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MECHANICAL BEHAVIOUR OF GRAPHITE AND ITS ADJACENT ZONE TO MATRIX INTERFACE IN CAST IRON 总被引:1,自引:0,他引:1
HE Zerong JI Song LIN Guoxiong Fuzhou University Fuzhou China professor Department of Mechanical Engineering Fuzhou University Fuzhou China 《金属学报(英文版)》1993,6(11):364-368
The micro-processes on tensile deformation and fracture of graphite-matrix interface incast iron with ferrite matrix under SEM have been observed in situ.It was revealed thatthe graphite in cast iron would not be regarded as a cavity and not formed yet notchstress concentration.A new explanation on the effect of graphite on strength of cast ironwas suggested. 相似文献
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The occurrence of internal ductile failure in cold-headed products presents a major obstacle in the fast expanding cold heading (CH) industry. This internal failure may lead to catastrophic brittle fracture under tensile loads despite the ductile nature of the material. Comprehensive testing and investigation methodologies were used to this work to reveal the complicated interplay of process and material parameters contributing in the initiation and propagation of internal ductile failure in six CH quality AISI steel grades.The metallurgical and microscopic investigations showed that internal ductile failure occurs progressively by void nucleation and growth mechanisms with increasing plastic strain inside the highly localized adiabatic shear bands (ASBs). The void nucleation occurs by decohesion at second-phase particles, inclusion–matrix interfaces, grain boundaries and by particle or inclusion cracking. Therefore, the number and morphology of any inclusions and second-phase particles are key factors in material formability.The metallurgical investigations showed that under compressive loading conditions, the nature of the metal flow pattern promotes different rates of material flow around the inclusions and stringers which supports decohesion and void nucleation since the early stages of deformation. At advanced stages of deformation, the metal flow pattern contributes to the ASB localization in supporting void growth and coalescence along the band leading to narrow void sheets.All tested materials in this work experienced ductile failure by void nucleation and coalescence, forming cracks along the ASBs. The ductile failure of each material was the result of the contribution of all the mechanisms of void nucleation at the inclusion–matrix interface, second phase–matrix interface and at the grain boundaries. However, the level of contribution of each mechanism in the final ductile failure varied depending on material properties and their microstructure. 相似文献
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In this study, microdeformation of soft particles in metal matrix composites was investigated. For that purpose, two different types of ductile iron having different metal matrix microstructures were tensile stressed and compressed. It was found that graphite nodules do not change their shape during tensile stressing, only decohesion occurs on the graphite and metal matrix interface. However, compression testing at room temperature resulted in a significant change in graphite shape. A relationship between average aspect ratio of graphite after various degrees of compression and bulk material deformation was devised. Furthermore, graphite nodule and metal matrix microdeformation was compared to bulk material deformation. This implies that deformation process is not homogenous. It strongly depends on metal matrix microstructure and can be represented with deformation ratio (K). Deformed or deformed and cracked graphite nodules were found on the fractured surface of compressed specimens, indicating that the graphite does not turn into powder during compression at room temperature, as other previous researches found. 相似文献
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R. Cao J. X. Wen H. J. Liu J. H. Chen 《Journal of Materials Engineering and Performance》2018,27(7):3374-3385
The notch sensitivity of tensile specimens of TiAl and K418 alloys has been investigated, and the notch strength has been quantitatively analyzed. The fracture surface of the specimens has also been observed and analyzed by scanning electron microscopy (JEOL-6700F). By comparing notch rod tensile specimens and special notch rod tensile specimens, it was found that the basic nature, fracture driving force, and fracture criteria of the brittle TiAl alloy and the ductile K418 alloy are different. The final fracture of the K418 alloy is controlled by strain and not by stress, and the specimens do not exhibit notch sensitivity. However, the final fracture of the TiAl alloy is controlled by stress, and the specimens exhibit small notch sensitivity. For the special notch rod tensile specimens, the K418 specimens do not exhibit notch sensitivity. However, for the TiAl alloy, when the notch depth reaches 10% of the specimen diameter, the tensile strength decreases, and when the notch depth reaches 20% of the specimen diameters, the tensile strength sharply decreases. 相似文献
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The tensile fracture characteristics of austempered ductile irons with dual matrix structures and different ausferrite volume
fractions have been studied for an unalloyed ductile cast iron containing (in wt.%) 3.50 C, 2.63 Si, 0.318 Mn, and 0.047 Mg.
