Thermal expansion and dimensional stability of unidirectional and orthogonal fabric M40/AZ91D composites

Unidirectional (60%, volume fraction) and orthogonal (50%, volume fraction) M40 graphite fibre reinforced AZ91D magnesium alloy matrix composites were fabricated by pressure infiltration method. The coefficients of thermal expansion (in the temperature range of 20-350 ℃) and dimensional stability (in the temperature range of 20-150 ℃) of the composites and the corresponding AZ91D magnesium alloy matrix were measured. The results show that coefficients of thermal expansion of the composites in longitudinal direction decrease with elevating temperature. The coefficients of thermal expansion (CTE) for unidirectional M40/AZ91D composites and orthogonal M40/AZ91D composites are 1.24×10-6 ℃-1 and 5.71×10-6 ℃-1 at 20 ℃, and 0.85×10-6 ℃-1 and 2.75×10-6 ℃-1 at 350 ℃, respectively, much lower than those of the AZ91D alloy matrix. Thermal cycling testing demonstrates that the thermal stress plays an important role on residual deformation. Thus, a better dimensional stability is obtained for the AZ91D magnesium alloy matrix composites. More extreme strain hysteresis and residual plastic deformation are observed in orthogonally fabric M40 reinforced AZ91D composite, but its net residual strain after each cycle is similar to that of the unidirectional M40/AZ91D composite.     

热暴露对SiCf/Ti-6Al-4V复合材料热膨胀行为的影响

研究了热暴露对SiCf/Ti-6Al-4V复合材料热膨胀行为的影响。对基体合金、制备态复合材料、经900 ℃热暴露10, 25及75 h的复合材料的热膨胀行为进行了实验测量。结果表明,无论纵向和横向,热暴露的复合材料其热膨胀系数均高于制备态复合材料。且无论纵向和横向,经900 ℃热暴露10和75 h的复合材料热膨胀系数均高于经900 ℃热暴露25 h的复合材料。这是因为经900 ℃热暴露25 h的复合材料拥有较为合适的界面反应。另外,在对复合材料纵向热膨胀系数进行预测时,因为Schapery模型在整个加热过程中都考虑了热拉伸残余应力对复合材料热膨胀系数的影响,因此在CTE曲线的高温阶段,预测值低于实验值     

Effect of thermal stresses on the thermal expansion and damping behavior of ZA-27/aluminite metal matrix composites

When the fabrication of a metal matrix composite (MMC) involves its cooling from a high temperature, plastic-elastic residual deformation fields can be generated within and around the particle due to the differential thermal expansion between the particle and matrix metal. The present investigation is concerned with the effect of thermal residual stresses on the thermal expansion and damping behavior of aluminite particulate-reinforced ZA-27 alloy MMCs. Composites were prepared by the compocasting technique with 1, 2, 3, and 4 wt.% of aluminite reinforcement. Thermal expansion and damping properties have been studied experimentally as a function of temperature over a temperature range 30 to 300 °C both in the heating and cooling cycle. The thermal expansion studies exhibited some residual strain, which increased with the increase in the weight percent of the reinforcement. The damping capacity of both the composites and matrix alloy is found to increase with the increase in temperature during the heating cycle, whereas in the cooling cycle, damping behavior exhibits a maximum, which becomes more pronounced with the increase in the weight percentage of the reinforcement. The appearance of the maximum may be linked with dislocation generation and motion as a result of plastic deformation of the matrix at the metal/reinforcement interface. This phenomenon is attributed to the thermal stresses generated as a result of coefficient of thermal expansion (CTE) mismatch between the composite constituent phases. The thermal stresses have been estimated in both the cases using simple models.     

Thermal expansion behavior of a β-LiAlSiO<Subscript>4</Subscript>/Cu composite

A copper matrix composite reinforced by β-LiAlSiO₄ with negative thermal expansion coefficient was fabricated using vacuum hot-pressed sintering technique. The thermal expansion behavior of the composite was investigated, and the average residual stress in the matrix was analyzed by a simple model. The results indicate that the residual stress in the matrix affects the thermal expansion properties. After heat treatment, the coefficient of thermal expansion (CTE) of the composite decreases greatly. The CTE of the composite after thermal cycling between 50–350°C is the lowest.     

