Fiber damage during the consolidation of PVD Ti6Al4V coated nextel 610TM alumina fibers

Titanium matrix composites reinforced with sol-gel synthesized α-alumina fiber tows have attracted interest as a potentially low cost continuous fiber reinforced metal matrix composite system. We have conducted a detailed investigation of fiber damage during high temperature consolidation of PVD Ti6Al4V metallized sol-gel alumina fiber tows. Using both hot isostatic pressing and interrupted vacuum hot press consolidation cycles, the two principal mechanisms of fiber damage have been experimentally identified to be microbending/fracture and fiber matrix reaction. A time dependent micromechanics model incorporating the evolving geometry and mechanical properties of both the fibers and matrix has been formulated to simulate the fiber bending/failure mechanism in a representative unit cell and explore the effect of fiber strength loss due to reaction with the matrix. This model has been used to design a process cycle that minimizes damage by exploiting the enhanced superplastic deformation of the initially nanocrystalline PVD Ti6Al4V matrix.     

An experimental and theoretical investigation of the rapid consolidation of continuously reinforced,metal-matrix Composites

The feasibility of the rapid consolidation of T-14Al-21Nb/SCS-6 foil/fiber/foil composites using a forging approach was established as an alternative to slower and more expensive processes such as those based on hot isostatic pressing (HIP) or vacuum hot pressing (VHP). A firm basis for the technique was developed through theoretical analyses of temperature transients, forging pressures, and fiber fracture. These analyses demonstrated that there exists an optimal forging speed at which the consolidation stresses are a minimum. It was also shown that the flow stress of the encapsulation material relative to that of the densifying layup is an important consideration in achieving full consolidation during forging. Specifically, the difference in flow stress between the two materials influences the magnitude and sign of the in-plane (secondary) stresses that are developed during forging and therefore the rate of pore closure during the latter stages of the process. With regard to fiber fracture, analyses were performed to estimate the axial and tangential stresses during rapid consolidation. The theoretical work was validated by experimental trials using the Ti-14Al-21Nb matrix/silicon carbide fiber system. Measured forging pressures were in good agreement with predictions. Fiber fracture observations indicated that tangential tensile stresses developed in the fiber control failure; a forging window to avoid such failures was thus developed. Finally, it was demonstrated that matrix microstructures and mechanical properties similar to those of conventionally consolidated Ti-14Al-21Nb/silicon carbide composites can be achieved by the forge-consolidation technique.     

A fiber fracture model for metal matrix composite monotape consolidation

The consolidation of plasma sprayed monotapes is emerging as a promising route for producing metal and intermetallic matrix composites reinforced with continuous ceramic fibers. Significant fiber fracture has been reported to accompany the consolidation of some fiber/matrix systems, particularly those with creep resistant matrices. Groves et al. [Acta metall. mater.42, 2089 (1994)] determined the predominant mechanism to be bending at monotape surface asperities and showed a strong dependence of damage upon process conditions. Here, a previous model for the densification of monotapes [Elzey and Wadley, Acta metall. mater.41, 2297 (1993)] has been used with a stochastic model of the fiber failure process to predict the evolution of fiber fracture during either hot isostatic or vacuum hot pressing. Using surface profilometer measured roughness data for the monotapes and handbook values for the mechanical properties of different matrices and fibers, this new model is used to elucidate the damage dependence on process conditions, monotape surface roughness, and the mechanical properties both of the fiber and matrix. The model is used to investigate the “processibility” of several currently important matrix and fiber systems and to identify the factors governing this. An example is also given of its use for the simulation of a representative consolidation process cycle. This approach to the analysis of a complex, nonlinear, time-varying process has resulted in a clear understanding of the causal relationships between damage and the many process, material and geometric variables of the problem and identified new strategies for its elimination.     

The effects of hot pressing parameters on the strength of aluminum/stainless steel composites

Stainless steel fiber reinforced aluminum matrix composites have been fabricated by hot pressing under different conditions of temperature, pressure, and time. The variation in tensile strength of these composites has been studied in detail, and the hot pressing parameters have been optimized in order to fabricate composites having maximum strength at any fiber volume fraction. Microprobe analysis and Scanning Electron Microscopy of these composites have revealed interesting features of fiber/ matrix interface. These features have been found helpful in explaining the dependence of strength of the composites on hot pressing parameters.     

