Thermal Expansion of Al Matrix Composites Reinforced with Hybrid Micro-Aiano-sized Ai2O3 Particles简

The thermal expansion behavior of aluminum matrix composites reinforced with hybrid （nanometer and micrometer） Al2O3 particles was measured between 100 and 600℃ and compared to theoretical models. The results revealed that the nanoparticle concentration had significant effect on the thermal expansion behavior of the composites. For the composites with lower nanoparticle concentration, their coefficient of thermal expansion （CTE） is determined by a stress relaxation process. While for the composites with higher nanoparticle concentration, their CTE is determined by a percolation process.     

Thermal Expansion of Al Matrix Composites Reinforced with Hybrid Micro-/nano-sized Al_2O_3 Particles

The thermal expansion behavior of aluminum matrix composites reinforced with hybrid(nanometer and micrometer) Al_2O_3 particles was measured between 100 and 600℃ and compared to theoretical models.The results revealed that the nanoparticle concentration had significant effect on the thermal expansion behavior of the composites.For the composites with lower nanoparticle concentration,their coefficient of thermal expansion(CTE) is determined by a stress relaxation process.While for the composites with higher nanoparticle concentration,their CTE is determined by a percolation process.     

Ablative and Mechanical Properties of C/C—ZrC Composites Prepared by Precursor Infiltration and Pyrolysis Process

C/C-ZrC composites with continuous ZrC matrix were prepared by precursor infiltration and pyrolysis process using zirconium-containing polymer.Ablation properties of the composites were investigated by oxyacetylene flame with heat flux of 2380 and 4180 kW/m~2,respectively.The results showed that C/C-ZrC composites exhibited excellent ablation resistance under the heat flux of 2380 kW/m~2 for 120 s and a tree-coral-like ZrO_2protective layer formed after ablation.However,when the heat flux increased to 4180 kW/m~2,the maximum temperature of ablated surface reached 2500 ℃ and a strong degradation of ablation resistance was observed due to the weak bonding between the formed ZrO_2 layer and the composites.The flexural strength of C/C-ZrC composites was 110.7 ± 7.5 MPa.There were a large number of carbon fiber bundles pull-out,and the composites exhibited a pseudo-plastic fracture behavior.     

Thermal Shock Behaviour of Alumina—Iron Composites

Thermal shock behaviour was investigated for two morphologically different composites comprising an alumina matrix and 20 vol.pct Fe particles for a wide range of quenching temperature differences(ΔT=100-800℃)and compared to a monolithic alumina.The retained strength and critical quenching temperature difference,ΔTc,of the two composites were a significant improvement over the values for the respective monolithic alumina.Crack lengths and densities were shown to be greater for the alumina than for the two composites at all quenching temperature differences .The thermal shock resistance parameters for monolithic alumina and the two composites were calculated according to their mechanical and physical properties.The calculated results agree well with the experimental one and indicate possible explanations for the differences in thermal shock behaviour.     

Influence of cryogenic thermal cycling treatment on the thermophysical properties of carbon/carbon composites between room temperature and 1900℃

Influence of cryogenic thermal cycling treatment(from-120℃ to 120℃ at 1.3 × 10~(-3) Pa) on the thermophysical properties including thermal conductivity(TC), thermal diffusivity(TD), specific heat(SH) and coefficient of thermal expansion(CTE) ranging from room temperature to 1900℃ of carbon/carbon(C/C)composites in x-y and z directions were studied. Test results showed that fiber/matrix interfacial debonding, fiber pull-out and microcracks occurred after the cryogenic thermal treatment and they increased significantly with the cycle number increasing, while cycled more than 30 times, the space of microdefects reduced obviously due to the accumulation of cyclic thermal stress. TC, TD, SH and CTE of the cryogenic thermal cycling treated C/C composites were first decreased and then increased in both directions(x-y and z directions) with the increase of thermal cycles. A model regarding the heat conduction in cryogenic thermal cycling treated C/C composites was proposed.     

Mechanical Properties and Fracture Behavior of Mg-Al/AlN Composites with Different Particle Contents

In this study, magnesium matrix composites reinforced with different loading of AlN particles were fabricated by the powder metallurgy technique. The microstructure, bending strength and fracture behavior of the resulting Mg-Al/Al N composites were investigated. It showed that the 5 wt% AlN reinforcements led to the highest densification and bending strength. The total strengthening effect of AlN particles was predicted by considering the contributions of CTE mismatch between the matrix and the particles,load bearing and Hall–Petch mechanism. The results revealed that the increase of dislocation density,the change of Mg17Al12 phase morphology, and the effective load transfer were the major strengthening contributors to the composites. The fracture of the composites altered from plastic to brittle mode with increasing reinforcement content. The regions of clustered particles in the composites were easy to be damaged under external load, and the fracture occurred mainly along grain boundaries.     

