Cansas-Ⅱ SiCf/SiC复合材料的高温拉伸蠕变行为

连续碳化硅纤维增强碳化硅复合材料(SiC_f/SiC)是发展先进航空发动机的关键材料,航空发动机长时服役要求材料具有优异的高温蠕变性能。本工作研究了平纹编织Cansas-Ⅱ碳化硅纤维增强碳化硅复合材料(2D-SiC_f/SiC)在空气中的高温蠕变行为,蠕变温度为1200～1400℃,应力水平为80～140MPa。利用扫描电子显微镜(SEM)观察了2D-SiC_f/SiC复合材料的微观组织和断口形貌,使用能谱分析仪(EDS)进行了成分分析。结果表明:当蠕变应力低于比例极限应力(σ_PLS)时, 2D-SiC_f/SiC的蠕变断裂时间超过500h,稳态蠕变速率为1×10^–10～5×10^–10/s,蠕变行为由基体和纤维共同控制。当蠕变应力高于σ_PLS时,复合材料的基体、纤维和界面均发生氧化,蠕变断裂时间显著降低,稳态蠕变速率提高一个数量级,蠕变行为主要由纤维控制。     

化学气相渗透2D-SiCf/SiC复合材料的蠕变性能及损伤机理

研究了采用化学气相渗透工艺制备2D-SiC_f/SiC复合材料的真空蠕变性能, 蠕变温度为 1200、1300和1400 ℃, 应力水平范围为100~140 MPa。用扫描电子显微镜(SEM)和高分辨透射电子显微镜(TEM)分别观察分析了2D-SiC_f/SiC复合材料的蠕变断口形貌和微观结构。结果表明, 2D-SiC_f/SiC复合材料的主要蠕变损伤模式包括基体开裂、界面脱粘和纤维蠕变。桥接裂纹的纤维发生蠕变并促进了基体裂纹的张开、位移增大, 进一步导致复合材料蠕变断裂, 在复合材料蠕变过程中起决定性作用。2D-SiC_f/SiC复合材料的蠕变性能与SiC纤维微观结构的稳定性密切相关。在1200 ℃/100 MPa时, 纤维晶粒没有长大, 复合材料的蠕变断裂时间大于200 h; 蠕变温度为1400 ℃时, 纤维晶粒明显长大, 2D-SiC_f/SiC复合材料蠕变断裂时间缩短至8.6 h, 稳态蠕变速率增大了三个数量级。     

C/SiC复合材料应力氧化失效机理

研究了干氧和湿氧两种气氛、疲劳和蠕变两种应力下C/SiC复合材料在1300℃的应力氧化行为. 试验结果和断口形貌SEM分析表明: C/SiC复合材料在疲劳应力下比在蠕变应力下具有更强的抗氧化能力和更长的持续时间; 干氧环境中的蠕变试样以C纤维氧化失效为主; 水蒸气的存在加剧了SiC基体的氧化, 并且使受蠕变应力的C/SiC复合材料以SiC基体氧化失效为主.     

平纹编织SiCf/SiC复合材料的中温蠕变断裂时间及损伤机制

碳化硅纤维增强碳化硅复合材料(SiC_f/SiC)是制造下一代航空发动机热结构件的关键材料,中等温度(～800℃)下,SiC_f/SiC的蠕变断裂时间t_u显著下降。为此,研究了平纹编织SiC_f/SiC (2D-SiC_f/SiC)在空气中500～1 000℃的蠕变性能及损伤机制,应力水平为100～160 MPa。利用SEM、TEM和EDS分析了断口形貌、微观组织和化学成分。结果表明：2D-SiC_f/SiC的t_u与温度和应力水平有关。相同温度下,2D-SiC_f/SiC的t_u随着应力增加而变短。当温度为800℃、蠕变应力大于基体开裂应力(PLS)时,2D-SiC_f/SiC发生中温脆化现象,其t_u下降。2D-SiC_f/SiC的中温脆化机制为基体开裂、BN界面氧化和SiO₂替代BN界面导致的强界面/基体结合。2D-Si...     

