SiC晶须对SiCw/6061Al复合材料拉伸和压缩变形行为的影响

通过试验研究了SiCw/6061Al复合材料和6061Al基体的拉伸和压缩变形行为.结果表明,SiCw/6061Al复合材料和6061Al的拉伸变形行为相同,而压缩变形曲线上出现应力峰,这与不同应力状态下SiC晶须的转动有关.拉伸时SiC晶须逐渐转向与外力平行方向使SiCw/6061Al复合材料的应力增大,而压缩时SiC晶须转向与外力垂直方向使其应力减小,而不是由动态再结晶引起.     

原位生成TiBw/Ti复合材料的微观组织及高温压缩变形过程的演化规律

用粉末冶金原位合成法制备了TiB晶须增强钛基复台材料。利用扫描电镜和X射线衍射方法对经挤压变形后复合材料的微观结构进行了分析。在复合材料中观察到反应生成的TiB晶须。复合材料经过热挤压变形后,TiB晶须沿挤压方向定向排列。对挤压态TiBw／Ti复合材料进行高温压缩变形,TiB晶须在热压缩变形过程中发生转动或折断现象，变形温度越低，晶须折断现象越严重。     

晶须增强铝基复合材料的热压缩变形行为研究

采用试验和数值模拟相结合的方法研究了纵向、横向和倾斜分布的晶须增强铝基复合材料热压缩变形行为.研究表明:SiCw/6061Al复合材料300℃压缩的应力-应变行为与晶须取向角密切相关;在热变形过程中,纵向分布晶须折断严重,导致复合材料表现为应变软化行为;而倾斜于压缩方向30°的晶须折断和转动明显,引起相应复合材料应变软化;横向分布晶须没有转动而折断程度很小,使复合材料呈现出加工硬化行为.     

晶须取向对SiCw/6061Al复合材料热压缩变形行为的影响

采用SEM 和 Magiscan-2A 图像分析系统研究了晶须取向对SiCw/6061Al复合材料在300℃压缩变形行为的影响.结果表明:晶须取向影响着晶须折断程度和转动角度; 随着晶须取向角的增加,晶须转动和折断行为所导致的软化效果下降.同时晶须取向也影响复合材料的热压缩应力-应变曲线的形状.在热压缩变形过程中,晶须取向角为0°和30°的复合材料表现出明显应变软化现象, 晶须取向角为45°的复合材料无明显软化现象.晶须取向角为90℃的复合材料表现出应变硬化现象.     

晶须转动对铝基复合材料热压缩变形行为的影响

采用平面应变有限元方法研究了晶须转动对晶须增强铝基复合材料热压缩变形行为的影响.数值结果表明:晶须转动不仅引起晶须承载能力的下降,而且也影响基体的塑性变形行为,同时复合材料在热压缩变形过程中表现出明显的应变软化行为.数值分析研究进一步证明:晶须转动是复合材料出现应变软化的重要原因.     

SiCW/ AZ91 镁基复合材料及AZ91镁合金的高温变形行为

利用Gleeble-1500 对SiC_W/ AZ91 复合材料和AZ91 镁合金在温度为423～723 K、应变速率为0.002～0.25s-1 、最大应变量为60 %的条件下进行高温压缩变形行为的研究。测试了其真应力-应变曲线, 观察了变形后的显微组织。结果表明: 晶须的转动和折断导致复合材料的应变软化现象较合金明显; 复合材料和合金的应变速率敏感指数(m) 和表观激活能(Q) 均随温度的升高而增大; 晶须的加入细化了晶粒, 使复合材料的m 值比合金高; 同时晶须的加入也限制了位错交滑移和晶界的迁移, 因此复合材料的Q 值比合金高; 压缩变形过程中, 合金和复合材料发生了动态回复和动态再结晶。      

SiCw／Al复合材料的强度分析

用简化应力的分析方法推导了SiCw／Al复合材料的强度与SiC晶须偏轴角α的关系，并通过试验进行验证．结果表明，计算的SiCw／Al复合材料强度与实测值比较接近，其误差归因于压缩变形过程中SiC晶须发生了转动和折断，改变了SiC晶须的偏轴角和平均长径比．     

