C/SiCN复合材料的疲劳行为

研究了碳纤维增强非晶SiCN陶瓷基复合材料(C/SiCN)的真空拉-拉疲劳行为,对比分析了预稳定化热处理C/SiCN(HTCS)与未热处理C/SiCN(NHTCS)在疲劳后的质量、显微结构和电阻变化。结果表明,对于NHTCS试样,非晶SiCN基体的晶化造成大量气体挥发,质量损失明显;显微组织结构变化表现为基体脱落、纤维拔出、纤维断裂、裂纹扩展直至失效;电阻变化率先降后升,变化规律与材料的组织结构变化密切相关。对于预稳定化处理的HTCS试样,其质量变化、电阻变化的程度相对较小,体现出较好的预稳定化效果。     

锰对ZA27合金压缩蠕变的影响

为了解质量分数为0.5%Mn对铸态ZA27合金压缩蠕变行为的影响,采用自制的试验装置研究了ZA27-Mn合金和ZA27合金第一阶段的压蠕变量和稳态蠕变速率.结果表明:ZA27-Mn合金第一阶段的压蠕变量和稳态蠕变速率低于ZA27合金,而两种合金的压蠕变均符合于lnt=C-nlnσ Q/RT,ZA27-Mn合金的应力指数(n)和蠕变激活能(Q)分别为3.83和82.76 kJ·mol-1,而ZA27合金的应力指数和蠕变激活能分别为3.46和81.09kJ·mol-1,材料结构常数C不同,合金压蠕变不同,ZA27-Mn合金的压蠕变抗力高于ZA27合金;加入0.5%Mn对ZA27合金蠕变机制并无影响,均是锌的点阵自扩散和位错的攀移机制.     

奥氏体耐热钢Sanicro25蠕变行为和断裂特征

用OM、SEM和TEM等方法研究了超超临界电站用Sanicro25钢的蠕变机制。结果表明，这种钢的最小蠕变速率随着温度的升高和应力的增大而提高。根据最小蠕变速率特征得出表观应力指数为7.6～8.2，表观激活能为496.7～531.8 kJ/mol。在蠕变过程中在晶内弥散析出的纳米级Cu-rich相和MX相阻碍位错运动，导致蠕变门槛值应力的出现。用线性外延法求出的门槛值应力，随着温度的升高而减小。用门槛值将蠕变本构方程修正为■，可将不同温度下的最小蠕变速率归一化；同时确定真实应力指数(n=5)和真实表观激活能(Q=286.6 kJ/mol约等于γ-Fe自扩散激活能)，从而判别出实验参数下材料的蠕变机制为点阵自扩散协助的位错攀移。     

V替代Nb对FINEMET非晶合金结晶动力学的影响

利用标准的单辊甩带技术在大气环境下制备了Fe73.5Si13.5B9Cu1Nb3-xVx(x=1,2)非晶条带。利用非等温DSC测量研究了非晶条带的初始结晶动力学行为。采用Kissinger方法计算了非晶条带的初始结晶激活能,其数值分别是302kJ/mol(x=1)和364kJ/mol(x=2)。Avrami因子n的计算结果分别为2.35(x=1)和1.61(x=2),Avrami因子的计算结果表明,非晶条带非等温初始结晶的机理为扩散控制的低维生长,且其形核率不断降低。在793、823和853K下分别对非晶条带进行真空等温退火1h,从而在非晶基体中形成纳米晶相。XRD分析结果表明,823和853K下真空等温退火1h后,x=1和2非晶条带中析出的α-Fe(Si)相的平均晶粒尺寸分别为12.8、14.0、13.3和14.2nm。     

真空气氛下非晶硅碳氮(SiCN)陶瓷的高温晶化行为

以六甲基二硅氮烷为单一前驱体,采用电热裂解化学气相沉积技术制备了SiCN陶瓷. 借助X射线衍射仪、透射电子显微镜研究了真空环境中非晶SiCN陶瓷在1300～1900℃范围内的晶化行为,并根据研究结果,运用分解-结晶机理解释了其晶化过程. 非晶SiCN陶瓷在低于1300℃开始发生分解,形成富Si-C区域,并最终发生β-SiC结晶. 其结晶度随热处理温度的升高而愈加明显.在1700℃处理时发生β-SiC→α-SiC相变. 分解形成的N-难以与Si-结合形成富Si-N区域,最终以N₂形式溢出,在整个热处理温度范围没有出现氮气下热处理时存在的Si₃N₄结晶.     

