A first report on fracture toughness ofbcc iron alloys as influenced by solutes: opposite effects of silicon and cobalt

The effect of solutes on resistance to fracture of body centred cubic iron single-phase solid-solution alloys has been investigated. TheJ-integral method has been used for the measurement of ductile fracture toughness. TheJ _IC values so determined quantitatively indicate the extent of degradation in fracture toughness due to the addition of hardening solute silicon. Cobalt addition results in alloy softening. The measuredJ _IC values clearly demonstrate the toughening effect of cobalt addition as a solute, which result renders the case of Fe-Co solid-solution alloys interesting.     

Toughening of epoxies through thermoplastic crack bridging

The fracture toughness and toughening mechanism of two epoxy matrices containing varying concentrations of pre-formed polyamide-12 particles was investigated. The pre-formed thermoplastic modifier was used to keep the physical and morphological characteristics of the second phase constant while varying the matrix intrinsic toughness to simplify the interpretation of toughening results. We observed that these particles toughened the epoxies through a crack bridging mechanism involving large plastic deformation of the second phase.This mechanism was found to be effective independent of the potential of the matrix for plastic deformation since the increasing fracture toughness was accomplished without significant amounts of plastic deformation in the epoxy matrix. A quantitative model was adapted to account for the increase in toughness due to the crack bridging mechanism. From this model, it was possible to determine the factors which are most important when attempting to toughen a material through thermoplastic crack bridging. A better understanding of the specific factors which influence the efficiency of the crack bridging mechanism enables the fracture properties of brittle materials to be further improved with thermoplastic addition. This was shown to be very important when attempting to enhance the toughness of materials which are believed to be un-toughenable by conventional rubber modification, or materials whose other mechanical properties suffer from the addition of elastomeric materials.     

Damage tolerance assessment by bend and shear tests of two multilayer composites: Glass fibre reinforced metal laminate and aluminium roll-bonded laminate

The damage tolerance of an aluminium roll-bonded laminate (ALH19) and a glass fibre reinforced laminate (GLARE) (both based on Al 2024-T3) has been studied. The composite laminates have been tested under 3-point bend and shear tests on the interfaces to analyze their fracture behaviour. During the bend tests different fracture mechanisms were activated for both laminates, which depend on the constituent materials and their interfaces. The high intrinsic toughness of the pure Al 1050 layers present in the aluminium roll-bonded laminate (ALH19), together with extrinsic toughening mechanisms such as crack bridging and interface delamination were responsible for the enhanced toughness of this composite laminate. On the other hand, crack deflection by debonding between the glass fibres and the plastic resin in GLARE was the main extrinsic toughening mechanism present in this composite laminate.     

微量含Si杂质对W－7Ni－3Fe高比重合金性能的影响

研究了掺杂到原料钨粉中的微量含Ｓｉ杂质（ＳｉＯ２０和Ｎａ２ＳｉＯ３）对Ｗ－７Ｎｉ－３Ｆｅ高比重合金力学性能的影响。结果表明，原料钨粉中的微量含Ｓｉ杂质（掺杂量＜０．０１％）可使合金的抗拉强度，延伸率及冲击韧性有所提高。通过对合金试样断口形貌的扫描电镜（ＳＥＭ）观察及时试样断口表面的Ｘ射线光电子能谱（ＸＰＳ）分析发现，在未掺杂试样的断口表面存在少量的薄膜状ＷＯ２，掺杂微量含Ｓｉ杂质可以消除ＷＯ２薄膜。     

Microstructure and fracture behavior of Al‐Si‐Mg alloy prepared with recycled alloy

In this paper, microstructure and fracture behavior of Al‐Si‐Mg alloy prepared with recycled alloy were studied. The results show that some casting defects such as oxidation slags, pores and iron‐rich intermetallic phases exist in the alloy prepared with recycled alloy. Stress concentration forms in the tips of the pores and iron‐rich intermetallic phases in the process of stretching. Cracks are easy to initiate near slags due to the weakening of the adhesion between the oxidized slag and the aluminum matrix. In addition, silicon phases play a hindering role on the movement of dislocations in the process of crack propagation and merging of cracks which is strengthening the alloy prepared with recycled alloy to some extent. Crack propagation is mainly along the direction of casting defects, and the cracks connect with each other until failure of material. Moreover, the crack originate from silicon phase and extend along the eutectic structure of alloy no‐containing recycled alloy, and a large plastic deformation occurred in the α‐aluminum dendrites.     

