Effects of heat treatment on the mechanical properties at elevated temperatures of plain-woven SiC/SiC composites

The effects of heat treatment on the mechanical properties of plain-woven SiC/SiC composites at 927 °C and 1200 °C in argon were evaluated through tensile tests at room temperature and at elevated temperature on the as-received and heat-treated plain-woven SiC/SiC composites, respectively. Heat treatment can improve the mechanical properties of composites at room temperature due to the release of thermal residual stress. Although heat treatment can damage the fiber, the effect of this damage on the mechanical properties of composites is generally less than the effect of thermal residual stress. Heat treatment will graphitize the pyrolytic carbon interface and reduce its shear strength. Testing temperature will affect the expansion or contraction of the components in the composites, thereby changing the stress state of the components. This study can provide guidance for the optimization of processing of ceramic matrix composites and the structural design in high-temperature environments.     

Slag resistance mechanism of MgO–Mg2SiO4–SiC–C refractories containing porous multiphase aggregates

The slag resistance of MgO–SiC–C (MSC) refractories should be improved because of the mismatch in the thermal expansion coefficient between the aggregates and matrix, as well as the defects caused by the affinity between periclase and slag. In this study, MgO–Mg₂SiO₄–SiC–C (MMSC) refractories were prepared using porous multiphase MgO–Mg₂SiO₄ (M-M₂S) aggregates to replace dense fused magnesia aggregates. Compared to MSC, the slag penetration index of MMSC decreased by 43.5%. The structure of the porous aggregates increased the surface roughness, and the multiphase composition of the aggregates decreased the mismatch of the thermal expansion coefficient with the matrix, thus reducing debonding between the aggregates and matrix. The aggregates and matrix in the MMSC formed an interlocking structure, which bound them more tightly to improve the slag resistance. The slag viscosity at different depths from the initial slag/refractory interface was calculated using the Ribond model. The M-M₂S aggregates increased Si_xO_y^z− in the slag, which increased the slag polymerization and slag viscosity. The aggregates and matrix in the MMSC reacted with the slag to form high melting point phases, which reduced the channel of the slag. In addition, the penetration depth and velocity derived from the Washburn Equation were modified for the CaO–SiO₂–Al₂O₃–MgO–FeO slag and magnesia based refractory to accurately evaluate slag penetration.     

服役工况及喷丸强化对航空齿轮钢接触疲劳性能的影响

目的 针对不同服役工况和表面处理下航空齿轮钢接触疲劳性能不明确的问题,探究不同工况参数及喷丸强化对AISI 9310航空齿轮钢接触疲劳性能的影响。方法 基于双圆盘滚动接触疲劳试验台,对AISI 9310航空齿轮钢圆盘滚子试件在2种接触应力、3种滑差率的服役工况,以及渗碳磨削、喷丸强化2种工艺下开展滚动接触疲劳性能研究。使用白光干涉仪、X射线衍射应力仪和显微硬度仪对喷丸强化前后的表面形貌、残余应力和显微硬度进行表征,探究喷丸处理后的表面完整性与滚动接触疲劳性能之间的规律。结果 当接触应力由2.5 GPa增至3.0 GPa时,AISI 9310试件的滚动接触疲劳寿命降低了64.7%;当滑差率由10%增至20%、30%时,试件的滚动接触疲劳寿命分别下降了18.9%和42.8%,同时寿命分散性有所降低。此外,研究发现喷丸强化使试件表面残余压应力提升了104.3%,并形成了残余压应力层,表面显微硬度提升了4.8%,同时使试件的沟壑状刀痕转变为凹凸不平的弹坑,表面性状宽高比提升至0.931 0,更有利于润滑油的储存,从而使得试件的滚动接触疲劳寿命提升了87.8%。结论 获取了不同接触应力、滑差率和...     

Micromechanical model of time-dependent damage and deformation behavior for an orthogonal 3D-woven SiC/SiC composite at elevated temperature in vacuum

This paper presents a micromechanical model to predict the time-dependent damage and deformation behavior of an orthogonal 3-D woven SiC fiber/BN interface/SiC matrix composite under constant tensile loading at elevated temperature in vacuum. In-situ observation under monotonic tensile loading at room temperature, load–unload tensile testing at 1200 °C in argon, and constant load tensile testing at 1200 °C in vacuum were conducted to investigate the effects of microscopic damage on deformation behavior. The experimentally obtained results led to production of a time-dependent nonlinear stress–strain response model for the orthogonal 3-D woven SiC/SiC. It was established using the linear viscoelastic model, micro-damage propagation model, and a shear-lag model. The predicted creep deformation was found to agree well with the experimentally obtained results.     

Vibration based damage detection of rotor blades in a gas turbine engine

This paper describes the problems concerning turbine rotor blade vibration that seriously impact the structural integrity of a developmental aero gas turbine. Experimental determination of vibration characteristics of rotor blades in an engine is very important from fatigue failure considerations. The blades under investigation are fabricated from nickel base super alloy through directionally solidified investment casting process. The blade surfaces are coated with platinum aluminide for oxidation protection. A three dimensional finite element modal analysis on a bladed disk was performed to know the likely blade resonances for a particular design in the speed range of operation. Experiments were conducted to assess vibration characteristics of bladed disk rotor during engine tests. Rotor blade vibrations were measured using non-intrusive stress measurement system, an indirect method of blade vibration measurement utilizing blade tip timing technique. Abnormalities observed in the vibration characteristics of the blade tip timing data measured during engine tests were used to detect the blade damage. Upon disassembly of the engine and subsequent fluorescent penetrant inspection, it was observed that three blades of the rotor assembly were identified to have damaged. These are the blades that exhibited vibration abnormalities as a result of large resonant vibration response while engine tests. Further, fractographic analysis performed on the blades revealed the mechanism of blade failures as fatigue related. The root cause of blade failure is established to be high cycle fatigue from the engine run data history although the blades were put into service for just 6 h of engine operation.     

