Oxidation protection of NiCoCrAlY coatings on γ-TiAl

The effect of NiCoCrAlY overlay coatings on the oxidation resistance of γ-TiAl was studied at 900 ℃ in static air. To hinder the interdiffusion of the elements, the Al/Al2O3 layer was added between the coating and the alloy. The results show that the TiAl alloy exhibits poor oxidation resistance. NiCoCrAlY coating can not effectively protect the γ-TiAl substrate from high temperature oxidation because of the serious interdiffusion between the coating and the substrates. With Al/Al2O3 diffusion barrier, the NiCoCrAlY coating exhibits excellent oxidation protection on γ-TiAl alloy.     

陶瓷与金属扩散连接的研究现状

陶瓷与金属的连接技术是材料工程领域的热点研究课题,而扩散连接方法被认为是陶瓷与金属连接最适宜的方法。本文综述了陶瓷与金属扩散连接的研究现状,重点介绍了界面反应研究、残余应力分析和连接工艺研究等内容,并在此基础上指出了研究中所存在的问题。     

Fe3Al合金与Q235钢扩散焊界面的析出相

采用扫描电镜(SEM)、能谱分析(EDS)、X射线衍射(XRD)和电子探针(EPMA)对Fe3Al/Q235扩散焊界面的析出相形貌进行观察和成分测定,研究微观析出相的形成以及工艺参数(T,t,p)的影响.结果表明,Fe3Al/Q235扩散焊界面靠近Fe3Al一侧形成FeAl(Cr)析出相,是引起扩散焊接头脆性断裂的关键.扩散焊温度T、保温时间t与析出相区宽度y符合抛物线规律y2=3.5(t-t0)exp[-5.6×104/(RT)].在降低加热温度和保温时间的条件下,增加焊接压力可促进Fe3Al/Q235界面原子的扩散,避免脆性析出相的形成,保证Fe3Al/Q235扩散焊接头的性能.     

12Cr1MoV钢高温时效过程中组织结构的演变

用Larson-Miller参数对12CrlMoV钢进行了在不同温度与时间的高温时效模拟，研究了其显微组织和亚结构的演变。光学显微镜、扫描电镜及透射电镜的观察分析表明，在时效过程中的主要组织结构变化是珠光体的消散与分解，碳化物从固溶体中脱落，碳化物粗化并向晶界转移以及晶粒长大。同时，位错密度下降并结成位错网络。这些持续而缓慢的变化符合热力学驱动力与扩散动力学规律。     

Numerical modeling of high-temperature corrosion processes

Numerical modeling of the diffusional transport associated with high-temperature corrosion processes is reviewed. These corrosion processes include external scale formation and internal subscale formation during oxidation, coating degradation by oxidation and substrate interdiffusion, carburization, sulfidation and nitridation. The studies that are reviewed cover such complexities as concentration-dependent diffusivities, cross-term effects in ternary alloys, and internal precipitation where several compounds of the same element may form (e.g., carbides of Cr) or several compounds exist simultaneously (e.g., carbides containing varying amounts of Ni, Cr, Fe or Mo). In addition, the studies involve a variety of boundary conditions that vary with time and temperature. Finite-difference (F-D) techniques have been applied almost exclusively to model either the solute or corrodant transport in each of these studies. Hence, the paper first reviews the use of F-D techniques to develop solutions to the diffusion equations with various boundary conditions appropriate to high-temperature corrosion processes. The bulk of the paper then reviews various F-D modeling studies of diffusional transport associated with high-temperature corrosion.     

Determination of isothermal section of Fe-Ti-Zr ternary system at 1173 K

Phase relations in the Fe-Ti-Zr ternary system at 1173 K were investigated by means of diffusion-triple approach together with electron probe microanalysis（EPMA） technique. A series of tie lines and tie-triangles were determined and the isothermal section at 1 173 K was established, which consists of four three-phase fields： β（Ti, Zr）＋FeZr2＋FeTi, FeZr2＋FeTi＋Fe2Zr, FeTi ＋Fe2Zr ＋Fe2Ti and Fe2Zr ＋Fe2Ti ＋Fe. The results show that the largest solubility of Ti in Fe2Zr is about 11.3 %（mole fraction） and the solid solubility of Ti in FeZr2 is about 26.9%, the solid solubility of Zr in Fe2Ti is about 8.1% and the solid solubility of Zr in FeTi is 7.2%. The binary compound FeZr2 is nearly a linear compound. No ternary compound is found.     

Determination of phase equilibria in the Ni-Ta-Zr system at 1223 K

The phase equilibrium relations of the ternary Ni-Ta-Zr system at 1223 K were investigated by means of diffusion triple and electron probe microanalysis (EPMA) techniques. Two ternary compounds were found and a series of tie lines and tie triangles have been determined. The maximum solubility of Ta in NiZr2 is about 13.5% (atom fraction). The corresponding tentative isothermal section at this temperature was constructed based on current experimental information.     

连接温度、压力和保温时间对Ti3SiC2／Ni扩散连接接头性能的影响

利用Gleeble 3500热模拟实验机，在800～1100℃、10～90min和6-20MPa条件下对Ti3SiC2和Ni进行真空扩散连接。通过正交实验，研究了连接温度、连接压力和高温保温时间对试样连接强度的影响，优选出最佳工艺参数。结果表明，扩散连接工艺参数显著影响Ti3SiC2／Ni接头的剪切强度。在1000℃、10min和20MPa实验条件下，获得的Ti3SiC2／Ni接头的剪切强度达到（121±7）MPa，接近Ti3SiC2陶瓷的剪切强度。     

Simultaneous Internal Oxidation and Nitridation of Ni–Cr–Al Alloys

A series of Ni–Cr–Al alloys was subjected to thermal cycling to 1100°C in air for up to 260 1-hr cycles. All alloys exhibited poor corrosion resistance. Repeated scale spallation led to subsurface alloy depletion in aluminum and, to a lesser extent, chromium. This caused transformation of the prior alloy three-phase structures (-Cr+-NiAl+-Ni) to single-phase -nickel solution. Destruction of the external scale allowed gas access to this metal, which was able to dissolve both oxygen and nitrogen. Inward diffusion of the two oxidants led to development of a complex internal-precipitation zone: Al₂O₃ and Cr₂O₃ beneath the surface, followed by Al₂O₃, then AlN, then AlN+Cr₂N, and, finally, AlN alone in the deepest region. This distribution is shown to reflect the relative stabilities of the precipitates and the higher permeability of nitrogen. Diffusion-controlled kinetics were in effect initially, but mechanical damage to the internal-precipitation zone led to more rapid gas access and approximately linear kinetics in the long term.     

热浸镀铝钢扩散层的AlFe3C0.5相

采用740℃热浸镀铝方法,在20碳钢表面制备热浸镀铝层。镀铝层经850℃扩散处理4 h后,采用SEM和XRD确定扩散层的相组成,采用TEM研究组成相的晶体学取向关系。分析AlFe3C0.5相的形成原因。结果表明：扩散层与基体之间存在Fe3Al、FeAl和点阵常数为0.377 nm的针叶状AlFe3C0.5相;针状AlFe3C0.5与Fe3Al相之间存在的晶体学取向关系是（220）Fe3Al//（1^-11）AlFe 3C0.5、[11^-0]Fe3Al//[1^-23^-]AlFe 3 C0.5。