Interface and thermal expansion of carbon fiber reinforced aluminum matrix composites

Two kinds of unidirectional PAN M40 carbon fiber(55%,volume fraction) reinforced 6061Al and 5A06Al composites were fabricated by the squeeze-casting technology and their interface structure and thermal expansion properties were investigated.Results showed that the combination between aluminum alloy and fibers was well in two composites and interface reaction in M40/5A06Al composite was weaker than that in M40/6061Al composite.Coefficients of thermal expansion(CTE) of M40/Al composites varied approximately from(1.45-2.68)×10-6 K-1 to(0.35-1.44)×10-6 K-1 between 20 °C and 450 °C,and decreased slowly with the increase of temperature.In addition,the CTE of M40/6061Al composite was lower than that of M40/5A06Al composite.It was observed that fibers were protruded significantly from the matrix after thermal expansion,which demonstrated the existence of interface sliding between fiber and matrix during the thermal expansion.It was believed that weak interfacial reaction resulted in a higher CTE.It was found that the experimental CTEs were closer to the predicted values by Schapery model.     

预制体对C／C复合材料热膨胀系数的影响

以天然气为前驱气体,整体碳毡和2D针刺碳毡为预制体,采用热悌度化学气相渗积技术制备了两种C／C复合材料,其表观密度均为1．74g／cm^3。借助光学显微镜和扫描电子显微镜观察了热解碳基体的生长特征和微观形貌,采用热膨胀仪测量了两种材料的热膨胀系数（CTE）,研究了由不同预制体增强C／C复合材料的CTE,解释了造成材料不同方向CTE差异的主要原因。结果表明,随着温度升高,材料A和B的CTE是逐渐升高的,且Z向CTE值均大于XY向。当两种材料在Z向的纤维体积分数接近时,随着XY方向纤维体积分数增大,材料在Z向的CTE增大,在XY向的CTE降低,两种材料存XY和Z向的CTE旱如下分布：αB—z〉dAz〉αA—xy〉αBxY-C／C复合材料的CTE主要取决于纤维体积分数和排市、碳基体及材料中的孔隙分布情况,前者起决定作用。     

可计及温度与层状结构影响的超高温陶瓷基复合材料涂层残余热应力理论表征模型

目的创建可计及温度与层状结构共同影响的超高温陶瓷基复合材料涂层与基体层因热不匹配导致的残余热应力的理论表征模型。方法基于经典的层合板理论与超高温陶瓷基复合材料热物理性能参数对温度的敏感性研究,引入温度和层状结构对涂层与基体层所受残余热应力的影响,形成各层残余热应力温度相关性的理论表征方法,并以ZrB_2-SiC复合材料涂层为例,利用该理论方法系统地研究了各种控制机制对残余热应力的影响及其随温度的演化规律。结果超高温陶瓷基复合材料涂层与基体层所受的残余热应力随着温度的变化而变化,涂层热膨胀系数与基体层热膨胀系数差别越大,变化幅度越大。当涂层材料热膨胀系数大于基体层材料热膨胀系数时,涂层材料遭受残余拉应力,基体层材料遭受残余压应力;随着涂层厚度的增加,涂层所受拉应力减小,而基体层所受压应力增大;当涂层材料热膨胀系数小于基体层材料热膨胀系数时,涂层材料遭受残余压应力,基体层材料遭受残余拉应力;随着涂层厚度的增加,涂层所受压应力减小,而基体层所受拉应力增大。低温下,各层所受残余热应力对层厚与每层材料组成的变化比较敏感,随着温度的升高,敏感性降低。结论对于涂层材料,应设计涂层材料的热膨胀系数小于基体层材料的热膨胀系数,使涂层遭受残余压应力,这不仅能够降低材料表面产生裂纹的危险,同时可以抑制表面已有缺陷的扩展。同时应当设计相对较小的涂层厚度,以增大涂层所受的残余压应力,降低基体层所受的残余拉应力,有效提高整体材料在不同温度下的强度性能。     

