Finite element modelling of the effect of non-metallic inclusions in metal forming processes

The presence of non-metallic inclusions can result in material failure during the metal forming process, or lead to a serious deterioration of the quality of the final product. Understanding the effects of inclusions during metal forming is therefore an important step towards predicting the behaviour of inclusions and subsequently minimising their consequences. To achieve this understanding the authors incorporated non-metallic inclusions into a finite element (FE) simulation of metal forming. The chosen metal forming process was rod drawing, the chosen inclusion material aluminium oxide (Al₂O₃) and the chosen inclusion shape spherical. Real rod drawing experiments were also designed with Al₂O₃ spheres embedded in a steel rod in order to compare experimental and simulated results. Specifically, from the experiments carried out the changes of the rod around the inclusion were investigated and the findings compared with the finite element simulation results of an equivalent model. The FE simulation of the experiments considered specifically the fracturing of brittle inclusions. A concept allowing the fracturing of brittle inclusions by means of finite element method is described. Experimental results from fourpoint bending tests, and tensile tests, for Al₂O₃ ceramic bodies were used to calibrate the simulation. Further experiments involved the crushing of Al₂O₃ spheres where the force necessary to achieve crushing was measured and compared with the results of the calibrated FE simulation. It is demonstrated that the intended FE method for the simulation of brittle fracture of inclusions can be used with good accuracy.     

Ductile rupture of A508 steel under nonradial loading

This study deals with the effect of loading path on the strain to failure of a C-Mn-Ni-Mo steel. The tests are carried out at 373 K on axisymmetric, notched tensile specimens calculated by the finite element method. Specimen geometries containing different notch radii and leading to widely different stress triaxiality ratios are investigated. The effect of nonradial load path is studied using relatively sharply notched specimens. It is shown that a simple linear damage rule does not account for the experimental results. A model based on the critical cavity void growth rate calculated from the Rice and Tracey model is shown to give results consistent with the experiments provided that the effect of prestrain on constitutive equation and on the stress triaxiality ratio is properly taken into account. The experimental results are also compared to a model based on continuum damage mechanics.     

Strengthening mechanisms in Al2O3/SiC nanocomposites

A strengthening mechanism merely arising from internal (residual) microstresses due to thermal expansion mismatch is proposed for explaining the high experimental strength data measured in Al₂O₃/SiC nanocomposites. Upon cooling, transgranular SiC particles undergo lower shrinkage as compared to the surrounding matrix and provide a hydrostatic “expansion” effect in the core of each Al₂O₃ grain. Such a grain expansion tightens the internal Al₂O₃ grain boundaries, thus shielding both weakly bonded and unbonded (cracked) grain boundaries. It is shown that the shielding effect by intragranular SiC particles is more pronounced than the grain-boundary opening effect eventually associated with thermal expansion anisotropy of the Al₂O₃ grains, even in the “worst” Al₂O₃-grain cluster configuration. Therefore, an improvement of the material strength can be found. However, a large stress intensification at the grain boundary is found when intergranular SiC particles are present, which can produce a noticeable wedge-like opening effect and trigger grain-boundary fracture. The present model enables us to explain the experimental strength data reported for Al₂O₃/SiC nanocomposites and confirms that the high strength of these materials can be explained without invoking any toughening contribution by the SiC dispersion.     

Al2O3/BaZrO3双陶瓷界面微观组织演化及机理

以Al₂O₃为背层(硅溶胶为粘结剂), 电熔BaZrO₃作为面层材料(钇溶胶为粘结剂), 1550℃烧结后制成50 mm×25 mm×5 mm的Al₂O₃/BaZrO₃双陶瓷试样。通过光学显微镜(OM)、扫描电子显微镜(SEM)、X射线衍射仪(XRD)和EDS等手段观察了BaZrO₃层和Al₂O₃/BaZrO₃界面的显微结构, 研究了BaZrO₃与Al₂O₃的界面反应。结果表明, 面层由BaZrO₃基体和分布其上的大小10 μm左右的Y稳定的ZrO₂晶粒组成; Al₂O₃/BaZrO₃界面发生反应形成厚约300 μm的过渡层, 界面反应生成物有BaOAl₂O₃、ZrO₂和BaO·Al₂O₃·2SiO₂。界面从单纯的BaZrO₃/Al₂O₃双陶瓷结构演变为BaZrO₃、ZrO₂、BaO·Al₂O₃、BaO·Al₂O₃·2SiO₂和Al₂O₃等多种物相组成的复杂结构。反应过程中Al元素基本不迁移扩散, BaZrO₃中Ba元素向Al₂O₃所在的位置扩散形成BaO·Al₂O₃, 残留物形成一层条状ZrO₂, 而BaO·Al₂O₃·2SiO₂围绕着EC95(Al₂O₃+5%SiO₂)粉体颗粒周围生成。     

Formation of MgAl2O4 in Al2O3-(Al-4wt.%Mg) composites

The Al₂O₃ particles are introduced into the Al-4wt.%Mg melt by the “vortex” method. After being cast, Al₂O₃-(Al-4wt.%Mg) composites are remelted at 700, 750, 800 and 850°C for different residence times to investigate the formation of MgAl₂O₄ (spinel).

