High temperature bending creep behavior of a multi-cation doped α/β-SiAlON composite

SiAlON ceramics with high hardness and high toughness can be made through designing α/β-SiAlON composites. An important advantage of α-SiAlON phase is that the amount of intergranular phase is reduced by the transient liquid phase being absorbed into the matrix of α-SiAlON phase during sintering. But, the thermal stability of the α-SiAlON phase is an important concern for α/β-SiAlON composites especially at high temperatures. The use of different types of single or multiple cations during fabrication directly affects resultant microstructures and mechanical behavior of α/β-SiAlON composites. In this study, the creep behavior of a multi-cation (Y, Sm and Ca) doped α/β-SiAlON composite, in which aluminum-containing nitrogen melilite solid solution phase was designed as intergranular phase, was investigated by four-point bending creep tests under stresses from 50 to 150 MPa and at temperatures from 1300 °C to 1400 °C in air. The stress exponent was determined to be 1.6 ± 0.13 at 1400 °C and the creep activation energy was calculated to be 692 ± 37 kJ/mol⁻¹. Grain boundary sliding coupled with diffusion was identified as the rate-controlling creep mechanism for the α/β-SiAlON composite.     

Study of creep cavitation in stainless steel weldment

Abstract

A study of creep cavities near reheat cracking in AISI Type 316H austenitic stainless steel headers, removed from prolonged high temperature operation in nuclear power plants, is reported. It is shown how application of scanning electron microscopy (SEM), cryogenic fractography and small angle neutron scattering (SANS) can be applied, in a complementary way, to observe and quantify creep cavitation. Creep cavities in the vicinity of the crack are found to be mainly surrounding inter-granular carbides. Trends in the size and area fraction of creep cavities relative to the crack path are quantified using optimised metallography. The SANS technique is found to be a very suitable method of quantifying creep cavitation within the size range up to 600 nm averaged over a larger gauge volume. It is shown that the cavity size distribution peaks in the region 100–300 nm, and this correlates closely with the quantitative SEM observations.     

Effect of long time service exposure on microstructure and mechanical properties of gas turbine vanes made of IN939 alloy

In the present study, the effects of long-term service exposure have been investigated on microstructure and mechanical properties of gas turbine vanes made of IN939 superalloy. The major microstructural changes for the investigated service-exposed vanes include the formation of continuous grain boundary carbides and the transformation (degeneration) of MC carbides located at the grain boundaries. The brittle σ phase, which is predicted to be stable on the basis of thermodynamic calculations, is not observed in the microstructure of service-exposed vanes. The microstructural changes during service lead to a loss in room temperature ductility as well as in creep properties of the alloy.     

A research on the creep properties of titanium matrix composites rolled with different deformation degrees

(TiB + La₂O₃)/Ti composites were in situ synthesized and deformed with different deformation degrees. The influence of TiB whisker orientation and grain refinement on the creep properties of titanium matrix composites (TMCs) are discussed. The creep test reveals that the steady state creep rate of TMCs first decreases and then increases with the increase of deformation degree, which can be attributed to competing effects: TiB whisker rotating to the rolling direction, α plate grain boundary hindering and pinning dislocations can all decrease the creep rate, however, dislocation movement on the α plate grain boundary and dislocation emitting from the α plate grain boundary can both increase the creep rate.     

A Prediction Method of the Behavior of Adhesively Bonded Structures under Cyclic Shear Loading Based on a Characterization of the Viscous Aspects of the Adhesive in an Assembly

The capabilities of structural bonding are more and more used. Estimating the abilities of an adhesive to endure repetitive loadings and to keep stable its mechanical properties along service life is an essential point to analyze in order to conduct fatigue assessments. The aim of this study is to develop a predictive tool for describing the fatigue behavior of an adhesive in an assembly under cyclic loadings. The approach developed analyzes the influence of viscosity on the mechanical behavior of an adhesive in an assembly based on monotonic and creep test results. Thanks to the evaluation of viscous phenomena, it is possible to predict the cyclic response of the adhesive. The experimental approach uses a unique bonded joint designed to limit the stress concentrations and with a maximum stress state in the center of the adhesive. In this paper, following the strategy developed under monotonic loading, experimental results under cyclic loading are presented for different types of loading using several load ratios and amplitudes. These results underline that the evolution of viscous deformations depends on the loading type. Under shear loading and for a ductile structural adhesive, the experimental results are well described using a viscoelastic–viscoplastic constitutive model with nonlinear viscous parameters. This model makes it possible to analyze the influence of different parameters on the mechanical response of bonded joints under cyclic shear loadings.     

