烧结工艺对梯度结构硬质合金梯度层组织和厚度的影响

本文用两步烧结方法制备了梯度结构硬质合金,用扫描电子显微镜(SEM)和能谱分析研究了不同的烧结工艺和基体中Co含量对梯度层元素浓度分布、梯度层厚度及显微组织的影响。实验结果表明,烧结温度和保温时间对脱β层厚度有很大影响,基体中高的Co含量有利于梯度层厚度的增加。     

真空-压力两步烧结制备的表层脱立方相梯度硬质合金的组织与性能

本文通过真空-压力两步烧结制备了脱立方相梯度硬质合金,并对材料的组织和性能做了研究。研究发现,相比于一步真空烧结制备的脱立方相梯度硬质合金,真空-压力两步烧结制备的梯度硬质合金脱立方相层更厚,合金内部的立方相晶粒尺寸更大。梯度硬质合金脱立方相层中的平均WC晶粒尺寸比内部的更大,这与脱立方相层中Co含量更高以及内部含Ti立方相的存在有关。梯度硬质合金中过渡层的微观硬度高于合金内部,而脱立方相层的硬度最低,微观硬度变化与Co、Ti等元素含量变化紧密相关。压力烧结对表面脱立方相层的致密化作用明显,使得脱立方相层的孔隙减少,梯度合金相对密度达到99.6%。脱立方相层厚度增加和孔隙缺陷减少促进了梯度硬质合金横向断裂强度的提高。     

立方相成分对梯度合金结构的影响——理论计算及实验研究

通过理论计算和实验研究了x(Ti)/x(Ta+Nb)比值对梯度层厚度、表层化学成分分布的影响。利用DICTRA扩散动力学软件对合金表层各相含量以及各组元成分随距离的分布进行了计算机模拟;采用SEM观察梯度层的显微结构,利用EDS对不同成分合金进行了合金表面组元成分分布测定。结合理论计算与实验结果,分析了烧结过程中尤其是固相烧结阶段N分解程度对梯度厚度的影响,以及立方相成分对梯度厚度、表层化学成分分布的影响。计算结果和实验结果表现出良好的一致性。计算结果表明其它成分一定时,梯度厚度随着x(Ti)/x(Ta+Nb)比例的升高而下降;实验结果显示N含量相同时,x(Ti)/x(Ta+Nb)比例越高,烧结过程中N损失越大;梯度层内液相含量随着x(Ti)/x(Ta+Nb)比例增大而升高,同时,梯度层毗邻区Co含量及Ti含量受梯度层厚度及立方相成分的影响。     

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。     

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.     

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.     

渗碳工艺对WC-Co梯度硬质合金的梯度结构和硬度的影响

对缺碳硬质合金采用渗碳处理制备梯度硬质合金,利用显微组织分析和维氏硬度测试等方法,研究渗碳工艺对梯度硬质合金的梯度结构和硬度的影响。结果表明：渗碳处理后随着渗碳时间延长,梯度层厚度增大,长时间渗碳还会出现梯度结构消失现象;渗碳时表面层WC晶粒长大,且渗碳时间越长晶粒长大越严重;渗碳后梯度硬质合金的表面硬度明显提高;渗碳后合金的表面硬度明显高于烧结态合金的表面硬度;随着渗碳时间的延长,合金表面硬度先增大后减小;合金的硬度在截面上沿梯度方向呈连续梯度变化,合金表面层因WC含量较高、钴含量较低而具有较高的硬度,中间层因钴含量较高、WC含量较低,其硬度较低。     

梯度硬质合金梯度层形成的计算机模拟及验证

建立了多组元梯度硬质合金W-C-Co-Ti-Ta-Nb-N体系的热力学和扩散动力学数据库,采用Thermo-Calc相图热力学计算软件描述了梯度烧结过程中出现的各相及其成分,并采用DICTRA扩散动力学软件对WC-Ti(C,N)-Co,WC-Ti(C,N)-TaC-Co和WC-Ti(C,N)-NbC-Co合金梯度层的各相体积分数及各组元成分随距离的分布进行了计算机模拟,模拟结果与实验结果吻合良好.在热力学和扩散动力学数据库基础上制备了WC-(Ti,W)C-Ti(C,N)-(Ta,Nb)C-Co合金,采用SEM观察梯度层的显微结构,利用EDS分析合金组元分布,并对合金梯度层的形成进行了计算模拟,模拟结果与实验结果吻合良好.     