原位转化碳纤维增韧氧化铝复合材料的制备工艺

以聚丙烯腈预氧化纤维为先驱纤维,使其在真空烧结过程中原位转化生成碳纤维来增韧氧化铝陶瓷材料.利用热重–差热分析和X射线衍射研究了聚丙烯腈预氧化纤维的相结构和化学结构以确定制备复合材料的升温烧结工艺,并探讨了加压方式和聚丙烯腈预氧化纤维含量对复合材料组织结构和性能的影响.研究发现聚丙烯腈预氧化纤维在差热曲线上444℃左右的放热峰和X射线衍射图谱中17左右的衍射峰是由预氧化阶段残留的未充分氧化的聚丙烯腈分子引起的;而1073℃左右的吸热峰和25.5左右的衍射峰说明预氧化纤维在加热烧结过程中已开始向碳纤维转变.热压烧结制备的复合材料的力学性能明显优于无压烧结.随着聚丙烯腈预氧化纤维含量的增加,复合材料的密度和显微硬度降低,而断裂韧性则先升高后降低,当聚丙烯腈预氧化纤维体积分数为20%时,复合材料的断裂韧性最大,达9.39MPa·m^1/2,说明原位碳纤维的生成提高了复合材料的断裂韧性,其增韧机制主要为纤维拔出和脱黏.     

Strength degradation of SiC fiber during manufacture of titanium matrix composites by plasma spraying and hot pressing

Titanium matrix composites (TMCs) reinforced with Sigma 1140+ SiC fiber have been manufactured by a combination of low pressure plasma spraying (LPPS spray/wind) and simultaneous fiber winding, followed by vacuum hot pressing (VHP). Fiber damage during TMC manufacture has been evaluated by measuring fiber tensile strength after fiber extraction from the TMCs at various processing stages, followed by fitting of these data to a Weibull distribution function. The LPPS spray/wind processing caused a decrease in mean fiber strength and Weibull modulus in comparison with as-received fibers. A number of fiber surface flaws, primarily in the outer C layer of the fiber, formed as a result of mechanical impact of poorly melted particles from the plasma spray. Coarse feedstock powders promoted an increase in the population of fiber surface flaws, leading to significant reduction in fiber strength. The VHP consolidation promoted further development of fiber surface flaws by fiber bending and stress localization because of nonuniform matrix shrinkage, resulting in further degradation in fiber strength. In the extreme case of fibers touching, the stress concentration on the fibers was sufficient to cause fiber cracking. Fractographic studies revealed that low strength fibers failed by surface flaw induced failure and contained a large fracture mirror zone. Compared with the more widely investigated foil-fiber-foil route to manufacture TMCs, LPPS/VHP resulted in less degradation in fiber strength for Sigma 1140+ fiber. Preliminary results for Textron SCS-6 fiber indicated a much greater tolerance to LPPS/VHP damage.     

Fabrication of metal matrix composites of

Fine fibrous titanium carbide (TiC) was processed through the self-propagating high-temperature synthesis (SHS) method and employed to fabricate aluminum matrix composites. Two consol-idation methods were investigated: (1) combustion synthesis of TiC fiber/Al composites directly using titanium powders and carbon fibers ignited simultaneously with varying amounts of the matrix metal powder and (2) combustion synthesis of TiC using titanium powders and carbon fibers followed by consolidation into different amounts of the metal matrix powder, Al,via hot isostatic pressing (HIP). In the former method, when the amount of the Al in the matrix was increased, the maximum temperature obtained by the combustion reaction decreased and the propagation of the synthesis reactions became difficult to maintain. Preheating was required for the mixture of reactants with more than approximately 5 mole pct aluminum matrix powders in order to ignite and maintain the propagation rate. Microstructural analysis of the products from the Al/C/Ti reaction without preheating shows that small amounts of an aluminum carbide phase (AI4C3) are present. In the second method, following separation of the individual fibers in the TiC product, dense composites containing the SHS products were obtained by HIP of a mixture of the TiC fibers and Al powders. No ternary phase was formed during this procedure. Formerly Graduate Research Assistant, Department of Chemical Engineering, Michigan Technological University, is with Particle Technology, Inc., Hanover, MD 21076. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17–21, 1991, under the auspices of the TMS Powder Metallurgy Committee.     