Nd2Fe14B/(α-Fe,Fe3B)复合纳米晶的生长动力学研究

采用X射线衍射分析(XRD)研究了Fe-Co-Nd-Dy-B非晶合金晶化过程中α-Fe、Fe3B纳米晶的生长动力学.根据纳米晶生长达到稳定状态所需的时间常数tE与退火温度Ta的关系,计算了α-Fe和Fe3B纳米晶的生长激活能为Egα-Fe=95±2 kJ/mol和E Fe3B=133±13 kJ/mol.该值远小于α-Fe和Fe3B两相的表观晶化激活能Ecα-Fe=555 kJ/mol和EcFe3B=481 kJ/mol.这表明Fe-Co-Nd-Dy-B非晶合金晶化过程中α-Fe和Fe3B纳米晶的形成主要由成核所控制.     

电弧堆焊铁基非晶/纳米晶复合涂层的组织及性能研究

以铁基非晶合金Fe41Co7Cr15Mo14C15B6Y2作为焊芯制备低氢型非晶堆焊焊条,利用手工电弧堆焊,调控堆焊工艺参数,在Q235钢上制备两种不同非晶/纳米晶组分的复合堆焊层。利用XRD/SEM/TEM探索不同堆焊工艺下的结构组织演变及非晶/纳米晶的组成比例变化,研究了不同比例非晶/纳米晶复合堆焊层的晶化特征、硬度和耐磨性变化。实验结果表明,堆焊层为铁基非晶/纳米晶复合涂层,与基体达到了良好的冶金结合;涂层中非晶相含量最高可达47.44%,纳米晶粒尺寸为10~48 nm,堆焊层的最高硬度达1 226HV1,其耐磨性可达Q235钢的8倍;两组堆焊层的晶化激活能分别为Ex(150 A)=107.476 kJ/mol,Ex(160A)=58.104 kJ/mol;随着堆焊热输入的增加,堆焊层中非晶相的含量降低,纳米晶粒尺寸增大,堆焊层的晶化温度、热稳定性、硬度和耐磨性有所降低。     

低频脉冲磁场处理非晶Fe78Si9B13合金纳米晶化的DSC研究

用低频脉冲磁场处理非晶Fe78Si9B13合金,采用差示扫描热分析(DSC)的方法测定了不同升温速率下非晶Fe78Si9B13合金的DSC曲线,利用Kissinger方程计算出了晶化过程的激活能.结果表明,晶化激活能由处理前的433.6kJ/mol降到200kJ/mol以下,提高了晶核形核速率,抑制了晶核长大速率,从而实现了非晶合金的低温纳米晶化.     

多元纳米复合氧化锌压敏陶瓷的晶粒生长

研究了多元纳米复合ZnO电压敏粉体在高温下的烧结行为.应用晶粒生长的动力学方程Gn-G0=K0texp(-Q/RT),确定了晶粒生长的动力学指数n和激活能Q.实验结果表明,随着烧结温度的提高和保温时间的延长,ZnO压敏陶瓷的晶粒不断长大,其动力学指数n=3.2,激活能Q=(185±28)kJ/mol.     

Fe83Zr7B9Mn1非晶合金中α-Fe纳米晶化的HRTEM和结晶动力学研究

通过高分辨透射电子显微镜(HRTEM)和差示扫描量热法(DSC)研究了Fe83Zr7B9Mn1非晶合金中α-Fe纳米晶的结晶过程和非等温结晶动力学。通过HRTEM观察和X射线衍射分析,确定结晶析出相是α-Fe,在经过903K热处理之后,合金内部晶粒尺寸长大至20nm以上。对重叠放热峰进行了分峰拟合。利用Flynn-Wall-Ozawa方法确定了结晶过程的表观激活能Ea=260.3kJ/mol。进一步研究发现Fe83Zr7B9Mn1非晶合金中α-Fe纳米晶非等温结晶过程不适合用Johnson-Mehl-Avrami(JMA)模型描述,而可以利用esták-Berggren自催化模型进行描述。通过计算,α-Fe纳米晶结晶过程的指前因子以及转化函数分别为:lnA=30.6以及f(α)=α0.37(1-α)1.08。     

Primary and Anelastic Creep of a Near α-Ti Alloy and Their Dependencies on Stress and Temperature