烧结碳势对梯度硬质合金组织结构的影响

结合热力学计算,通过两次烧结法,先在含氮气氛下预烧结,然后在不同碳势的气氛下梯度烧结,分别制备了WC-Ti(C,N)-7.5%Co、WC-Ti(C,N)-10%Co、WC-Ti(C,N)-7.5%Co-3%Nb和WC-Ti(C,N)-10%Co-3%Ta共4组成分的梯度硬质合金。通过对其成分分布和微观组织结构的表征,研究了烧结碳势对不同组分的梯度硬质合金基体合金表面无立方相层厚度的影响。结果表明:烧结碳气氛与合金碳势差较大时能形成表面无立方相层,烧结气氛的碳势越高,表面无立方相层越薄;在气氛与合金碳势差过小时不形成梯度,而是形成均质结构;Co含量提高,更容易形成梯度,表面无立方相层加厚;加入了Ta或Nb之后,立方相增多,梯度形成效果更明显,但无立方相层变薄。     

烧结工艺对脱β层厚度及性能的影响

改变硬质合金烧结工艺控制的几个关键因素,通过测量合金的理化性能,利用扫描电镜分析合金的内部结构,研究了烧结气氛(真空,N_2)、烧结温度、烧结压力对硬质合金梯度结构和机械性能的影响。结果表明,对于含氮硬质合金的梯度烧结,适时引入一定量的氮气可抑制合金中含氮物质的早期分解,可用氮压来控制梯度增长速率,烧结气氛中氮气压力适宜控制在100～200 mbar;随着烧结温度的提高,合金的致密度和脱β层梯度厚度增加明显,合金抗弯强度增加;随着烧结压力的增大,合金脱β层梯度厚度变薄。     

Microstructure and mechanical properties of functionally gradient cemented carbides fabricated by microwave heating nitriding sintering

A new method is presented for the fast preparation of functionally graded cemented carbide materials by microwave heating nitriding sintering. The influence of composition and sintering temperature on the mechanical properties, microstructure, and phase composition of the materials was studied. Results showed that functionally graded cemented carbides with the desired mechanical properties can be obtained rapidly by microwave heating nitriding sintering. A gradient layer with a Ti(C, N)-enriched surface layer, and underneath a Co-enriched layer formed on the top of the hard alloy substrate. The nitriding process had little effect on the microstructure of the matrix. A lower surface roughness, and the similar layer thickness as seen in conventional heating nitriding was obtained by microwave heating nitriding sintering in a short period of time. The thickness of the gradient layer increased with increasing temperature. The high Ti content in the raw material was beneficial to the formation of the gradient layer; however, the Co content had little effect on the gradient layer thickness when it increased from 6% to 10%.     

梯度硬质合金及耐磨涂层的计算模拟和实验验证

建立了多组元硬质合金W-C-Co-Ti-Cr-Ta-Nb-N体系热力学及动力学基因库。利用所建立的热力学及扩散动力学基因库,模拟了WC-Ti(C,N)-TaC-Co硬质合金梯度层形成过程,计算所得各相体积分数及组元成分与实验结果相吻合。采用SEM和EDS等方法对不同N气氛下梯度烧结所获得的WC-Ti(C,N)-Co梯度硬质合金进行了合金表面组元成分分布测定,并对样品梯度层的形成进行了模拟,模拟能很好地描述实验结果。基于第一原理计算和实验对广泛应用的三元Ti-Al-N耐磨涂层体系的结构、力学、热力学性质和调幅分解曲线,以及调幅分解析出立方二元氮化物的性能进行了研究。计算结果与已有实验值符合较好,可为高性能硬质合金和多元涂层的开发设计提供理论指导。最后提出了硬质合金及耐磨涂层研发的基因框图。     

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...     

The influence of carbon content on formation of carbo-nitride free surface layers in cemented carbides

To increase crack propagation resistance in cemented carbide cutting tools, it is sometimes of interest to create tough surface zones in the substrates. A way to do this is to use so-called gradient sintering in the manufacturing of the cutting tool. In this sintering process a nitrogen and titanium containing cemented carbide is sintered in a nitrogen free atmosphere. The difference in nitrogen activity between atmosphere and cutting tool during sintering will create an outward nitrogen diffusion. Due to thermodynamical coupling between nitrogen and titanium, this gives rise to an inward titanium diffusion, which creates a surface zone depleted of hard cubic carbo-nitrides, and enriched in ductile binder phase. By varying the carbon content of the material, the nitrogen activity is affected, and this in turn affects the surface zone formation.

