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。     

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。     

梯度硬质合金梯度层形成过程模拟分析

以梯度硬质合金W-C-Co-Ti-Ta-Nb-N体系为研究对象,分别建立了动力学和热力学数据库,通过计算机模拟分析了梯度层形成过程中相体积分数与合金组元成分分布,并与文献实验结果进行了对比。结果表明,模拟结果和实验结果基本吻合。     

梯度硬质合金梯度层形成过程模拟分析

以梯度硬质合金W-C-Co-Ti-Ta-Nb-N体系为研究对象,分别建立了动力学和热力学数据库,通过计算机模拟分析了梯度层形成过程中相体积分数与合金组元成分分布,并与文献实验结果进行了对比。结果表明,模拟结果和实验结果基本吻合。     

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。     

基于热、动力学计算模拟研发梯度硬质合金

对于材料研发和过程参数优化来说,掌握材料体系的热、动力学信息并基于CALPHAD(相图计算)方法进行计算模拟是一种强有力且高效的方式。CALPHAD计算结果的准确性在很大程度上取决于热力学和动力学数据库的质量。基于前期建立的热力学数据库(CSUTDCC1)和动力学数据库(CSUDDCC1),对硬质合金研发过程中所关心的烧结"碳窗口"、立方相组分等信息进行了计算模拟。研究了几种在真空和不同N_2分压下烧结的梯度硬质合金,采用SEM和EPMA研究了梯度层的微观结构和元素浓度分布,并通过热、动力学数据库进行了计算模拟,计算模拟结果与实验数据吻合。本工作探讨了热、动力学计算模拟在梯度硬质合金的设计和研发中的应用示范,为新型高性能梯度硬质合金的开发提供理论支撑。     

梯度硬质合金梯度层形成的计算机模拟及验证

结合相图热力学计算,使用DICTRA软件计算模拟Co-W-Ti-C-N、Co-W-Ti-Nb-C-N和Co-W-Ti-Ta-C-N体系梯度硬质合金梯度层形成过程,对比计算模拟和实测的梯度硬质合金中Co含量的距离变化曲线。通过分析各相体积分数及组元成分随距离的分布研究烧结时间、烧结温度、Co含量和Ti含量对梯度层厚度的影响。结果表明:计算模拟与实验数据吻合较好。延长烧结时间、升高烧结温度和增加Co含量均会促进梯度层厚度的增加,而增加Ti含量则会抑制梯度层厚度的增加。     

硬质合金中立方相成分对梯度区形成的影响

在实验中，利用扫描电镜(SEM)，电子探针显微分析(EPMA)和透射电镜法(TEM)对具有一个立方碳化物硬质相被消耗而富集了塑性粘结相的表面区的硬质合金切削刀具刀片进行了研究。根据多元扩散方程组的解连同计算结果，利用各个相的热力学描述将实验结果与模拟数据作了比较。研究中的材料为WC-Ti(C，N)-Co，     

Fe-Al-Si三元扩散偶中合金元素扩散行为的动力学计算

建立了Fe- 6.8%Al-1.0%Si/Fe和Fe-6.3%Al-0.9%Si/Fe两组扩散偶,分别对其进行1 050 ℃×3 h和1 000 ℃×64 h的退火处理,而后采用EPMA技术对合金元素在不同扩散偶中的成分分布进行测定;同时结合Fe-Al-Si体系的热力学与动力学性质,采用Thermo-Calc & DICTRA软件中的移动界面模型,对Fe、Al和Si元素随温度与时间的浓度变化情况进行了模拟计算,计算结果与实验测定的成分分布数据吻合得较好,从而验证了计算中所采用热力学与动力学参数的有效性,并可为模拟相关合金体系中相组成的演变行为提供计算依据.     