残余应力对W-Cu复合材料导热性能的影响

用熔渗法制备的W-Cu复合材料在小批量生产时，发现熔渗结束后样品的热导率数据分散不稳定，受熔渗时摆放位置的影响很大。将熔渗结束后的样品退火，采取不同的方式冷却。用X射线衍射法测量其残余应力，用热脉冲法测量其热导率，研究了残余应力对材料导热性能的影响。结果表明：随冷却速度的加快，残余应力值增大，热导率降低。钨和铜的热膨胀系数相差较大，从高温冷到室温时两相收缩程度不一样，冷却速度过快时，在界面处产生残余应力，使材料的热导率降低。分析了残余应力对材料导热性能的影响及机制。     

Effect of thermal-cooling cycle treatment on thermal expansion behavior of particulate reinforced aluminum matrix composites

Two micron SiC particles with angular and spherical shape and the sub-micron Al2O3 particles with spherical shape were introduced to reinforce 6061 aluminium by squeeze casting technology.Microstructures and effect of thermal-cooling cycle treatment(TCCT) on the thermal expansion behaviors of three composites were investigated.The results show that the composites are free of porosity and SiC/Al2O3 particles are distributed uniformly.Inflections at about 300 °C are observed in coefficient of thermal expansion(CTE) versus temperature curves of two SiCp/Al composites,and this characteristic is not affected by TCCT.The TCCT has significant effect on thermal expansion behavior of SiCp/Al composites and CTE of them after 3 cycles is lower than that of 1 or 5 cycles.However,no inflection is observed in Al2O3p/Al composite,while TCCT has effect on CTE of Al2O3p/Al composite.These results should be due to different relaxation behavior of internal stress in three composites.     

Process study, microstructure, and matrix cracking of SiC fiber reinforced MoSi2 based composites

SiC fiber reinforced SiAlON-MoSi₂ composites have been manufactured by a concurrent fiber winding and low pressure plasma spraying (LPPS) technique to produce a multilayer, circumferentially fiber reinforced composite ring. The LPPS parameters for SiAlON-MoSi₂ powder were optimized by a two-level experimental design approach followed by further optimization, which provided a smooth sprayed surface, low matrix porosity, and high deposition efficiency. The microstructure of SiAlON-MoSi₂ matrix consisted of a lamellar structure built up of individual splats and a uniform distribution of discontinuous SiAlON splats throughout the MoSi₂ matrix. The spray/wind composites exhibited 2% porosity and well-controlled fiber distribution. High temperature consolidation led to the formation of a thick reaction zone at the fiber-matrix interface by a chemical reaction between C coating and MoSi₂. Matrix cracking occurred in SiC _f (15 vol.%)/MoSi₂ after cooling from 1500 to 25 °C and was attributed to the large tensile residual stresses in the matrix developed on cooling because of coefficient of thermal expansion (CTE) mismatch between matrix and fiber. The addition of 40 vol.% SiAlON into the MoSi₂ effectively eliminated the matrix cracking by reducing the matrix-fiber CTE mismatch. Predictions of matrix cracking stress on the basis of residual stresses in the composites showed that the maximum permissible fiber volume fraction to avoid matrix cracking was 6% for SiC _f /MoSi₂ and 23% for SiC _f /SiAlON(40 vol.%)-MoSi₂.     

Thermal expansion behavior of a β-LiA1SiO4/Cu composite

A copper matrix composite reinforced by β-LiAlSiO4 with negative thermal expansion coefficient was fabricated using vacuum hot-pressed sintering technique. The thermal expansion behavior of the composite was investigated, and the average residual stress in the matrix was analyzed by a simple model. The results indicate that the residual stress in the matrix affects the thermal expansion properties. After heat treatment, the coefficient of thermal expansion (CTE) of the composite decreases greatly. The CTE of the composite after thermal cycling between 50-350℃ is the lowest.     

Thermal properties of pressureless melt infiltrated AlN–Si–Al composites

Thermal properties of AlN-Si-Al composites produced by pressureless melt infiltration of Al/Al alloys into porous α-Si₃N₄ preforms were investigated in a temperature range of 50-300 °C. SEM and TEM investigations revealed that the grain size of AlN particles was less than 1 μm. In spite of sub-micron grain size, composites showed relatively high thermal conductivity (TC), 55-107 W/(m.K). The thermal expansion coefficient (CTE) of the composite produced with commercial Al source, which has the highest TC of 107 W/(m.K), was 6.5×10^?6 K^?1. Despite the high CTE of Al (23.6×10^?6 K^?1), composites revealed significantly low CTE through the formation of Si and AlN phases during the infiltration process.     