SiCp/Al Composites Fabricated by Modified Squeeze Casting Technique

A cost effective method was introduced to fabricate pure aluminum matrix composites reinforced with 20% volume fraction of 3.5 μm SiC particles by squeeze casting followed by hot extrusion. In order to lower volume fraction of the composites, a mixed preform containing pure aluminum powder and the SiC particles was used. The suitable processing parameters for the infiltration of pure aluminum melt into the mixed preform are： melt temperature 800℃, preform temperature 500℃, infiltration pressure 5 MPa, and solidification pressure 50 MPa. Microstructure and properties of the composites in both as-cast and hot extruded states were investigated. The results indicate that hot extrusion can obviously improve the mechanical properties of the composite.     

Oxidation and Thermal Shock Behavior of a Glass-Alumina Composite Coating on K38G Superalloy at 1000℃

The glass-alumina composite coatings were successfully prepared on the K38G superalloy substrates.Their isothermal oxidation and thermal shock behavior at 1000 ℃ were characterized.With a post-annealing process at 850 ℃,the composite coatings possessed an improved protective effect for the alloy substrates from isothermal oxidation and a higher resistance to thermal shock.Crystallization from the glass matrix and interfacial reaction between the matrix and alumina inclusions,which caused the composites more refractory and tough,accounted for this improvement.The micromechanisms for the formation of oxidation results of spinel ZnCr_2O_4 were also discussed.     

Thermal—Mechanical Fatigue Behavior and Life Analysis of Cast Ni—base Superalloy K417

In-phase(IP) and out-of-phase(OP)thermal-mechanical fatigue(TMF) behavior of cast Ni-base superalloy K417 was studied.All experiments were carried out under total strain control with temperature cycling between 400-850℃.Both in-phase and out-of-phase TMF specimens exhibited cyclic hardening followed by cyclic softening at the minimum temperature.Besides,they cyclically hardened in the early stage of life followed by cyclic softening at the minimum temperature.Besides,they cyclically hardened in the early stage of life followed by cyclic softening at the maximum temperature.OP TMF life was longer than of IP TMF.Various damage mechanisms operating in different controlled strain ranges and phasing were discussed.A few life prediction methods for isothermal fatigue were used to handle TMF fatigue and their applicability to superalloy K417 was evaluated.The SEM analysis of the fracture surface showed that transgranular fracture was the principal cracking mode for both IP and OP TMF.Oxidation was the main damage mechanism in causing shorter fatigue life for IP TMF compared with OP TMF.     

Experimental Study on the Uniaxial Cyclic Deformation of 25CDV4.11 Steel

The strain cyclic characteristics and ratcheting behavior of 25CDV4.11 steel were studied by the experiments under uniaxial cyclic loading with relatively high cyclic number and at room temperature. The cyclic hardening/softening feature of the material was first observed under the uniaxial strain cycling with various strain amplitudes. Then, the ratcheting behavior of the material was researched in detail, and the effects of stress amplitude and mean stress on the ratcheting were discussed under uniaxial asymmetrical stress cycling. Comparing with the experimental results of SS316L stainless steel, it is concluded that the material exhibits remarkable cyclic softening feature, and then a special ratcheting behavior is caused. Some conclusions useful to establish corresponding constitutive model are obtained.     

金属基复合材料的热循环行为

本文参阅了大量文献，对国外金属基复合材料的热循环行为的研究现状进行了综述与评价，主要讨论了热循环对复合材料的性能与结构的影响，并初步探讨了热循环造成破坏的机制。     

Thermal cycling and isothermal oxidation behavior of quadruple EB‐PVD thermal barrier coatings

Increase of energy efficiency by increasing the turbine inlet temperature is the main driving force for further investigations regarding new thermal barrier coating materials. Today, thermal barrier coatings consisting of yttria stabilized zirconia are state of the art. In this study, thermal barrier coatings consisting of 7 weight percent yttria stabilized zirconia (7YSZ) and pyrochlore lanthanum zirconate (La₂Zr₂O₇) were deposited by electron beam physical vapor deposition. Regarding thermal cycling and isothermal oxidation behavior different layer architectures such as mono‐, double‐ and quadruple ceramic layers were investigated. The thermal shock behavior was examined by thermocycle tests at temperatures in the range between T = 50 °C ‐1,150 °C. Additionally, the isothermal oxidation behavior at a temperature of T = 1,150 °C with dwell times of t= 50 h and t = 100 h was studied in the present work. The conducted research concerning the behavior of various thermal barrier coating systems under thermal cycle and isothermal load highlights the potential of multilayer thermal barrier coatings for operating in high temperature areas.     