Effect of fiber volume fraction on the fracture behavior of Nb-1 Wt pct Zr/218W composites at elevated temperatures

Nb-1 wt pct Zr/218W long-fiber composite monotapes, nominally containing 0 to 70 vol pct of 218 tungsten fibers, were fabricated by arc spraying the Nb-1 pct Zr matrix onto the tungsten fibers. The monotapes were consolidated by hot pressing and hot isostatic pressing techniques. Tensile tests conducted between 1400 and 1600 K, under engineering strain rates varying between 1.5×10⁻⁵ and 1.5×10⁻³ s⁻¹, demonstrated that composites containing 70 vol pct of fibers had the highest strength-to-density ratio. Microstructural observations of specimens tested at 1400 K revealed that composites containing less than 50 vol pct of fibers showed extensive matrix cavitation, fiber-matrix debonding, and necking of the fibers. Above 50 vol pct, the composite matrix was less prone to cavitation, with an increasing tendency toward shear deformation of the fibers as the fiber volume fraction increased. No fiber damage was observed at 1400 K away from the fractured end, but significant fiber damage was observed at higher temperatures. A phenomenological model is presented to rationalize these observations. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Mechanical interrogation of interfaces in monofilament model composites of continuous SiC fiber-aluminum matrix

The interfacial region between continuous SiC fiber and aluminum alloy matrix, in monofilament metal matrix composites (MMC), has been characterized. The study utilized two SiC fibers, produced by Textron (SCS-2) and Sigma (Σ) and two aluminum alloys: A11100 and A16061. Characterization methods employed included: optical and electron microscopy (i.e. SEM and EPMA), mechanical testing of as-received and heat treated single fiber samples and monitoring of acoustic emission (AE) during tensile tests. In addition, interfacial shear strength (ISS) was experimentally determined by t wo different techniques, indentation and fragmentation. Indentation tests were carried out in the temperature range of 0–320°C. Results indicate that load is transferred from the matrix to the fiber, primarily by frictional stresses. The friction between fiber and matrix during loading of composite stems residual compressive thermal stresses, which result from the temperature differential between consolidation and testing temperatures, and the difference between the thermal expansion coefficients of the fiber and matrix. Consolidation had no effect on fiber strength and no reaction zones formed. Exposure for 7000 h at 6000δC had no significant effect on ISS. Fiber fractures were accompanied by intense AE. The breaks produced a small piece between adjacent larger fragments, and gave rise to characteristic AE with distinct waveforms for the two different fibers (Σ and SCS-2).     

Effect of fiber volume fraction on the fracture behavior of Nb-1 wt pct Zr/218W composites at elevated temperatures

Nb-1 wt pct Zr/218W long-fiber composite monotapes, nominally containing 0 to 70 vol pct of 218 tungsten fibers, were fabricated by arc spraying the Nb-1 pct Zr matrix onto the tungsten fibers. The monotapes were consolidated by hot pressing and hot isostatic pressing techniques. Tensile tests conducted between 1400 and 1600 K, under engineering strain rates varying between 1.5×10⁻⁵ and 1.5×10⁻³ s⁻¹, demonstrated that composites containing 70 vol pct of fibers had the highest strength-to-density ratio. Microstructural observations of specimens tested at 1400 K revealed that composites containing less than 50 vol pct of fibers showed extensive matrix cavitation, fiber-matrix debonding, and necking of the fibers. Above 50 vol pct, the composite matrix was less prone to cavitation, with an increasing tendency toward shear deformation of the fibers as the fiber volume fraction increased. No fiber damage was observed at 1400 K away from the fractured end, but significant fiber damage was observed at higher temperatures. A phenomenological model is presented to rationalize these observations. L.J. GHOSN, formerly Researcher with Case Western Reserve University, Cleveland, OH 44115 This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Effects of thermal residual stresses and fiber packing on deformation of metal-matrix composites

The combined effects of thermal residual stresses anmd fiber spatial distribution on the deformation of a 6061 aluminum alloy containing a fixed concentration unidirectional boron fibers have been analyzed using detailed finite element models. The geometrical structure includes perfectly periodic, uniformly spaced fiber arrangements in square and hexagonal cells, as well as different cells in which either 30 or 60 fibers are randomly placed in the ductile matrix. The model involves an elastic-plastic matrix, elastic fibers, and mechanically bonded interfaces. The results indicate that both fiber packing and thermal residual stresses can have a significant effect on the stress-strain characteristics of the composite. The thermal residual stresses cause pronounced matrix yielding which also influences the apparent overall stiffness of the composite during the initial stages of subsequent far-field loading along the axial and transverse direction. Furthermore, the thermal residual stresses apparently elevate the flow stress of the composite during transverse tension. Such effects can be traced back to the level of constraint imposed on the matrix by local fiber spacing. The implications of the present results to the processing of the composites are also briefly addressed.     

