冷却速率对Ti40.9Zr30.4Nb4.2Cu7Ni1.7Be15.8内生型非晶合金复合材料组织及性能的影响

探究了冷却速率对Ti_40.9Zr_30.4Nb_4.2Cu₇Ni_1.7Be_15.8内生型非晶合金复合材料组织结构及力学性能的影响。采用铜模铸造法制备内生型非晶复合材料,通过控制复合材料的尺寸来控制冷却速率,尺寸越大冷却速率越慢。利用XRD、SEM、DSC、万能力学试验机等对非晶合金复合材料的组织结构及力学性能等进行表征。结果表明,随冷却速率的降低,枝晶相发生熟化且尺寸增大;压缩试验证明,冷却速率显著影响复合材料的压缩性能,随冷却速率的降低,加工硬化能力变强,抗压强度显著提高,最高达到1921 MPa,但屈服强度略有降低;拉伸试验证明,冷却速率对非晶复合材料的屈服强度和抗拉强度等影响不大,最高抗拉强度达到1469 MPa。     

Molecular dynamics simulation of intrinsic and extrinsic mechanical properties of amorphous metals

Mechanical response of amorphous metal Ni₄₀Zr₆₀ under applied tensile loading is investigated using large-scale atomistic simulations. To obtain intrinsic properties, homogeneous samples with atomically smooth surface are used while samples with deliberately introduced surface notches of varying depths and root radii are used to test extrinsic effects. It is found that the notch-free samples show strength close to the theoretical fracture strength and extremely large ductility. Apparent strain hardening and strain rate sensitivity are also observed in these studies. It is argued that the free volume generation and localized shear displacement are responsible for the mechanical properties. Therefore, the presence of surface imperfections can greatly reduce the strength and ductility. The results suggest that to retain and improve the intrinsic mechanical properties of amorphous metals, surface treatment may be needed, which has been practiced in oxide glass industry for centuries but received little attention in metallic glass community.     

铍含量对钛基非晶复合材料摩擦行为的影响

通过真空电弧熔炼制备了一系列钛基非晶复合材料和钛基非晶合金,研究了合金中铍元素的含量对整体合金摩擦行为的影响。随着合金中铍元素的减少,非晶复合材料中的枝晶体积分数逐渐增加,整体合金的摩擦系数降低,但是合金的磨损率升高。所有材料的磨损表面都展现出了磨粒磨损的磨损机制,并且磨屑的尺寸随着枝晶体积分数的升高而逐渐降低。     

Micromechanics constitutive model for predicting the stress–strain relations of particle toughened bulk metallic glass matrix composites

Based on the secant modulus and extended Mori-Tanaka method for dual ductile phases, a micromechanics model is proposed to predict the monotonic mechanical behaviors of bulk metallic glass matrix composites (BMGCs) toughened by particles. In this model, the deformation behaviors of the BMG matrix and particles are described by the use of the free volume model and the simple Ludwik flow equation, respectively, and Weng's homogenization frame is adopted to bridge the constituents and the composites. As compared to the existing relevant models, the present model is much more convenient for applying, and more readily to be extended. The developed model is applied with stain-controlled loading, and is verified by modeling the monotonic stress–strain relations of particle toughened BMGCs. The predictions were in good agreement with the experiments from the literature, which confirms that the developed analytical model is capable of successfully describing the mechanical properties, such as yield strength, stress hardening and strain softening elongation, of composites.     

短纤维增强金属基复合材料拉伸应力场的有限元数值分析

运用空间轴对称弹塑性有限元方法,研究了短纤维增强金属基复合材料拉伸应力场分布。研究表明,基体和纤维的应力分布及基体塑性行为具有明显的不均匀性,材料参数（纤维长径比,纤维体积分数,纤维根间距和基体应变硬化指数）以不同方式通过影响应力传递基体约束变形和基体应变硬化进而影响应力场分布。     

