = 0) to a hypoeutectic microstructure(x = 0.25), then to a full eutectic microstructure(x = 0.45) and finally to a hypereutectic microstructure(0.5 x 1.2). The compressive test results show that the Nb0.45(x = 0.45) alloy with a full eutectic microstructure possesses the highest compressive fracture strength of 2558 MPa and a fracture strain of 27.9%. The CoCrFeNi alloy exhibits an excellent compressive ductility, which can reach 50% height reduction without fracture. The Nb0.25 alloy with a hypoeutectic structure exhibits a larger plastic strain of 34.8%. With the increase of Nb content, increased hard/brittle Laves phase leads to a decrease of the plasticity and increases of the Vickers hardness and the wear resistance. The wear mass loss, width and depth of wear scar of the Nb1.2(x = 1.2) alloy with a hypereutectic structure are the lowest among all alloy systems, indicating that the wear resistance of the Nb1.2 alloy is the best one.     

Compressive properties of high-pressure die cast IP 75 alloy with strengthening Laves phase

The NiAl-2Ta-7.5Cr-0.5Nb alloys (IP 75 alloy) were prepared by high-pressure die cast (HPDC), and tested for compressible strength and fracture behavior in the temperature range 300-1373 K. The fine structures with a homogeneous distribution of Laves phase at the boundary regions created by high-pressure die cast led to improvements in both the compressible yield strength and fracture strain. The high temperature (1373 K) 0.2% compressible yield strength of the HDC IP 75 alloy (160 MPa) is larger than that of the IP 75 alloys prepared by other processes. The room-temperature compressible fracture strain of the HDC IP 75 (14%) is also superior to the IP 75 alloy (5%) prepared by an ingot-casting process. The effects of size refinement and the more homogenous distribution of Laves phase and the formation of a ductile Cr-rich phase due to a rapid solidification contribute to the increments of the compressible yield strength and the fracture strain of the HPDC alloy.     

Fabrication of Ti–Nb–Ag alloy via powder metallurgy for biomedical applications

Ti and some of its alloys are widely used as orthopedic implants. In the present study, Ti–26Nb–5Ag alloys were prepared by mechanical alloying followed by vacuum furnace sintering or spark plasma sintering (SPS). The microstructure and mechanical properties of the Ti–Nb–Ag alloys were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX), compressive and micro-hardness tests. The effect of different sintering methods on the microstructure and properties of Ti–Nb–Ag alloy was discussed. The results showed that the titanium alloy sintered by vacuum furnace exhibited a microstructure consisting of α, β and a small amount of α″ martensite phase; whilst the SPS sintered alloy exhibited a microstructure consisting of α, β and a small amount of α″ martensite phase, as well as a nanostructured Ag homogeneously distributed at the boundaries of the β phases. The Ti–Nb–Ag alloy sintered by SPS possessed fracture strength nearly 3 times of the alloy sintered by vacuum furnace.     

放电等离子烧结制备Nb-Si-Ti-Al-Hf-Cr合金的显微组织及力学性能

以预合金化的粉末尺寸D50为3.3μm的NbSS固溶体相细粉末,粉末尺寸D50分别为22.1μm和23.5μm的Nb5Si3和Cr2Nb化合物粉末为原料,采用放电等离子烧结技术制备NbSS/Nb5Si3两相合金和NbSS/Nb5Si3/Cr2Nb三相合金,研究显微组织形貌、室温和高温力学性能及高温氧化性能。结果表明:两相合金的显微组织由NbSS基体和呈均匀岛状分布的Nb5Si3组成,三相合金中NbSS有相互连接成基体的趋势,而Nb5Si3和Cr2Nb相也以块状散布在NbSS中。NbSS/Nb5Si3两相合金和NbSS/Nb5Si3/Cr2Nb三相合金的室温断裂韧性值KQ分别达到15.1MPa·m1/2和11.3MPa·m1/2,室温下合金中NbSS相以韧窝型断裂为主,对Nb-Si基合金的室温断裂韧性有利,而Nb5Si3和Cr2Nb相为脆性断裂。1250℃时NbSS/Nb5Si3/Cr2Nb合金的压缩强度高于NbSS/Nb5Si3合金,但当温度上升到1350℃时两者强度出现反转。Cr2Nb相对合金高温抗氧化性能有利,1250℃下静态氧化100h时NbSS/Nb5Si3合金的氧化增重为233mg/cm2,大于NbSS/Nb5Si3/Cr2Nb合金的175mg/cm2。     