The primary creep behaviour of a high temperature near -Ti alloy Ti6242Si has been investigated in the temperature range from 500 to 625°C, and the stress range from 80 to 450 MPa. The results are analysed in terms of the dependencies of stress on strain (strain hardening) and on strain rate (strain rate sensitivity). Furthermore, full unloading experiments were conducted in order to gain additional information as to the nature of primary creep. It is shown that primary creep can be described by an athermal component, strain hardening, with a mean strain hardening coefficient of 0.37, and a thermally activated component, strain rate sensitivity, with a strain rate sensitivity coefficient suggesting a mechanism based on climb controlled recovery. This is confirmed by the activation energy of 259 kJ/mol determined at different stresses, which is similar to the activation energy of Ti self diffusion in -Ti. The anelastic strain obtained on full unloading was analysed in its fast stage in a similar way. The kinetics of anelastic creep and its activation energy are in many aspects very similar to those of primary creep. It is thought that, in the stress and temperature range investigated, primary creep is to a relatively high extent anelastic in nature, and is controlled by the climb controlled bow out of pinned dislocation segments, particularly dislocations pinned at lath boundaries.     

Creep behaviour of carbon containing Fe3Al alloy

Abstract

Tensile creep of a Fe–16 wt-%Al–0·5 wt-%C alloy was investigated over a temperature range of 773 to 873 K and stress range of 80 to 200 MPa. Creep curves exhibited very limited primary and secondary creep regimes. An extended tertiary creep regime was observed for all the test conditions. Stress dependence of minimum creep rate can be represented by a power-law equation with stress exponents being in the range 4 to 5. The activation energy for creep was found to be ～340 kJ mol^?1. The observed stress exponent and activation energy for creep suggest that creep is controlled by dislocation climb. Creep fracture in Fe₃Al–C alloy is predominantly by transgranular ductile mode by nucleation, growth and coalescence of microvoids formed at FeAlC_0·5 particle/matrix interface by decohesion as well as fracture of elongated particles. Extended tertiary creep observed in the alloy was analysed in the light of the mechanisms proposed for nickel based superalloys.     

原位合成TiB2/ ZL109 复合材料的高温蠕变行为

采用原位合成方法制备了TiB₂ 超细颗粒增强ZL109 复合材料, 对材料进行了高温拉伸蠕变实验。实验结果表明, 复合材料在高温恒应力条件下, 表现出高的名义应力指数和高的名义蠕变激活能, 优于纯Al 和ZL109 合金, 而且比常规外加颗粒复合材料具有更好的高温蠕变性能。引入门槛应力概念, 复合材料的蠕变实验结果能够用微观结构不变模型来解释, 说明复合材料的蠕变受到基体点阵扩散的控制。复合材料的蠕变断裂行为可以用Monkman2Grant 经验公式来描述, 蠕变断裂特征为延性断裂。     

Impression creep behaviour of TiB2 particles reinforced steel matrix composites

Impression creep behaviour of the powder metallurgy processed steel matrix composites was investigated under constant stress at different temperatures in the range of 873–973?K. By using the power-law relationship, the estimated activation energy for unreinforced steel was found to be 149?kJ?mol^?1 and steel reinforced with 2 and 4?vol.-% TiB₂ was found to be 298 and 338?kJ?mol^?1, respectively indicating better creep resistance of the reinforced steel matrix composites. Dislocation creep is the dominant creep mechanism based on the calculated values of stress exponent and activation energy. Hence, this method can be used to assess the potential of steel matrix composites for use as structural materials for high-temperature application.     

Creep behaviour of AISI 316H stainless steel under stress-varying creep loading conditions: primary creep regeneration

Primary creep regeneration (PCR) is an important reported observation from creep under stress-varying conditions for several alloys. For a specimen deforming in the secondary creep regime, a stress reversal leads to an enhanced creep rate upon reloading due to reactivation of the primary creep regime (i.e. PCR). This paper focuses on an investigation of the PCR phenomenon during stress-varying creep loading for AISI 316H stainless steel at 650°C. The experimental observations clarify the influence of different parameters (e.g. forward creep stress level, reverse stress magnitude and forward and reverse accumulated inelastic strain) on the extent of PCR activation. In addition, a correlation between the extent of PCR activation and inelastic strain accumulation during the reverse loading period was found, which was employed to develop an empirical–phenomenological model for prediction of the creep behaviour of the alloy after stress transients (e.g. stress reversals).     