钨增强块体非晶复合材料的研究进展

钨增强块体非晶复合材料属于一种新型材料,具有高强度、高硬度和耐磨损等优质性能,而且还具有非常优越的穿甲性能,在国防军事上有广阔的应用前景。所以国内外学者对钨增强块体非晶复合材料的制备方法、准静态和动态的力学性能和侵彻穿甲方面进行了广泛的研究,研究表明,钨增强块体非晶复合材料的性能与增强相钨的形态、体积分数、直径、环境温度、应变速率等密切相关。本文综述了钨增强块体非晶复合材料的国内外研究现状,并对其制备方法、力学性能和侵彻穿甲性能3个方面在不同影响因素下的差异性进行了总结和归纳,同时对未来的研究工作和方向进行了展望。     

Microstructure and tensile properties of high-strength high-ductility Ti-based amorphous matrix composites containing ductile dendrites

In the present study, two Ti-based amorphous matrix composites containing ductile dendrites dispersed in an amorphous matrix were fabricated by a vacuum arc melting method, and deformation mechanisms related to the improvement of strength and ductility were investigated by focusing on how ductile dendrites affected the initiation and propagation of deformation bands, shear bands or twins. Ti-based amorphous matrix composites contained 70–73 vol.% coarse dendrites of size 90–180 μm, and had excellent tensile properties of the yield strength (1.2–1.3 GPa) and elongation (8–9%). The Ta-containing composite showed strain hardening after yielding, and reached fracture without showing necking, whereas necking occurred straight after yielding without strain hardening in the Nb-containing composite. The improved tensile elongation and strain hardening behavior was explained by the homogeneous distribution of dendrites large enough to form deformation bands or twins, the role of β phases surrounding α phases to prevent the formation of twins, and deformation mechanisms such as strain-induced β to α transformation.     

On the microstructure–tensile property correlations in bulk metallic glass matrix composites with crystalline dendrites

Bulk metallic glass (BMG) matrix composites with crystalline dendrites as reinforcements exhibit a wide variance in their microstructures (and thus mechanical properties), which in turn can be attributed to the processing route employed, which affects the size and distribution of the dendrites. A critical investigation on the microstructure and tensile properties of Zr/Ti-based BMG composites of the same composition, but produced by different routes, was conducted so as to identify “structure–property” connections in these materials. This was accomplished by employing four different processing methods—arc melting, suction casting, semi-solid forging and induction melting on a water-cooled copper boat—on composites with two different dendrite volume fractions, V_d. The change in processing parameters only affects microstructural length scales such as the interdendritic spacing, λ, and dendrite size, δ, whereas compositions of the matrix and dendrite are unaffected. Broadly, the composite’s properties are insensitive to the microstructural length scales when V_d is high (～75%), whereas they become process dependent for relatively lower V_d (～55%). Larger δ in arc-melted and forged specimens result in higher ductility (7–9%) and lower hardening rates, whereas smaller dendrites increase the hardening rate. A bimodal distribution of dendrites offers excellent ductility at a marginal cost of yield strength. Finer λ result in marked improvements in both ductility and yield strength, due to the confinement of shear band nucleation sites in smaller volumes of the glassy phase. Forging in the semi-solid state imparts such a microstructure.     

Effects of residual stress on elastic plastic behavior of metallic glass bolts formed by cold thread rolling

Metallic glass (MG) has unique mechanical properties, combining high strength and low Young's modulus. By using MG to fabricate fasteningbolts, high resistance against bolt loosening is expected. However, MG components are considered brittle because MG exhibits poor ductility when subjected to uniaxial loading at room temperature. We have developed hexagonal cap bolts made of zirconium-based MG by cold thread rolling. The MG bolts showed a 1.6% plastic strain with a tensile strength of more than 1550 MPa. In addition, the load–strain curve was similar to that of a strain hardening material, although MG itself is free of strain hardening. In this study, we attempted to clarify the reasons for these characteristics, which are advantageous for bolts in terms of toughness and reliability. Various experiments and numerical analysis indicated that residual stress plays an important role in the behavior.     

