QT400-18L球墨铸铁的低温冲击断裂行为

在室温和低温下对QT400-18L球墨铸铁件进行Charpy冲击试验,采用金相显微镜、扫描电镜对冲击断口形貌进行分析,利用透射电镜对断口附近位错密度进行观察分析,探讨了球墨铸铁低温冲击断裂的机制。结果表明:温度对冲击韧性影响较大,随试验温度的降低,冲击吸收的能量显著减小。观察发现,低温冲击断口表现出两种不同的解理断裂刻面:"连续型"和"不连续型"河流形貌。球墨铸铁中裂纹优先在石墨与铁素体基体界面形成,其次在碳化物与基体界面形成。在低温冲击载荷作用下,活动位错源数量少,发生解理断裂。     

热处理过程中球墨铸铁的组织及石墨碳变化行为的研究

引言 球墨铸铁的机械性能及断裂抗力主要取决于球状石墨及基体组织。球状石墨的球径、间距、数量、圆整度以及基体组织则随不同的热处理制度而变化。此外,基体的碳含量对球铁的强韧化处理起着重要的影响。为提高球墨铸铁的使用性能,可通过强韧化     

粉末冶金铍铝合金的高低温力学性能研究

对粉末冶金铍铝合金进行了-100~500℃的拉伸性能测试,分析了其力学性能随温度变化的规律,并采用扫描电镜获取不同温度下的断口形貌。结果表明,铍铝合金的抗拉强度和屈服强度均随温度升高而降低,延伸率随温度升高先上升再下降。随温度升高铍铝合金的断裂机制发生转变:低温时表现为由Be相的解理断裂及Al相的韧性断裂构成的混合型断裂,随温度升高转变为铍铝两相界面的开裂和Al相的韧性断裂。采用ABAQUS有限元软件模拟铍铝合金在高低温拉伸过程中的内部应力场分布,揭示了铍铝合金高温拉伸断裂模式发生转变的力学机制。     

热轧7075/AZ31B复合板的显微组织及结合性能

为了研究热轧铝/镁复合板结合强度的变化规律,本文综合考虑压下率、轧制温度和轧制速度等多种轧制参数,单道次热轧制备了7075 Al/AZ31B Mg复合板。结果表明:在复合板轧制过程中由于热和强变形作用组织发生了动态再结晶,且增大轧制速度有助于镁基体产生完全动态再结晶。在相同轧制温度下,铝镁复合板结合强度均随压下率增加先升高后降低;强度升高是由于界面元素扩散宽度的增大和镁合金近界面晶粒组织的细化所致,强度降低是由于大变形导致镁基体近界面处产生裂缝,以及塑性功产生热量过多使得镁基体温度升高导致的镁侧晶粒长大所致。对复合板进行拉剪实验,铝镁结合界面剪切强度较低时,断裂发生在复合界面处且成脆性断裂特征,强度较高时断口形貌呈韧性断裂特征,断裂发生在镁基体侧。      

奥氏体-贝氏体球墨铸铁断裂的微观过程及强韧化机理

用扫描电子显微镜及微拉伸台对奥氏体－贝氏体球墨铸铁裂纹萌生，扩展的微观过程进行了跟踪观察。结果发现：受拉时微裂纹首先在石墨－基体界面上萌生，并沿界面扩展，基体中的裂纹多数是沿贝氏体铁素体－奥氏体界面扩展，不同取向的基体组织可使裂纹偏转或分叉，主裂纹扩展过程中前方始终存在石墨－基体界面的开裂。此外，还根据实验结果进一步分析了奥氏体－贝氏体球墨铸铁的强韧化机理。     

奥氏体-贝氏体球墨铸铁中石墨与基体界面的疲劳强度

采用系统分析的方法，通过3点弯曲疲劳实验，跟踪监测了奥氏体-贝氏体球墨铸铁试样的疲劳损伤过程。实验结果表明，奥氏体-贝氏体球墨铸铁中石墨球与基体组织界面有一定的疲劳强度；在不同的疲劳载荷作用下，该处疲劳开裂的时间和程度存在差异，并对疲劳裂纹的萌生和扩展有不同的影响。     

中厚板粗轧机工作辊辊颈断裂失效分析

采用宏观断口分析、金相检验及试件的应力拉伸试验,对轧辊辊颈断裂进行失效原因分析。结果表明,此轧辊在制造过程中出现石墨球化和孕育不良等铸造缺陷,组织中石墨出现枝晶状及少量团状石墨,导致轧辊辊颈强度明显下降,在上机使用过程中即发生断裂。     