烧结Nd-Fe-B合金断裂机制和增韧方法

Nd-Fe-B合金磁性能的提升扩大了其应用范围,但也存在韧性差、脆性大、抗震和抗冲击能力差的缺点,使其在机械加工过程中易开裂,降低了成品率和加工精度,提高了生产成本,也限制了其在高精度仪器仪表、高速电机等领域的应用。因而如何提高其力学性能逐渐成为研究的重点,由此需要探索清楚其断裂机制,并采用适当而有效的方法进行该合金的增韧。基于此目的,综述了近年来Nd-Fe-B合金断裂机制方面的研究成果,分析了其主要是沿晶断裂,只是缺陷的存在会增加穿晶断裂的几率。同时,指出了元素的添加和改善制备工艺是合金增韧的两条途径,不过目前并没有从根本上改变其力学性能差的缺点,所以需要在未来的研究中进一步深入,寻求更为行之有效的增韧方式。     

Influence of temperature on impact fracture behavior of an alloy steel

In this paper, the influence of temperature on impact toughness and fracture behavior of alloy steel (AISI Classification 8320) is presented and discussed. Impact toughness decreased with a decrease in test temperature. The extrinsic influence of temperature on impact toughness–fracture resistance relationships is rationalized in light of the conjoint and mutually interactive influences of intrinsic microstructural features, local stress states and macroscopic fracture behavior.     

晶须增韧补强陶瓷基复合材料的若干关键技术研究(Ⅲ):纤维独石结构和层状结构的设计

为了进一步提高晶须增韧陶瓷基复合材料的断裂韧性，提出了纤维独石结构和层状结构的复合材料的设计思路，这两类特殊结构的复合材料都具有非常高的断裂韧性和断裂功。在这两类材料的设计中，都基于一种多级增韧机制的思想，即在材料中引进相对较弱的界面将一个个结构单元（纤维或薄层）结合起来，结构单元之间的弱界面层作为一级增韧机制，是这两类复合材料具有很高断裂韧性的主要增韧机制；在结构单元内部，晶须增韧体作为二级增韧机制；长柱状的基体晶粒作为三级增韧机制。这三级增韧机制的协同作用，使得这类复合材料具有非常高的断裂韧性和断裂功。     

A transformation toughening white cast iron

An experimental white cast iron with the unprecedented fracture toughness of 40 MPa m1/2 is currently being studied to determine the mechanisms of toughening. This paper reports the investigation of the role of strain-induced martensitic (SIM) transformation. The dendritic microconstituent in the toughened alloy consists primarily of retained austenite, with precipitated M7C3 carbides and some martensite. Refrigeration experiments and differential scanning calorimetry (DSC) were used to demonstrate, firstly, that this retained austenite has an "effective" sub-ambient MS temperature and, secondly, that SIM transformation can occur at ambient temperatures. Comparison between room temperature and elevated temperature KIc tests showed that the observed SIM produces a transformation toughening response in the alloy, contributing to, but not fully accounting for, its high toughness. SIM as a mechanism for transformation toughening has not previously been reported for white cast irons. Microhardness traverses on crack paths and X-ray diffraction (XRD) on fracture surfaces confirmed the interpretation of the KIc experiments. Further DSC and quantitative XRD showed that, as heat-treatment temperature is varied, there is a correlation between fracture toughness and the volume fraction of unstable retained austenite. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of rare earth cerium on the impact toughness,corrosion and wear resistance of CrNiMnMo alloyed lining steel

The effects of the addition of rare earth cerium on the impact corrosion and wear resistance of CrNiMnMo alloyed lining steel were studied using metallurgical microscopy, scanning electron microscope, x-ray powder diffraction, a hardness tester, an impact tester, and an impact corrosion abrasion tester. The properties of CrNiMnMo alloyed lining steel with the addition of cerium were compared with that of CrNiMnMo steel without the addition of cerium after being heat treated by same processes. The results show that CrNiMnMo alloyed lining steel with the addition of rare earth cerium consisted of lath martensite with a low residual austenite phase after being quenched and tempered. The lath of the martensite was fine and uniform. The impact toughness and the corrosion resistance of the steel lining board improved greatly with the addition of cerium and the impact toughness increased by 22 % with the fracture mode of being ductile. The CrNiMnMo alloyed lining steel with cerium showed the highest resistance to impact corrosion abrasion compared with the alloy without cerium.     