3000℃氧-乙炔重复烧蚀后4D-炭/炭复合材料的剩余强度和热物理性能(英文)

主要研究炭/炭(C/C)复合材料在高温使用环境下的烧蚀、剩余强度以及热物理性能。实验采用四维C/C复合材料,在3000℃下进行重复氧-乙炔烧蚀实验,每次烧蚀时间为30s,烧蚀后快速冷却。结果表明:材料的烧蚀率与烧蚀时间呈线性关系;在总烧蚀时间相同的情况下,单步烧蚀比逐步烧蚀的烧蚀率高。经过最初几步烧蚀实验后,材料的烧蚀率保持恒定。试样烧蚀面的粗糙度对试样的烧蚀率有重要影响。随着烧蚀次数的增加,材料的压缩强度、热导率和热膨胀系数逐步降低,而比热容几乎保持不变。     

热膨胀性能对炭/炭复合材料在制动摩擦过程中热应力场的影响

测量以纯树脂炭、粗糙层热解炭和光滑层热解炭为基体的3种炭/炭复合材料的热膨胀系数,并采用有限元分析软件,模拟这3种炭,炭复合材料在飞机正常着陆能量条件下的热应力场,研究热膨胀系数对炭,炭复合材料热应力场及其摩擦性能的影响.结果表明:3种炭/炭复合材料在z方向上的热膨胀系数大于在X和y方向的,且热膨胀系数均随着温度的升高而逐渐增大,其中,基体为粗糙层热解炭的炭/炭复合材料的热膨胀系数最小,纯树脂炭试样的次之,光滑层热解炭试样的最大;在制动过程中,炭/炭复合材料摩擦表面产生的热应力与材料的热膨胀系数相关,材料的热膨胀系数越大,产生的热应力越大;过大的热应力使纯树脂炭试样具有相对稳定的摩擦曲线,在较大热应力的作用下,光滑层热解炭试样的摩擦曲线不稳定,影响其摩擦性能.     

WC_p/2024Al复合材料热学性能研究

利用粉末冶金法制备了WC颗粒体积分数分别为8%、11.8%、16.7%的WCp/2024Al复合材料,采用扫描电子显微镜、热膨胀分析仪、热导率测试仪等多种手段研究不同WC体积分数、挤压比和热处理对复合材料热膨胀系数（CTE）、热导率和微观结构的影响。结果表明：复合材料的热膨胀系数随WC体积分数的增大而明显降低,随挤压比的增大而提高,经过T4态热处理后,复合材料内应力的降低和第三相的析出导致其热膨胀系数降低,热膨胀系数的实测值与kermer模型的计算值相近。复合材料的热导率随WC体积分数的增大而降低。     

Microstructure and properties of high-fraction graphite nanoflakes/6061Al matrix composites fabricated via spark plasma sintering

30–50 wt.% graphite nanoflakes (GNFs)/6061Al matrix composites were fabricated via spark plasma sintering (SPS) at 610 °C. The effects of the sintering pressure and GNF content on the microstructure and properties of the composites were investigated. The results indicated that interfacial reactions were inhibited during SPS because no Al₄C₃ was detected. Moreover, the agglomeration of the GNFs increased, and the distribution orientation of the GNFs decreased with increasing the GNF content. The relative density, bending strength, and coefficient of thermal expansion (CTE) of the composites decreased, while the thermal conductivity (TC) in the X?Y direction increased. As the sintering pressure increased, the GNFs deagglomerated and were distributed preferentially in the X?Y direction, which increased the relative density, bending strength and TC, and decreased the CTE of the composites. The 50wt.%GNFs/6061Al matrix composite sintered at 610 °C under 55 MPa demonstrated the best performance, i.e., bending strength of 72 MPa, TC and CTE (RT?100 °C) of 254 W/(m·K) and 8.5×10^?6 K^?1 in the X?Y direction, and 55 W/(m·K) and 9.7×10^?6 K^?1 in the Z direction, respectively.     

