Static and cyclic creep behaviour of SiC whisker reinforced aluminium composite

Abstract

Static and cyclic creep tests were carried out in tension at 573–673 K on a 20 vol.-%SiC whisker reinforced aluminium (Al/SiC_w ) composite. The Al/SiC_w composite exhibited an apparent stress exponent of 18·1–19·0 at 573–673 K and an apparent activation energy of 325 kJ mol^-1 for static creep, whereas an apparent stress exponent of 19·6 at 623 K and an apparent activation energy of 376 kJ mol^-1 were observed for cyclic creep. A cyclic creep retardation (CCR) behaviour was observed for the Al/SiC_w composite. The steady state creep rate for cyclic creep was three orders of magnitude lower than that for static creep. Furthermore, the steady state creep rates of the composite tended to decrease continuously with increasing percentage unloading amount. The static creep data of the Al/SiC_w composite were rationalised by the substructure invariant model with a true stress exponent of 8 together with a threshold stress. The CCR behaviour can be explained by the storage of anelastic strain delaying non-recoverable creep during the onload cycles.     

Creep and microstructure in powder metallurgy 15 vol.% SiC<Subscript>p</Subscript>–2009 Al composite

The creep behavior and microstructure of powder metallurgy (PM) 15 vol.% silicon particulate-reinforced 2009 aluminum alloy (SiCp–2009 Al composite) and its matrix PM 2009 Al were investigated over six orders of magnitude of strain rate and at temperatures in the range 618–678 K. The results show that the creep behavior of PM 15% SiC_p–2009 Al composite resembles that of PM 2009 Al with regard to (a) the variations in both the apparent stress exponent and the apparent activation energy for creep due to applied stress, (b) the value of the true stress exponent, (c) the value of the true activation energy for creep, (d) the interpretation of creep in terms of a threshold stress, and (e) the temperature dependence of threshold stress. This resemblance implies that deformation in the matrix governs deformation in the composite. Analysis of the creep data in terms of creep rate against an effective stress shows that the creep behaviors of the composite and unreinforced alloy are consistent with the operation of viscous glide creep at low stresses. A comparison between the creep data of the composite and those of the unreinforced matrix revealed that the composite exhibited more creep-resistant characteristics than its matrix over the entire range of applied stresses.     

Creep behavior in an Al-Fe-V-Si alloy and SiC whisker-reinforced Al-Fe-V-Si composite

High temperature strengthening mechanisms in discontinuous metal matrix composites were examined by performing a close comparison between the creep behavior of 15vol. pct SiCw/8009Al and that of its matrix alloy, 8009Al. Both the alloy and composite exhibit a single-slope behavior with anomalously high values of apparent stress exponent and high apparent activation energy. The presence of SiC whiskers does not remarkably influence these two kinds of dependence of creep rates but reduces the creep rates by about two orders of magnitude. Transmission electron microscopy examination of the deformation microstructure reveals the occurrence of attractive dislocation/particle interaction. The creep data were analyzed by the threshold stress approach and by the dislocation-climb theories based on attractive interaction between dislocations and dispersoids. All data can be rationalized by a power-law with a stress exponent of 5 and a creep activation energy close to that for the self-diffusion in aluminum. The threshold stress decreases linearly with increasing temperature. General climb together with the attractive but not strong interactions between the dislocations and dispersoids is suggested to be the operative deformation mechanism. The contribution of SiC whiskers to the creep strength of 8009 Al composite can be evaluated quantitatively when the shear-lag model is applied. However, the effects of whisker length and whisker orientation distributions must be considered. Two probability density functions are used for modelling the distribution of whisker length and whisker orientation.     