铸态TiAl-Nb合金的组织结构与蠕变性能

通过蠕变性能测试和组织形貌观察,研究了铸态TiAl-Nb合金在近890~910℃温度区间的蠕变行为。结果表明,铸态TiAl-Nb合金的组织结构主要由层片状γ/α_2两相组成,不同取向γ/α_2两相层片状组织之间存在不规则锯齿状形态的晶界,该锯齿状非层片晶界由单一γ相组成。在高温蠕变期间,合金具有较好的蠕变抗力和较长的蠕变寿命;合金在蠕变期间的变形机制是大量位错以位错列的形式剪切层片状γ/α_2两相,其中,大量位错在基体中滑移,发生反应可形成位错网,可促进位错的攀移,减缓应力集中,改善合金的蠕变抗力。与α_2-Τi_3Al相比,γ-TiAl相有较弱的强度。因此,蠕变期间合金中的裂纹易于在与应力轴呈45°角、且与层状结构相平行的晶界处萌生与扩展,直至蠕变断裂是合金在蠕变期间的断裂机制;其中,与层状结构相平行的断口呈光滑表面,而与层状结构呈一定角度的断裂表面存在撕裂棱,为较高强度的α_2-Ti_3Al相阻碍裂纹扩展所致。     

W-4Re-0.27HfC合金的1500~1700℃拉伸蠕变性能及损伤机理

本文研究了粉末冶金制备的W-4Re-0.27HfC合金的拉伸蠕变行为,测试环境为真空,蠕变温度为1500~1700℃,蠕变应力为40~60MPa。采用SEM、EBSD和TEM观察其微观组织,表征晶粒尺寸和位错等组织在蠕变过程中的演变规律。结果表明,W-4Re-0.27HfC合金的稳态蠕变速率范围为1′10-7~5′10-6,较纯钨(W)低两个数量级。W-4Re-0.27HfC合金抗蠕变性能优于纯W主要原因是弥散分布的HfC颗粒钉扎位错和Re取代W原子产生晶格畸变阻碍位错运动,降低位错迁移率。蠕变温度为1500℃时,W-4Re-0.27HfC的蠕变机制以位错滑移为主,伴随有晶界滑动。随着温度升高,位错攀移成为主要蠕变机制。HfC颗粒塞积位错,导致HfC/基体界面结合变差,HfC颗粒剥落出现孔洞,合金蠕变性能下降。     

基于孔洞长大理论的多轴蠕变设计模型及其工程应用

多轴蠕变是高温构件失效的重要原因,工程设计中必须予以考虑.本文介绍了以孔洞长大理论为基础的多轴蠕变设计方法;探讨了孔洞长大的基本原理;讨论了基于该理论的5个常用模型;分析了这些模型在R5、ASME III、RCC-MR 及VGB-R 509L 中的应用,指出了高温构件多轴蠕变设计的困难,并对进一步的工作提出了建议.     

常压烧结Sialon材料高温性能的研究

研究了添加剂为Y2O3、Al2O3、AlN的Sialon材料的高温性能。晶化处理后,其断裂韧性由4MPa·m1／2提高到6MPa·m1／2;高温强度到1100℃仍不明显降低;抗热震临界温差达△Tth＝473℃。其高温断裂为蠕变机制,符合裂纹准静态扩展机理。裂纹准静态扩展是热震损伤的控制因素。     

轨底坡对轮轨滚动接触行为的影响

为了探讨轨底坡与轮轨滚动接触行为的内在联系,针对高速客运专线轮轨接触状况,即轮对踏面为LMA和钢轨为国产CN60,利用改进的三维接触几何程序、Kalker的三维弹性体非赫兹滚动接触理论及其CONTACT数值程序,分析比较轨底坡对轮轨接触几何参数、接触斑形状、接触斑滑动量、摩擦功、接触应力等的影响.通过分析计算可得,轨底坡对轮轨滚动接触行为有很大影响.尤其当轮对相对轨道横移在6 mm～9 mm范围内变化时,随着轨底坡从1/20到1/40的逐渐减小,轮对左右轮滚动半径差和等效锥度增大,这说明在1/40轨底坡下,轮对恢复对中的能力更好,更有利于曲线通过.1/40轨底坡对应的最大正压力、等效应力等较小,其对应的摩擦功较大.1/20轨底坡的情况刚好相反.综合考虑各种因素,对于LMA-CN60轮轨接触副,1/40轨底坡较1/20轨底坡的接触状态更好.数值结果为轨底坡的设计提供重要的参考依据.