裂解碳涂层对碳纤维增强碳化硅复合材料力学性能的影响

采用先驱体裂解－热压烧结方法制备出Cf/SiC复合材料,探讨了裂解碳涂层和烧结温度对复合材料纤维／基体界面和力学性能影响,烧结温度为1800度时,由于其中由富碳界面相构成的纤维／基体界面相使纤维／基体界面结合适中,具有较好的力学性能。     

不连续纤维增强Ti基复合材料蠕变形变与断裂

研究了ＴｉＣ颗粒和ＴｉＢ_２晶须增强的Ｔｉ－６Ａｌ－４Ｖ复合材料高温蠕变形变与断裂行为，ＴｉＣ增强的复合材料比基体合金的蠕变速率低一个数量级，ＴｉＢ_２增强的复合材料比基体合金低二个数量级。蠕变速率与应力的关系曲线呈两个阶段：在低应力阶段，蠕变应力指数为２～４，蠕变激活能为１２６～１８８ｋｌ／ｍｏｌ；在高应力阶段，应力指数为７～９，激活能为２４３～２７６ｋｌ／ｍｏｌ。复合材料与基体合金的应力指数和激活能相同。用透射电镜和扫描电镜研究了形变和断裂后的位错结构、裂纹和断口形貌。     

T300碳纤维热处理对Cf/SiC复合材料性能的影响

以聚碳硅烷先驱体浸渍裂解工艺制备T300碳纤维增强3D Cf/SiC复合材料,研究了T300碳纤维预先热处理对材料性能的影响.结果表明,热处理能够弱化Cf/SiC复合材料中纤维-基体界面结合,减少碳纤维在复合过程的损伤,显著提高复合材料性能.纤维经热处理后制备的Cf/SiC复合材料弯曲强度和断裂韧性分别从未经处理的154MPa,4.8MPa·m1/2提高到437MPa,20.4 MPa·m1/2.     

威碳纤维及其SiC基复合材料的性能分析

以聚碳硅烷(PCS)为先驱体,国产光威(Gw)碳纤维为增强体,采用先驱体浸渍-裂解工艺(PIP)制备了Cf/SiC复合材料.结果表明,所制备Gw碳纤维复合材料的力学性能优异,抗弯强度达到405.3MPa,断裂韧性15.7 MPa·m1/2.并对GW纤维制备复合材料的表面和断口进行了显微形貌分析,复合材料断口纤维拔出较多,Gw碳纤维在复合材料中很好地发挥了补强增韧作用.     

不同碳纤维表面状态对Cf/SiC复合材料性能的影响

采用XPS对两种不同的碳纤维表面进行了分析，制备了束丝Cf/SiC复合材料，并采用束丝拉伸强度表征其性能。结果表明，碳纤维表面主要有C、O两种元素存在，其中碳主要有C-C和C-O两种存在方式，并且两种纤维的Ols/Cls有明显的不同，当氧含量高时，纤维在经历高温处理后强度下降幅度较大，所制备的Cf/SiC复合材料性能较差。     

平面应力作用下复合材料的蠕变分析

本文对受平面应力作用下的复合材料的细观蠕变进行了研究,分析了纤维与基体的性能,纤维取向以及纤维体积百分含量等对复合材料蠕变性能的影响,从细观角度提出了控制复合材料蠕变的具体措施.     

Constitutive description of creep behavior of M-4Al-1Ca alloy

Creep behavior of an advanced magnesium alloy AX41 (4 wt.% Al, 1 wt.% Ca, Mg balanced) was investigated in temperature interval from 343 to 673 K and stresses from 2 to 200 MPa. Compressive creep experiments with stepwise loading were used in order to obtain stress dependence of the creep rate in interval from 10⁻⁹ to 10⁻³ s⁻¹ for a given temperature. All stress dependences can be well described by the Garofalo sinh relationship with natural exponent n = 5. An analysis of the parameters of this relationship has shown that lattice diffusion controls creep at all experimental conditions. While climb-controlled creep mechanism is decisive at lower stresses and higher temperatures, glide-controlled mechanisms act at higher stresses and lower temperatures. A typical power-law breakdown is observed at intermediate stresses and temperatures. __________ Translated from Problemy Prochnosti, No. 1, pp. 36–39, January–February, 2008.     