超声化学原位合成纳米Al_2O_3/6063Al复合材料组织及高温蠕变性能

为研究纳米颗粒增强铝基复合材料的高温蠕变特性,基于6063Al-Al2(SO4)3体系,采用超声化学原位合成技术,制备出不同Al2O3体积分数(5%、7%)的纳米Al2O3/6063Al复合材料,通过高温蠕变拉伸试验测试其高温蠕变性能,利用XRD、OM、SEM及TEM分析其微观形貌。结果表明:施加高能超声可显著细化增强体颗粒并提高其分布的均匀性,所生成的Al2O3增强颗粒以圆形或近六边形为主,尺寸为20~100nm;纳米Al2O3/6063Al复合材料的名义应力指数、表观激活能和门槛应力值与基体相比大幅提高,均随着增强体体积分数的增加而提高,表明纳米Al2O3/6063Al复合材料的抗蠕变性能提高;纳米Al2O3/6063Al复合材料的真应力指数为8,说明复合材料蠕变机制符合微结构不变模型,即受基体晶格扩散的控制;纳米Al2O3/6063Al复合材料的高温蠕变断口特征以脆性断裂为主,高应力下形成穿晶断裂,低应力下形成沿晶断裂和晶界孔洞;纳米Al2O3/6063Al复合材料的主要强化机制为位错强化与弥散强化。     

Creep Behavior of Mullite Short Fiber Reinforced ZL109 Alloy Composites at High Temperature

The creep behavior of AI203.SIO2 fiber reinforced ZL109 composites has been investigated at four temperatures ranging from 553 to 623 K. The results show high stress exponent and highapparent creep activation energy. A good correlation between the normalized creep rate and normalized effective stress means that the true stress exponent of minimum creep strain rate of the composite is very close to 5, and the minimum creep strain rate is matrix lattice diffusion     

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。     

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。     

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.     

Static and cyclic creep behaviour of 2024/SiCp and its matrix alloy at high temperature

Abstract

The tensile creep and tensile-tensile cyclic creep behaviour of 2024/SiC_p composite and its matrix alloy have been investigated and analysed at high temperature. It was found that the creep threshold stress of the composite may not be caused by SiC_p alone: the matrix alloy also contributes to the threshold stress. The higher threshold stress of the composite compared with that of the matrix alloy can be explained in terms of load transfer in the composite and the value of threshold stress for the matrix alloy. A direct comparison between the composite and its matrix alloy indicates that only below a critical stress does the composite show a creep resistance higher than that of its matrix alloy. The two materials shown cyclic creep retardation in the tested stress range, cyclic creep showing a higher stress exponent and higher apparent activation energies in comparison with static creep. An analysis based on anelasticity is introduced to explain this result. The relationships between rupture lifetimes and applied stress, creep rate, and unloading amount show that the creep fracture mechanism is dominant in the present test condition.     

Creep behaviour of AA 6061 metal matrix composite alloy and AA 6061

Abstract

The tensile creep properties of a pure AA 6061 matrix and an AA 6061 matrix reinforced with 22% of irregularly shaped Al₂O₃ particles (metal matrix composite) are presented for a temperature of 573 K and initial stresses between 15 and 70 MPa (where 70 MPa is about one-half of the yield stress). The metal matrix composite (MMC) was fabricated by a stir casting process and both materials were extruded. All the specimens were overaged before testing. The MMC exhibits a higher secondary creep rate for the whole range of loads. A stress exponent of n ≈ 1 for stresses from 15 to 25 MPa for the unreinforced material indicates the dominating diffusional creep mechanism. A stress exponent of n ≈ 3 is found from 25 to 50 MPa concluding dominating dislocation creep for the unreinforced material. This mechanism is found to be dominating for the MMC from as low as 15 MPa to 50 MPa (n ≈ 3). Although the secondary creep rate of the reinforced samples is higher than that of the unreinforced, the exposure time is longer for the MMC at stress levels below 20 MPa. The transition between the secondary and the tertiary creep stage occurs earlier in the unreinforced material. Thus, the 1% creep limit of the unreinforced alloy is reached only in the tertiary creep stage, whereas it can be applied as a conservative design criterion for the composite in the whole stress range. Furthermore, the MMC promises at low stress levels higher creep lifetime than the unreinforced alloy. Creep damage in the tertiary stage of the MMC was found to be as a result of void nucleation resulting in particle decohesion from the matrix. Relatively high tertiary creep strains are produced by necking of the unreinforced samples.     

