The fracture toughness of niobium-based,in situ composites

The fracture resistance of Nb-Cr-Ti alloys orin situ composites of three different compositions, Cr₂Nb, and a Nb-10Siin situ composite was studied at ambient temperature. The crack-tip deformation and fracture behaviors were characterized using near-tip measurement techniques and fractographic analyses. The relevant fracture and toughening mechanisms were identified and related to the microstructure. Despite fracture by a combination of cleavage and slip band decohesion, the Nb solid-solution alloy exhibited a resistance-curve behavior with a relatively high toughness and local ductility. The source of toughness was modeled and explained in terms of a cracking process that involved alternate slip band decohesion and cleavage. Thein situ composites, on the other hand, exhibited cleavage fracture but considerably lower toughness with little or no resistance-curve behaviors. The difference in the fracture behavior appears to arise from two factors: (1) the presence of a high constraint in the Nb solid-solution matrix in thein situ composites, and (2) the lack of plastic flow associated with cleavage of the constrained Nb solid-solution matrix.     

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.     

Effect of fiber orientation on the fracture toughness of brittle matrix composites

The effective fracture toughness of brittle matrix materials can be increased through the addition of short, poorly bonded fibers which bridge the growing crack. The orientation distribution of the fibers is likely to be biased, and not in an ideal random or aligned state. A micromechanical model is formulated for the post-cracking behavior using the force-displacement relation for an arbitrary fiber bridging a crack, the fiber orientation density function, and the fiber location density function. This model is then used to determine an effective traction law for the bridging fibers, as well as the steady state bridging toughness increment. In most cases, the results may be placed in the form of a product of the aligned fiber results times a modifying integrated orientation factor. The friction shear st ress on fiber pull-out is allowed to vary during pull-out, modeling the effects of matrix breakdown, fiber surface smoothing or wear debris accumulation. Results are presented for a variety of representative planar and three-dimensional fiber orientation states.     

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。     

MoSi2复合材料断裂韧性的测量及评价

对压痕法和单边切口梁法测量MoSi2基复合材料的断裂韧性进行了研究和分析,提出了压痕法测量MoSi2基复合材料断裂韧性的计算修正式K1C=0.0336(l/α)-1/2(Eφ/H)2/5 Hα1/2/φ.结果表明单边切口梁法的测量结果随试样切口宽度的增加而增大,压痕法测量的结果随计算式和试验载荷的不同而不同.使用上述修正式将缩小压痕法与SENB法的测量偏差.     

Effects of thickness and precracking on the fracture toughness of particle-reinforced al-alloy composites

The effect of specimen thickness on the fracture toughness of a powder metallurgically processed 7093 Al/SiC/15p composite was evaluated in different microstructural conditions. The fracture toughness in the underaged condition increased significantly with a decrease in specimen thickness, even at thicknesses well below the value specified by ASTM-E 813 for a valid J _Ic test. The influence of thickness was considerably lower in the peak-aged (PA) condition. This relative insensitivity is believed to be due to the low strain to failure associated with severe flow localization in the PA condition. The effect of precracking on the fracture toughness of discontinuously reinforced aluminum (DRA) was also evaluated. The dependence of fracture toughness on specimen thickness and precracking is explained in terms of the hydrostatic stress, which has a strong influence on the plastic straining capability of the DRA material. The fracture toughness was modeled using a critical strain formulation, with the void growth strain dependent on hydrostatic stress through the Rice and Tracey model. The predicted toughnesses for the thick and thin specimens were in good agreement with the experimental data.     

A formal approach to optimizing the groove design

A single formula is derived for the broadening in grooves of different shapes. On that basis, the groove design may be optimized so as to maximize the efficiency of deformation, by means of optimal control theory. Numerical results of optimization are presented.     

The effects on fracture toughness of ductile-phase composition and morphology in Nb-Cr-Ti and Nb-Siin situ composites

Niobium-chromium alloys, both single and two phase, were alloyed with titanium in order to enhance fracture toughness and fatigue crack growth resistance. The selection of titanium as an alloying element and the relationship of electronic bonding to toughness are examined. The results indicated that toughness increased with a decreasing number of D +s electrons. Titanium was found to increase the toughness of solid-solution Nb-Cr alloys from ≈8 to 87 MPa√m, while for the twophase “insitu composites,” toughness was increased from ≈5 to 20 MPa√m, although this is less than expected. Fracture toughness of the composites correlated nonlinearly with the volume fraction of the phases. The evidence suggests that the toughness of the composites is decreased due to fracture of the intermetallic particles and constraint on matrix deformation imposed by the intermetallic. Fracture characteristics of the Nb-Cr-Ti materials are compared to those of Nb-Cr and Nb-Si materials.     

Microstructural analysis of fracture toughness variation in 2XXX-series aluminum alloy composites reinforced with SiC whiskers

The effects of local microstructure on fracture properties in powder-metallurgy (P/M)-processed 2124/SiC/15w and 2009/SiC/15w composites are analyzed in this study. Ductility and fracture toughness of the 2009/SiC/15w, in which dispersoid-forming elements such as manganese and iron were nearly absent, were greater than in the 2124/SiC/15w, while its tensile and yield strengths were somewhat less. Microstructural examination and fracture parameter analysis revealed that the improved fracture toughness of the 2009/SiC/15w compared to the 2124/SiC/15w was due to the increase in the critical microstructural distance,l* when manganese-containing particles are absent. 2009/SiC/15w was also heat-treated in T4P and overaged (OA) conditions. The OA 2009 composite showed lower fracture toughness than the 2009-T4P composite and the critical fracture strain of the OA condition was much lower, too. Detailed fractographic analyses indicated that interface precipitates facilitate premature SiC whisker failure in the OA condition. Formerly Research Assistant, Department of Materials Science and Engineering Pohang University of Science and Technology, Pohang, 790-784 Korea     

硬质合金断裂韧度的测试研究现状

断裂韧度是用来衡量硬质合金在实际工况条件下使用寿命最为重要的力学性能指标之一,因而有关硬质合金断裂韧度的研究一直备受硬质合金领域研究者和使用者的关注。评述了国内外有关硬质合金断裂韧度测试方面的研究成果,重点介绍几种常用的硬质合金断裂韧度测试方法的特点及研究现状,同时对硬质合金主要断裂力学模型的研究工作进行简要介绍。     

高硅带钢激光焊缝抗断裂能力的研究

带钢在酸洗、轧制过程中带钢焊缝需承受很高的拉伸应力、弯曲应力和轧制力.在这些力的作用下,带钢焊缝内部的缺陷可能会扩展,从而造成断裂,严重影响线连续生产.因此希望能够找到一种有效的评价带钢焊缝抗断裂能力的方法.通过线切割缺口试样拉伸试验方法研究了带钢不同焊缝的抗断裂能力,并对焊缝抗断裂能力进行了评价.