Ductile-phase toughening in V-V3Si in situ composites

This article describes the room-temperature fracture behavior of ductile-phase-toughened V-V₃Si in situ composites that were produced by arc melting (AM), cold-crucible induction melting (IM), and cold-crucible directional solidification (DS). Composites were produced containing a wide range of microstructures, interstitial impurity contents, and volume fractions of the ductile V-Si solid solution phase, denoted (V). The fracture toughness of these composites generally increases as the volume fraction of (V) increases, but is strongly influenced by the microstructure, the mechanical properties of the component phases, and the crystallographic orientation of the (V) phase with respect to the maximum principal stress direction. For eutectic composites that have a (V) volume fraction of about 50 pct, the fracture toughness increases with decreasing “effective” interstitial impurity concentration, [I]=[N]+1.33 [O]+9 [H]. As [I] decreases from 1400 ppm (AM) to 400 ppm (IM), the fracture toughness of the eutectic composites increases from 10 to 20 MPa √m. Further, the fracture toughness of the DS eutectic composites is greater when the crack propagation direction is perpendicular, rather than parallel, to the composite growth direction. These results are discussed in light of conventional ductile-phase bridging theories, which alone cannot fully explain the fracture toughness of V-Si in situ composites.     

Strengthening of composites due to microstructural changes in the matrix

The addition of discontinuous silicon carbide (SiC) to aluminum (Al) alloys can result in a five-fold increase in the yield stress. The magnitude of the increase is obviously a function of the volume fraction and the particle size of the SiC. Previously, it was proposed that the strength increase due to SiC addition to Al alloys was the result of change in the matrix strength, i.e. an increase in dislocation density and a reduction of subgrain size. The data obtained from a series of experiments indicate that dislocation density increases with an increase in volume fraction of SiC and decreases with an increase in particle size. The subgrain size decreases as the volume fraction increases and increases as the particle size increases. There is a good correlation between the microstructural changes in the matrix and the changes in the yield stress of the composites.     

On the actual contribution of crack deflection in toughening platelet-reinforced brittle-matrix composites

The effectiveness of crack deflection as a single toughening mechanism is evaluated in the case of brittle-matrix reinforced by high aspect-ratio platelets. Microstructural parameters characterizing size, morphology, orientation and distribution of the second phase including aspect ratio, volume fraction and interparticle distance, were experimentally obtained by means of image analysis techniques on a model composite. Introducing “true” microstructural parameters in the fracture mechanics formulation of the strain energy release rate for a deflected crack, it is demonstrated that no appreciable toughening can be obtained due to the geometrical local misalignment of the crack front even in the case of increasing aspect-ratio or volume fraction of second phase. It is concluded that crack deflection processes may play an important role only as precursors for other operative mechanisms, being, in the general case, the deflection of the crack path only phenomenologically related to toughening.     

Delineating brittle-phase embrittlement and ductile-phase toughening in Nb-based in-situ composites

The fracture toughness of Nb-based in-situ composites typically decreases with increasing volume fractions of hard intermetallic phases, despite the presence of a ductile niobium solid-solution phase in the microstructure. For composites with a continuous intermetallic matrix, the fracture toughness can be more than double that of the monolithic intermetallics, but is still low in absolute terms, indicating that the solid-solution phase is not very effective in inducing ductile-phase toughening. The lack of enhancement of the fracture resistance appears to arise from an embrittlement effect instigated by the brittle phases in the microstructure, whose nondeformability results in a high plastic constraint acting on the ductile phase. In this article, an analytical model is developed for treating both brittle-phase embrittlement and ductile-phase toughening in terms of constituent properties and microstructural variables. The model is then used to (1) delineate brittle-phase embrittlement and ductile-phase toughening in Nb-based in-situ composites, and (2) design fracture-resistant in-situ composites based on Nb-Ti-Cr, Nb-Ti-Al, and Nb-Ti-Si systems.     

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.     