The results show that MgAl₂O₄ is the unique interface of the Al₂O₃-(Al---Mg) composites held at 700–850°C. Fine MgAl₂O₄ crystals grow on the surface of the Al₂O₃ particle but, as the holding temperature and the residence time increase, some spinels will form themselves into pyramidal shape. The MgAl₂O₄ grows not only at the matrix-particle interface but also on the surface of the composite specimens. The formation reactions of interfacial MgAl₂O₄ are as follows: Mg(1) + 2Al(1) + 2O₂(g) = MgAl₂O₄(s)3Mg(1) + 4Al₂O₃(s) = 3MgAl₂O₄(s) + 2Al(1) Both of them are equally important.     

Al_2O_3颗粒镀铜对铜基粉末冶金摩擦材料Al_2O_3-Fe-Sn-C/Cu摩擦磨损性能的影响

通过对陶瓷摩擦组元的表面进行化学镀铜来改善铜基粉末冶金摩擦材料中陶瓷相与基体间的结合效果,从而提高材料摩擦磨损性能。分别采用镀铜Al2O3颗粒和未镀铜Al2O3颗粒与铜粉和铁粉等经混合、压制、加压烧结制备Al2O3-Fe-Sn-C/Cu摩擦磨损试样。测试并分析了摩擦材料的微观结构、力学性能及摩擦磨损性能。结果表明:摩擦组元镀铜可使硬质颗粒与铜基体结合紧密;摩擦材料的布氏硬度增加了12%,弹性模量提高了约7%,摩擦系数提高了5%~10%,线磨损量降低了20%~50%;表面镀铜后的Al2O3颗粒不易脱落,摩擦系数稳定性提高了13%~23%。研究结果表明,摩擦组元表面镀铜可提高材料的综合性能。     

残余应力对涂覆Al2O3涂层的ZrO2陶瓷的强度和裂纹扩展阻力的影响

本研究在ZrO₂基体表面涂覆一薄层Al₂O₃涂层, 利用基体与涂层之间热膨胀系数不匹配, 在Al₂O₃-ZrO₂预应力陶瓷(简称A_CZ_S预应力陶瓷)表层引入压应力。采用维氏压痕法评价残余应力对A_CZ_S预应力陶瓷的表层和基体中裂纹扩展阻力的影响。理论分析结合实验结果表明: 表层的压应力使得A_CZ_S预应力陶瓷的裂纹扩展阻力增大, 最终导致强度和损伤容限提高; 且A_CZ_S预应力陶瓷表层的压应力和裂纹扩展阻力随着基体截面积与涂层截面积比值的增加而增大。当ZrO₂基体表层的Al₂O₃涂层厚度为40 μm时, 表层压应力使A_CZ_S预应力陶瓷的弯曲强度达到(1207±20) MPa, 相比于同种工艺下制备的ZrO₂陶瓷强度提高了32%, 同时也是Al₂O₃强度的3倍。此外, A_CZ_S预应力陶瓷也表现出很好的抗热震性能。     

Processing, microstructure and properties of C40 Mo(Si,Al)2/Al2O3 composites

The C40 Mo(Si_0.75Al_0.25)₂/Al₂O₃ composites were prepared by spark plasma sintering (SPS) of mechanically alloyed (MA) powders. The Mo(Si_0.75Al_0.25)₂/0–20 vol.% Al₂O₃ materials, showing micron and submicron composite structure, possess a hardness of 13.9–14.6 GPa but a poor toughness of 1.78–1.80 MPa m^1/2. The addition of 30 vol.% Al₂O₃ leads to the formation of the micron C40 Mo(Si_0.75Al_0.25)₂/Al₂O₃ composite with an intergranular distribution of Al₂O₃, that results in a drop of the hardness to 10.2 GPa and an improvement of the toughness to 3.67 MPa m^1/2. The transition of the cleavage facets to the intergranular fracture with the addition of Al₂O₃ is assumed as the main toughening mechanism.     

