无η相梯度WC-Co硬质合金的微观结构与性能

采用气氛烧结法制备了无η相梯度WC-Co硬质合金,并通过扫描电子显微镜观察渗碳处理前后样品的微观形貌和WC晶粒尺寸,X射线能谱仪测量Co含量的分布,X射线衍射仪分析样品的相结构,显微硬度计测量样品表面、心部的硬度,X射线应力仪测量样品烧结表面的残余应力,钴磁测量仪和矫顽磁力计测量样品的钴磁值、矫顽磁力,对梯度硬质合金的微观结构和性能进行了分析。结果表明,通过渗碳处理可以制备出无η相的梯度WC-Co硬质合金,WC晶粒有长大的趋势,但长大幅度较小;渗碳处理后合金钴磁值增大,矫顽磁力减小;梯度硬质合金的硬度呈梯度分布,表面硬度显著高于渗碳处理前的硬质合金的平均硬度;梯度硬质合金烧结表面的残余应力为压应力,残余应力数值低于渗碳处理前的硬质合金。     

WC-Co梯度硬质合金的制备及渗碳对其组织的影响

采用光学金相检测、扫描电镜分析、能谱分析等方法对WC-6Co硬质合金渗碳处理后的成分和梯度组织结构进行分析.结果表明:对硬质合金渗碳处理后可形成显微组织和钨、钴含量的梯度分布,其特征是合金表层和次表层的η相已经完全消失,属正常的WC γ两相组织,合金的芯部依然是含η相的三相组织,中间形成了一个富钴层;碳原子的扩散和液相钴的流动是形成梯度的原因;在各渗碳温度下,合金的梯度结构厚度均随渗碳时间的增加而增加;在渗碳时间和渗碳温度相同的情况下,合金的梯度层厚度均随合金初始总碳含量的增加而增厚.渗碳处理后外表面的WC晶粒可能会产生一定的粗化现象.     

渗碳时间对梯度硬质合金显微组织和抗弯强度的影响

采用预烧结-后续渗碳的方法制备钴相呈梯度分布的硬质合金,通过对试样显微组织的观察和抗弯强度的测试,研究渗碳时间对梯度硬质合金显微组织和抗弯强度的影响。结果表明:试样富钴层钴相含量随渗碳时间的延长而增加,试样的抗弯强度随渗碳时间出现了峰值现象,即当渗碳时间少于140min时,试样的抗弯强度随渗碳时间的增加而增加,在渗碳处理140min时出现最大值,当渗碳160min后,试样的抗弯强度开始下降。分析认为,富钴层中金属钴的良好塑性变形能力能有效地吸收来自外部裂纹扩展的能量,提高合金的抗弯强度,同时当渗碳时间过长时(超过140min),WC与η相晶粒出现了聚集长大,造成钴相分布不均匀,并局部形成Co池,导致试样抗弯强度的下降。对于直径为10mm的矿用梯度球齿,其合理的渗碳时间应控制在120～140min。     

基体碳含量对梯度结构硬质合金微观结构与性能的影响

Co相梯度结构硬质合金与传统硬质合金(WC-Co)相比具有良好的硬度和韧性组合.本文通过固体渗碳烧结处理制备出了Co相梯度结构硬质合金,研究了贫碳量对固体渗碳后硬质合金中Co相梯度结构、力学性能的影响,探索了Co相梯度结构的形成机制.结果表明,贫碳合金碳含量越低,η相体积分数越大,渗碳时需要消耗更多的活性碳原子,渗碳烧...     