铸态Mg-4Al-4Si-0.75Sb合金的显微组织与力学性能

采用重力铸造法制备Mg-4Al-4Si-0.75Sb(AS44-0.75Sb)(质量分数/%,下同)镁合金,研究铸态合金的显微组织和室温力学性能。结果表明:铸态AS44-0.75Sb合金主要由α-Mg基体、β-Mg17Al12相、Mg2Si相和Mg3Sb2相组成;加入0.75Sb后形成高熔点的Mg3Sb2相,显著改善了Mg2Si相的形貌,使粗大的骨骼状Mg2Si转变为相对细小的汉字状Mg2Si。铸态合金的硬度HV为65.9,屈服强度为136.4MPa,抗拉强度为172.3MPa,伸长率为3.3%;拉伸断裂形式为准解理脆性断裂。     

形状记忆合金的力学及界面参数和体积分数对大块金属玻璃基复合材料增韧的影响

采用考虑塑性的超弹性材料模型和基于损伤塑性的准脆性材料模型,建立了三维单胞有限元模型,模拟了形状记忆合金颗粒增韧大块金属玻璃基复合材料的单调拉伸行为。讨论了形状记忆合金的力学参数、体积分数、界面厚度和界面材料参数对金属玻璃增韧效果的影响。结果表明:提高形状记忆合金的相变应变和马氏体塑性屈服应力将显著提高形状记忆合金颗粒增韧大块金属玻璃基复合材料的拉伸失效应变;形状记忆合金弹性模量超过50.0GPa、马氏体塑性屈服应力超过1.8GPa后,复合材料的拉伸失效应变变化不大。能同时兼顾失效应变和失效应力的形状记忆合金体积分数为15%左右。复合材料界面弹性模量和界面屈服应力的增加将提高复合材料的失效应力,但对失效应变影响不大;复合材料界面厚度的增加在提高失效应变的同时,也降低了复合材料的失效应力。     

Synthesis and mechanical behavior of ternary Mg–Cu–Dy in situ bulk metallic glass matrix composite

In situ Mg phase reinforced Mg₇₀Cu₁₇Dy₁₃ bulk metallic glass (BMG) matrix composite with diameter of 3 mm was fabricated by conventional Cu-mold casting method. The results show that the Mg-based BMG matrix composite exhibits some work hardening except for initial elastic deformation, a high fracture compressive strength of 702 MPa, which is 1.125 times higher than single-phase Mg₆₀Cu₂₇Dy₁₃ BMG and some plastic strain of 0.81%. The improvement of the mechanical properties is attributed to the fact that the Mg phase distributed in the amorphous matrix of the alloy has some effective load bearing and plastic deformation ability to restrict the expanding of shear bands and cracks and produce its own plastic deformation, which was proved by the shear deforming and fracturing mode and the fracture surfaces characterized by the vein pattern, severe remelting, and the very rough and bumpy region of the alloy.     

复相Al3Ti基合金的高温强化

报导了对Ａｌ３Ｔｉ结合合金进行复相强化的研究,利用适量的Ｎｂ合金化,在Ｌｌ２Ａｌ３Ｔｉ基体中形成分散的第二相,其室温和高温强度显著提高,韧性也有改善。改变制备工艺,使合金发生重有序和析出过程,形成具有高度弥散微粒的复相细晶组织,合金的室温和高温强度进一步提高。并探讨了其强韧化作用机理。     

Tensile behavior of bulk nanostructured and ultrafine grained aluminum alloys

In the present study, data on tensile behavior of bulk nanostructured aluminum alloys processed via consolidation of mechanically milled powders and severe plastic deformation are analyzed. High strength and low strain hardening were observed in bulk nanostructured and ultrafine-grained Al alloys. The ductility of aluminum alloys decreases with decreasing grain size. The high amount of intercrystalline components may have an influence on tensile properties of bulk nanostructured materials when grain sizes are less than 100 nm. The high strength in bulk nanostructured Al-Mg alloy may be attributed to contributions arising from grain size strengthening, the presence of high dislocation densities, Orowan strengthening, precipitation hardening and solid-solution hardening. The large and sudden stress drops in the stress-strain curves of cryomilled Al alloys are most probably indicative of the dislocation annihilation in the vicinity of or breakaway from the strong pinning role of dispersoids.     

