The effect of niobium morphology on the fracture behavior of MoSi2/Nb composites

The morphology of the niobium reinforcement added to MoSi₂affected the fracture behavior (and hence toughness) of MoSi₂/20 vol pct Nb composites. The addition of discontinuous random niobium in the form of particles or short fibers deflected cracks that propagated through the MoSi₂ matrix. However, this did not result in any improvements in toughness (estimated from the area under flexural stress-displacement curves), as matrix cracks preferentially prop-agated through the Nb/MoSi₂ interphase region. The addition of aligned niobium fibers, ori-ented perpendicular to the direction of matrix crack propagation, directly participated in the fracture of the composite. Depending on the diameter of Nb embedded in the MoSi₂matrix, these fibers either fractured in a brittle manner or ruptured in a ductile manner. Small (400-μn) diameter continuously aligned Nb fibers fractured by brittle cleavage during testing. Therefore, the addition of these fibers was not as effective in improving the toughness of MoSi₂as the addition of larger (800-μm) diameter continuously aligned Nb fibers, which ruptured in a ductile manner. It was observed that the larger diameter fibers had separated from the matrix through the propagation of cracks in the reaction zone adjacent to the fibers and that these cracks formed prior to yielding of these fibers. In contrast, the smaller diameter fibers remained well bonded to the matrix and, thus, were constrained by the MoSi₂matrix from yielding. This resulted in brittle fracture behavior of the Nb fiber. There appeared to be an effect of aspect ratio on the fracture of the ductile embedded fibers. Shorter length 400-μm-diameter fibers separated from the matrix, behavior similar to the continuous 800-μm-diameter fibers and not the continuous 400-μm-diameter fibers. Formerly Graduate Research Assistant, Materials Engineering Department, Rensselaer Polytechnic Institute.     

Abrasive wear behavior of MoSi2SiC and MoSi2ZrO2 composites

The volume wear behavior of MoSi₂/SiC and MoSi₂/ZrO₂ composites was evaluated using 150 grit SiC particles in a pin-on-drum abrasion test. The addition of SiC whiskers or particles reduced the volume wear of the composite relative to monolithic MoSi₂ by about a factor of two, with the SiC whisker containing composite having a slightly lower volume wear rate than the SiC particulate reinforced composite. The addition of partially-stabilized (PS)-ZrO₂ particles lowered the volume wear of the composite relative to MoSi₂. The addition of unstabilized (US)-ZrO₂ or fully-stabilized (FS)-ZrO₂ particles to the MoSi₂ matrix had little effect of the volume wear relative to the unreinforced matrix. The difference in wear behavior of the ZrO₂ reinforced composites may be associated with the ability of the PS-ZrO₂ particles to transform reducing the fragmentation process during abrasion.     

Bridge toughening enhancement in double-notched MoSi2/Nb model composites

Single-ply composites containing both laminate and continuous Nb fiber reinforcement coated with A1₂O₃ debond coatings in an MoSi₂ matrix are used as model systems for investigating bridge toughening concepts for various precrack configurations. When cracks are introduced symmetrically on either side of the ductile phase with zero crack offset spacing (S = 0), a minimum amount of energy is expended in plastic deformation and the local rupture process in the metal, as measured by the area of the force displacement curve in tension. For asymmetric precracks introduced on either side of the ductile reinforcement, as the offset spacing,S, was varied from 1 to 20R (R being the ductile phase half-thickness), the overall extension continuously increased within the bridging ligament. The effective ligament gage length was nearly equal to the crack spacing in the limiting case of a weak interface. However, the ductile Nb phase developed a Nb₅Si₃ reaction layer on its surface which was strongly bonded to the Nb and was found to undergo periodic cracking, leading to numerous shear bands within the ductile phase. This unique and previously unreported mode of metal deformation in shear loading has been analyzed using a simple geometric model. The results indicate that the profusion of shear bands is the primary source of toughening enhancement in the case of asymmetric crack geometry, which was not recognized in prior work of this type. This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium as a part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

原位SiC颗粒增强MoSi_2基复合材料室温断裂韧度的效果与机制

以钼、硅、碳粉末为原料,采用湿法混合和原位反应热压一次复合工艺制备纯MoSi2及含原位SiC颗粒体积分数为40%的SiCP/MoSi2复合材料试样,并研究其显微结构和室温断裂韧度.结果表明,原位SiC使MoSi2基体晶粒得到明显细化,消除了脆性SiO2玻璃相,并阻碍SiCP/MoSi2复合材料断裂时的裂纹扩展而造成裂纹的偏转和桥接,最终使SiCP/MoSi2的室温断裂韧度比纯MoSi2有了大幅度的提高,达到4.91 MPa.m1/2.     