变形温度对SiCW/6061Al复合材料压缩变形期间基体金属流动和晶须重新取向的影响

借助观察和分析变形前后晶须取向和试样形状的变化，研究了晶须呈定向排列的SiCw/6061Al复合材料压缩变形期间金属流动和晶须重新取向的现象。研究结果表明，由于晶须的定向排列，SiCw/6061Al复合材料压缩变形期间基体金属的流动和晶须的重新取向强烈地依赖于变形温度。在较高温度压缩变形时，基体金属可以更均匀地流动，此时晶须是否发生重新取向与变形时环状和柱状试样的应变场有关；在较低温度变形时晶须不发生重新取向。     

SiCW对TiB2／25SiCW陶瓷材料高温增韧效果的影响

采用热压工艺制备了ＴｉＢ２／ＳｉＣＷ陶瓷复合材料，结果表明：在ＴｉＢ２基体中添加体积分率为２５％的ＳｉＣ晶须，可显著的提高材料的断裂韧性和抗弯强度，实验表明，ＴｉＢ２／２５ＳｉＣＷ陶瓷材料的断理解韧在１０００℃内随温度的升高而增大，其原因是由于的升高使晶须径向残余压应力松驰，晶须拔出所需要的力ＦＰ减小，温度越高，晶须拔出越容易，能够被拔出的晶须量增多，拔出功增大，在低落曙下晶须以脆性断裂为主，在高     

SiCw／Al2O3复合材料残余应力分析

利用Ｅｓｈｅｌｂｙ模型计算了碳化硅晶须补强氧化铝复合材料中的晶须含量对残余应力的影响，与Ｘ－ｒａｙ衍射测量的数据进行比较，两者结果非常接近。随着晶须含量的增加，基体Ａｌ２Ｏ３中残余张应力增加，而碳化硅晶须中的残余压应力降低。     

Compressive stress-strain response of directionally aligned SiC<Subscript>w</Subscript>/Al composite

A SiC_w/6061Al composite was fabricated through a squeeze-casting route and hot extruded to obtain a composite with directionally aligned whiskers. Based on observed changes in whisker orientation and length before and after deformation, compressive deformation behaviour of the directionally aligned SiC_w/Al composite was investigated. It is found that when the compressive temperature is much lower than the solidus of the matrix alloy, the compressive flow stress of the directionally aligned composite is increased with compressive strain first and then decreased. When the compressive temperature equals the solidus of the matrix, however, the compressive flow stress of the directionally aligned composite is increased monotonously with compression strain. During compression, whisker rotation and breakage occurred, and the higher the compressive temperature, the easier the whisker rotation and hence the smaller the degree of whisker breakage. When the compressive strain was quite high, the degree of whisker breakage was serious even at the temperature as high as the solidus of the matrix. Analyzing changes in whisker orientation and breakage before and after compression indicates that the decreased compressive flow stress with compressive strain is the result of the decreased load carrying ability of whiskers caused by whisker rotation and breakage. Compared with whisker rotation, whisker breakage has a bigger contribution to the decreased compressive flow stress. No strain softening in the composite compressed at 580°C can be thought to be a result of the very low strengthening effect of whiskers at such a high temperature. From the point of view of whisker breakage, to get higher properties of SiC_w/Al composite parts made by means of plastic forming, too high plastic strain should not be suffered by SiC_w/Al composites during the plastic forming.     

Study of the whisker rotation in metal matrix composite

Based on a simplified model of whisker reinforced metal matrix composite and by using the theory of the analytic function, the whisker rotation in the tensile process of the composite has been formulated. Through in-situ observation of a tensile process in SiC whisker reinforced aluminium (SiC_w/Al) composite by a scanning electron microscope (SEM), the formula of the whisker rotation has been verified. Lastly the effect of the whisker rotation on the strength of SiC_w/Al composite has been discussed.     