Crystal morphology and growth mechanism of reinforcements synthesized by direct melt reaction in the system Al–Zr–O

In-situ Al₃Zr and Al₂O₃ particulates reinforced aluminum matrix composites were fabricated by the direct melt reaction (DMR) technique in the system Al–Zr–O. Microstructures of the composites and crystal morphology of in-situ formed Al₃Zr and Al₂O₃ particulates were analyzed by scanning-electron microscope (SEM) and transmission electron microscope (TEM). Results indicated that in-situ formed Al₃Zr and Al₂O₃ particles were finer and well distributed in aluminum matrix. Al₃Zr particulates with a tetragonal structure are mainly in the shape of polyhedron. A few of them are rectangular. The length/width ratio of the rectangular Al₃Zr is less than 2.0 and the maximum size is 4 μm. In addition, submicro Al₂O₃ particles with a hexagonal structure were also found in this system. Furthermore, it is found that twin may appear in the Al₃Zr crystal. The twin plane is (1 ). The twinning direction is [2 1].     

纳米Al_2O_3/聚酰亚胺复合薄膜的介电与力学性能

采用原位聚合与热亚胺化的方法,成功制备了一系列不同纳米Al_2O_3粒子质量分数的纳米Al_2O_3/聚酰亚胺(PI)复合薄膜。通过SEM、TEM、XRD、FTIR、LCR数字电桥、高压电源及电子万能材料试验机对纳米Al_2O_3/PI复合薄膜的微观结构、介电性能及力学性能进行了表征和测试。结果表明:纳米Al_2O_3粒子在均匀地分散在PI基体中;当纳米Al_2O_3粒子质量分数为8%时,纳米Al_2O_3/PI复合薄膜的击穿强度和拉伸强度均达到了最大值;纳米Al_2O_3/PI复合薄膜的介电常数随纳米Al_2O_3质量分数的增加而增加。     

ZrO_2增韧Al_2O_3陶瓷的力学性能和增韧机制

对通过热压烧结法制备的3种陶瓷99.5vol%Al2O3(AD995)、ZrO2(15vol%)/Al2O3和ZrO2(25vol%)/Al2O3的力学性能和增韧机制进行了实验和理论研究。基于复合材料细观力学理论并考虑ZrO2的相变特性,建立了描述ZrO2/Al2O3陶瓷力学性能的本构模型。结果表明:ZrO2的加入细化了基体Al2O3晶粒,ZrO2/Al2O3陶瓷的致密性得到提高;3种陶瓷试件的破坏呈现小变形到脆性破坏的特点,压缩加载下试件应力-应变曲线近似为线性关系;AD995陶瓷的断裂韧性为5.65 MPa·m1/2,ZrO2(25vol%)/Al2O3陶瓷的断裂韧性为8.42 MPa·m1/2,提高了近50%;随ZrO2增韧相含量的增加,ZrO2/Al2O3陶瓷的弹性模量降低而断裂韧性增加,这一变化趋势与实验结果有良好的一致性。     

Effect of Al2O3 particles on mechanical properties of directionally solidified intermetallic Ti–46Al–2W–0.5Si alloy

The effect of Al₂O₃ particles on microhardness and room-temperature compression properties of directionally solidified (DS) intermetallic Ti–46Al–2W–0.5Si (at.%) alloy was studied. The ingots with various volume fractions of Al₂O₃ particles and mean ₂–₂ interlamellar spacings were prepared by directional solidification at constant growth rates ranging from 2.78×10⁻⁶ to 1.18×10⁻⁴ ms⁻¹ in alumina moulds. The ingots with constant volume fraction of Al₂O₃ particles and various mean interlamellar spacings were prepared by directional solidification at a growth rate of 1.18×10⁻⁴ ms⁻¹ and subsequent solution annealing followed by cooling at constant rates varying between 0.078 and 1.889 K s⁻¹. The mean ₂–₂ interlamellar spacing λ for both DS and heat-treated (HT) ingots decreased with increasing cooling rate according to the relationship λ∝ ^−0.46. In DS ingots, microhardness, ultimate compression strength, yield strength and plastic deformation to fracture increased with increasing cooling rate. In HT ingots, microhardness and yield strength increased and ultimate compression strength and plastic deformation to fracture decreased with increasing cooling rate. The yield stress increased with decreasing interlamellar spacing and increasing volume fraction of Al₂O₃ particles. A linear relationship between the Vickers microhardness and yield stress was found for both DS and HT ingots. A simple model including the effect of interlamellar spacing and increasing volume fraction of Al₂O₃ particles was proposed for the prediction of the yield stress.     