Sb-Ba复合随流孕育对大断面球铁低温冲击性能的影响北大核心CSCD

采用随流孕育的方式向铁液中分别加入普通孕育剂75SiFe和含Sb和Ba孕育剂进行孕育处理,浇注尺寸规格为180 mm×180 mm×200 mm的大断面球墨铸铁试块。运用光学显微镜(OM)、扫描电子显微镜(SEM)、拉力试验机、冲击试验机等检测方法,研究Sb-Ba复合随流孕育对大断面低温球墨铸铁组织和性能的影响。结果表明,与用普通孕育剂75SiFe相比,运用含Sb和Ba复合孕育剂对铁液进行随流孕育能显著改善大断面低温球墨铸铁微观组织和力学性能。该工艺可提高试样的石墨球化级别、改善石墨形态、增加石墨球数量、细化石墨球,可有效避免孕育衰退,使铸件组织和力学性能达到较佳匹配。冲击试验表明,低温韧性得到了明显提高,其(-40℃)冲击功平均值不低于为12 J。     

62Be-38Al合金高、低温力学性能及断裂机制

采用万能拉伸试验机、金相显微镜及扫描电镜对62Be-38Al铍铝合金在-100～500℃下的高、低温力学性能及断裂机制进行探究。结果表明,铍铝合金的抗拉强度与屈服强度随温度的升高而降低,延伸率则呈现先升再降的变化趋势,在300℃时达到最大值。同时,随着测试温度的上升,铍铝两相的界面结合强度逐渐低于铍颗粒的解理断裂强度,断口组织中相界面断裂增加,铍解理面减少。当温度升至500℃时,铝相软化,铍铝相界面的结合强度大幅降低,伴随着合金强度塑性的急剧下降,其断口呈沿晶断裂。另外,随温度的升高,合金的形变强化指数单调下降,-150℃时为0.22,400℃时为0.08。     

包覆法制备Gp/SiC 复合材料的显微结构和性能

以不同粒径的石墨颗粒和SiC粉体为原料,采用SiC粉体包覆石墨颗粒的方法,于2000℃热压制备了石墨/碳化硅(Gp/SiC)复合材料.利用扫描电子显微镜(SEM,EDS)分析了材料的金相和断口显微结构.研究表明,石墨粒径较小且质量分数较少的复合材料比石墨粒径较大且质量分数较多的复合材料在热压工艺中更致密.石墨颗粒呈岛状紧密地镶嵌在SiC基体中,石墨与SiC界面处C和Si的扩散不明显.复合材料的相对密度、抗折强度,断裂韧性和硬度随石墨粒径和质量分数的减少而增加.断口形貌表明SiC陶瓷基体为脆性,石墨为韧性断裂.当石墨粒径为125μm、SiC与石墨的质量比为3.5时,复合材料的综合性能最佳,开口气孔率为0.3%,相对密度为97.9%,抗折强度为75±15 MPa,断裂韧性为5.4±0.5 MPa.m1/2,硬度为26.8±3GPa.     

Ductility Loss in Ductile Cast Iron with Internal Hydrogen

Hydrogen-induced ductility loss in ductile cast iron (DCI) was studied by conducting a series of tensile tests with three different crosshead speeds. By utilizing the thermal desorption spectroscopy and the hydrogen microprint technique, it was found that most of the solute hydrogen was diffusive and mainly segregated at the graphite, graphite/matrix interface zone, and the cementite of pearlite in the matrix. The fracture process of the non-charged specimen was dominated by the ductile dimple fracture, whereas that of the hydrogen-charged specimen became less ductile because of the accompanying interconnecting cracks between the adjacent graphite nodules. Inside the hydrogen-charged specimen, the interspaces generated by the interfacial debonding between graphite and matrix are filled with hydrogen gas in the early stage of the fracture process. In the subsequent fracture process, such a local hydrogen gas atmosphere coupled with a stress-induced diffusion attracts hydrogen to the crack tip, which results in a time-dependent ductility loss.     

Damage mechanisms in a cast ductile iron and a Al2O3p /Al composite

Mechanical behavior and damage mechanisms of an Al₂O₃ particulate-reinforced Al matrix composite (Al₂O_3p /Al) prepared by pressure infiltration are investigated and compared with those of a cast ductile iron. In addition to low cost and reduced weight, the composite has a Young’s modulus comparable to the ductile iron. However, its fracture toughness is lower than that of the ductile iron. Interface debonding between the graphite and ferrite is responsible for the crack initiation behavior of the ductile iron. The crack in the ductile iron is arrested by the ductile ferrite phase surrounding the graphite, leading to high fracture toughness. For the Al₂O_3p /Al composite, the dominating crack initiation mode is particulate cracking. Interface debonding and zigzag cracking of particulates are additional fracture modes. The high content of Al₂O₃ particulates and the high thermal and elastic incompatibilities between the Al matrix and Al₂O₃ particulates result in brittle fracture and low fracture toughness for the composite. Possible ways to increase the fracture toughness of the Al₂O_3p /Al composite material are also outlined.     

