The effect of graphite shape on the tensile deformation behaviour in hot-swaged and hot-rolled ferritic spheroidal graphite cast iron

The shape of the graphite nodules in ferritic spheroidal graphite (SG) cast iron is changed by hot-swaging and hot-rolling, when symmetric ellipsoidal graphite and asymmetric ellipsoidal graphite can be obtained, respectively. For the symmetric ellipsoidal graphite, the graphite shape ratio tends to decrease with increasing reduction ratio. The increase of flow stress with increasing reduction ratio can be explained by the decrease in the stress concentration induced by the graphite with increasing reduction ratio. On the other hand, the graphite shape ratio is more complex for the asymmetric ellipsoidal graphite than the symmetric ellipsoidal graphite specimens. The graphite shape ratio tends to increase and decrease with increasing reduction ratio in two transverse directions. As a result, a balance exists in the specimen between increasing and decreasing stress concentration, showing that the tensile flow stress is independent of reduction ratio.

To understand the effect of the graphite shape ratio on the tensile flow stress during tensile deformation, specimens with different carbon contents and graphite shape ratios were used in this study. Furthermore, the triaxial parameter (k₁) has been defined such that the larger stress concentration effect will have the larger k₁ value. Based on the present experimental results, the graphite shape ratio tends to decrease with increasing reduction ratio or tensile strain, so that the stress concentration effect and triaxial parameter (K₁) tend to decrease with increasing reduction ratio and tensile strain. Therefore, the effect of the graphite nodules in ferritic SG cast iron on the tensile flow stress is not only reduction of the effective area fraction of the matrix but also development of an uneven triaxial stress field.     

大型饼类锻件变形规律及夹杂性裂纹产生过程研究（续）

大型饼类锻件变形规律及夹杂性裂纹产生过程研究（续）（清华大学１０００８４）马庆贤，曹起骧，谢冰（第一重型机械集团公司１６１０４２）谢云岫，宋士丹，吴书勤４夹杂性裂纹产生过程的模拟研究４．１实验条件大型钢锭的解剖实验表明：低熔点非金属夹杂物（如硫化物、...     

Shape memory behaviour in auxetic foams: Mechanical properties

Shape memory is the property of a material to remember its original shape despite subsequent plastic deformation. The exposure to specific temperature profiles induces a shape memory effect on auxetic (negative Poisson’s ratio) foam specimens, returning them to their initial dimensions. This behaviour is a one-way effect, and it is a property of the polyurethane (PU) constituent of the foam. The foam specimens were transformed from conventional Poisson’s ratio to auxetic, returned to conventional and once again to auxetic under multiple mechanical and thermal loading. At each stage the foams were mechanically characterized under cyclic tensile and compressive loading. The manufacturing route adopted for the auxetic PU specimens involved a multiaxial compression of the native foam, heating of the compressed specimens above the T_g of the foam polymer, and cooling under running water. Twenty specimens, divided equally between two varieties of PU-based foam, were fabricated. Two different compression ratios were used during auxetic conversion for both kinds of foam, and the same final temperature of 135 °C was adopted. Tensile and compressive cyclic tests were performed in order to measure the tangent modulus (i.e. stiffness), Poisson’s ratios and energy dissipated per unit volume. Remarkable differences before and after the memory effect took place were found, showing mechanical behaviour unique to each of these phases. The shape memory effect plays an important role in the mechanical behaviour of the auxetic foams.     

Bulk deformation of Ti-6.8Mo-4.5Fe-1.5Al (timetal LCB) alloy

Recently, a low-cost near-β titanium alloy (Timetal LCB Ti-6.8Mo-4.5Fe-l.5Al wt %) containing iron and molybdenum has been developed. This alloy is cold formable in the β microstructure and can be aged to high strengths by precipitating the a phase. Due to its combination of cold formability and high strength, the alloy is a potential replacement for steel components in the automotive industry. The current study was undertaken to evaluate the cold bulk forming characteristics of Timetal LCB for use in lightweight automotive applications. Room-temperature compression tests conducted over a strain-rate range of 0.01 to 5/s indicate that the bulk cold compression of the alloy is affected by two factors: the microstructure and the length-to-diameter aspect ratio of the specimen. In the aged condition, when the microstructure has a-phase particles distributed along flow lines in the β-phase matrix, the alloy has the propensity for shear failure when deformed in compression in a direction parallel to the flow lines. In the solution-heat-treated condition, the microstructure consists of β grains with athermal ω phase. In this condition, the alloy can be cold compressed to 75 % reduction in height using specimens with aspect ratio of 1.125, but fails by shear for a larger aspect ratio of 1.5. Plastic deformation of the material occurs initially by single slip in most grains, but changes to multiple slip at true plastic strains larger than about 0.15. At a slow strain rate, the deformation is uniform, and the material work hardens continuously. At high strain rates, shear bands develop, and the localized deformation and temperature rise due to deformation heating leads to flow softening during compression. Although there is a considerable rise in temperature (200 to 500 °) during deformation, precipitation of the a phase was not observed.     