Fatigue crack growth in fiber-reinforced metal-matrix composites

Fatigue crack growth in fiber-reinforced metal-matrix composites is modeled based on a crack tip shielding analysis. The fiber/matrix interface is assumed to be weak, allowing interfacial debonding and sliding to occur readily during matrix cracking. The presence of intact fibers in the wake of the matrix crack shields the crack tip from the applied stresses and reduces the stress intensity factors and the matrix crack growth rate. Two regimes of fatigue cracking have been simulated. The first is the case where the applied load is low, so that all the fibers between the original notch tip and the current crack tip remain intact. The crack growth rate decreases markedly with crack extension, and approaches a “steady-state”. The second regime occurs if the fibers fail when the stress on them reaches a unique fiber strength. The fiber breakage reduces the shielding contribution, resulting in a significant acceleration in the crack growth rate. It is suggested that a criterion based on the onset of fiber failure may be used for a conservative lifetime prediction. The results of the calculations have been summarized in calibrated functions which represent the crack tip stress intensity factor and the applied load for fiber failure.     

Fatigue in selectively fiber-reinforced titanium matrix composites

Many applications of the Ti alloy matrix composites (TMCs) reinforced with SiC fibers are expected to use the selective reinforcement concept in order to optimize the processing and increase the cost-effectiveness. In this work, unnotched fatigue behavior of a Ti-6Al-4V matrix selectively reinforced with SCS-6 SiC fibers has been examined. Experiments have been conducted on two different model panels. Results show that the fatigue life of the selectively reinforced composites is far inferior to that of the all-TMC panel. The fatigue life decreases with the decreasing effective fiber volume fraction. Suppression of multiple matrix cracking in the selectively reinforced panels was identified as the reason for their lack of fatigue resistance. Fatigue endurance limit as a function of the clad thickness was calculated using the modified Smith-Watson-Topper (SWT) parameter and the effective fiber volume fraction approach. The regime over which multiple matrix cracking occurs is identified using the bridging fiber fracture criterion. A fatigue failure map for the selectively reinforced TMCs is constructed on the basis of the observed damage mechanisms. Possible applications of such maps are discussed.     

The mode I fracture resistance of unidirectional fiber-reinforced aluminum matrix composites

The mode I fracture resistance has been measured for Al and Al/4 Mg matrix composites, unidirectionally reinforced with ceramic fibers, prepared using a squeeze casting technique. Effects of SiC particle additions have also been investigated. The Al/4 Mg system had a high toughness, whereas the Al matrix system had a relatively low fracture resistance. In all cases, the addition of particulates slightly decreased the resistance to crack growth. The fracture resistance was simulated by a ductile bridging model with plastic dissipation occurring within a zone governed by the fiber spacing. The tensile strength of these composites has been estimated, based on the resistance behavior and microstructure.     

The effect of nb morphology on the cyclic oxidation resistance of MoSi2/20 vol pct Nb composites

The effect of Nb morphology on the 1200 °C cyclic oxidation resistance of MoSi₂/20 vol Pct Nb composites was investigated. Niobium was incorporated into MoSi₂ as particles, random short fibers, and continuous aligned fibers. After oxidation, it was found that all the composites had lost weight and essentially disintegrated. This was attributed to spalling of both the Nb₂O₅ scale and the MoSi₂ matrix. The spalling of the matrix was a result of cracks originating in the oxidized Nb and propagating through the MoSi₂ matrix. These cracks arose from two sources: (1) the volume expansion associated with the transformation of Nb to Nb₂O₃ and (2) the difference in thermal expansion between Nb₂O₅ and MoSi₂. However, it was found that the addition of smaller diameter Nb reinforcements tended to retard the disintegration of the composites. This was attributed to the effect of reinforcement size on CTE mismatch cracking. D.E. Alman, Formerly Graduate Student, Materials Engineering Department, Rensselaer Polytechnic Institute     