Effects of Spark-Plasma Sintering Treatment on Cold-Sprayed Copper Coatings

Cold-spray is well known as an effective coating technique to make thick metallic coatings. However, cold-sprayed metallic coatings usually have low tensile strengths due to low adhesion strength between particles, and low ductility due to low adhesion strength between particles and work hardening. Spark-plasma sintering (SPS) is a pressure-sintering technique that employs a large pulsed direct current. Compared to annealing heat treatment (AHT), SPS is expected to effectively improve the adhesion strength between particles in cold-sprayed metallic coatings. In order to investigate the effects of SPS, cold-sprayed Cu coatings were treated by both SPS and AHT under a wide range of temperatures. The microstructures and mechanical properties of the treated specimens were investigated primarily by scanning electron microscopy, electron backscatter diffraction analysis, hardness tests, and tensile tests. Despite comparable values for porosity, crystal grain size, plastic strain distribution, hardness, and yield stress, the tensile strength and ductility of the specimen treated by SPS at 400 °C (SPS400) were significantly higher than those of the specimen treated by AHT at 450 °C. Based on these results, it was determined that SPS treatment is more effective in improving the adhesion strength between the particles in cold-sprayed Cu coatings than AHT.     

High Temperature Deformation Behavior of High Strength and Toughness Ti-Ni Base Bulk Metallic Glass CompositesEI北大核心CSCD

Room-temperature brittleness and strain-softening during deformation of bulk metallic glasses, and limited processability of shape memory alloys have been stumbling blocks for their advanced functional structural applications. To solve the key scientific problems, a new shape memory bulk metallic glass based composite, through the approach using transformation-induced plasticity (TRIP) effect of shape memory alloys to enhance both ductility and work-hardening capability of metallic glasses, and superplasticity of bulk metallic glass in supercooled liquid region to realize near net forming, was developed in this work. And the Ti-Ni base bulk metallic glass composites (BMGCs) rods were prepared by the levitation suspend melting-water cooled Cu mold process. Microstructure, thermal behavior, mechanical properties and high temperature deformation behavior of the alloy were investigated. The results show that the as-cast alloy microstructure consists of amorphous matrix, undercooled austenite and thermally-induced martensite. Besides, the size of the crystal phase precipitated on the amorphous matrix in-creases from the surface to the inside. The alloy exhibits excellent comprehensive mechanical properties at room temperature. The yield strength, fracture strength and the plastic strain of alloy are up to 1286 MPa, 2256 MPa and 12.2%, respectively. Under compressive loading in the supercooled liquid region, the composite exhibits approximate Newtonian behavior at lower strain rate in higher deformation temperature, and the optimum deformation temperature is T>480 degrees C and the intersection part with supercooled liquid region (SLR). When the temperature is 560 degrees C and the strain rate is 5x10(-4) s(-1), the stress sensitivity index m and the energy dissipation rate Psi are 0.81 and 0.895, respectively. Furthermore, the volume of activation is quantified to characterize the rheological behavior.     

Structural bulk metallic glasses with different length-scale of constituent phases

Bulk metallic glass composites containing constituent phases with different length-scales are prepared via an in situ method by copper mold casting homogeneous Zr–Ti–Nb–Cu–Ni–Al melts. The phase formation and the microstructure of the composite materials are investigated by X-ray diffraction, optical, scanning and transmission electron microscopy, and microprobe analysis. The composition of the melt as well as the cooling conditions realized during casting determine the type and the morphology of the phases present in the composite. The mechanical properties of composite materials with quasicrystalline or ductile bcc phase reinforcements are tested in uniaxial compression at room temperature, showing that the deformation is controlled by the type of the constituent phases and their morphology. Ductile phase-containing metallic glass composites demonstrate improved work hardening and ductility compared to monolithic metallic glasses. Similar results are obtained for composites with ductile bcc phase dendrites embedded in a nanocrystalline matrix. The improved ductility of the composites is due to the presence of the ductile second phase, which counteracts catastrophic failure by shear localization.     

Ductile necking behavior of nanoscale metallic glasses under uniaxial tension at room temperature

Glasses are normally brittle materials with no tensile ductility at room temperature. Using in situ, quantitative nanomechanical tests inside a transmission electron microscope, we demonstrate that certain nanoscale metallic glass samples are exceptions to this general rule. Such metallic glasses can be intrinsically ductile, capable of elongation and necking under uniaxial tension, in lieu of catastrophic fracture caused by severe shear banding. Beam-off tests confirm that the ductile behaviors are not artifacts due to electron-beam effects during the in situ tests. Additional experiments indicate that ductile necking gives way to fast shear banding failure at increased samples sizes and elevated strain rates. The observed spread-out shear transformations delaying strain localization and severe shear banding are explained in terms of the propensity for participation in deformation, while the tendency towards necking is attributed to the lack of strain hardening mechanism and inadequate strain rate hardening.     

