新型颗粒增强金属玻璃复合材料的拉伸增韧机制

利用有限元方法探究了颗粒体积分数、颗粒的应变硬化指数、颗粒的间距以及网状结构对新型非晶合金复合材料即金属玻璃基复合材料（Metallic Glass Composites,MGCs）强度和韧性的影响。结果表明：随着颗粒应变硬化指数的增大,复合材料的强度和韧性都有很大提高,颗粒体积分数的增大、颗粒间距的变小和网状结构排布也将提高复合材料的韧性。这些都有利于设计出有较好韧性的复合材料。     

金属玻璃基复合材料的微结构效应

基于自由体积理论和Ramberg-Osgood模型,并利用ABAQUS软件,建立颗粒随机分布代表性体积单元模型,模拟了Ti_(64.5)Zr_(14.5)V_(18.5)Cu_(2.5)颗粒增韧Ti基金属玻璃基复合材料在单轴拉伸状态下的微结构效应,讨论了颗粒的体积分数、团聚数目、长径比、定位取向和界面对金属玻璃韧性的影响。结果表明:提高颗粒体积分数能显著提高复合材料的塑性,但部分牺牲了复合材料的强度;增大颗粒长径比能够增强复合材料的塑性和屈服强度;使颗粒的取向与荷载方向成90°或0°,不仅增强了复合材料的塑性,而且与其他排布相比也增强了复合材料的强度;减少团聚数目至2个以下,能明显减少金属玻璃基复合材料的塑性和强度的损失,使团聚中颗粒与荷载成90°,却能改善复合材料的塑性和强度;在颗粒增韧金属玻璃基复合材料中加入零厚度界面,能观察到在主剪切带上颗粒和基体在界面处脱粘,得到与实验现象更加吻合的结果。通过上述的研究能够很好地理解复合材料的微结构效应,并有利于材料的设计。     

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。     

激光冲击处理对金属微结构及其性能的影响

论述了激光冲击处理的基本原理,分析了激光诱发的冲击波在材料中的传播过程.激光冲击处理会使金属材料表层发生塑性变形而产生很高的残余压应力;冲击区的显微组织中会产生高密度位错,有时亦有孪晶与相变产生;金属材料表层的塑性变形、高残余压应力、高密度位错、孪晶以及相变,这些因素的共同作用使得金属材料表面硬度、疲劳寿命获得很大的提高.     

金属拉伸试验应力速率对屈服强度的影响

本文通过８组Ａ３钢试件研究了应力速率对屈服强度的影响。其结果指出：ＧＢ２２８－８７规定的应力速率３－３０Ｎ／ｍｍ＾２Ｓ＾－１的上下限均可考虑适当提高。这样不仅不会影响屈服极限的实验精度，同时也能提高工作效率。文中建议修改ＧＢ２２８－８７规定的应力速率的范围。     

玻璃微珠表面处理对LDPE 复合材料拉伸性能的影响

应用Inst ron 材料试验机, 于室温下考察了玻璃微珠含量及其表面处理对填充低密度聚乙烯复合物拉伸性能的影响。结果表明, 复合材料的弹性模量(E_c) 随着微珠体积分数的增加而增大, 屈服强度(R_yR ) 变化不大, 而断裂强度(R_bR )、断裂应变(E_bR ) 和断裂能(E _bR ) 则减小; 在相同的实验条件下, 微珠表面经硅烷偶联剂预处理的填充体系的E c、R_yR 和R_bR 稍高于未作表面预处理的复合材料; 对于E_bR 和E _bR , 两体系之间的差异甚微。      

梯度分布对密度梯度金属空心球阵列动力学性能的影响

梯度及梯度排布是密度梯度多孔材料设计中的关键性问题。该文基于分层梯度金属空心球阵列模型,重点讨论了不同冲击速度下,梯度分布对金属空心球阵列动力学性能的影响。研究结果表明：在低速冲击下,梯度金属空心球阵列的动力学响应只取决于梯度的大小而不敏感于梯度的空间分布;而在中高速冲击下,梯度金属空心球阵列的动力响应不仅取决于梯度的大小,而且必须考虑梯度的空间分布。一定冲击速度下,梯度分布决定了材料内部的应力分布和材料的能量吸收过程,但吸收的总能量由材料的相对密度决定。考虑到对被冲击防护结构的保护,从冲击端高密度向远端低密度的梯度分布更有优势。该文的结论为实现多孔材料冲击动力学性能的多目标优化设计提供了新的设计思路。     