Thermal fatigue behavior of squeeze cast, discontinuous alumina-silicate fiber-reinforced aluminum alloy (A356) composite

Microstructural damage mechanisms owing to thermal cycling and isothermal exposure at elevated temperature are studied for a short alumina-silicate fiber-reinforced aluminum alloy (A356) composite produced by pressure casting. The tensile strength of the metal matrix composite is found to degrade considerably in each case. An X-ray double-crystal diffraction method is employed to study the mechanisms of recovery in the matrix. The fractal dimension of the X-ray “rocking curves” for individual grains in the composite reflects the substructure formation owing to the rearrangement of dislocations into subdomain walls. Recovery by polygonization is more pronounced in the case of thermal cycling than for equivalent isothermal exposure. The residual stresses in the matrix that provide the fiber clamping force undergo more relaxation in the case of isothermal exposure. The two competing damage mechanisms, thermally activated recovery by polygonization and relaxation of clamping stresses in the matrix, result in identical strength degradation (25%) for both thermal cycling and isothermal exposure.     

铸钢表面镍基合金渗层的热疲劳行为

采用铸渗技术在铸钢ZGCr5Mo试样表面制备了镍基合金渗层,渗层的厚度为0.6—1.2 mm,考察了镍基合金渗层的热疲劳行为,用扫描电镜(SEM)、X射线衍射(XRD)对热疲劳循环后的渗层表面进行了形貌观察与成分分析。结果表明:在热循环次数低于20次时,表面仅发生了氧化现象,当热循环次数超过90次时,渗层表面出现微裂纹,随着循环次数的增加,在渗层与基体的侧表面上出现贯穿渗层与基体的微裂纹以及在渗层与基体的界面处的表面氧化膜层出现了平行于渗层表面以及发散的微裂纹;随着镍基合金渗层厚度的增加,出现微裂纹的热循环次数略有降低。表面的氧化膜层主要为镍、铬的氧化物以及镍铬的复杂氧化物。     

Thermal expansion behavior of aluminum alloys reinforced with alumina planar random short fibers

The thermal expansion behavior of two aluminum alloys (Al-4%Cu and Al-12%Si) reinforced with alumina planar random short fibers has been studied, both experimentally and theoretically. The metal matrix composites (MMCs) were manufactured by pressure infiltration of molten metal into short fiber preforms with a planar random distribution of fibers. Dilatometric testing was used to investigate the influence of fiber volume fraction and architecture, and the effects of thermal cycling between 25 °C to 560 °C. Thermal expansion measurements showed that, by increasing the fiber content in the composites, both the thermal strains and the effective coefficient of thermal expansion (CTE) were reduced in the whole temperature range. Furthermore, the thermal strains of MMCs increased almost linearly up to a critical temperature, T _cr, where the metallic matrix began to yield macroscopically due to internal thermal stresses. For temperatures higher than T _cr the thermal strains of MMCs showed a marked hysteresis during heating/cooling cycles due to the elasto-plastic response of the metallic matrix. In this temperature range, the thermal expansion curves deviated appreciably from linearity and the planar (in the plane of fibers) and transverse (normal to the plane of fibers) responses were very different: while the planar CTE was strongly reduced, the transverse CTE increased sharply with temperature, being even larger than the CTE of the unreinforced alloy. Thermal cycling produced a net dimensional change of composites during the first 2-3 cycles but, on the subsequent cycles, the permanent deformation disappeared almost completely and the successive thermal expansion curves were identical. Experimental results were compared to the theoretical predictions of an analytical model based on the Eshelby's equivalent inclusion method, and an excellent agreement was obtained.     