High strength ductile Cu-base metallic glass

Usually, bulk metallic glasses exhibit strength values superior to conventional crystalline alloys, often combined with a large elastic limit and rather low Young's modulus. This combination of properties renders such alloys quite unique when compared to commercial materials. However, the major drawback for engineering applications is their limited room temperature ductility and toughness due to the localized deformation processes linked to shear banding, where high plastic deformation is accumulated in a very narrow region without contributing to macroscopic deformation, work hardening or yielding. In this work we report on a new class of metallic glass in a simple Cu-base alloy. Addition of 5 at.% Al increases the glass-forming ability of binary Cu₅₀Zr₅₀. The resulting Cu_47.5Zr_47.5Al₅ glass exhibits high strength (2265 MPa) together with large room temperature ductility up to 18%. After yielding a strong increase in the flow stress is observed during deformation. The structure of the metallic glass exhibits atomic-scale heterogeneities that enable easy nucleation and continuous multiplication of shear bands. The interaction and intersection of shear bands increases the flow stress of the material with further deformation, leading to a ‘work hardening’-like behavior and yields a continuous rotation of the shear angle up to fracture resulting in a high compressive ductility.     

Deformation in metals after low-temperature irradiation: Part II – Irradiation hardening,strain hardening,and stress ratios

Effects of irradiation at temperatures ⩽200 °C on tensile stress parameters are analyzed for dozens of body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close packed (hcp) pure metals and alloys, focusing on irradiation hardening, strain hardening, and relationships between the true stress parameters. Similar irradiation-hardening rates are observed for all the metals irrespective of crystal type. Typically, irradiation-hardening rates are large, in the range 100–1000 GPa/dpa, at the lowest dose of <0.0001 dpa and decrease with dose to a few tens of MPa/dpa or less at about 10 dpa. However, average irradiation-hardening rates over the dose range of 0 dpa−D_C (the dose to plastic instability at yield) are considerably lower for stainless steels due to their high uniform ductility. It is shown that whereas low-temperature irradiation increases the yield stress, it does not significantly change the strain-hardening rate of metallic materials; it decreases the fracture stress only when non-ductile failure occurs. Such dose independence in strain-hardening behavior results in strong linear relationships between the true stress parameters. Average ratios of plastic instability stress to unirradiated yield stress are about 1.4, 3.9, and 1.3 for bcc metals (and precipitation hardened IN718 alloy), annealed fcc metals (and pure Zr), and Zr-4 alloy, respectively. Ratios of fracture stress to plastic instability stress are calculated to be 2.2, 1.7, and 2.1, respectively. Comparison of these values confirms that the annealed fcc metals and other soft metals have larger uniform ductility but smaller necking ductility when compared to other materials.     

Plasticity improvement of (Cu43Zr48Al9)98Y2 bulk metallic glass composites by dispersed Ta particles

(Cu₄₃Zr₄₈Al₉)₉₈Y₂-based bulk metallic glass composites (BMGCs) with dispersed Ta particles (3vol.%, 6vol.%, 9vol.%) were successfully fabricated through suction casting. The thermal properties, microstructure, and mechanical properties of the BMGCs were systematically investigated. Ta particles are homogeneously dispersed in the amorphous matrix. Ta particle reinforced BMGCs exhibit similar thermal properties and glass-forming ability with the (Cu₄₃Zr₄₈Al₉)₉₈Y₂ base BMG. Compression test results show that the BMGC with 9vol.% Ta particles has superior mechanical performance with up to 15.7% compressive plastic strain, 2,216 MPa yield strength, and 2,260 MPa fracture strength at room temperature. These homogeneously distributed Ta particles act as discrete obstacles in the amorphous matrix, restricting the highly localized shear band. This results in the formation of multiple shear bands around the Ta-rich particles, which lowers the stress concentration, allowing the shear band to propagate further and improve plasticity.