等离子蒸发制备的纳晶镍的压缩变形机理

与传统粗晶金属材料相比,纳晶金属具有特殊的变形机理。为了探索纳晶金属的变形机理,本文以等离子蒸发结合热压烧结制备的块体纳晶镍为研究对象,进行了准静态压缩力学性能测试,并且利用XRD和TEM技术对试样压缩前和压缩后的微结构演化进行了研究。结果表明：块体纳晶镍表现出较高的压缩强度和较好的韧性,且强度和韧性均具有率相关性。同时,纳晶镍压缩变形后晶粒尺寸较压缩前减小,但其微应变增加。结合纳晶镍的力学行为和压缩过程中微结构的演化,本文预测晶界位错运动和晶界滑移的联合机制是块体纳晶镍压缩过程中塑性变形的主要机理。     

灰口铸铁热变形后的组织与性能

采用包覆压缩工艺在Gleeble3500热模拟试验机上实现了灰口铸铁的大塑性变形。热变形试样在石墨化退火后用光学显微镜和图像分析仪观察了显微组织和石墨形态，采用显微硬度测试以及小试样拉伸方法研究了压下量对灰口铸铁力学性能的影响．用扫描电镜观察了拉伸断口微观形貌。结果表明，灰口铸铁大塑性变形后，石墨片趋于平行分布，试样的显微硬度、拉伸强度和伸长率均有不同程度的提高，经过80％热变形的试样其显微硬度增加到217HV，拉抗强度提高到249MPa，伸长率增加到5．2％；扫描电镜观察表明，拉伸断口有分层特征。     

块体材料连续变断面循环挤压形状参数的优化

采用连续变断面循环挤压(Continuous Variable Cross-section Recycled Extrusion,CVCE)法对矩形截面的铸态铅块进行挤压和回复镦粗变形,研究了原始坯料高径比、挤压角度等形状参数对块体变形后的外部形状和内部金属流动规律的影响。结果表明:经CVCE工艺镦粗后的试样,由于金属的径向流动和侧面金属翻平的共同作用,导致上底面面积增大,这种变形自上而下依次渗透并逐层减少;坯料在竖直方向上受到挤压而产生的变形有传递的作用,自上而下,各层的压缩量逐渐减少。当挤压角度为6°、原始坯料高径比为1.83时,循环挤压回复到原始形状的效果最好,且制品表面质量更好,内部金属流动较为均匀。     

定向石墨灰口铸铁的拉伸行为

提出了一种新的灰口铸铁(GCI)热加工工艺,即包覆压缩(CCC)。采用该工艺,经大于45%热变形压缩,制备了定向石墨灰口铸铁。经80%热变形的GCI拉伸性能显著提高,抗拉强度从117MPa提高到249MPa,伸长率从0提高到5.2%;热变形量超过45%的GCI,拉伸断口有分层现象,出现了一些韧性断裂的特征。     

Densification effects on the electrical behavior of uniaxially compacted bismuth nanowires

Densification of bismuth nanowires following uniaxial compression was employed to form nanostructured bulk pellets. The bismuth nanowires were first synthesized and purified using a modified template-based vapor deposition technique, resulting in nanowire diameters of either 20 or 250 nm, depending on the type of template employed. Characteristics of the compression process were studied with regards to material properties such as electrical conductivity and porosity. To understand the material deformation, pellets were compressed at 100, 500 and 3000 MPa. In comparison, similarly formed microparticle pellets were also examined. The porosity of the pellets was as low as 10% for the microparticle-based samples but at least 30% for the nanostructured materials. This discrepancy was attributed to the difference in mechanical yield strength, shape and aspect ratio of the particles. Hall coefficient measurements revealed that electrons were the dominant carriers with similar concentrations (∼10²⁰ cm⁻³) across the samples. Electrical conductivity of the pellets was found to be a function of compression level, but the measurements yielded lower values compared to bulk properties. Subsequent analyses revealed that factors such as porosity, particle surface oxide breaching due to deformation and particle-particle interfacial resistance contribute to the observed behavior. Moreover, the nanowire-based pellets exhibit anisotropy in electrical conductivity due largely to the fact that the nanowires align preferentially along a plane perpendicular to the compression process.     