Interlaminar mechanical properties of nano- and short-aramid fiber reinforced glass fiber-aluminum laminates: a comparative study

Delamination damages limit the application potential of Fiber metal laminates, hence improving the interlaminar mechanical properties has always been a research focus and challenge in this field. The toughening effect of two fillers, i.e., nano-aramid fibers (ANFs) and short-aramid fibers (ASFs), which are used at the interface of glass fiber-aluminum laminates, have been investigated. Chemical pretreatments of aluminum alloy surface were conducted to ensure the better adherence between the fiber composites and metal sheets. Results revealed that Mode-I and Mode-II fracture toughness of the laminates could be simultaneously improved when using the two fillers at the interface of glass fiber-aluminum laminates. Attributed to the better dispersion of ANFs in epoxy matrix, the toughening performance of ANFs is better than that of ASFs in this case. The mechanism of interlaminar toughening was revealed with electron microscopic observation of fracture morphology. Meanwhile, the finite element analysis based on bilinear cohesive zone model was adopted to predict the increased interlaminar tensile and shear strength.

     

Fracture behaviour in tempered martensite embrittlement of medium-carbon alloy steels

The effect of alloying additions (nickel and silicon) on the fracture behaviour in tempered martensite embrittlement (TME) has been studied in commercial alloy steels. The fracture behaviour is analysed using the fractographs of the impact specimens tested at various temperatures. In 4140-Ni(4340) steel, where nickel-addition increases the intrinsic matrix toughness, the intergranular brittle type of THE is observed. In 4140-Si(4140 + 2Si) steel, where silicon-addition decreases the intrinsic matrix toughness, the intergranular brittle type of TME is also observed. The occurrence of the intergranular brittle type of TME is attributed to the activation of coarse grain-boundary carbides at the grain boundaries which the relatively high impurity content of commercial alloy steel renders impurer (i.e. weaker), despite relatively low intrinsic matrix toughness in 4140-Si steel.     

Strengthening and toughening of cordierite by the addition of silicon carbide whiskers,platelets and particles

Cordierite containing silicon carbide of different morphologies has been fabricated to produce composites with superior properties to the unreinforced cordierite. It has been demonstrated that the morphology of the silicon carbide affects the densification and the mechanical properties of the composites they constitute. The densification process was controlled by the degree of mutual contact of the silicon carbide phase within the cordierite matrix and by the rigidity of the resulting networks. The operative toughening mechanisms and their relative contributions were also dependent on the morphology of the silicon carbide. Fracture strength of the composites was governed by the relative contributions of an improvement in fracture toughness and by the magnitude of the critical flaw size.     

高强铝合金断裂韧性的研究现状及展望

通过对三种裂纹模式的概述，引出了铝合金断裂韧性的概念及断裂判据，重点评述了影响高强铝合金断裂韧性的内在与外在因素，以及断裂韧性与其它力学性能间的关系，最后提出了几个在今后的研究及应用中需要重点解决的问题。     

变质处理对ZG85Cr12MoNi组织和性能的影响

ＺＧ８５Ｃｒ１２ＭｏＮｉ是一种耐磨性能优良的合金钢，但其组织中共晶碳化物量较多，脆性较大，应用于承受冲击的工况下易发生断裂，为提高其抗断裂能力，研究了钾、钠、稀土和钛复合变质处理对其组织和性能的影响。结果表明，ＺＧ８５Ｃｒ１２ＭｏＮｉ经钾、钠、稀土和钛复合变质处理后，组织细化，共晶碳化物由条块状变成团球状且分布均匀，冲击韧性和断裂韧性大幅度提高，热疲劳性能也明显改善；作轧机导卫板用使用安全，寿命比高铬铸铁和高镍铬合金分别提高３００％和３５．６％。     

Influence of high aluminium content on the mechanical properties of directionally solidified multicrystalline silicon

The purpose of this research is the mechanical characterisation of multicrystalline silicon crystallised from silicon feedstock with a high content of aluminium for photovoltaic applications. The mechanical strength, fracture toughness and elastic modulus were measured at different positions within the multicrystalline silicon block to quantify the impact of the segregation of impurities on these mechanical properties. Aluminium segregated to the top of the block and caused extensive micro-cracking of the silicon matrix due to the thermal mismatch between silicon and the aluminium inclusions. Silicon nitride inclusions reduced the fracture toughness and caused failure by radial cracking in its surroundings due to its thermal mismatch with silicon. However, silicon carbide increased the fracture toughness and elastic modulus of silicon.     