挤压管状SiCw/Al复合材料中残余应力及其高温松弛行为

利用X射线应力测量方法,研究挤态20vol%SiCw/6061Al复合材料中残余应力,发现复合材料中存在较大的残余应力,而且各方向的残余应力分布很不均匀,动态测量去应力退火期间复合材料残余应力的高温松弛过程,证实高温状态下残余应力按幂指数的方式发生松弛,基于蠕变机制,分析残余应力搞温松弛行为,结果表明复合材料应力指数及应力松弛激活能明显高于基体合金。     

Impression creep behavior of Al–1.9%Ni–1.6%Mn–1%Mg alloy

In the present study, microstructure and creep behavior of an Al–1.9%Ni–1.6%Mn–1%Mg alloy were studied at temperature ranging from 493 to 513 K and under stresses between 420 and 530 MPa. The creep test was carried out by impression creep technique in which a flat ended cylindrical indenter was impressed on the specimens. The results showed that microstructure of the alloy is composed of primary α(Al) phase covered by a mantle of α(Al)+Ni₃Al intermetallic compound. Mn segregated into Al_xMn_yNi_z or Al₆Mn phases distributed inside the matrix phase. It was found that the stress exponent, n, decreases from 5.2 to 3.6 with increasing temperature. Creep activation energies between 115 kJ/mol and 151 kJ/mol were estimated for the alloy and it decreases with rising stress. According to the stress exponent and creep activation energies, the lattice and pipe diffusion- climb controlled dislocation creep were the dominant creep mechanism.     

High Temperature Creep behaviour of a Si_3N_4 Whisker Reinforced Al-Fe-V-Si Composite

The creep behaviour of β-Si3N4 whisker reinforced Al-8.5Fe-1.3V-1.7Si composite has been investigated at the temperature 773 and 823 K. The results are characterized by high stress exponent and high apparent creep activation energy The creep data can be interpreted based on the incorporation of a threshold Stress and a load transfer coefficient into the power-law creep equation. A good correlation between the normalized creep rate and normalized effective stress is available which demonstrates that the creep behaviour of both the alloy and the composite is controlled by the matrix lattice self-diffusion in AI. EXamination on microstructure shows that edge dislocations exist at the interfaces between two adjacent whiskers and the intedeces emit edge dislocations in parallel paired-columns.     

Creep behavior of an Al-6061 metal matrix composite produced by liquid metallurgy processing

Constant stress creep tests were conducted on an Al-6061 metal matrix composite reinforced with alumina microspheres and produced using liquid metallurgy processing. By introducing a threshold stress into the creep analysis, it is concluded that creep occurs by viscous glide in the matrix with a stress exponent of ≈ 3 and an activation energy of ≈125 kJ mol⁻¹. The threshold stress is probably associated with the presence of fine spinel crystals which have been identified in the matrix of the composite.     

Compressive creep of Fe3Al-type iron aluminide with Zr additions

High-temperature creep of a Fe₃Al-type iron aluminide alloyed by zirconium was studied in the temperature range 873–1073 K. The alloy contained (wt.%) 31.5% Al, 3.5% Cr, 0.25% Zr, 0.19% C (Fe balance). It was tested in two states: (i) as received after hot rolling and (ii) heat treated (1423 K/2 h/air). Creep tests were performed in compression at constant load with stepwise loading: in each step, the load was changed to a new value after steady state creep rate had been established. Stress exponent and activation energy of the creep rate were determined and possible creep mechanisms were discussed in terms of the threshold stress concept. A rapid fall of the stress exponent and of the threshold stress with the increasing temperature indicates that creep is impeded by the presence of precipitates only at temperature 873 K. The results were compared with the results of long-term creep tests in tension performed recently on the same alloy. __________ Translated from Problemy Prochnosti, No. 1, pp. 117–120, January–February, 2008.     

Cyclic Creep Deformation and Fracture of a Cr-Mo Rotor Steel at Elevated Temperatures

Static creep and cyclic creep tests were carriedout on a Cr-Mo rotor steel from 0.5 to 0.6 T_m.Cyclic creep retardation occurred under the testconditions.With an increase of stress frequency,cyclic creep strain rate decreases and rupture timeincreases.The cyclic creep strain rate for the stresswave of a square shape is higher than that of a tri-angle shape.The apparent stress exponent of cycliccreep and the apparent activation energy of cycliccreep are both higher than those of static creep.Theminimum strain rate is inversely proportional torupture time for both static and cyclic creeps at dif-ferent stresses,temperatures,frequencies and waveshapes.The cyclic creep retardation mechanism wasexplored by the observation and analysis of the dis-location structure and fracture surface.     