具有梯度结构的涂层硬质合金刀片

研究了Co和Ti(CN)含量对梯度硬质合金的力学性能、梯度结构的影响.测试了梯度硬质合金刀片的切削性能.结果表明:随着合金钴含量的增多,合金梯度层厚度增厚,梯度结构越明显;合金的强度提高,磁饱和提高,磁力、硬度和密度减小.随着合金Ti(CN)含量的增多,合金梯度层厚度有变薄的趋势;随合金的硬度提高,合金的强度和密度减小.切削试验表明:具有梯度结构涂层硬质合金刀片的切削性能比无梯度结构涂层硬质合金刀片的切削性能优良.达到同一磨损高度hB=0.15 mm时,前者的切削寿命较后者提高了近一倍.同时随着合金钴含量的增多,硬质合金刀片的切削性能提高.     

碳含量对矿用硬质合金梯度结构形成的影响

采用两步法(预烧结-渗碳处理),制备出了具有钴相梯度结构的硬质合金.研究了渗碳处理前合金中η相的形态和含量与合金中碳含量的关系,讨论了渗碳处理温度以及碳含量对矿用梯度合金梯度结构形成的影响,并对梯度硬质合金的梯度形成机理进行了探讨.结果表明:在本实验条件下,初始碳含量与渗碳温度是影响梯度结构的主要参数;在其它条件相同的情况下,初始碳含量越高,梯度结构越明显,厚度越大;渗碳温度越高,梯度结构越不明显.研究还发现,上述参数对梯度结构的作用主要是影响碳原子的扩散和液相的流动.     

WC粒度对梯度硬质合金组织和性能的影响

由于不同材料的热膨胀系数不同,涂层在冷却过程中可能因为热应力不同而产生裂纹,表面富粘结相的梯度硬质合金基体因粘结相含量高,韧性好,能有效吸收裂纹扩展时的能量,延长涂层刀具的使用寿命。为了研究WC晶粒度对梯度硬质合金的组织及性能的影响,制备了三种WC粒度的硬质合金。采用XRD和SEM对梯度硬质合金的相成分、微观组织进行了分析。实验结果表明,三种WC晶粒度的硬质合金表面均形成了梯度层。随着WC晶粒度的增大,梯度层厚度减小,抗弯强度和断裂韧性增大。三种梯度硬质合金表层显微硬度分布趋势相似。当WC晶粒度较小时,梯度硬质合金无梯度的合金芯部断裂形式均以沿晶断裂方式为主,随着WC晶粒度的增加,穿晶断裂方式增多;梯度表层出现了Co相变形和撕裂形貌,存在蜂窝状的韧性花样。     

烧结温度对梯度硬质合金表层组织结构和力学性能的影响

梯度硬质合金表层组织结构的变化直接影响着CVD涂层硬质合金刀具的加工性能,而烧结温度的变化将显著影响梯度硬质合金表层和芯部组织结构。本文研究了1 410、1 440℃和1 460℃3种烧结温度对梯度硬质合金表层（0～100μm）组织结构和力学性能的影响规律。采用SEM和EBSD对梯度硬质合金表层组织结构、梯度区和合金芯部粒度进行了分析,研究了不同温度下表层组织结构的变化对梯度硬质合金抗弯强度和显微硬度的影响。结果表明：随着温度升高,表面Co含量明显下降,梯度层厚度增加。合金硬质相（WC和立方固溶体相）晶粒度随烧结温度升高而明显长大,梯度区内WC晶粒度大于芯部WC晶粒度,并且二者差异随烧结温度的升高而加大,芯部立方固溶体的生长速度高于WC。随烧结温度的升高,合金的磁力下降了10.3%,合金的断裂韧性和抗弯强度分别提高了8.5%和7.7%,相比去除梯度层试样,3个烧结温度下保留梯度层合金试样的抗弯强度提高了7.6%～9.8%。从表面至芯部合金的显微硬度先降低至最低点后逐渐上升至接近芯部硬度,梯度层内HV最低点约为芯部HV的87%。随烧结温度的升高,梯度区和芯部的HV都呈现下降趋势。     