纳米级氮化钒最佳纳米化尺度的计算研究

为了研究纳米氮化钒添加到钢中的性能优势,通过有关纳米级氮化钒颗粒所含的晶胞数、原子数以及表面原子数的计算,得到相关纳米氮化钒结构参数的表征,并结合纳米微粒的特性对纳米氮化钒的最佳纳米化尺度进行模拟计算。结果表明,纳米级氮化钒的最佳纳米化尺度为200 nm,此时颗粒内的总原子数和平行于(001)面原子数达到平衡。     

Design of multiscalar metallic multilayer composites for high strength,high toughness,and low CTE mismatch

We propose a new class of multilayer composites that consists of alternating tough and strong layers. Both the tough and the strong layers are metallic, effectively reducing the coefficient of thermal expansion (CTE) mismatch problem that often plagues metal-ceramic composites. The high-strength layers are themselves very fine-scale metallic multilayer composites. The high strengths result from Orowan strengthening of these very fine-scale layers. We present detailed analyses of the flow stress, toughness, and thermal stability of these multiscalar metallic multi-layer composites (M³C) as a guide for microstructural optimization. The dominant term in the flow stress is proportional to the volume fraction of the strong layers and scales inversely with thickness of the very fine-scale layers that make up the strong layer. The toughness is dominated by the plastic flow of the tough layers and is proportional to the volume fraction and flow stress of the tough layers, as modified by plastic constraint. The thermal stability of M³Cs is discussed in the context of solubility, length scales, and interdiffusivity of the two metals. Preliminary results suggest that M³Cs do exhibit an unusual combination of high toughness and strength.     

An investigation of toughening in NiAl composites reinforced with yttria-partially stabilized zirconia particles

The results of an investigation of toughening mechanisms in NiAl composites reinforced with yttria-partially stabilized zirconia polycrystals are presented. Different yttria stabilization levels in the zirconia, between 0 and 6 mole pct are employed. It is shown that substantial improvements in fracture toughness are obtained in all the composites reinforced with partially stabilized zirconia particles. The phase contents and microstructures of the composite systems are characterized by X-ray diffraction and transmission electron microscopy (TEM) techniques. Crack tip deformation was also studied using crack tip TEM analysis, and laser Raman spectroscopy was used to estimate the size of the transformation zone. The results show that transformation toughening is significant only in the 2 mole pct yttria-stabilized zirconia composite. Toughening is also shown to occur via slip phenomena within the NiAl grains in the near-tip regions of the composites reinforced with 2, 4, or 6 mole pct yttria-stabilized zirconia particles.     

Distribution of metallic phase in carbonado diamond polycrystalline composites

The structure of carbonado synthetic diamonds is studied by scanning electron microscopy. It is stated that the surface of the carbonado is coated by coarse diamond crystallites reaching 200 μm and having an open growth texture. Metal-catalyst (nickel) drops with submicrometer sizes were observed on the surface of the crystals. It is revealed that the synthetic carbonado structure is made of interpenetrating carcasses of diamond and metalloceramic phases.     

Large scale bridging in brittle matrix composites

The influence of the bridging zone length on the resistance curve behavior of three brittle-matrix composites is examined. The experimental measurements are correlated with models of crack bridging (taking into account the finite specimen dimensions) and compared with the resistance curves expected when small-scale bridging conditions prevail. The results demonstrate that the resistance curves of composite materials strongly depend on both the absolute length of the bridging zone and the length of the bridging zone relative to the total crack length and the specimen width. The latter effects are due to large-scale bridging. The results suggest that the resistance curves of toughness measurements obtained from small test specimens may overestimate the true behavior and thus, caution must be exercised in interpreting some of the recently published data. The implications for future resistance curve measurements are discussed.     

Fiber fragmentation during processing of metallic matrix composites

Fiber fragmentation is a problem frequently encountered during the processing of metallic matrix composites. In this study, we examine the fragmentation of continuous fibers during a common composite consolidation process based on hot pressing in an open-ended channel die with fibers aligned parallel to the die walls. During the latter stages of consolidation, flow of the matrix along the die cavity may occur such that the resulting load transfer to the fibers can cause their fracture even in the absence of bending. This study analyzes the combination of conditions necessary for both matrix flow along the die cavity and the shear-lag loading of the fibers to a level that causes fragmentation. In order to validate the analysis, we model the fragmentation of fibers during elevated temperature hot pressing of Ni-base composites by the room-temperature consolidation of degraded sapphire fibers in a tin matrix. The observed fiber fragmentation behavior is in good agreement with theoretical predictions. The analysis also indicates that this mode of fiber fragmentation is confined either to low volume-fraction fiber composites or to the ends of panels of high volume-fraction fiber composites.     