原位自生纳米Al2O3/Al-Zn-Cu复合材料的力学性能

将纳米ZnO粉末和Al粉球磨后冷压成Al-ZnO预制块,然后将其加到Al-Zn-Cu熔体中进行Al-ZnO原位反应,制备出纳米Al₂O₃颗粒增强Al-Zn-Cu基复合材料。能谱面扫描分析和透射电镜观察结果表明,复合材料由纳米Al₂O₃颗粒和Al₂Cu析出相两种颗粒/析出相组成。纳米Al₂O₃颗粒通过异质形核和晶界钉扎,细化了Al-Zn-Cu合金晶粒组织和Al₂Cu析出相。原位纳米Al₂O₃颗粒的生成提高了基体合金的拉伸性能,轧制+热处理使Al₂O₃/Al-Zn-Cu复合材料的拉伸强度比相同处理的基体合金提高约100%,总伸长率提高约98%。     

MnTe and ZnTe grown on sapphire by molecular beam epitaxy

We report on growth of MnTe layers by molecular beam epitaxy on Al₂O₃ substrates and of ZnTe layers on hybrid MnTe/Al₂O₃ substrates. The aim of our work was to prepare hexagonal phases of epitaxial thin films of these two materials. In the case of MnTe, the hexagonal NiAs-type phase was prepared by depositing the film directly on Al₂O₃ substrates. On the other hand, the crystal structure of ZnTe layers grown on hybrid MnTe/Al₂O₃ substrates was found to depend on the layer thickness: layers thinner than 0.05 μm grew in a metastable hexagonal wurtzite structure, but with further increases of the thickness, the cubic zinc blende phase of ZnTe tended to appear. The structural properties of MnTe and ZnTe layers were characterized by high energy electron and X-ray diffraction methods. Electrical properties of MnTe films were assessed by the Hall effect measurements. The topography and microstructure were analyzed by atomic force microscope. The Néel temperature and magnetic domains structure of antiferromagnetic hexagonal MnTe layers were obtained from neutron experiments.     

The effect of second principal stress on the fatigue propagation of mode I surface crack in Al2O3/Al alloy composites

Using a plate made of A2017-T6 metal matrix composites reinforced with 10 volume % and 20 volume % Al₂O₃ particles and Al alloy possesses the same composition as matrix alloy, the crack propagation rate da/dN of a mode I surface crack by the simultaneous action of plane bending and cyclic torsion are studied. And the effects of crack tip opening stress σ_top, crack opening displacement COD, biaxial stress ratio C (=second principal stress/first principal stress) and the surface roughness of crack section are examined. When stress intensity factor range ΔK is lower than the specific level, da/dN decreases with the increase of volume fraction of Al₂O₃ in C=0 and C=−0.55. But, da/dN of Al alloy becomes minimum in C=−1 and the effect of Al₂O₃ particles disappears. σ_top rises with the increase of volume fraction of Al₂O₃ particles and the decline of C. On the other hand, COD doesn’t always rise with the decline of C. These phenomena can be explained by the residual compressive stress formed at the surface layer of the specimen by the fatigue test and the surface roughness of crack section.     

Spectra and laser properties of Er, Yb:phosphate glasses

The effects of Al₂O₃, Yb₂O₃, Er₂O₃ and OH on spectral properties of P₂O₅·Na₂O·SrO·Al₂O₃·Yb₂O₃·Er₂O₃ erbium phosphate glass were studied. 5, 8, 13 mol% Al₂O₃, 4, 5, 6, 7 and 8 mol% Yb₂O₃ and 0.05, 0.2, 0.4 mol% Er₂O₃ were used. It was found Al₂O₃ improves fluorescent lifetime of Er³⁺ ions, but the integrated absorption cross-section of Er³⁺ ions decreases with the increase of Al₂O₃ concentration. Evaluating from energy transfer efficiency of Yb³⁺ to Er³⁺ and spectral parameters of Yb³⁺ and Er³⁺, lower Al₂O₃ content, 6 mol% Yb₂O₃ and 0.2–0.4 mol% Er₂O₃ are preferred for LD pumped microchip laser application. OH groups in glass greatly affect fluorescent intensity and lifetime of Er³⁺, Yb³⁺:phosphate glass. The OH absorption coefficient at 3000 cm⁻¹ should be <1 cm⁻¹ for laser applications. Pumped with a 2 W, 974 nm InGaAs laser diode, CW laser centered at 1530 nm with slope efficiency of 10.6% and maximum output of 43 mW was achieved in our 2 mm thick Er³⁺, Yb³⁺:phosphate glass at room temperature.     