Ballistic Impact Properties of Zr-Based Amorphous Alloy Composites Reinforced with Woven Continuous Fibers

This study aims at investigating ballistic impact properties of Zr-based amorphous alloy (LM1 alloy) matrix composites reinforced with woven stainless steel or glass continuous fibers. The fiber-reinforced composites with excellent fiber/matrix interfaces were fabricated without pores and misinfiltration by liquid pressing process, and contained 35 to 41 vol pct of woven continuous fibers homogeneously distributed in the amorphous matrix. The woven-STS-continuous-fiber-reinforced composite consisted of the LM1 alloy layer of 1.0 mm in thickness in the upper region and the fiber-reinforced composite layer in the lower region. The hard LM1 alloy layer absorbed the ballistic impact energy by forming many cracks, and the fiber-reinforced composite layer interrupted the crack propagation and blocked the impact and traveling of the projectile, thereby resulting in the improvement of ballistic performance by about 20 pct over the LM1 alloy. According to the ballistic impact test data of the woven-glass-continuous-fiber-reinforced composite, glass fibers were preferentially fragmented to form a number of cracks, and the amorphous matrix accelerated the fragmentation of glass fibers and the initiation of cracks. Because of the absorption process of ballistic impact energy by forming very large amounts of cracks, fragments, and debris, the glass-fiber-reinforced composite showed better ballistic performance than the LM1 alloy.     

Infiltration of fiber preforms by an

Preforms of 20 vol pct SAFFIL alumina fibers are infiltrated with Al-4.4 wt pct Cu-0.3 wt pct Mg using a horizontal die casting machine. Fiber preform temperature is varied from 973 to 673 K. Solute distribution, fiber volume fraction, and matrix microstructure are characterized using optical metallography and electron microprobe analysis. Increases in fiber volume fraction are observed in the composites downstream of the infiltration path. We propose that these result from locking of the compressed fibers by solid metal present during infiltration. With this as- sumption, we find good agreement between theory presented in Parts I [1] and II [2] for solute concentration, fiber volume fraction distributions, as well as matrix microstructure and exper- iments. With an initial preform temperature of 673 K, freckles are found in the composite, which are interpreted to result from the combined effects of pressure and significant enrichment in solute at the infiltration front.     

Evolution of fiber fragmentation in a short-fiber-reinforced metal-matrix model composite during tensile creep deformation—An acoustic emission study

The objective of this work is to obtain deeper insight into the damage evolution occurring during creep in short-fiber-reinforced metal-matrix composites. Uniaxial tensile creep experiments were performed on a model composite with a lead (Pb) matrix. This system was chosen because it allowed the performance of all creep tests at room temperature, thus facilitating the detection of fiber fragmentation by acoustic emission measurements. By this experimental approach, for the first time, quantitative information about the spatial and temporal evolution of microfractures in creeping metal-matrix composite of this kind was obtained. The acoustic emission results show that fiber fragmentation sets in early in the creep life and continues to operate up to macroscopic failure, thus affecting the creep behavior in all stages including the steady-state regime. During the whole creep process, the fracture sites are homogeneously distributed in the specimen volume. These findings largely support the micromechanical damage model proposed by Dlouhy and co-workers, in which the creep process in short-fiber-reinforced metal-matrix composites is described as an interplay of work hardening and recovery in the matrix as well as fragmentation of the fibers.     

纤维分散对C/C-SiC复合材料力学性能的影响

利用温压-原位反应法制备短炭纤维增强C/C-SiC复合材料,研究纤维分散对复合材料力学性能的影响.结果表明: 利用分散短炭纤维制备的C/C-SiC复合材料,其抗弯强度和抗压强度分别达到56.6MPa和89.3MPa.该材料纤维之间孔隙少,纤维与基体接合界面多,弯曲时有纤维拔出,为假塑性断裂行为.压缩时无纤维拔出,为脆性断裂行为.最后,利用LI V C提出的束丝数学模型证明了纤维分散有利于提高C/C-SiC复合材料的力学性能.     