Case studies on numerical simulations of the thermomechanical behaviour of Al SiC whisker composites

Simulations concerning the thermomechanical behaviour of a SiC whisker-reinforced aluminium alloy are carried out with the finite-element method. A representative three-dimensional unit cell with an overlapping whisker arrangement is derived by geometrical idealisations. Special importance is placed on the material model which should be as true to life as possible because the mechanical behaviour of the composite is decisively influenced by the inelastic behaviour of the matrix. Investigations showed pronounced inhomogeneous residual stresses in the matrix of the composite as a result of the cooling process during manufacture. These stresses have a considerable influence on mechanical behaviour, especially under transverse loading. The composite is anisotropic with higher stiffness and strength in the longitudinal direction than in the transverse direction. The elastic modulus of the aluminium alloy clearly increases due to the whisker reinforcement. However, the yield strength is limited by sharp stress concentrations which result from the cylindrical shape of the whiskers and especially the sharp edge between the shell and the upper surface. Furthermore, high hydrostatic stresses develop in the matrix in the regions of the whisker ends at tensile loads which can lead to damage and eventually to failure of the composite at low fracture strains. At cyclic loads, ratchetting can be observed and, as residual stresses decrease in the course of the cycles, the strength increases significantly.     

Effects of extrusion and heat treatment on the microstructure and tensile properties of in situ TiBw/Ti6Al4V composite with a network architecture

In situ TiB whisker reinforced Ti6Al4V (TiBw/Ti64) composites with a network architecture were extruded and heat treated in order to further improve their mechanical properties. The microstructure results show that the equiaxed network architecture was extruded to column network architecture and TiB whisker to alignment distribution. The transformed β phase is formed and the residual stress generated during extrusion obviously decreases after water quenching and aging processes. The tensile test results show that the strength, elastic modulus and ductility of the composites can be significantly improved by the subsequent extrusion, and then, the strength can be further improved by water quenching and aging processes after hot extrusion deformation. The elastic modulus of the as-sintered composites with a novel network microstructure follows the upper bound of Hashin-Shtrikman (H-S) theory before extrusion, while that of the as-extruded composites with a column network microstructure agrees well with the prediction from Halpin-Tsai equation.     

Residual stresses in fibre- and whisker-reinforced aluminium alloys during thermal cycling measured by X-ray diffraction

Residual stresses have been determined using X-ray diffraction in two different metal matrix composites, viz. a squeeze-cast Al-2%Mg matrix with 10, 20 or 26 vol.% Al₂O₃ fibres and an extruded AA 6061 alloy with 25 vol.% SiC whiskers. The composites have been studied after thermal cycling between 240 or 250 °C and room temperature succeeded in some cases by quenching to liquid nitrogen temperature. On the squeeze-cast composite, stresses were measured at room temperature and in situ at 240 °C. X-ray stress determinations were compared with the stress values calculated by a modified Eshelby model for equivalent inclusions. By the model, the stresses can be accurately predicted for both composite systems. Thermally induced plastic relaxation reduces the residual stresses. The degree of reduction depends on the reinforcement volume fraction, the difference in coefficient of thermal expansion between the phases and the magnitude of the temperature drop. At elevated temperature the stresses are less responsive to reiterated quenching and heating.     

Protrusion and whisker growth on tin coated copper substrate under stresses

Abstract

Tin thin films were coated on copper substrates using chemical method. Tin coated copper specimens under continuously applied compressive and tensile stresses were oxidised at 100°C for 8 days. Protrusion and whisker growth behaviour of tin coated copper specimens was characterised by scanning electron microscope, X-ray diffractometer and transmission electron microscope. The results show that continuously applied compressive stress on tin coated copper specimens would enhance the growth of whisker while tensile stress would reduce whisker growth. A SnO₂ layer was formed during oxidation and intermetallics of Cu₃Sn and Cu₆Sn₅ were formed between SnO₂ layer and copper substrate, which built up the residual compressive stresses in the coated tin layer. Continuously applied stress on the specimens can change the magnitude of residual compressive stress in coated tin layer, which affects the driving force for protrusion and whisker formation and growth.     

Modeling of effects of thermomechanical processing on elevated-temperature mechanical properties of in situ (TiB + TiC)/Ti-1100 composite

An in situ (TiB + TiC)/Ti-1100 composite was prepared by reacting B₄C and Ti. The effect of the amount of deformation during thermomechanical processing (TMP) on the microstructure, orientation of TiB whiskers, and the mechanical properties of the composite were investigated. Improvements in the composite tensile strength from TMP are discussed in terms of grain refinement and TiB whisker rotation. A model is suggested to predict the TiB whisker orientation factor for the composite after TMP with various amounts of deformation. Based on the effect of grain refinement and rotation of the TiB whiskers, the yield strengths of the composite after TMP with various amounts of deformation were modeled. The modeled values agreed well with the test results.