双相结构功能梯度WC-Co合金的微观组织结构与小负荷维氏硬度

用光学显微镜观察了采用两步法工艺制备的双相结构功能梯度WC-Co硬质合金的微观组织结构，并测量了其小负荷维氏硬度。结果表明：合金芯部的η相呈点状弥散分布于硬质相WC颗粒之间和硬质相WC颗粒与Co粘结相之间；在3相区，由于超细η相的存在，WC晶粒大部分已失去其多角特征，出现晶粒圆化现象；DP合金前驱体在后续渗碳处理过程中，合金表层的部分Co在碳势差的作用下向合金内部发生迁移，使合金中间过渡层及其附近区域中Co粘结相呈现梯度变化；合金表层与合金中间过渡层在由原来的3相组织变为两相组织的同时，合金中细小的WC晶粒明显减少。WC晶粒的均匀度明显增加；合金截面的小负荷维氏硬度出现由高变低再由低变高的规律性变化，与合金内部微观组织结构的变化相对应。     

基于钴、碳迁移动力的梯度硬质合金制备与材料特性研究

功能梯度硬质合金(FG WC-Co)与WC-Co相比能够提供优异的硬度和断裂韧性组合。基于液相Co从高Co含量向低Co含量、从高C含量向低C含量迁移的钴、碳迁移动力,本文设计了富碳高钴的WC-15%Co和贫碳低钴的WC-6%Co双层样品。采用扫描电镜(SEM)、X射线衍射(XRD)等分析手段,研究了双层硬质合金的微观结构与力学性能。结果表明双层样品中Co相从富碳高钴层向贫碳低钴层迁移形成梯度结构,并且两层间C含量差别越小,Co浓度梯度越大;高钴层中WC晶粒尺寸比低钴层的大,并且两层间C含量差别越小,WC晶粒尺寸差别越大;随着两层间C含量差别减小,矫顽磁力(Hc)减小,钴磁(Com)增加;硬度从1 260 HV至1 408 HV逐渐升高,从WC-15%Co面加压测的抗弯强度2 612 MPa高于从WC-6%Co面加压测的抗弯强度2 388 MPa。     

Fracture in strain gradient elasticity

Recent experiments have shown that the microscale material behavior is very different from that of bulk materials, and displays strong size effects when the characteristic length associated with the deformation is on the order of microns. Conventional continuum theories, however, can not predict this size dependence because they do not have an intrinsic length in their constitutive models. A new continuum theory, namely the strain gradient theory, has been proposed to investigate the deformation of solids at the microscale. For materials undergoing plastic deformation, the basis of strain gradient theory is the dislocation theory in materials science, and strain gradient plasticity has agreed remarkably well with experiments. For elastic materials with microstructures, it has also been established that the material behavior can be represented by an elastic strain gradient theory. A general approach to investigate fracture of materials with strain gradient effects is established. Both the near-tip asymptotic fields and the elastic full-field solutions are obtained in closed form. Due to stain gradient effects, stresses ahead of a crack tip are significantly higher than those in the classical K field. The plastic zone size surronunding a crack tip is estimated by elastic near-tip fields, as well as by the Dugdale model. It is established that the plastic zone is, in general, much more round and larger than that estimated from the classical K field.     

Effect of powder particle size on gradient formation and grain growth in ultrafine crystalline gradient cemented carbide

Sinter-HIP combining vacuum and pressure in one-step sintering process has been applied to prepare ultrafine crystalline cemented carbide with a surface gradient layer enriched in binder. The effect of powder particle size on gradient formation and grain growth has been examined. The results show that the gradient layer thickness increases with decreasing WC and Ti(C,N) powder particle size. The number of abnormal WC grains increases with decreasing WC powder particle size. The formational mechanism of the gradient cemented carbide with ultrafine grains is discussed through analyzing the decomposability of nanoscale Ti(C,N), atomic diffusion and grain growth during one-step Sinter-HIP process.     