Fatigue and fracture behavior of bulk metallic glass

Tensile, compressive, cyclic tension-tension, and cyclic compression-compression tests at room temperature were systematically applied to a Zr_52.5Cu_17.9Al₁₀Ni_14.6Ti₅ bulk metallic glass for comprehensive understanding of its damage and fracture mechanisms. Under tensile loading, the metallic glass only displays elastic deformation followed by brittle shear fracture. Under compressive loading, after elastic deformation, obvious plasticity (0.5 to 0.8 pct) can be observed before the final shear fracture. The fracture strength under compression is slightly higher than that under tension. The shear fracture under compression and tension does not occur along the maximum shear stress plane. This indicates that the fracture behavior of the metallic glass does not follow the Tresca criterion. The fracture surfaces show remarkably different features, i.e., a uniform vein structure (compressive fracture) and round cores coexisting with the radiating veins (tensile fracture). Under cyclic tension-tension loading, fatigue cracks are first initiated along localized shear bands on the specimen surface, then propagated along a plane basically perpendicular to the stress axis. A surface damage layer exists under cyclic compression-compression loading. However, the final failure also exhibits a pure shear fracture feature as under uniaxial compression. The cyclic compression-compression fatigue life of the metallic glass is about a factor of 10 higher than the cyclic tension-tension fatigue life at the same stress ratio. Based on these results, the damage and fracture mechanisms of the metallic glass induced by uniaxial and cyclic loading are elucidated.     

Interfacial modification and impact properties of Nb/MoSi2 laminate composites by the addition of ZrO2, NbSi2, and SiC particles

In this study, the results of a thermodynamical estimation indicate that the impact properties of Nb/MoSi₂ laminate composites can be improved by suppressing the interfacial reaction. The effects of addition of SiC, NbSi₂, and ZrO₂ on the impact properties and interfacial reaction of Nb/MoSi₂ laminate composites are also examined. Laminate composites, which comprise alternate layers of matrix mixture and Nb foil, were fabricated by the hot press process. Addition of ZrO₂ particle is clearly demonstrated to increase both the impact value and the sintered density of Nb/MoSi₂ laminate composites. The thickness of the interfacial reaction layer in Nb/MoSi₂ laminate composites dramatically decreases with increasing volume fraction of ZrO₂ particle. The suppression of the interfacial reaction is caused by the formation of ZrSiO₄ in the MoSi₂-ZrO₂ matrix mixture. In addition, it appears that the reduction of the reaction layer promotes the plastic deformation of Nb foil and the interfacial delamination in laminate composites.     

Fatigue crack growth resistance of unidirectional fiber-reinforced titanium metal-matrix composites under transverse loading

The transverse fatigue crack growth resistance of unidirectional 8 and 35 pct 1140+/Ti-6-4 fiber-reinforced composites has been investigated. It has been found that, at a low fiber volume fraction, the transverse fatigue crack growth resistance of metal-matrix composites (MMCs) is improved with respect to the monolithic matrix alloy. This occurs because “holes” from debonded interfaces can trap the crack and reduce the average fatigue crack growth rates by periodically increasing the effective crack-tip radius. However, an increase of fiber volume fraction from 8 to 35 pct decreases the fatigue crack growth resistance dramatically, due to the significant increase of the frequency of interaction and coalescence between the main crack, the debonded interfaces, and microcracks.     

Fatigue and fracture of damage-tolerant In Situ titanium matrix composites

In an effort to engineer damage-tolerant ingot metallurgy (IM) in situ titanium matrix composites with attractive mechanical properties, the fatigue and fracture properties of a range of high-modulus titanium alloys reinforced with TiB whiskers were examined. The strengthening effects due to elastic whisker reinforcement are quantified using shear lag and rule-of-mixture models. The effects of alloy composition and microstructure on the fatigue behavior of the in situ titanium composites will also be discussed.     

Fatigue and fracture behavior of a fine-grained lamellar TiAl alloy

The fatigue and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pct boron addition was studied by performing tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully lamellar microstructure with approximately 150-μm grain size and 1-μm lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile transition temperature (BDTT), while fracture and fatigue tests were performed at 25 °C and 815 °C. Fracture toughness tests were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained, fully lamellar microstructure were identified and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness, and fatigue resistance were identified and related to pertinent microstructural variables.