金属Cu诱导层对热丝法制备多晶硅薄膜微结构的影响

采用热丝化学气相沉积法(HWCVD),在金属铜诱导层上成功制备出横向晶粒尺寸在1μm左右、垂直晶粒尺寸达20μm的柱状多晶硅薄膜,其晶化率在95%以上.使用XRD、Raman光谱、扫描电子显微镜(SEM)等分析测试手段研究了灯丝温度在1500～1800℃之间变化时,金属铜诱导层对多晶硅薄膜的微观形貌、结晶性及晶体学生长方向的影响规律.结果表明:金属铜诱导层的引入,在一定温度范围内改善了晶粒尺寸,改变了多晶硅薄膜的择优取向,降低了薄膜的晶化温度,提高了晶化率.     

金属拉伸速率的控制

焊接熔敷金属的拉伸试验时，拉伸速度过快，会使材料的σｓ点上升；过慢又会较严重的出现“鱼目”，降低材料的延伸率。按ＧＢ２２８－８７的要求，对ＷＥ－６０拉力试验机，设计出了用指针在单位时间走过的格数（０．６格￣６格／秒），从而较简便地解决了老式拉力试验机速率控制问题。     

Effect of concentration on the structure of isothermally-annealed CuZr metallic glasses

To clarify the influence of isothermal annealing on Cu_xZr_100-x (x?=?36, 44, 50, 56, and 64) metallic glasses, the annealing process was conducted using molecular dynamics (MD) simulation. The local structure is analysed by means of the radial distribution functions, Voronoi tessellation and the nearest-neighbour correlation index, C_ij. The results show that Cu-centered < 0 0 12 0 > and Zr-centered < 0 0 12 4 > dramatically increases after an isothermal annealing treatment and presents a strong tendency of being the nearest neighbour with each other, as the Cu concentration is over 50%. These two kinds of clusters are formed the Cu₂Zr Laves phase which can be directly observed in the Cu-rich glasses.     

Cooling rate dependent as-cast microstructure and mechanical properties of Zr-based metallic glasses

In this article, wedge chill casting is used to provide a variation of the cooling rate for the study of cooling rate-dependent as-cast microstructure and mechanical properties of Zr-based metallic glasses with and without yttrium doping. In-situ formed crystalline phases and crystallization sequence are realized in the context of the cooling rate. Macro-hardness is studied via the Brinell ball indentation and the contact morphology is discussed on the basis of microstructure analysis.     

Effect of Er on properties of Zr-based bulk metallic glasses

(Zr_0.6336Cu_0.1452Ni_0.1012Al_0.12)_100-xEr_x(x?=?0～6) bulk metallic glasses were fabricated by copper mould suction casting method, and the effect of Er on properties was studied. The compressive plastic strain (ε_p) and compressive strength (σ_max) at room temperature increases first and then decreases with the increase of Er content. The compressive plastic strain (ε_p) of the specimen is 35.1% when x?=?2.6, which is about eight times than that of the specimen with x?=?0. The compressive strength (σ_max) is 2513?MPa, which is much higher than that of the specimen with x?=?0. It indicates that Zr-based bulk metallic glasses could be strengthened and toughened by adding Er. The thermal stability decreases gradually and the glass-forming ability increases first and then decreases with the increase of Er content.     