Through-thickness texture gradient of tantalum sputtering target

For sputtering targets, through-thickness texture is critical for assuring reliable sputtering performance.In this paper, through-thickness texture gradient of a rolled and subsequently annealed tantalum sputtering target was investigated. The results show that by carefully controlling the rolling process, shear-related components at the surface of the rolled sample could be avoided. Both the rolled sample and the annealed target develop a through-thickness texture gradient with the formation of a stronger {1 1 1}fiber in the center layer compared with that in the surface layer.     

羟基磷灰石生物活性梯度涂层材料的界面特点

用透射电子显微镜对钛合金基体等离子喷涂羟基磷灰石 (HA)生物活性梯度涂层的界面进行了观察与分析。结果表明 :经热处理后生物活性梯度涂层的结晶程度明显提高 ,涂层中存在HA晶体、β Ca3(PO4 ) 2 晶体以及中间相CaTiO3 晶体。涂层和基体的界面结合为冶金化学结合 ,HA涂层和基体Ti间存在过渡相ZrO2 ,过渡相ZrO2 的存在有利于提高涂层和基体之间的界面结合力。     

梯度结构钢铁材料的研究进展

随着社会发展的需求,人们对钢铁材料的性能要求不断提高.鉴于梯度结构可以解决传统强化手段造成的强度提高而塑/韧性下降的问题,近年来关于梯度结构钢铁材料的研究不断增多.本文简要综述了梯度结构钢铁材料的发展历史和制备加工方法,分析了梯度结构对钢铁材料强塑性、耐磨性、腐蚀性和疲劳性能等的影响,最后对面临的一些基础科学问题和工业...     

W-Cu梯度功能材料的热物理性能

对采用不同粒度配比和热压制备的W-Cu梯度功能材料的热物理性能进行研究.结果表明:梯度材料的整体热导率较高,达到226.4 W/(m·K),高于过渡层W/Cu33的热导率,低丁散热层W/Cu50的热导率;封接层具有低的线性热膨胀系数,аRT-100℃=6,82×10-6/℃,满足与BeO基板材料封接匹配的要求;低温热压条件下制备的W-Cu梯度功能材料各梯度层的热膨胀系数具有良好的可控性和可设计性能,其实测值与理论值十分接近,其误差值低于6%;耐热冲击温度达到800℃以上,热疲劳性能可达500℃水淬50次以上.     

Higher-order gradient effects in micropillar compression

Deformation behavior in single-slip oriented micropillars subjected to compressive loading is analyzed using a three-dimensional finite element method incorporating a higher-order gradient crystal plasticity theory in which the spatial gradients of the geometrically necessary dislocation densities are assumed to give rise to backstress that impedes or promotes dislocation motion, depending on its sign. In experimental studies, it has been frequently observed that the gauge portion of micropillar samples is split into a shear zone and two nearly unstrained dead zones under compressive loading. Here it is shown that effects of a higher-order gradient are essential for the formation of this particular deformation mode.     

Hot corrosion in a temperature gradient

Hot corrosion is the accelerated attack of materials at elevated temperatures that is induced by a thin film of fused salt. Many materials that experience hot corrosion are exposed to high heat fluxes and temperature gradients. The hot corrosion of metals can be described by the dissolution of a protective metal oxide into the fused salt and the subsequent precipitation of the metal oxide in a nonprotective form. Rapp and Goto have established a criterion for the sustained hot corrosion of a pure metal that the solubility gradient of the protective oxide in the salt film must be negative at the oxide/salt interface. The influence of a temperature gradient on the metal oxide solubility gradient (and the predicted hot corrosion susceptibility) is discussed. Solubility data for NiO in Na₂SO₄ are used to illustrate the effects of a temperature gradient. The effects of a temperature gradient were determined by examining various combinations of dissolution reactions, oxidants, and transport kinetics. Based on these results, and the thinness of the molten salt films, temperature gradients are not expected to be of importance to hot corrosion fluxing mechanisms.