Thermal stress development in fibrous composites

In this work, the thermal stress development in anisotropic fiber-reinforced polymer composites is investigated for temperatures below the glass transition temperature of the resin. By applying two independent experimental methodologies, it was found that the initial thermal (residual) strain in the reinforcing fibers is compressive of about − 0.04% at ambient temperatures. This is due to the mismatch of the thermal expansion coefficient between the polymer matrix and fiber, as the material is cooled down from the processing temperature. However, on reheating the composites the compressive stress in the fiber gradually diminishes and becomes zero at 50 °C. Further heating to 100 °C introduces tensile strains in the fiber of maximum of 0.13%. The conformity of these results to analytical models that relate the composite thermal strain to the thermal expansion coefficients of fiber and resin, as well as, the fiber volume fraction, is examined. Finally, the possibility of tailoring the sign (positive, negative or, even, zero) of the composite thermal expansion coefficient of certain advanced composites by simply varying the thermal expansion of the polymer matrix, is discussed.     

Thermal Stress Behavior of Diffusion-bonded SiC/6061Al Laminates during Thermal Cycling

1.IntroductionInrecentyears,therehasbeenrenewedinter-estinthethermalstressinmultilayeredstructurescomprisingbymetallic,ceramicorpolymericmate-rials,whichareextensivelyusedtoachievedesir-ablefunctionsinintegratedcircuits,semiconductorlasers,opticaldevices,andmultichipmodulesforelec-tronicpackaging['~lol.Inadvancedstructuralmateri-alssuchasfiber-reinforcedcomposites,layersofcoat-ingsonthesurfaceofreinforcingfibersareoftenusedtoenhanceadherence,toserveasdiffusionbarriers,ortoavoiddegradationdueto…     

Thermoschockbeständigkeit und das Verhalten von SiC-faserverstärkten Glas-Matrix-Verbundwerkstoffen bei einer thermischen Wechselbeanspruchung

The damage evolution of commercially available SiC-Nicalon? fiber-reinforced glass matrix composites under thermal shock and thermal cycling conditions in oxidizing atmospheres was investigated. The thermal shock tests involved quenching the samples from high temperatures (590–710°C) to room temperature in a water bath. For the thermal cycling tests the samples were quickly alternated between high temperature (T=700°C) and room temperature air for different number of cycles. Both destructive and non-destructive techniques were employed to characterize the samples and to detect differences in behavior for the various thermal loading conditions. In thermally shocked samples, damage in the form of matrix microcracks was induced by quenching from intermediate temperatures, e.g. 660°C. The extent of damage increased with the number of thermal shock cycles, as detected by a decrease in the Young’s modulus and a simultaneous increase in the internal friction measured non-destructively be a mechanical force resonance technique. In thermally cycled samples, material degradation was ascribed to porosity formation in the matrix as a consequence of the extended exposures at high temperatures. With increasing number of cycles, also interfacial oxidation was detected. An attempt was made also to explore the possibility of healing the induced microcracks in thermally shocked samples by an optimized post-thermal shock heat-treatment (annealing) schedule, exploiting the viscous flow of the glass matrix.     

T300/LD2热循环时产生滞后环原因及改善方法探讨

T300/LD2复合材料在热循环时产生滞后环并留下很大的残留应变。本文对T300/LD2试样在热循环时产生滞后环和残留应变的原因进行分析和探讨。实验结果表明,基体在热循环时大量的塑性变形是产生滞后环和残留应变的主要原因,并通过在界面预设不同应力和缩短热循环温度区间来研究界面应力状态和热循环温度区间跟热循环时滞后环和残留应变之间的关系。提出了改善残留应变和滞后的方法(1)尽量减少复合材料的使用环境中的温度变化,(2)根据材料的使用环境,经过各种预处理使复合材料在工作时基体处于弹性变形状态,(3)提高基体的屈服强度。     

Thermal cycling response behavior of ceramic matrix composites under load and displacement constraints

Thermal cycling response of a two-dimensional carbon fiber reinforced SiC matrix composite (2D C/SiC) to load constraint (LC) and to displacement constraint (DC) in an oxidizing environment was investigated. During thermal cycling between 700 and 1200 °C, a constraint strain with a 0.208% range and a constraint stress with a 180 MPa range were, respectively, generated on the composites in LC and DC. It was found that with increasing cycles, the constraint strain increased in LC and the constraint stress decreased in DC. After 50 cycles, in contrast to the as-received composite materials, the as-cycled composites suffered greater loss in mechanical properties: the residual strength and failure strain are 204 MPa and 0.49% for the LC tested samples, and 223 MPa and 0.64% for the DC tested samples, respectively. Microstructural observations indicated that the LC could develop thermal microcracks and assist in oxidizing the internal fibers, whereas the DC reduced crack propagations and fiber oxidation because of decreasing tensile and increasing compressive stresses.