Novel Ti-base nanostructure-dendrite composite with enhanced plasticity

Single-phase nanocrystalline materials undergo inhomogeneous plastic deformation under loading at room temperature, which results in a very limited plastic strain (smaller than 0-3%). The materials therefore display low ductility, leading to catastrophic failure, which severely restricts their application. Here, we present a new in situ-formed nanostructured matrix/ductile dendritic phase composite microstructure for Ti-base alloys, which exhibits up to 14.5% compressive plastic strain at room temperature. The new composite microstructure was synthesized on the basis of the appropriate choice of composition, and by using well-controlled solidification conditions. Deformation occurs partially through dislocation movement in dendrites, and partially through a shear-banding mechanism in the nanostructured matrix. The dendrites act as obstacles restricting the excessive deformation by isolating the highly localized shear bands in small, discrete interdendritic regions, and contribute to the plasticity. We suggest that microscale ductile crystalline phases might therefore be used to toughen nanostructured materials.     

Microstructural evolution,dislocation density and tensile properties of Al-6.5Si-2.1Cu-0.35Mg alloy produced by different casting processes

Al-Si-Cu-Mg foundry alloys are used in casting process technologies.However,their strength proper-ties remain low due to their microstructural characteristics and porosity.In this work,the microstruc-tural characteristics,dislocation densities,and mechanical properties of Al-Si-Cu-Mg cast alloys prepared through different casting methods were studied experimentally.Four casting processes,namely,gravity casting (GC),rheocasting (RC),thixoforming (Thixo),and Thixo with heat treatment,were used.The GC and RC samples had mainly dendritic α-Al phase microstructures and exhibited coarse Si particles and intermetallic compounds in their interdendritic regions.By contrast,the Thixo and heat-treated Thixo(HT-Thixo) samples exhibited microstructural refinement with uniformly distributed α-Al globules,fine fibrous Si particles,and fragmented intermetallic compounds among α-Al globules.The accumulation of dislocation densities increased in the Thixo sample as the strain was increased due to plastic deforma-tion.Furthermore,the ultimate tensile strength and yield strength of the HT-Thixo sample increased by 87％ and 63％,respectively,relative to those of the GC sample.The cleavage fracture displayed by the GC and RC samples led to brittle failure.Meanwhile,the Thixo and HT-Thixo samples presented dimple-based ductile fracture.     

The Role of Particles in Fatigue Crack Propagation of Aluminum Matrix Composites and Casting Aluminum Alloys

Fatigue crack propagation （FCP） behaviors were studied to understand the role of SiC particles in 10 wt pct SiCp/A2024 composites and Si particles in casting aluminum alloy A356. The results show that a few particles appeared on the fracture surfaces in SiCp/Al composites even at high △K region, which indicates that cracks propagated predominantly within the matrix avoiding SiC particles due to the high strength of the particles and the strong particle/matrix interface. In casting aluminum alloy, Si particle debonding was more prominent.Compared with SiCp/Al composite, the casting aluminum alloy exhibited lower FCP rates, but had a slight steeper slope in the Paris region. Crack deflection and branching were found to be more remarkable in the casting aluminum alloy than that in the SiCp/Al composites, which may be contributed to higher FCP resistance in casting aluminum alloy.     

Microstructural Modulations Enhance the Mechanical Properties in Al–Cu–(Si,Ga) Ultrafine Composites

Adding small amounts of Si or Ga (3 at.%) to the eutectic Al₈₃Cu₁₇ alloy yields an ultrafine bimodal eutectic composite microstructure upon solidification. The as‐solidified alloys exhibit a distinct microstructural length‐scale hierarchy leading to a high fracture strength of around 1 GPa combined with a large compressive plastic strain of up to 30% at room temperature. The present results suggest that the mechanical properties of the ultrafine bimodal eutectic composites are strongly related to their microstructural characteristics, namely phase evolution, length‐scales, and distribution of the constituent phases.     

Microstructure and room temperature fracture toughness of cast Nbss/silicides composites alloyed with Hf

The effect of Hf addition on microstructure and room temperature fracture toughness of cast Nb-16Si alloy was investigated. The Hf addition changes significantly the microstructural morphology of Nb-16Si alloys, which includes microstructure refinement and disappearance of eutectic colonies. Fracture toughness of the alloys improves with increasing Hf content. The improvement in fracture toughness is mainly attributed to the microstructural change by Hf addition. The Hf addition leads to a transition of Nb solid solution fracture manner from brittle cleavage to plastic stretching.     