Thermo-mechanical behaviors of the expanded graphite-phase change material matrix used for thermal management of Li-ion battery packs

In this paper, blocks for the thermal management of Li-ion battery are prepared. The blocks are made of paraffin wax, which is used as a phase change material (PCM), and graphite flakes. The process starts by compacting expanded graphite into the desired modular shapes and then impregnating it into molten paraffin wax. The modular pieces were assembled together, followed by finishing operations to achieve a desired packaging geometry.Thermo-mechanical properties of the produced phase change material–expanded graphite (PCM/EG) composites have been studied. The tests include thermal conductivity, tensile compression and bursting test. The results showed that as mass fraction of paraffin wax increases in the composite material, the thermal conductivity, tensile strength, compression strength, and burst strength were improved while tested at low operating temperatures. In contrast, the results showed reverse behaviors when tested at relatively high operating temperature.     

Effect of room-temperature compression on microstructure of ductile cast iron subjected to hot plastic deformation

The change in the microstructure of ductile cast iron subjected to hot plastic deformation has been investigated after the fracture of the samples induced by compression (upset forging) at room temperature. It has been shown that compression-induced tangential stresses cause shear deformation, which results in the shear fracture of test samples at an angle of 40°–50° to the longitudinal axis of a sample. It has been established that the fracture is accompanied by the formation of a narrow zone of severe plastic deformation of ductile cast iron, which is located on both sides of the major fracture. In this zone, the initial microstructure undergoes significant changes due to the plastic flow of the matrix and graphite inclusions.     

铜包铝线材室温拉变形后的显微组织和力学性能

研究经室温拉变形后的纯铜包覆铝合金的不同线径的线材的显微组织及力学性能。结果表明:显微组织自原始的等轴晶变为细长条纤维状,纤维直径与形变量近似地成反比,纤维长度与形变量的平方近似地成正比;经室温拉变形的铜包铝线的极限抗拉强度随形变量增大而增大,与形变量平方根呈直线关系;延伸率随形变量增大逐渐降低,但延伸率波动较大。根据原始纯铜和合金铝的极限抗拉强度值,可以用复合材料强度的混合法则近似地预测不同线径的铜包铝线的极限抗拉强度。     

Microstructure and Room Temperature Compressive Deformation Behavior of Cold-Sprayed High-Strength Cu Bulk Material

This study investigated the room temperature compressive deformation behavior of Cu bulk material manufactured by cold spray process. Initial microstructural observation identified a unique microstructure with grain size of hundreds of nm in the particle interface area and relatively coarse grains in all other areas. Room temperature compressive results confirmed cold-sprayed Cu to have a yield strength of 340 MPa, which is similar to materials manufactured by severe plastic deformation process such as equal channel angular press. In addition, strain softening phenomenon, which is rarely found in room temperature compressive deformation, was observed. According to such unique characteristics, continuous microstructure evolution and surface fractures according to the strain (ε _t = 0.3/0.6/0.9) of the material were observed, and considerations were made for deformation and fracture behavior. Microstructural observation after compressive deformation confirmed that average grain size decreased as the strain increased, and the fraction of the low-angle boundary, which has an indirect relationship with dislocation density, showed a tendency to decrease in ε _t = 0.3-0.6 region where the strain softening phenomenon occurs. Based on the results described above, this study was able to identify the possibility of manufacturing cold-sprayed Cu bulk material for structural material and its room temperature deformation behavior.     