In this report, Ti(C,N)–(Ti,W)C–WC–Co, Ti(C,N)–NbC–WC–Co, and Ti(C,N)–TaC–WC–Co cemented carbides were studied. All three materials were produced in series with varied carbon content, in order to study the effect of carbon on gradient surface zone formation.     

Fabrication of Mo-Ti functionally graded material

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Fabrication of WC-Co/(Ti,W)C graded cemented carbide by spark plasma sintering

The WC-Co/(Ti, W)C graded cemented carbide was prepared by spark plasma sintering. The substrate is WC-8Co, and the hard layer is (Ti, W)C solid-solution. The effects of sintering temperature and holding time on the microstructure and properties of graded cemented carbide were analyzed. The hard layer is mainly formed by dissolving WC in the Co-phase and then by solid-solution reaction with TiC. As the sintering temperature increases, the migration rate of WC increases. When the holding time is 5 min, the thickness and the W content of the (Ti, W)C solid-solution hard layer increases with the increasing of sintering temperature. The thickness of the (Ti, W)C solid-solution can reach 51 ± 2 μm at the sintering temperature of 1700 °C for the holding time of 5 min. The hardness of hard layer surface increases first and then decreases with the increasing of sintering temperature. The Vickers hardness is the highest at 1600 °C, which can reach HV_0.221.53GPa. As the holding time increases, the thickness of the solid-solution hard layer increases, but the rate of growth decreases. As the thickness increases, the difference in the W element concentration between the solid-solutions of the same pitch decreases along the layer depth direction, and W element concentration in the entire hard layer increases. The oxidation behavior of graded cemented carbide at 400 °C and 600 °C was investigated. The (Ti, W)C hard layer has superior oxidation resistance relative to the WC-Co substrate.     

初始成分对梯度硬质合金微观组织及脱β层厚度的影响

采用普通市售中颗粒Ti(C,N)粉末,以一步烧结法制备脱β层梯度硬质合金;利用显微组织分析和图像分析等手段,研究合金初始成分对其微观组织及脱β层厚度的影响规律。结果表明:当Ti(C,N)含量低于1.6%(质量分数)时,随着Ti(C,N)含量的增加,脱β层厚度明显增大,而当Ti(C,N)含量超过1.6%时,脱β层厚度呈缓慢缩小的趋势;随着钴含量的增加,脱β层的厚度迅速增大,但当钴含量达到10%(质量分数)左右时,在脱β层与芯部的界面处钴相聚集现象严重;总碳含量为6.51%(质量分数)的合金中WC晶粒度较大且呈规则的多边形,在1 450℃、2 h梯度烧结工艺下制备的脱β层厚度可达38μm左右,而总碳含量为6.23%的合金中WC晶粒度较小且呈等轴化趋势,同时脱β层的厚度仅为17μm左右。     

Migration of Point Defects in the Field of a Temperature Gradient

The influence of the temperature gradient over the thickness of the cladding of a fuel element of a fast-neutron reactor on the migration of point defects formed in the cladding material due to neutron irradiation has been studied. It has been shown that, under the action of the temperature gradient, the flux of vacancies onto the inner surface of the cladding is higher than the flux of interstitial atoms, which leads to the formation of a specific concentration profile in the cladding with a vacancy-depleted zone near the inner surface. The experimental results on the spatial distribution of pores over the cladding thickness have been presented with which the data on the concentration profiles and vacancy fluxes have been compared.     

A new approach to fabrication of gradient WC–Co hardmetals

WC–Co hardmetals with gradient structure comprising neither η-phase nor grain growth inhibitors were produced for the first time by regulating the WC re-crystallisation and carbon content in their near-surface layer and core. Hardmetals with low Co contents in the surface region were obtained by selective carburisation of the near-surface zone of green articles with the original low carbon content and their consequent liquid-phase sintering. The surface region of such gradient hardmetals has a hardness of up 150 Vickers units higher and fracture toughness significantly superior than those of the core. Gradient hardmetals with high Co contents in the surface region were obtained by selective decarburisation of the near-surface zone of green articles with the original high carbon content and their consequent liquid-phase sintering. The new approach for fabrication of gradient WC–Co materials appears to be a unique tool for increasing both the hardmetal hardness and fracture toughness.