     

Twin-Induced Plasticity of an ECAP-Processed TWIP Steel

The TWIP steels show high strain hardening rates with high ductility which results in high ultimate tensile strength. This makes their processing by equal channel angular pressing very difficult. Up to now, this has only been achieved at warm temperatures (above 200 °C). In this paper, a FeMnCAl TWIP steel has been processed at room temperature and the resulted microstructure and mechanical properties were investigated. For comparison, the material has also been processed at 300 °C. The TWIP steel processed at room temperature shows a large increase in yield strength (from 590 in the annealed condition to 1295 MPa) and the ultimate tensile strength (1440 MPa) as a consequence of a sharp decrease in grain size and the presence within the grains of a high density of mechanical twins and subgrains. This dense microstructure results also in a loss of strain hardening and a reduction in ductility. The material processed at 300 °C is more able to accommodate deformation and has lower reduction in grain size although there is a significant presence of mechanical twins and subgrains produced by dislocation activity. This material reaches an ultimate tensile strength of 1400 MPa with better ductility than the room temperature material.     

合金成分与热处理对6xxx系铝合金力学性能的影响

研究合金成分（Mg,Si,Cu）和热处理（自然时效和预时效）对6xxx系铝合金力学性能的影响。结果表明：合金成分与热处理不仅影响材料的成形性能,而且影响材料的烘烤硬化性能;提高合金中Si含量或Si／Mg比或添加0．3％Cu,可显著提高材料的韧性和成形性能,而预时效将减低材料的韧性和成形性能。对所研究合金的强度、韧性、加工硬化、应变敏感性、成形极限和烘烤硬化性进行了比较和总结。     

Effect of the volume fraction of the ex-situ reinforced Ta additions on the microstructure and properties of laser-welded Zr-based bulk metallic glass composites

To investigate the effect of the volume fraction of the ex-situ reinforced Ta additions on the weldability of Zr–Cu–Ag–Al bulk metallic glass composites (BMGCs), in this study, different Ta contents (0–6 vol%) of BMGCs are welded using the Nd:YAG pulsed laser technique with preselected welding parameters. After welding, the microstructure (including the parent material (PM), weld fusion zone (WFZ) and heat-affected zone (HAZ)), mechanical and thermal properties of the test samples are investigated.The test results show, for all BMGC welds, the micro-sized Ta particles in the PM, WFZ and HAZ to be covered by a crystallized interfacial layer (IL), ZrCu. For both un-welded and laser-welded BMGCs, as the Ta contents increase, the glass transformation temperature (T_g) increases, which in turn reduces the glass formation ability (GFA) indices, ΔT_x, γ and γ_m. However, when compared to that of un-welded BMGC, the GFA index, ΔT_x, of the laser-welded BMGCs is slightly improved. However, the γ, and γ_m of the BMGC welds seem not to be affected.In addition, due to the characteristics of the rapid thermal cycle of the laser welding process, two smaller sizes of Ta, nano-sized (mainly on the surface of WFZ) and sub micro-sized Ta, are found in the WFZ. These sub-micro-sized Ta particles normally locate near the micro-sized Ta, which tends to slightly reduce the hardness in this area.Furthermore, an increase in the volume fraction of Ta (0–6 vol%) in the BMGCs does not encourage the formation of the harmful crystalline phase in the amorphous matrix after the laser welding process. It is observed that, other than the IL (ZrCu) on the micro-sized Ta particles, no other type of crystalline is observed in the amorphous matrix of the laser-welded BMGCs.     

Mechanical properties of Pd–Cu–Si bulk metallic glass

In present work, Pd_77.5Cu₆Si_16.5 bulk metallic glass balls with diameter up to 6 mm have been prepared by fluxing and water quenching method. It has been found that the Pd–Cu–Si glassy alloy exhibits a compressive plastic strain of about 11.4%, together with strain hardening characteristics. A large localized shear band, accompanied by a shear step of about 220 μm in size, has been clearly observed on the deformed specimen. The related shear plane is estimated to have an angle of 42 degrees with respect to the loading axis. The good ductility of the glassy alloy is believed to be partially attributed to strain hardening and the higher resistance of the glassy alloy to crack nucleation and propagation due to its large Poisson's ratio.