Strengthening and toughening of Mg-Cu-(Y, Gd) bulk metallic glasses by minor addition of Be

Mg₆₅Cu₂₅Re₁₀ and Mg₆₅Cu₂₄Be₁Re₁₀ (Re = Y, Gd) bulk metallic glasses (BMGs) were successfully fabricated by conventional Cu-mold casting method. By the addition of 1 at.% Be, the compressive strengths of Mg₆₅Cu₂₄Be₁Y₁₀ and Mg₆₅Cu₂₄Be₁Gd₁₀ alloys are increased from 760 MPa and 860 MPa to 930 MPa and 1025 MPa, respectively. The fracture morphology is changed from nanometer scale corrugation to micrometer scale dimple and vein pattern, indicating that the addition of minor Be obviously improves the toughness of the alloys. The fracture morphologies with different size of plastic zone in Mg-based BMGs provide probability for understanding the fracture mechanism of BMGs.     

Fatigue of metallic glasses

The fatigue behaviour of Ni₄₉Fe₂₉P₁₄B₆Si₂, Ni₄₈Fe₂₉P₁₄B₆Al₃ and Pd_77.5Cu₆Si_16.5 metallic glasses is examined. In the finite lifetime regime the relationship between stress amplitude ( _a), fracture stress ( _f), mean stress ( _m) and cycles to failure (N _f) is _a=A( _f–_m) (2N _f) ^b , whereA andb are 16.9 and –0.40 respectively for reduced gauge section Ni₄₉ strips (for _m 140 kg mm^–2) and 27.0 and –0.44 for Pd base wires. These results are unusual in thatA 1. Consequently, a sharp discontinuity exists near _a( _f– _m) –1. In a simple tensile test failure occurs at _f(=_y) and 2N_f=1; for peak stresses only a percent or so less than _f the sample will withstand hundreds of cycles of stress. For uniform cross-section glassy metal filaments, a fatigue limit is observed at stress ratios ( _a/ _f) in the vicinity of 0.07 to 0.15. The fatigue limit for reduced section specimens is a factor of 2 higher. Fatigue failure of the Ni-Fe strips may occur under partially or fully plane stress or plane strain conditions, depending on sample thickness and stress. Final failure of the Pd_77.5Cu₆Si_16.5 wires always occurs by general yielding of the remaining section.     

Effect of processing conditions on physical properties of transition metal-metalloid metallic glasses

The influence of processing conditions on the physical properties of transition metal-metalloid based metallic glasses was studied. It was shown that if the melt superheat and cooling rate are changed, the quenched-in stresses and the local environment of the constituent atoms are modified causing remarkable changes in several magnetic, mechanical and chemical properties. In addition, the relaxation characteristics and thermal behaviour were also drastically changed by the processing.     

Effect of microstructure on thermal expansion behaviour of nanocrystalline metallic materials

Materials properties, among which thermodynamic ones, are influenced by microstructural features. This is so also in the case of nanocrystalline materials, featuring average grain size below 100 nm. A reduced grain size involves that significant fractions of atoms are localised in grain boundary regions and this has remarkable effects on the resulting thermodynamic properties, like heat capacity, transition temperatures, coefficient of thermal expansion, etc. In the present work we consider the thermal expansion behaviour of ball-milled nanocrystalline metallic powders using dilatometric measurements. High-energy ball-milling, that is capable to achieve extremely high deformation degrees, induces in the milled powders microstructural defects, like vacancies, antisites, dislocations and planar faults. Another effect of milling is the reduction of the crystallite size, that, in the long run, may reach the nanometric range. In view of the microstructural changes that can be brought about by milling and of the numerous transformations occurring during the dilatometric runs, a comparative study has been conducted on intermetallic, NiAl and Ni₃Al, and on a pure metal, nickel, powders. The results emerging from the experimental investigation are quite complex, owing to the complex defect structures that are present in the ball-milled powders. It turns out that the thermal expansion coefficient of the nanocrystalline powders increases as the average grain size is reduced. However, when the average grain size achieves very low values, the strain relaxation of the crystalline lattice and the rearrangement of grain boundary regions result in a reduction of the thermal expansion coefficient. Another aspect that emerges from the dilatometric curves is the interplay between recrystallization and reordering, i.e. the re-establishment of the long-range order in the intermetallic powders, that had been partially or fully eliminated by ball-milling.