CNTs添加对Cu-Al2O3复合材料耐电弧侵蚀性能的影响

Cu-Al₂O₃复合材料具有优异的传导性能和力学性能,在耐磨材料领域具有广阔的应用前景。为进一步提升电摩擦条件下复合材料的耐电弧侵蚀性能,本文采用内氧化法与粉末冶金法相结合制备了不同碳纳米管(CNTs)含量的CNTs/Cu-Al₂O₃复合材料,观察了CNTs/Cu-Al₂O₃复合材料中增强相的分布及其与基体界面结合情况,研究了添加不同含量CNTs对Cu-Al₂O₃复合材料传导性能和力学性能的影响,重点探究了CNTs/Cu-Al₂O₃复合材料的耐电弧侵蚀机制。结果表明：原位生成的纳米Al2O3颗粒钉扎位错及对CNTs分布具有调控作用,使CNTs弥散分布在铜基体中。与Cu-Al₂O₃复合材料相比,CNTs/Cu-Al₂O₃复合材料燃弧时间和燃弧能量明显降低,波动更平稳。在电弧侵蚀过程中,...     

SiC对反应烧结多孔Al2TiO5的物相及组织性能影响

本研究以γ-AlOOH、TiO₂和SiC为原料, 通过无压反应烧结制备了Al₂TiO₅多孔材料, 分析比较了SiC粒度和含量对合成产物的物相组成、显微组织、抗压强度、孔隙率和孔径分布的影响。结果表明: 反应产物的物相组成为Al₂TiO₅、Al₆Si2O₁₃、TiC、SiO₂和Al₂O₃, 还有少量未反应的TiO₂。SiC与TiO₂反应生成TiC和SiO₂, TiC颗粒弥散分布于多孔材料壁面或者骨架中, 而SiO₂进一步与γ-AlOOH分解出的Al2O3反应生成Al₆Si₂O₁₃晶须, 晶须交错分布于Al₂TiO₅颗粒之间或者孔洞中, 与TiC颗粒一起提高复合材料的抗压强度, 特别是采用小粒径SiC时, 对抗压强度的改善效果更加显著; 添加大粒径SiC后, 改变原有颗粒堆积状态, 可提高复合材料的孔隙率。但当SiC含量超过5wt%时, 因为生成较多低熔点的SiO₂, 部分填充于多孔材料的孔隙中, 部分则分布于Al₂TiO₅晶粒之间, 既减小孔隙率, 又降低晶粒间结合强度和试样的抗压强度。     

Modeling and optimizing of factors affecting erosion–corrosion of AA6063–(TiC/Al2O3) hybrid composites by experimental design method

In this article, modeling and optimizing of factors affecting erosion–corrosion wear of aluminum alloy A6063 reinforced with (Al₂O₃/TiC) particles have been determined by experimental design method. The erosion–corrosion wear characteristics and mechanism of AA6063–(TiC/Al₂O₃) with experimental parameters namely; type and concentration of corrosive media in the slurry, erosion speed and time have been investigated. Two models for reinforced and unreinforced alloys were applied to describe the influences of these factors on the erosion behavior of alloys. The erosion–corrosion mechanisms of the AA6063–(TiC/Al₂O₃) were dominated by particles erosion wear in alkaline slurry, and by the interaction of particle erosion wear and medium corrosion in acidic slurry. The results of experimental work are coinciding with that of calculated ones confirming the successful modelization.     

Microstructures, densification and mechanical properties of TiC–Al2O3–Al composite by field-activated combustion synthesis

Dense TiC–Al₂O₃–Al composite was prepared with Al, C and TiO₂ powders by means of electric field-activated combustion synthesis and infiltration of the molten metal (here Al) into the synthesized TiC–Al₂O₃ ceramic. An external electric field can effectively improve the adiabatic combustion temperature of the reactive system and overcome the thermodynamic limitation of reaction with x < 10 mol. Thereby, it can induce a self-sustaining combustion synthesis process. During the formation of Al₂O₃–TiC–Al composite, Al is molten first, and reacted with TiO₂ to form Al₂O₃, followed by the formation of TiC through the reaction between the displaced Ti and C. Highly dense TiC–Al₂O₃–Al with relative density of up to 92.5% was directly fabricated with the application of a 14 mol excess Al content and a 25 V cm⁻¹ field strength, in which TiC and Al₂O₃ particles possess fine-structured sizes of 0.2–1.0 μm, with uniform distribution in metal Al. The hardness, bending strength and fracture toughness of the synthesized TiC–Al₂O₃–Al composite are 56.5 GPa, 531 MPa and 10.96 MPa m^1/2, respectively.