Nucleation of Bainite in Bainite Ductile Cast Iron

The bainite ductile cast iron with given comppasition was quenched to get bainite strueture. The nucleating position of bainite and the distrihution of alloying elements in the matrix were measured. The results show thai the bainite nucleates at the interface between graphite and austenite during quenching. Based on the experimental results and thermodynamics, the nucleating tnechanism of bainite in ductile iron was analyzed.     

Damage effect on the fracture toughness of nodular cast iron: Part-II. Damage zone characterization ahead of a crack tip

In order to understand the fracture toughness of nodular cast iron, the damage zone was studied by Scanning Electron Microscope (SEM) observations of the polished surface of a CT 25 specimen before and after ductile tearing. Damage is defined as decohesion at the graphite/matrix interface. It is shown that the damage zone is very large in nodular cast iron (almost throughout the whole remaining ligament ahead of the crack tip), so linear elastic fracture mechanics (LEFM) are not valid for small specimens. The size of the damage zone was calculated analytically by introducing a damage initiation criterion which was based both on observations of the debonding of the interface between matrix and graphite nodules and on measurements of the pressure sensitivity of cast iron. To take into account the actual boundary conditions, the damage zone was also calculated by numerical modeling using the modified Gurson’s model and by considering the nodular cast iron as a porous material. The calculated results led to good agreement with the damage zone observations. Plane stress and plane strain calculations yielded nearly the same size plastic zone. This result is opposite to those obtained for fully dense materials.     

An investigation of the mechanical damping of ductile iron

Ductile iron has been suggested as a candidate material for a number of practical applications, including turbine castings, automotive components, and transportation and storage casks for hazardous and radioactive materials. The applications require the enhanced ductility resulting from the presence of spherical graphite nodules in the ductile ferrite iron matrix. Proper design of such components requires a knowledge of the mechanical properties, including how energy is absorbed and dissipated (mechanical damping) by the test material. This article is a study of the mechanical damping of a series of well-characterized ductile iron materials (four separate materials) as a function of strain amplitude, temperature over the range of -100 °C to +100 °C, and magnetic field. The major sources of damping were found to be dislocation motion in the graphite phase and magnetomechanical damping in the ferrite phase. The magnitude of the magnetomechanical damping was much larger than that due to dislocation motion. An additional goal of the investigation was to determine if any correlation existed between the measured mechanical damping and the fracture toughness of the ductile iron materials; no correlation was found. This article is based on a presentation given in the Mechanics and Mechanisms of Material Damping Symposium, October 1993, in Pittsburgh, Pennsylvania, under the auspices of the SMD Physical Metallurgy Committee.     

熔渗量对颗粒增强的高性能铁基耐磨材料性能影响的研究

为研制在某些特殊工况条件下用的高强度、高耐磨性的铁基耐磨材料，采用烧结一熔渗一热处理工艺研制出一种由Co—Cr—Mo—Si颗粒增强的铁基粉末冶金耐磨材料。结果表明：Co—Cr—Mo—Si硬颗粒单独存在于基体中，起颗粒强化的作用。未熔渗时，孔洞多，硬颗粒与基体界面清晰可见，结合强度低，材料性能较差。随着熔渗量的增多，材料的孔隙度减小，硬颗粒与基体界面结合强度好，材料性能明显提高。同时，材料的断裂主要通过铜相的撕裂，呈现明显的塑性断裂特征。因此，足够的熔渗量可获得各相界面结合较好的一种高性能铁基材料。     

Fracture Characteristics of Austempered Spheroidal Graphite Aluminum Cast Irons

The fracture characteristics of austempered spheroidal graphite aluminum cast iron had been investigated. The chemical content of the alloy was C 3.2, Al 2.2, Ni 0.8 and Mg 0.05 (in mass percent, %). Impact test samples were produced from keel blocks cast in CO2 molding process. The oversized impact samples were austenitized at 850 and 950 ℃ for 2 h followed by austempering at 300 and 400 ℃ for 30, 60, 120 and 180 min. The austempered samples were machined and tested at room temperature. The impact strength values for those samples austempered at 400 ℃ varied between 90 and 110 J. Lower bainitic structures showed impact strength values of 22 to 50 J. The fractures of the samples were examined using SEM. The results showed that the upper bainitic fracture revealed a honey Comb-like topography, which confirmed the ductile fracture behavior. The lower bainitic fractures of those samples austempered for short times revealed brittle fracture.     

Influence of casting size and graphite nodule refinement on fracture toughness of austempered ductile iron

Casting size affects the solidification cooling rate and microstructure of casting materials. Graphite nodules existing in the structure of ductile iron are an inherent and inert second phase that cannot be modified in subsequent heat-treatment processing. The matrix and the fineness of the second phase undoubtedly have some impact on the fracture toughness of the as-cast material, as does the subsequent heat treatment, as it alters the microstructure. This research applied austempering heat treatment to ductile iron of different section sizes and graphite nodule finenesses. The influence of these variables on the plane strain fracture toughness (K _IC ) of the castings so treated was compared to that of the as-cast state. Metallography, scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural observation.