Modelling the effect of oxidation on the creep behaviour of fibre-reinforced ceramic matrix composites

A creep-oxidation model is presented for continuous fibre-reinforced ceramic matrix composites at high temperature. The model includes the effects of interface and matrix oxidation, creep of the fibres and degradation of fibre strength with time. In particular, the influence of the glassy phases resulting from the oxidation of certain types of SiC based matrices is discussed. Model predictions are presented for the case of a woven Hi-Nicalon™/SiC composite and compared to experimental results at 1000 and 1100 °C. The fraction of broken fibres increases with time in an accelerated manner as a result of load transfer and fibre degradation. The model also predicts that a fraction of broken fibres of about 15% triggers the unstable failure of the composite.     

高速高温条件下刷式封严环涂层与刷丝摩擦副磨损行为

目的 研究高速高温刮削条件下Ni Cr-Cr₂C₃涂层和SG37合金刷丝组成的摩擦副的磨损行为。方法 利用自制的高速高温刮擦试验机,在不同进给速度和过盈量条件下进行封严环涂层与刷丝摩擦副的高速高温刮削试验。用轮廓测量仪测量刷式封严环表面涂层的磨痕深度。用超景深显微镜(OM)和扫描电镜(SEM)观察涂层及刷丝样品的原始形貌和磨痕形貌,并利用配备的EDS能谱仪分析磨痕表面元素成分,同时分析了摩擦过程中的磨损机理。结果 NiCr-Cr₂C₃涂层与SG37A合金刷丝在高速高温刮擦过程中发生了磨粒磨损、黏着磨损和刷丝材料向涂层的转移。刷丝之间也存在摩擦变形和磨损,这主要是由于刷丝与刷丝在磨损过程中发生了高温变形和挤压,由于“拖尾”现象的存在,使刷丝尖端的变形和挤压更加明显,高温下金属流动性的增加进一步促进了菱形的形成。通过分析不同过盈量和进给速度对磨痕深度的影响,发现过盈量和进给速度均对涂层磨痕深度存在影响,随过盈量和进给速度的增加,涂层磨痕深度增大。结论 高速高温的苛刻工况下,刷丝在磨损过程中具有显著的变形...     

FGH4102粉末高温合金等温热模拟压缩试验及动态再结晶组织研究

本文研究了新型第四代粉末高温合金FGH4102在等温热模拟压缩过程中的组织演变,对γ′相在动态再结晶过程中的作用进行了探讨。结果表明,热等静压态合金在1060~1120℃温度范围变形时,热加工性能较好。1140℃变形后试样容易发生开裂,合金热加工性能较差。合金在γ+γ′两相区变形时均发生了不同程度的动态再结晶,再结晶晶粒尺寸远小于热等静压态的晶粒尺寸。变形过程中,尺寸较大的γ′相起到促进动态再结晶的作用。变形参数对动态再结晶的影响非常显著。低温高应变速率变形时,γ′相促进动态再结晶形核占主导地位,再结晶晶粒比较细小;高温低应变速率变形时,晶粒长大逐渐占据主导地位,再结晶晶粒尺寸较大。     

Microstructure response of Amosic-3 SiC/SiC composites under self-ion irradiation

Amosic-3 SiC/SiC composites were irradiated at 300 °C using 6 MeV Si ions to peak doses of 13 and 55 displacements per atom (dpa). The loss of amorphous carbon packets and the growth of SiC grains were simultaneously observed in Amosic-3 SiC fibers, using a combination of transmission electron microscopy (TEM) and Raman spectroscopy. A mechanism based on the grain growth theory was proposed to expound the relationship between the loss of carbon packets and the growth of SiC grains. Small and curved SiC grains can absorb surrounding carbon packets to grow themselves; at some point, these grains further grow at the expense of adjacent small SiC grains until their grain boundary became straight. TEM images were found to support the above mechanism.     

Design and realization of interface strengthening to GNPs/YSZ nanocomposite coating

Interfacial strengthening of ceramic coatings on metal surfaces is a goal that has been pursued by researchers in this field, but remains unsolved. In this work, the interface strengthening between ceramic coating and metal substrate was designed and successfully realized. GNPs were modified with silica-sol at the molecular scale to disperse in YSZ uniformly and the GNPs/YSZ nanocomposite coatings with high bonding strength were produced due to in-situ generation of interfacial carbides. Results show that the bonding strength of 9 wt% GNPs/YSZ nanocomposite coating increased by up to ∼54% compared to nanostructured YSZ coating without NiCrAlY bond-coat, and even 17% higher than that of nanostructured conventional type YSZ coating with NiCrAlY bond-coat. This technique could be applied to reinforce the interface between various materials, showing great application potential.