真空感应重熔NiTi形状记忆合金室温拉伸力学性能

采用真空感应熔炼炉制备NiTi合金,并对重熔后的合金进行不同制度热处理。研究NiTi形状记忆合金边角料重熔合金不同温度时效后的室温拉伸力学性能。结果表明,NiTi形状记忆合金边角料重熔后,其室温屈服强度和抗拉强度均有不同程度的提高,断后延伸率略有增加,应力诱发马氏体相变对应的形状回复率降低;合金塑性变形能力随时效温度的升高而变差;合金中R相重新取向所提供的延伸率从总体上看比由单质原材料一次熔炼而成的合金要高。     

累积叠轧焊法制备超细晶纯铝的研究

采用累积叠轧焊方法在室温下对1060纯铝进行剧烈塑性变形，并分析1060纯铝变形前后内部微观组织结构的演变和力学性能的变化。实验结果表明，随着累积叠轧道次的增加，层界面复合越来越紧密，5道次后出现母材基体的相互渗透；材料的抗拉强度和硬度得到大幅度提高，伸长率在1道次时急剧下降，然后基本保持不变；晶粒尺寸急剧细化，等效真应变为6．4的条件下得到了平均晶粒尺寸为400nm的超细晶组织。     

Tensile properties of austempered ductile iron under thermomechanical treatment

A new processing method was investigated for improving the strength and elongation of austempered ductile iron (ADI) by grain refinement of parent austenite using thermomechanical treatment. The material was deformed at the austenitization temperature by single and multipass rolling before the austempering treatment. The effects of the amount of deformation, austenitization temperature, austempering temperatures, reaustenitization, and secondary deformation on the tensile properties were studied. The properties obtained using the method were compared with those of the ASTM standards. The effect of deformation on the graphite shape was also studied. Tensile strength/yield strength/elongation values were found to increase with increasing austenite deformation up to 40% and then to start decreasing. Tensile strength/yield strength and elongation values of 1700 MPa/1300 MPa/5% and 1350 MPa/920 MPa/15% can be achieved with this method in the ranges of variables studied.     

Microstructural features and mechanical properties of compacted graphite iron treated with calcium-magnesium based masteralloy

This paper presents some data on the microstructural features and mechanical properties of as-cast compacted graphite iron (CGI), produced through treatment with a special Ca-CaC₂-Mg masteralloy. Information on graphite morphology and matrix features, obtained using different numerical indices evaluated through a computer-based image-analyzing system and SEM observation of deep-etched specimens, shows that the graphite form is a mixture of ASTM types I, II, and III (compacted graphite), interspersed in a matrix with a 2:1 ratio of pearlite to ferrite. An average hardness (BHN) of 219, Charpy-V-Impact energy of 7.92 J, tensile strength of 354.5 MPa, and a corresponding elongation of 1.20% has been obtained for the as-cast iron. The fracture surfaces of tensile and room temperature impact tested specimens showed mixed modes of fracture.     

The influence of the reinforcing particle shape and interface strength on the fracture behavior of a metal matrix composite

A numerical analysis of the reinforcing particle shape and interface strength effects on the deformation and fracture behavior of an Al/Al₂O₃ composite is performed. Three-dimensional calculations are carried out for five elastic–brittle particles embedded into the elastic–plastic matrix, the reinforcing particle shape being varied from spherical to strongly irregular. It is shown that microstructural heterogeneity of the composite gives rise to a complex stress–strain state in the vicinity of particle boundaries and hence to near-interface areas undergoing tensile deformation both in tension and compression. Within the strain range under study, compressive strength is not achieved, either in compression or in tension, i.e., all cracks grow only under tensile stress. Particle fracture is found to occur by two mechanisms: interface debonding and particle cracking. Individual and combined effects of the particle shape, interface strength, and loading conditions on the fracture mechanisms are analyzed.     

An experimental study on the formability of intermetallic nickel aluminide

In this paper, the formability of ductile intermetallic Ni₃Al and limiting strains at room temperature are presented. Uniaxial tensile tests are carried out to identify uniform elongation, work hardening behavior, and fracture character. Forming limit diagrams are determined and the effect of biaxial deformation on limiting strains and failure modes are shown. The bulk formability is investigated by cold upset testing on continuous cast and extruded bars. Forming limit diagrams, both for bars and sheets, do not show a significant effect of thermomechanical processing and sheet thickness at the range of 0.30-0.46 mm. These observations are in agreement with common ductile metals where larger deformations are obtained in biaxial modes. Microcrack initiation at grain boundaries is shown to contribute to lower limiting strains in the plane strain region during sheet metal stretching and compression of intermetallic nickel aluminide.