Specimens were intercritically austenitized (partially austenitized) in two phase region (α + γ) at various temperatures for
20 min and then quenched into a salt bath held at austempering temperature of 365 °C for various times and then air cooled
to room temperature to obtain various ausferrite volume fractions. Conventionally austempered specimens with fully ausferritic
matrix and unalloyed as-cast specimens having fully ferritic structures were also tested for comparison. In dual matrix structures,
results showed that the volume fraction of proeutectoid ferrite, new (epitaxial) ferrite, and ausferrite [bainitic ferrite + high-carbon
austenite (stabilized or transformed austenite)] can be controlled to influence the strength and ductility. Generally, microvoids
nucleation is initiated at the interface between the graphite nodules and the surrounding ferritic structure and at the grain
boundary junctions in the fully ferritic microstructure. Debonding of the graphite nodules from the surrounding matrix structure
was evident. The continuity of the ausferritic structure along the intercellular boundaries plays an important role in determining
the fracture behavior of austempered ductile iron with different ausferrite volume fractions. The different fracture mechanisms
correspond to the different levels of ausferrite volume fractions. With increasing continuity of the ausferritic structure,
fracture pattern changed from ductile to moderate ductile nature. On the other hand, in the conventionally austempered samples
with a fully ausferritic structure, the fracture mode was a mixture of quasi-cleavage and a dimple pattern. Microvoid coalescence
was the dominant form of fracture in all structures. 相似文献
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利用金相显微镜、扫描电镜、透射电镜和万能拉伸试验机等测试手段,研究了原位合成TiB2(wt,8%)/6351Al复合材料在热挤压前后的显微组织及室温拉伸性能。结果表明,热挤压变形有助于增强颗粒在基体合金中均匀分布,热挤压后TiB2颗粒与基体界面结合良好,未发现界面处开裂;热挤压变形时TiB2颗粒周围的基体合金中形成复杂的位错;基体合金发生再结晶和回复形成完整的等轴晶和亚结构,显微组织得到细化,基体合金再结晶的主要形核方式为增强体颗粒引起位错塞积区形核,亚晶吞并长大形核及应变诱发晶界迁移形核。热挤压复合材料基体合金具有较强的[111]织构。与铸态相比,热挤压后复合材料的屈服强度Rp0.2、抗拉强度Rm、伸长率A及布氏硬度显著提高。复合材料断口特征由热挤压前的韧性和沿晶的混合断裂,转变为以韧性断裂为主。 相似文献
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AbstractGrey cast iron has been regarded, in some circles, as being brittle, low in reliability and one where the usual design equations cannot be applied. The reason is usually attributed to the presence of randomly distributed graphite flakes. This presentation discusses, both experimentally and theoretically, the notch strength, bending strength, and the scattering of tensile strength of grey cast iron, and suggests a new approach based on plasticity theory for the accurate evaluation of these characteristics. Experiments showed that grey iron has a very low notch sensitivity in circumferentially notched bars, but that strength decreases in edge-notched plates. At the temperature of liquid nitrogen, notch sensitivity is very high. In ductile iron, strength increases in notched specimens. These results have been interpreted by the stress distribution, taking into account non-elastic stress/strain behaviour, and using a fracture criteria with an over-stressed depth, δ· The δ is a necessary region for fracture and its minimum value is related to the graphite eutectic cell size. Nominal bending strength, which is about twice that of the tensile strength and which varies with the beam height and sectioned geometry, was also explained by these considerations. The scatter in strength, expressed by the coefficient of variation, was 2–9% in grey iron, a figure not as large as expected. The value was higher in the case of small test pieces, whilst the average strength was unchanged. These results were easily explained by the strength- hardness relationship and by a relationship derived from the strength theory of bundled threads, in which the fracture process is similar to that of cast iron.In the analyses, the fracture behaviour of cast iron was assumed to be non-elastic, and ductile rather than elastic and brittle. It is shown that reasonably accurate strength evaluation of cast-iron structures is possible by considering these characteristics. 相似文献