Creep Behavior of As-Cast Ti-48Al-2Cr Intermetallic Alloy for Aerospace and Automotive Applications

The creep properties of as-cast Ti-48Al-2Cr (at%) alloy which had been strengthen with addition of 2 at% Cr were investigated. Tensile creep experiments were performed in air at temperatures from 600-800°C and initial stresses ranging from 150 to 180 MPa. Stress exponent and activation energy were both measured. Data indicates that the alloy exhibits steady state creep behavior and the steady state creep rate is found to depend on the applied load and temperature. The measured power law stress exponent for steady state creep rate is found to be close to 3 and the apparent activation energy for creep is calculated to be 15.7 kJ/mol. The creep resistance of the present alloy was also compared with binary Ti-48Al (at%) to evaluate the effect of Cr addition on creep resistance of TiAl. It is concluded that addition of 2 at% of Cr does not have significant effect on the creep resistance of TiAl.     

Initial nucleation of amorphous Si–B–C–N ceramics derived from polymer-precursors

Nucleation behavior of amorphous Si–B–C–N ceramics derived from boron-modified polyvinylsilazane procusors was systematically investigated by transmission electron microscopy(TEM) combined with spatially-resolved electron energy-loss spectroscopy(EELS) analysis. The ceramics were pyrolyzed at1000?C followed by further annealing in N2, and SiC nano-crystallites start to emerge at 1200?C and dominate at 1500?C. Observed by high-angle annular dark-field imaging, bright and dark clusters were revealed as universal nano-structured features in ceramic matrices before and after nucleation, and the growth of cluster size saturated before reaching 5 nm at 1400?C. EELS analysis demonstrated the gradual development of bonding structures successively into SiC, graphetic BNCxand Si_3N_4 phases, as well as a constant presence of unexpected oxygen in the matrices. Furthermore, EELS profiling revealed the bright SiC clusters and less bright Si_3N_4-like clusters at 1200–1400?C. Since the amorphous matrix has already phase separated into SiCN and carbon clusters, another phase separation of SiCN into SiC and Si_3N_4-like clusters might occur by annealing to accompany their nucleation and growth, albeit one crystallized and another remained in amorphous structure. Hinderance of the cluster growth and further crystallization was owing to the formation of BNCxlayers that developed between SiC and Si_3N_4-like clusters as well as from the excessive oxygen to form the stable SiO_2.     

Al_2O_3-SiO_(2(sf))/AZ91D镁基复合材料蠕变本构方程

利用常应力拉伸蠕变试验法对体积分数为25%的硅酸铝短纤维(Al2O3-Si O2(sf))增强AZ91D镁基复合材料及其基体合金AZ91D在温度为473 K和573 K、外加应力为30～100 MPa下进行蠕变测试。根据应变和应变速率曲线,计算出复合材料的真应力指数、真蠕变激活能、真门槛应力、载荷转移因子和蠕变本构方程。TEM分析结果表明,复合材料蠕变后的门槛应力来源于短纤维表面上的MgO颗粒和Mg17Al12析出相对可动位错的钉扎作用,短纤维具有承载和传递载荷作用,从而提高了复合材料的抗蠕变性能。     

Effects of heat treatments on the creep properties of a hot pressed silicon nitride ceramic

Effects of heat treatment in an argon atmosphere at high temperatures for varying times on the creep properties of a Y₂O₃-Al₂O₃ (8-2 wt%) doped hot pressed silicon nitride (HPSN) ceramic were investigated. It was observed from the creep measurements that higher temperature, i.e. 1360C, and longer time, i.e. 8 h, heat treatment in an argon atmosphere improved the creep properties, (e.g. secondary creep rate) of this material. Heat treatment at a lower temperature of 1300C and for a shorter time of 4 h did not change the creep behaviour. Improvement of the creep properties was related to the crystallization of an amorphous grain boundary phase by heat treatment. Secondary creep rate parameters of the as-received material: stress exponent, n (2.95–3.08) and activation energy, Q (634–818 kJ mol^S–1), were in the range of values found by other investigators for various hot pressed silicon nitride ceramics.     

Creep of polymer composites

A model incorporating a modified thermal activation theory is presented to model and predict creep of polymer composites. Results are presented of the successful application of this model to predict creep of a unidirectional, continuous-carbon-fiber-reinforced polymer composite (AS4/3501-6) and its epoxy matrix, over a wide range of stress (10–80% of ultimate tensile strength) and temperature (295–433 K). From an analysis of model parameters, it is concluded that the reinforcing carbon fibers do not alter the creep mechanism but do alter the creep behavior of the epoxy matrix, resulting in reductions in creep rate and in the magnitude of creep.