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).     

金属蠕变律及蠕变行为研究

蠕变过程中 ,材料内部状态的不断演化 ,使得材料的蠕变行为发生改变。本文提出考虑损伤和硬化影响的蠕变律。利用该蠕变律讨论了 12 Cr1Mo V钢蠕变行为。分析结果表明 ,ε·c在蠕变过程中始终变化 ,第二阶段仅仅是ε·c相对稳定的阶段 ,其相对稳定程度和持续范围与载荷大小有关。在相同寿命分数下 ,不同应力水平引起的硬化状态也不相同     

Influence of bonding on the tensile creep behavior of paper in a cyclic humidity environment

It has been shown, as paper structure is improved through increased bonding (by increasing relative bonded area or specific bond strength), a fully efficient loaded structure can be achieved. Once fully efficient, further improvements in bonding become redundant and have no effect on some paper deformation behaviors; deformation is dictated only by the characteristics of the fibers. Although previous work had shown this was true for elastic modulus and short time duration stress-strain behavior, it has only recently been shown to be true for constant humidity tensile creep behavior. In this study, the goal was to ascertain if cyclic humidity tensile creep behavior (known as accelerated creep) would follow the same trend. To accomplish this, sheets were made at differing levels of relative bonded area and specific bond strength. This was done by applying two different wet pressing levels (to alter relative bonded area) and using bonder and debonder (to change specific bond strength). It was found that accelerated creep behavior of paper sheets is no different than constant humidity creep behavior; changing bonding does not influence accelerated creep if the sheet has a fully efficient loaded structure. If the sheet structure is inefficiently loaded (there is no redundancy in bonding), accelerated creep will be affected by bonding. However, it is proposed that the only reason accelerated creep is influenced by bonding when inefficiently loaded is because constant humidity creep behavior determines the accelerated behavior and it is influenced, in this case, by bonding.     

Proposal of a new concept on creep fracture remnant life for a precracked specimen

Abstract

The creep life time of a smooth specimen can be predicted using existing laws for creep deformation and steady state creep rate. When crack growth behaviour is involved, it is necessary to construct a law of creep crack growth rate to predict creep fracture life. Creep fracture life can be measured by integrating the law of creep crack growth rate. One example is the creep crack growth rate, represented by the parameter Q*. In this study, we investigated the applicability of this prediction method to creep fracture remnant life for a cracked specimen. The Ω criterion is proposed to predict creep fracture remnant life for a smooth specimen for creep ductile materials. In this study, the correlation between Q*_L derived from the paremeters Q* and Ω is investigated. The correlation between Q_L* and Ω provided a unified theoretical prediction law of creep fracture remnant life for high-temperature creep-ductile materials in the range from smooth to precracked specimens.     

循环频率对SiCw／6061Al复合材料高温循环蠕变的影响

在测试应力范围内，SiCW／6061Al复合材料在196℃显示循环蠕变加速而在350℃显示循环蠕变减速在0～0.5Hz范围内，随着频率的增加，温度较低时循环蠕变应力指数增加而激活能降低，温度较高时应力指数与激活能同时增加当频率增加到1Hz时则偏离上述趋势     

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.     

A three dimensional view of high temperature creep damage

Abstract

A three dimensional view of creep voids in a hydrogen reformer tube is presented. By separating and reconstructing various microstructural features present, the proper representation of creep voids in 3D can be fully studied. Useful measurements of parameters such as void volume, void-to-void distance, and grain boundary angles were obtained. The data presented here represent the initial collection of creep void information for use in various creep void nucleation and growth models. Additional data are currently being collected for material subjected to different service conditions.     

Measurements of Creep Internal Stress Based on Constant Strain Rate and Its Application to Engineering

This research is carried out on the basis of Constant Strain Rate （CSR） to measure creep internal stress. Measurements of creep internal stress are conducted on the material test machine by using the CSR method. A mathematical model of creep internal stress is also proposed and its application is presented in this paper.