表面无立方相层功能梯度硬质合金的研究进展

综述了目前应用于涂层基体的无立方相层含氮功能梯度硬质合金的研究进展;详细介绍无立方相层的形成热力学基础、梯度结构特征、机理和动力学研究进展以及力学性能和切削性能;重点评述C、N含量以及组分对无立方相层的影响规律;提出获取合金系统真实的热力学相图和动力学数据是今后研究工作的重点。     

Experimental Investigation and Computer Simulation of Gradient Zone Formation in WC-Ti(C,N)-TaC-NbC-Co Cemented Carbides

The thermodynamic and diffusion databases for multi-component W-C-Co-Ti-Ta-Nb-N cemented carbides have been developed through a combination of experimental, theoretical and assessment work. Using the CALPHAD approach, the thermodynamic database has been established based on the literature information and experimental investigations. The diffusion database contains the atomic mobility parameters for different diffusing elements in liquid, which were calculated by a modified Sutherland equation. According to the present thermodynamic and diffusion databases, a gradiently sintered WC-Ti(C,N)-(Ta,Nb)C-Co cemented carbides has been designed and prepared. Scanning electron microscopy was employed to investigate the microstructure of the gradient zone, and electron probe microanalysis was used to determine the concentration profiles and distributions of the elements. The gradient zone formation of the cemented carbides was then simulated by DICTRA, and the simulation results show a good agreement with the experimental results.     

Computational investigation of constitutional liquation in Al–Cu alloys

Constitutional liquation means the local eutectic melting of second-phase particles in a matrix at temperatures above the eutectic temperature and below the solidus of the alloy, which may occur in the heat-affected zone (HAZ) during welding. In the present paper, the constitutional liquation in the Al–Cu system was computationally investigated using the DICTRA program coupled with critically assessed thermodynamic and kinetic databases. Computer simulated results are in quantitative agreement with existing experimental data. The computational procedures for obtaining the critical heating rate to avoid constitutional liquation are demonstrated. The critical heating rate was found to be inversely proportional to the square of the precipitate size. The present computational procedures can be readily extended to predict the susceptibility of multicomponent commercial alloys to constitutional liquation during welding with available thermodynamic and kinetic databases.     

Integrated design of Nb-based superalloys: Ab initio calculations,computational thermodynamics and kinetics,and experimental results

An optimal integration of modern computational tools and efficient experimentation is presented for the accelerated design of Nb-based superalloys. Integrated within a systems engineering framework, we have used ab initio methods along with alloy theory tools to predict phase stability of solid solutions and intermetallics to accelerate assessment of thermodynamic and kinetic databases enabling comprehensive predictive design of multicomponent multiphase microstructures as dynamic systems. Such an approach is also applicable for the accelerated design and development of other high performance materials. Based on established principles underlying Ni-based superalloys, the central microstructural concept is a precipitation strengthened system in which coherent cubic aluminide phase(s) provide both creep strengthening and a source of Al for Al₂O₃ passivation enabled by a Nb-based alloy matrix with required ductile-to-brittle transition temperature, atomic transport kinetics and oxygen solubility behaviors. Ultrasoft and PAW pseudopotentials, as implemented in VASP, are used to calculate total energy, density of states and bonding charge densities of aluminides with B2 and L2₁ structures relevant to this research. Characterization of prototype alloys by transmission and analytical electron microscopy demonstrates the precipitation of B2 or L2₁ aluminide in a (Nb) matrix. Employing Thermo-Calc and DICTRA software systems, thermodynamic and kinetic databases are developed for substitutional alloying elements and interstitial oxygen to enhance the diffusivity ratio of Al to O for promotion of Al₂O₃ passivation. However, the oxidation study of a Nb–Hf–Al alloy, with enhanced solubility of Al in (Nb) than in binary Nb–Al alloys, at 1300 °C shows the presence of a mixed oxide layer of NbAlO₄ and HfO₂ exhibiting parabolic growth.     

Precipitation of paraequilibrium cementite: Experiments,and thermodynamic and kinetic modeling