薄壁金刚石钻头热压弹性石墨模具设计

根据材料热膨胀特性并结合热压特点，分析了钻头胎体、钢体和石墨模具在热压中的行为，给出了影响热压金刚石钻头胎体结合强度乃至于导致其开裂的主要因素。在此基础上，改进设计了新型石墨模具。推荐了模具设计的二个主要参数。     

Effect of laminate stacking sequence on the high frequency fatigue behavior of SCS-6 fiber-reinforced Si3N4 matrix composites

In many potential applications, continuous fiber-reinforced ceramic matrix composites (CFCMCs) will encounter cyclic fatigue loadings at high frequencies (25 Hz or higher). While most of the work in the area of fatigue of CFCMCs has concentrated on low frequency behavior, high frequency behavior is equally important. In CFCMCs, stress-strain hysteresis occurs during fatigue and is associated with energy dissipation in the composite. In addition to this, the repeated friction and sliding between fiber and matrix are responsible for a substantial temperature rise at the fiber/matrix interface. In this study, [0/90] and [±45] SCS-6 (silicon carbide)/Si₃N₄ composites made by hot pressing were investigated under high frequency fatigue loadings. The angle-ply laminate showed the same extent of heating as cross-ply laminates, but at much lower stress levels. Frictional heating was caused by sliding at the fiber/matrix interface. Temperature rise due to heat generation in the specimens correlated very well with damage in modulus as a function of fatigue cycles in the composites. Matrix microcracking was more predominant in the angle ply than in the cross-ply composite, due to the much lower stiffness of the angle-ply composite in the longitudinal loading direction.     

Thermomechanical behavior of TiNi shape memory alloy fiber reinforced 6061 aluminum matrix composite

The processing and thermomechanical behaviors of TiNi shape memory alloy (SMA) fiber-reinforced 6061 Al matrix smart composites are investigated experimentally and analytically. Optimum processing conditions of hot pressing temperature and pressure are identified. Composite yield stresses are observed to increase with an increase in the volume fraction of TiNi fiber and prestrain given to the composites. An analytical model for thermomechanical behavior of the composites is developed by utilizing an exponential type of SMA constitutive model. The model predicts an increase in the composite yield stress with an increase in prestrain. It is found that the key parameters affecting the composite yield stress are the fiber volume fraction, prestrain, and matrix heat treatment. The predictions are in a reasonably good agreement with the experimental results.     

Thermomechanical behavior of TiNi shape memory alloy fiber reinforced 6061 aluminum matrix composite

The processing and thermomechanical behaviors of TiNi shape memory alloy (SMA) fiber-reinforced 6061 Al matrix smart composites are investigated experimentally and analytically. Optimum processing conditions of hot pressing temperature and pressure are identified. Composite yield stresses are observed to increase with an increase in the volume fraction of TiNi fiber and prestrain given to the composites. An analytical model for thermomechanical behavior of the composites is developed by utilizing an exponential type of SMA constitutive model. The model predicts an increase in the composite yield stress with an increase in prestrain. It is found that the key parameters affecting the composite yield stress are the fiber volume fraction, prestrain, and matrix heat treatment. The predictions are in a reasonably good agreement with the experimental results.     

Control of interfacial reactions during liquid phase processing of aluminum matrix composites reinforced with INCONEL 601 fibers

A comprehensive investigation is made of the parameters affecting the extent of interface reactions during squeeze casting of composites consisting of a matrix of either pure Al or alloy AS13 reinforced with fibers of INCONEL 601. The process parameters are the preform thickness and temperature, the fiber volume fraction, the temperature and mass of the liquid metal, and the temperature of the die. Adjustment of these process parameters made possible the full control of reactions. It is found that reactions proceed mainly in the solid state after decomposition of the oxide barrier layer covering the fibers. A simple kinetic model is developed that enlightens the role of this barrier layer. No trace of reaction could be detected in composites processed using preoxidized preforms. Alloying Al with Si also induces a drastic reduction of reactivity. The high ductility of the composites attests to the processing quality. An original procedure is proposed for measuring the activation energy for initiation of reactions by differential thermal analysis.