Impression creep behaviour of magnesium alloy-based hybrid composites in the transverse direction

The creep behaviour of a creep-resistant AE42 magnesium alloy reinforced with Saffil short fibres and SiC particulates in various combinations has been investigated in the transverse direction, i.e., the plane containing random fibre orientation was perpendicular to the loading direction, in the temperature range of 175–300 °C at the stress levels ranging from 60 to 140 MPa using impression creep test technique. Normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at 175 °C at all the stresses employed, and up to 80 MPa stress at 240 °C. A reverse creep behaviour, i.e., strain rate increasing with strain, then reaching a steady state and then decreasing, is observed above 80 MPa stress at 240 °C and at all the stress levels at 300 °C. This pattern remains the same for all the composites employed. The reverse creep behaviour is found to be associated with fibre breakage. The apparent stress exponent is found to be very high for all the composites. However, after taking the threshold stress into account, the true stress exponent is found to range between 4 and 7, which suggests viscous glide and dislocation climb being the dominant creep mechanisms. The apparent activation energy Q_c was not calculated due to insufficient data at any stress level either for normal or reverse creep behaviour. The creep resistance of the hybrid composites is found to be comparable to that of the composite reinforced with 20% Saffil short fibres alone at all the temperatures and stress levels investigated. The creep rate of the composites in the transverse direction is found to be higher than the creep rate in the longitudinal direction reported in a previous paper.     

Effect of stress-temperature regimes on creep mechanisms in Ti-24Al-11Nb

The steady state creep rate (SSCR) of various microstructures of Ti-24Al-11Nb (a/o) has been determined. SSCR vs. stress and vs. temperature curves were determined to find Q_a, apparent creep activation energy, and to investigate n, power law stress exponent. At low stresses, apparent creep activation energies determined for all microstructures were found to be between 106 kJ mol⁻¹ and 156 kJ mol⁻¹, which agrees fairly well with the energy for self-diffusion in alpha titanium, and the energy of interdiffusion on the Al-rich side of the ₂ phase in Ti₃Al, both having a value of 150 kJ mol⁻¹. In many SSCR vs. stress curves, a slope change was observed in the stress range investigated. As temperature increased, slopes decreased towards unity, suggesting that different creep mechanisms, i.e. dislocation creep and diffusional creep, may be rate-controlling in different stress-temperature regimes.     

Creep rupture of a phenolic-alumina particulate composite

The creep and creep rupture behaviour of a phenolic-alumina particulate composite was determined in an aqueous environment. Flexural creep tests were carried out in which the loading-point displacement was measured as a function of applied stress and time. The material exhibits power-law creep behaviour in which the steady-state creep rate is a power function of the initial applied elastic stress. The creep exponent was found to be 5.3. The creep rupture behaviour can be explained using a modified Monkman-Grant relationship which provides a failure criterion that is independent of applied stress and stress state.     

20vol%体积分数纳米Al2O3颗粒增强铝基复合材料的高温压缩性能

为提高铝基材料的高温力学性能以满足其在573 K以上用于航空航天装备结构件的性能需求,采用高能球磨结合真空热压烧结工艺制备了体积分数高达20vol%的纳米Al₂O₃颗粒(146 nm)增强铝基复合材料,对其微观结构和高温压缩性能进行了研究。结果表明：纳米Al₂O₃颗粒均匀分散于超细晶铝基体中,且复合材料完全致密;该复合材料具有优异的高温压缩性能：应变速率为0.001/s时,473 K时压缩强度高达380 MPa,即使673 K时依然高达250 MPa,比其他传统铝基材料提高至少1倍;通过对其流变应力进行基于热激活的本构模型拟合可以发现,该复合材料具有高的应力指数(30)和表观激活能(204.02 kJ/mol)。这是由于高体积分数纳米颗粒能够有效钉扎晶界,并与铝基体形成热稳定的界面结合,显著提高复合材料的组织热稳定性,而且在变形过程中与晶界有效阻碍位错运动,显著提高复合材料的热变形门槛应力(在473～673 K时为190.6～328.4 MPa),其热变形过程可以由亚结构不变模型进行解释。