The precipitation of cementite prior to the precipitation of the strengthening M₂C phase is investigated using two model ultra-high strength (UHS) steels. The structure, microstructure and chemical composition of cementite are studied by analytical electron microscopy techniques. The structure of cementite precipitated during early stages of tempering at 755 and 783 K was confirmed by convergent beam electron diffraction. In an alloy containing 0.16 mass% C, the cementite particles were primarily plate shaped and interlath type, whereas in an alloy containing 0.247 mass% C both inter- and intralath particles were observed. Consistent with the earlier studies on tempering of Fe-C martensite, lattice imaging of cementite suggests microsyntactic intergrowth of M₅C₂ (Hägg carbide). Quantification of the substitutional elements in cementite confirms its paraequilibrium state with ferrite at the very early stage of tempering. Computational thermodynamic and kinetic tools, Thermo-Calc and dictra (diffusion controlled transformation) software, respectively, are used to model the precipitaton of paraequilibrium cementite in several multicomponent alloys. A thermodynamic model parameter describing the effect of Si on the stability of cementite is proposed. The model parameter is consistent with the following results: (a) that Si does not partition to cementite in Fe–Si–C and Co–Si–C alloys under orthoequilibrium conditions, and (b) there is a large driving force for the precipitation of paraequilibrum cementite in an Fe–0.41C–3Mn–2Si alloy where it has been experimentally verified. The nucleation driving forces for the precipitation of paraequilibrium cementite, and the two-phase (ferrite and cementite) paraequilibrium boundaries for multicomponent alloys are calculated using the Thero–Calc software systems. The results of growth simulations of cementite under paraequilibrium condition in multicomponent systems using the dictra software are also presented.     

Constitution of Ni–Al–Ti system studied by scanning electron microscopy

The Ni–Al–Ti system is one of the key model systems for technically important materials based on nickel and titanium aluminides. A recent critical literature review by Schuster stated a lack of experimental data apart from corners of the composition triangle. This work is focused on several alloys of Ni–Al–Ti system from the central part of the composition triangle, where numerous intermetallic phases coexist. The microstructure of alloys was studied and quantified after long term annealing at 1050 °C by means of scanning electron microscopy with an energy dispersive X-ray analysis and electron backscatter diffraction. Experimental results were compared with existing literature data. In order to compare experimental knowledge on phase equilibria with results of thermodynamic modelling, phase diagram was calculated using the software package ThermoCalc and two different thermodynamic databases.     

多元Al合金的热力学和热物理数据库的建立及凝固过程显微组织演变的模拟

介绍了相图热力学及热物理性能数据库的建立方法;综述了国内外学者构筑多元Al合金相图热力学和热物理性能数据库的研究工作。并介绍了使用相场方法,使用热力学和热物理性能数据库模拟多元Al合金凝固时显微组织演变的几个实例。最后展望了新一代热力学和热物理性能数据库建立及凝固过程显微组织演变定量模拟未来的可能发展方向。     

On the Potential for Improving Equilibrium Thermodynamic Databases with Kinetic Simulations

In this work, the possibilities for improving the accuracy of thermodynamic databases based on kinetic simulations are explored. With a new model for the simulation of precipitation kinetics in multi-component alloys, calculations are performed with all unknown parameters of the simulation obtained from independent thermodynamic and kinetic databases. Since no fitting parameters are used, the simulations are considered as having ‘predictive character’. The corresponding methodology is outlined. Based on the comparison of the predicted precipitation kinetics and experimental information, the potential for improving the accuracy of thermodynamic databases is explored. This article was presented at the Multi-Component Alloy Thermodynamics Symposium sponsored by The Alloy Phase Committee of the joint EMPMD/SMD of the Minerals, Metals, and Materials Society (TMS), held in San Antonio, Texas, March 12-16, 2006, to honor the 2006 William Hume-Rothery Award recipient, Professor W. Alan Oates of the University of Salford, UK. The symposium was organized by Y. Austin Chang of the University of Wisconsin, Madison, WI, Patrice Turchi of the Lawrence Livermore National Laboratory, Livermore, CA, and Rainer Schmid-Fetzer of the Technische Universitat Clausthal, Clauthal-Zellerfeld, Germany.     

Incorporating first-principles energetics in computational thermodynamics approaches

Computational thermodynamic approaches have become a valuable tool in the calculation of complex, multicomponent phase equilibria often found in industrial alloys. These methods rely on databases of free energies, obtained from an optimization process involving experimental thermodynamic and phase diagram data. However, many phases of practical interest (e.g., metastable precipitate phases) are absent from computational thermodynamics databases, due to insufficient information to perform the optimization process. We demonstrate that first-principles, density functional calculations provide a means to obtain thermodynamic functions of phases absent from current databases. Two examples illustrate this hybrid first-principles/computational-thermodynamics approach: (1) the famous metastable Cu-containing precipitate phase, Al₂Cu-θ′, often found in age-hardened aluminum alloys, and (2) a new assessment of thermodynamic data in the Al–Sr system. We show how first-principles input may be used in both binary and multicomponent industrial systems.