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Liu Jin-hai Yan Jian-shuai Zhao Xue-bo Fu Bin-guo Xue Hai-tao Zhang Gui-xian Yang Peng-hui 《中国铸造》2020,17(4):260-271
The quantity and morphology of spheroidal graphite have an important effect on the properties of ductile iron,and the characteristics of spheroidal graphite are determined by the solidification process.The aim of this work is to explore the precipitation and evolution of graphite nodules in hypoeutectic,eutectic,and hypereutectic ductile irons by thermal analysis,liquid quenching and metallographic technique.Results show that hypoeutectic ductile iron has the longest solidification time and the lowest eutectic temperature;eutectic ductile iron has the shortest solidification time;hypereutectic ductile iron has the highest eutectic temperature.After solidification is completed,hypoeutectic ductile iron has the lowest nodule count,nodularity and graphite fraction;eutectic ductile iron has the highest nodule count,nodularity and the smallest nodule diameter;hypereutectic has the highest nodule diameter and graphite fraction.The nucleation and growth of graphite nodules in hypereutectic ductile iron starts before bulk eutectic crystallization stage,however,the precipitation and evolution of graphite nodules of hypoeutectic and eutectic ductile irons mainly occur in the eutectic crystallization stage.The graphite precipitated in eutectic crystallization of hypoeutectic,eutectic,and hypereutectic ductile irons,are 61%,68% and 43% of total graphite volume fraction,respectively.Simultaneously,there are plenty of austenite dendrites in hypoeutectic and hypereutectic ductile irons,which are prone to shrinkage defects.Therefore,the eutectic ductile iron has the smallest shrinkage tendency. 相似文献
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采用扫描电镜分析了QT900-5(1#试样)和QT900-2(2#试样)的微观组织及断口形貌.结果表明,1#试样石墨球细小,且形状较为圆整;2#试样石墨球数量较少,比较粗大.对断口的扫描结果发现,1#试样的微观断裂特征为石墨球与基体形成圆整的韧窝;而2#试样石墨球表面粗糙,且极不规整,断口处出现部分团絮状的硅酸盐.对两组试样的湿化学分析结果表明,1#和2#试样残留Mg含量分别为0.056%和0.035%;而P含量分别为0.036%和0.051%.2#试样较低的Mg残留量和较高的P含量是其韧性较低的原因之一. 相似文献
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1 INTRODUCTIONComparedtoconventionaltechnologiessuchascastingandplasticforming ,semi solidmetalprocess ing (SSP)isakindofnewlyemergingmetalprocess ingtechnology ,beinghighlyefficientandlow con suming .So ,itisnecessarytoconductsometheoreti calandexperimentals… 相似文献
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Void growth and coalescence/linkage, which play significant roles during ductile fracture processes, are strongly influenced by stress triaxiality in a deforming solid. The stress state can be changed by cutting notches in a tensile sample. In the current paper, void growth and linkage of an artificial void array embedded in a notched model material was studied by X-ray computed tomography, coupled with in situ tensile deformation. The cross-sectional shape of the tensile specimens was square, and a pair of notches was cut along only one direction. Thus, the lateral principal stress does not have an isotropic distribution: the principal stress along the notch direction is considered to be higher. This technique allowed us to explore the entire process of growth and linkage events of a void array embedded in a metal matrix. The notch effect creates a marked acceleration in void growth, leading to a large reduction in the linkage strains, as compared with similarly fabricated unnotched samples. The standard models for coalescence could not provide consistent predictions of the measured notch effect. 相似文献