Fracture toughness of metal reinforced glass composites

The effect of hardness and strength of particulate reinforcements on the toughening of a glass matrix composite have been investigated. Spherical particles of two gold-based alloys were blended with a low-fusing glass powder; the mixture was hot-pressed, and disc-shaped specimens prepared for fracture toughness testing using the strength/flaw method. Scanning electron microscopy was used to examine fracture surfaces. It was found that the softer, more ductile alloy was a more effective toughening additive than the harder alloy.     

超声辅助微弧氧化Ti-13Nb-13Zr合金制备仿生涂层及其断裂力学性能

为了提高微弧氧化钛合金制备的脆性仿生涂层的断裂力学性能,利用超声辅助微弧氧化复合工艺在Ti-13Nb-13Zr合金表面制备了钙磷生物涂层。通过压痕法测试分析了涂层断裂韧性,采用扫描电镜和X射线衍射仪测试了涂层表面形貌和相组成,并与微弧氧化制备的涂层性能进行了比较,分析了增韧原因。结果表明,引入超声后,微弧放电电压下降了40V,涂层致密层明显增厚;相同电源占空比条件下,超声工艺所制备涂层的断裂韧性相比无超声工艺都有所提高。部分锐钛矿相TiO_2转变为金红石相的相变增韧,超声空化效应引起的涂层致密化和增厚效果,以及微裂纹的均匀分布,是促使涂层断裂力学性能提高的主要原因。该复合工艺实现了微弧氧化钛合金生物涂层的增韧。     

Fracture toughness assessment of silicon carbide-based ceramics and particulate-reinforced composites

The fracture toughness of sintered silicon carbide (α-SiC) and silicon carbide reinforced with particulate titanium diboride (TiB₂/SiC) has been evaluated using specimens in bending containing chevron notches and through-thickness precracks at ambient and elevated temperatures in air and in vacuum. Fracture toughness values measured from through-thickness precracked test-pieces are lower at all test temperatures. The particulate reinforcement has been shown to toughen the matrix significantly at room temperature only. At the test temperature of 1200°C the difference in toughness between the two materials is reduced and increasing the temperature to 1600°C further reduces this difference, to the extent that the two materials have values of fracture toughness which are indistinguishable. This provides strong evidence that the dominant toughening mechanism in the composite is the effect of thermal residual stresses which are relieved as the temperature is increased. Fractographic observations suggest that the bonding between the SiC and TiB₂ particulate is relatively weak because interfacial decohesion of particles is observed at all test temperatures. Nevertheless, surface roughness measurements indicate that there may also be a contribution to the toughness from increased crack deflection in the composite material at room temperature only.     

Toughening of a brittle thermosetting polymer: Effects of reinforcement particle size and volume fraction

Micron- and nanometer-sized aluminum particles were used as reinforcements to enhance the fracture toughness of a highly-crosslinked, nominally brittle, thermosetting unsaturated polyester resin. Both particle size and particle volume fraction were systematically varied to investigate their effects on the fracture behavior and the fracture toughness. It was observed that, in general, the overall fracture toughness increased monotonically with the volume fraction of aluminum particles, for a given particle size, provided particle dispersion and deagglomeration was maintained. The fracture toughness of the composite was also strongly influenced by the size of the reinforcement particles. Smaller particles led to a greater increase in fracture toughness for a given particle volume fraction. Scanning electron microscopy of the fracture surfaces was employed to establish crack front trapping as the primary extrinsic toughening mechanism. Finally, the effects of particle volume fraction and size on the tensile properties of the polyester-aluminum composite were also investigated. The measured elastic modulus was in accordance with the rule-of-mixtures. Meanwhile, the tensile strength was slightly reduced upon the inclusion of aluminum particles in the polyester matrix.