SiC晶须增韧Ti(C,N)基金属陶瓷复合材料的研究

主要研究了不同的SiC晶须(简称SiCw)加入量和真空烧结温度对Ti(C,N)基金属陶瓷复合材料性能的影响,并对SiCw的增韧机理进行了探讨.结果表明:在SiCw的质量分数为15%、真空烧结温度为1470℃时,SiCw增韧Ti(C,N)基金属陶瓷复合材料的综合力学性能最佳;材料中存在裂纹偏转、裂纹桥接和晶须拔出等增韧机理.     

Preparation of metallic glass based composites and measurement of the fracture toughness

采用铜模吸铸法制备了（Zr0.55Al0.1Ni0.05Cu0.30）100-xTix（x=0、2、4、6、8）板状哑铃型金属玻璃基复合材料试样。用X射线衍射（XRD）、岛津AG-10TA万能材料力学试验机和JSM-6700F场发射扫描电子显微镜（SEM）对试样的组织结构以及断裂韧性进行了测试。结果表明,当x=0、2、4时,试样为非晶-晶体复合材料,当x=6、8时,试样为晶体材料。表明通过调整Ti的含量可以制备出金属玻璃基复合材料。采用三点弯曲法测定了复合材料的断裂韧性,当x=0、2、4时,试样的断裂韧性KIC值分别为10.529、5.142和3.446MPa.m1/2。     

一类状态时滞系统的最优预见控制器设计

研究了一类状态时滞系统的最优预见控制器设计问题.首先通过差分将所研究的时滞系统转化为形式上不含时滞的一般系统,然后根据已有的无时滞系统的预见控制理论设计出系统的控制器,并且给出了所设计的控制器存在的充分条件.仿真实验说明了预见前馈补偿的有效性.     

Deformation behavior of metallic composites with brittle fibers during bending

Conclusions The aluminum-boron system was chosen for studying the mechanism and conditions of deformation of metallic composite materials reinforced with brittle fibers. It was established that, unlike that of an orthodox metallic material, the bending of a composite is accompanied by displacement of a neutral layer in the direction of compression. The causes and character of the displacement of this layer in the course of deformation were determined. The role of processing pads in bending was examined. A study was made of the fracture of fiber-reinforced composites on the attainment of critical values of strain.Translated from Poroshkovaya Metallurgiya, No. 11(239), pp. 80–85, November, 1982.     

Elasto-plastic load transfer in bulk metallic glass composites containing ductile particles

In-situ diffraction experiments were performed with high-energy synchrotron X-rays to measure strains in crystalline reinforcing particles (5 and 10 vol. pct W or 5 vol. pct Ta) of bulk metallic glass composites. As the composites were subjected to multiple uniaxial tensile load/unload cycles up to applied stresses of 1650 MPa, load transfer from the matrix to the stiffer particles was observed. At low applied loads, where the particles are elastic, agreement with Eshelby elastic predictions for stress partitioning between matrix and particles is found, indicating good bonding between the phases. At high applied loads, departure from the elastic stress partitioning is observed when the particles reach the von Mises yield criterion, as expected when plasticity occurs in the particles. Multiple mechanical excursions in the particle plastic region lead to strain hardening in the particles, as well as evolution in the residual strain state of the unloaded composite.     

Load ratio,notch, and scale effects for bridged cracks in fibrous composites

This paper considers the problem of Mode I matrix cracks growing under either monotonic or cyclic loading in the presence of unidirectional, bridging fibers coupled to the matrix by friction. Scaling relations facilitate the presentation of exhaustive solutions for specimens of various shapes. Particularly simple results are found numerically for the ratio of the net to the applied crack tip intensity factor in monotonic loading or the ratio of their ranges in cyclic loading. Material properties that control how finite specimen size and applied stress level determine the relative strength of bridging are identified for monotonic and cyclic loading. Bending is shown to be important in edge notch specimens, even under uniform remote loading. Conditions are found for fracture surface contact during cyclic loading.