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The fracture behavior of SiCp/A356 composite at room and high temperatures was studied.Under tensile stress condition at room temperature, the fracture is mostly a combination of the brittle fracture of SiC particles and ductile fracture of A356 matrix.As the tensile temperature increases, the composite changes the main fracture behavior to the separation fracture of the bonding surface between SiC particles and A356 matrix.When the tensile temperature reaches 573 K, the fracture behavior of the composites is almost the whole separation fracture of the bonding surface, which is the main strengthening mechanism at high temperature.Under the cycle stress condition at room and high temperatures, the main fracture behavior of the composites is always a combination of the brittle fracture of SiC particles and ductile fracture of A356 matrix.However, under the cycle stress at high temperature, cycle behavior of the composites changes from cycle hardening at room temperature to the cycle softening at high temperature. 相似文献
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采用光滑和缺口拉伸试样进行不同温度(950、1050和1150 ℃)和不同应变速率(0.5、1.0和5.0 s-1)的高温拉伸试验,研究了40CrNiMo钢在高温拉伸时的力学性能变化、微观组织演变以及塑性损伤形成机理,分析了不同应力三轴度对高温塑性损伤的影响。结果表明,提高变形温度或降低应变速率会降低峰值应力;应变速率从0.5 s-1增大至5 s-1,晶粒大小不均匀程度增加,材料更容易产生塑性损伤;变形温度从950 ℃提高到1150 ℃,晶粒尺寸增大近3倍;损伤经历形核、长大并形成微裂纹3个步骤,应力三轴度与缺口半径成负相关关系,应力三轴度的增大会加剧塑性损伤的发生,使得拉伸试件的断裂应变值降低。在车轴实际轧制过程中,在保证一定生产效率的前提下,可以通过尽可能减小楔横轧模具的成形角,并适当增大展宽角的方法,来降低材料塑性变形时内部各处的动态应力三轴度值,降低损伤发生的概率。 相似文献
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《International Journal of Cast Metals Research》2013,26(1-4):90-95
AbstractThe present study examined the influence of refinement of graphite nodules on microstructure and tensile properties of austempered ductile cast iron (ADI). A casting technique using deoxidation treatment enabled manufacture of thin walled castings made of ductile cast iron without ledebulite. The thin walled casting (t=2 mm) was subjected to extreme refinement of graphite nodules, where the number of graphite nodules was 1750 mm?2. Decrease in graphite nodule diameter resulted in refinement of ausferrite and γ-pool structures in ADI and rapid reaction of austempering. The significant increase in the number of graphite nodules resulted in a remarkable drop in the tensile strength and elongation of ADI. These results can be explained by the graphite nodule distance. 相似文献
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Hai Qing 《金属学报(英文版)》2014,(5):844-852
The influence of the shape and spatial distribution of reinforced particles on strength and damage of metal matrix composite (MMC) is investigated through finite element method under uniaxial tensile, simple shear, biaxial tensile, as well as combined tensile/shear loadings. The particle shapes change randomly from circular to regular n-sided polygon (3 ≤ n ≤ 10); the particle alignments are determined through a sequentially random number stream and the particle locations are defined through the random sequential adsorption algorithm. The ductile failure in metal matrix and brittle failure in particles are described through damage models based on the stress triaxial indicator and maximum principal stress criterion, respectively, while the debonding behavior of interface between particles and matrix is simulated through cohesive elements. The simulation results show that, under different loadings, interface debonding is the dominated failure mechanism in MMCs and plastic deformation and ductile failure of matrix also play very important roles on the failure of MMCs. 相似文献