界面改性对SiCf/Cu基复合材料热膨胀性能的影响

研究了SiCf/Cu基复合材料分别在有无Ti6Al4V界面改性涂层两种情况下的纵向热膨胀行为,并采用扫描电镜对热循环后的试样进行显微形貌观察。结果表明,界面结合强度对纤维增强金属基复合材料的纵向热膨胀行为有很大影响。对于没有Ti6Al4V涂层的复合材料,其热膨胀行为不稳定,在经历连续两次热循环后,其纵向均表现为正的残余应变,原因是基体发生了严重的界面脱粘、滑移和膨胀;而对于有Ti6Al4V涂层的复合材料,其纵向热膨胀系数明显减小,两次热循环后其尺寸保持稳定,纤维/基体界面结合也保持稳定。     

Thermal expansion behaviour of squeeze-cast aluminium matrix composites reinforced with PAN- and Pitch-based carbon fibres

Aluminium composites reinforced with the PAN- and pitch-based short carbon fibres were fabricated by squeeze casting, then the thermal expansion behaviour of the composites was investigated. Optical microscopy revealed that the fibres were in a random arrangement on the plane parallel to the pressed plane during the melt infiltration process. TEM observation and hardness test revealed that the PAN-based fibre bonds strongly with the aluminium matrix while the pitch-based fibre bonds poorly. The difference in the bonding strength affected the thermal strain response; the heating and cooling curve approximately traced the same paths during the heating-cooling cycles for the PAN-based fibre composite, while the curve did not trace the same path for the pitch-based fibre composite. The fibre-reinforcement decreased the coefficient of the thermal expansion (CTE) of matrices in the direction parallel to the pressed plane. For example, the CTE of the pure aluminium and its composite at 333 K were 23.0 and 19.0 × 10^?6/K, respectively.     

Numerical simulation on thermal expansion coefficient of 3D braided C/C composites简

To effectively get the thermal expansion coefficient(CTE) of three-dimensional(3D) braided C/C composites and study the variations, a VC?? program with graphical user interfaces was obtained, based on the yarn unit model and numerical analysis. With the limited basic properties of carbon fibers and carbon matrix, CTE of 3D braided C/C composites is obtained at 85 °C. The deviation between the simulated and experimental axial CTE of 3D braided C/C composites is no more than 11 %. The effects of different parameters(including the braiding angle of 3D braided preform, the fiber volume fraction and the porosity of 3D braided C/C composites, and the elastic modulus, Poisson's ratio and CTEs of carbon fibers and carbon matrix) were analyzed with the program. The results show that the axial CTE of C/C composites decreases with the increase of the braiding angle, the fiber volume fraction, and the porosity of 3D braided C/C composites. The transverse elastic modulus of carbon fibers has the greatest effect on the axial CTE among the studied mechanical parameters, followed by the elastic modulus and Poisson's ratio of carbon matrix.     

Reinforcement architectures and thermal fatigue in diamond particle-reinforced aluminum

Aluminum reinforced by 60 vol.% diamond particles has been investigated as a potential heat sink material for high power electronics. Diamond (CD) is used as reinforcement contributing its high thermal conductivity (TC ≈ 1000 W mK^?1) and low coefficient thermal expansion (CTE ≈ 1 ppm K^?1). An Al matrix enables shaping and joining of the composite components. Interface bonding is improved by limited carbide formation induced by heat treatment and even more by SiC coating of diamond particles. An AlSi7 matrix forms an interpenetrating composite three-dimensional (3D) network of diamond particles linked by Si bridges percolated by a ductile α-Al matrix. Internal stresses are generated during temperature changes due to the CTE mismatch of the constituents. The stress evolution was determined in situ by neutron diffraction during thermal cycling between room temperature and 350 °C (soldering temperature). Tensile stresses build up in the Al/CD composites: during cooling <100 MPa in a pure Al matrix, but around 200 MPa in the Al in an AlSi7 matrix. Compressive stresses build up in Al during heating of the composite. The stress evolution causes changes in the void volume fraction and interface debonding by visco-plastic deformation of the Al matrix. Thermal fatigue damage has been revealed by high resolution synchrotron tomography. An interconnected diamond–Si 3D network formed with an AlSi7 matrix promises higher stability with respect to cycling temperature exposure.