Failure behavior of Cu–Ti–Zr-based bulk metallic glass alloys

Microstructure fracture and mechanical properties of Cu-based bulk metallic glass alloys were investigated. Centrifugal casting into copper molds were used to manufacture basic Cu₄₇Ti₃₃Zr₁₁Ni₉, and modified Cu₄₇Ti₃₃Zr₁₁Ni₇Si₁Sn₁ alloys. Although the alloys show an amorphous structure, TEM images revealed the formation of nanoparticles. At room temperature compression tests reveal fracture strength of 2000 MPa, elastic modulus of 127 GPa, and 1.8% fracture strain for the unmodified basic alloy. Whereas the modified alloy exhibits a fracture strength of 2179 MPa, elastic modulus reaches 123 GPa, and 2.4% fracture strain. So, with the addition of 1 at.% Si and Sn, the fracture strength improves by 9% and the fracture strain improves by 25%, but the fracture behavior under compression conditions exhibits a conical shape similar to that produced by tensile testing of ductile alloys. A proposed fracture mechanism explaining the formation of the conical fracture surface was adopted. The formation of homogeneously distributed nano-size (2–5 nm) precipitates changes the mode of fracture of the metallic glass from single to multiple shear plane modes leading to the conical shape fracture surface morphology.     

Structure and properties of bulk nanostructured alloys synthesized by flux-melting

Nanomaterials can easily be prepared as thin films and powders, but are much harder to prepare in bulk form. Nanostructured materials are prepared mainly by consolidation, electrodeposition, and deformation. These processing techniques have problems such as porosity, contamination, high cost, and limitations in refining the grain size. Since most bulk engineering metals are initially prepared by casting, we developed a casting technique, flux-melting and melt-solidification, to prepare bulk nanostructured alloys. The casting technique has such advantages as simplicity, low cost, and full density. In our method, Ag–Cu alloys were melted in B₂O₃ flux, which removed most of the impurities, mainly oxides, in the melts. Upon solidifying the melt at a relatively slow cooling rate on the order of 10¹–10² K/s a large undercooling of ∼0.25 T _m (where T _m is the melting temperature) was achieved. This large undercooling leads to the formation of bulk nanostructured Ag–Cu alloys composed of alternative Ag/Cu lamella and nanocrystals, both ∼50 nm in dimension. Our liquid-processed alloys are fully dense and relatively free from contamination. The nanostructured Ag–Cu alloys have similar yield strength in tension and in compression. The as-quenched alloys have yield strength of 400 MPa, ultimate tensile strength (UTS) of 550 MPa, and plastic elongation of ∼8%. The UTS was further increased to ∼830 MPa after the as-quenched alloy rod was cold drawn to a strain of ∼2. The nanostructured Ag–Cu alloys show a high electrical conductivity (∼80% that of International Annealed Copper Standard), a slight strain hardening (strain-hardening coefficient of 0.10), and a high thermal stability up to a reduced temperature of 2/3 T _m. Some of these behaviors are different than those found in previous bulk nanostructured materials synthesized by solid state methods, and are explained based on the unique nanostructures achieved by our flux-melting and melt-solidification technique.     

Effect of friction stir welding on microstructure,tensile and fatigue properties of the AA7005/10 vol.%Al2O3p composite

Several studies have been recently focused on friction stir welding of aluminium alloys and some data are also reported on FSW of aluminium-based composites. The application of this solid state welding technique to particles reinforced composites seems very attractive, since it should eliminate some typical defects induced by the traditional fusion welding techniques, such as: gas occlusion, undesidered interfacial chemical reactions between the reinforcement and the molten matrix alloy, inhomogeneous reinforcement distribution after welding. The present work describes the effect of the FSW process on the microstructure and, consequently, on the tensile and low-cycle fatigue behaviour, of an aluminium matrix (AA7005) composite reinforced with 10 vol.% of Al₂O₃ particles (W7A10A). The microstructural characterization evidenced, in the FSW zone, a substantial grain refinement of the aluminium alloy matrix (due to dynamic recrystallization induced by the plastic deformation and frictional heating during welding) and a significant reduction of the particles size (due to the abrasive action of the tool). Tensile tests showed a high efficiency of the FSW joints (about 80% of the ultimate tensile strength). The low-cycle fatigue tests evidenced a fatigue life reduction for the FSW material respect to the base composite, particularly for high values of total strain range. The fracture mechanisms for the FSW specimens were those typical of metal matrix composites: interfacial decohesion, void nucleation and growth, as well as fracture of reinforcing particles, as shown by SEM analyses of the fracture surfaces.     

Microstructure,texture and mechanical behavior characterization of hot forged cast ZK60 magnesium alloy

structure with a significant reduction in casting porosity, while the texture changed to sharp basaMeasured mechanical properties of the forged alloy showed that strength did not change, howductility improved by 75%. The analysis of the fracture surface of the forged alloy under tension rea ductile fracture with dimple morphology, while the as-cast alloy displayed a brittle fracture with pores. This demonstrated that the reduction of casting defects and dendritic morphology, as well evolution of recrystallized grains, enhanced ductility, while partial dynamic recrystallization throuforging process resulted in only marginal modification of strength in the forged condition.. 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials SciTechno