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1.
黄群飞  何燕霖  罗娟娟  史文  李麟 《钢铁》2009,44(4):60-0
 通过Crussard Jaoul (C J)分析和n值的分段分析对不同马氏体含量双相钢的变形行为进行了研究。C J分析时采用连续数据绘制C J曲线,避免了取点拟合过程中的不连续性,n值的分段分析基于微分原理,直观给出了不同应变量下的n值。结合对两者的分析,认为只有较低马氏体含量的双相钢才具有双屈服特性,并且双相钢的第二阶段硬化的拐点只有在一定量的马氏体发生变形时才出现。同时在小应变量下双相钢中的应变硬化速率随马氏体含量的增加而升高,而在低马氏体含量下n值会出现最大值。  相似文献   

2.
A numerical analysis of the tensile test for sheet metals   总被引:1,自引:0,他引:1  
The strain hardening, strain-rate hardening, and plastic anisotropy properties of metal sheets are normally determined in a tensile test during the nearly uniform deformation prior to the maximum load. Beyond this point, strain nonuniformity leading to a neck is poorly understood in terms of interaction of these material properties with changes in strain-rate and stress-state within the neck, and the resulting load-extension plot. Satisfactory modeling of this problem has been achieved by using a rigid/plastic constitutive law including strain hardening and strain-rate hardening. Progressive cessation of deformation starting from elements in the specimen fillet region toward the center is demonstrated. This effect is shown to generate a strain peak (neck) at the gage length center. The predicted load-extension plots and strain distributions in the neck agree well with experiments conducted on a number of test materials. This work provides a quantitative measure of the influence of various material parameters on tensile ductility and identifies the proper constitutive law for input into mathematical models of more complex forming operations. Formerly with Research Laboratories, General Motors Corporation, Warren, Ml.  相似文献   

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The hardening or softening behavior of materials is one of the major factors affecting the deformation homogeneity in work-pieces during processing by equal channel angular pressing. In the present study, the difference between plastic flow behavior and deformation homogeneity of strain hardening and strain softening materials were investigated with the aid of finite element simulation. Results showed that localization of plastic strain occurred in the strain softening materials and fluctuations were observed in the strain distribution along work-piece. In strain hardening materials, a more uniform distribution of plastic strain was obtained compared to strain softening material.  相似文献   

7.
《Acta Metallurgica》1986,34(12):2443-2453
The influence of porosity on the deformation and fracture behavior of two alloys, powder-fabricated Ti and Ti-6Al-4V, with differing levels of matrix strain hardening has been examined both experimentally and analytically. A large strain elastoplastic finite element model based on a regular array of equal-sized spherical voids is used to predict bulk porosity effects; the analysis is in good agreement with the experimentally observed rates of void growth but underestimates the degradation of strength with increasing porosity. The effects of porosity on a local scale, especially as regards fracture, are examined by a model of a porous continuum which contains imperfections whose magnitude depends upon the maximum porosity path within the continuum. At critical values of strain these imperfections cause localization of plastic flow. The predicted values for the strains at localization are in good agreement with measured fracture strains. The analysis thus explicitly recognizes that a primary effect of pores on fracture is to localize deformation into narrow regions of high porosity (“imperfections”) which are present even in random distributions of pre-existing pores and which are the sites of macrofracture initiation.  相似文献   

8.
Strain Hardening of Hadfield Manganese Steel   总被引:7,自引:0,他引:7  
The plastic flow behavior of Hadfield manganese steel in uniaxial tension and compression is shown to be greatly influenced by transformation plasticity phenomena. Changes in the stress-strain (σ−ε) curves with temperature correlate with the observed extent of deformation twinning, consistent with a softening effect of twinning as a deformation mechanism and a hardening effect of the twinned microstructure. The combined effects give upward curvature to the σ−ε curve over extensive ranges of plastic strain. A higher strain hardening in compression compared with tension appears to be consistent with the observed texture development. The composition dependence of stacking fault energy computed using a thermodynamic model suggests that the Hadfield composition is optimum for a maximum rate of deformation twinning. Comparisons of the Hadfield steel with a Co-33Ni alloy exhibiting similar twinning kinetics, and an Fe-21Ni-lC alloy deforming by slip indicate no unusual strain hardening at low strains where deformation is controlled by slip, but an unusual amount of structural hardening associated with the twin formation in the Hadfield steel. A possible mechanism of anomalous twin hardening is discussed in terms of modified twinning behavior (pseudotwinning) in nonrandom solid solutions. Formerly Graduate Student at Massachusetts Institute of Technology  相似文献   

9.
最大塑性功原理在烧结体上限分析中的应用   总被引:1,自引:0,他引:1  
秦训鹏  华林 《粉末冶金技术》2006,24(4):248-250,255
最大塑性功原理又称第二塑性变分原理,在致密体塑性变形分析叶l是运用能量法进行变形力能计算的基础。本文从分析Drucker公设的亢要条件出发,证明了烧结体塑性变形最大塑性功原理的存在,片在该原理的荩础卜,推导出塑性变形的上限功率表达式,并以平面挤压条件下的上限法应用作为实例加以说明。  相似文献   

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The paper presents numerous measurements which confirm stages IV and V to be general ranges of cold work deformation. Analogous to stage II, stage IV exhibits a linear athermal hardening with constant strain rate sensitivity and activation enthalpy. In stage IV the dislocation cell size is constant, while the dislocation density growth rate is markedly reduced compared with stages II and III. Features of stage V are analogous to stage III, the increase of strain rate sensitivity (decrease of activation enthalpy) indicating the onset of thermally activated dislocation annihilation. In stage V, the mechanism is identified as dislocation climb from observing subgrain formation and saturation in density of deformation induced vacancies. Comparisons with recent investigations of stage IV and V at high temperatures suggest a common picture of low and high temperature deformation which only requires principles of storage and annihilation for both screw and edge dislocations.  相似文献   

12.
Mg alloys containing long-period stacking-ordered (LPSO) phases often display excellent mechanical properties. The underlying mechanism is yet unclear. In this work, in situ synchrotron X-ray diffraction was employed to study tensile deformation of a Mg97Y2Zn alloy that contains 18R-type LPSO phase. From lattice strain measurement, it is found that the LPSO phase has a similar elastic modulus as Mg. After material yielding, lattice strain in the Mg phase decreased, while lattice strain in the LPSO phase increased further. By analyzing the lattice strain evolution of different Mg peaks, basal slip and deformation twinning are identified as the dominant deformation mechanisms. This finding is further confirmed by surface slip trace analysis using electron backscattered diffraction (EBSD). Additional analysis of diffraction peak broadening indicates a continuous increase of dislocation density during plastic deformation. Based on the above results, it can be concluded that the interdendritic LPSO phase behaves like a reinforcing phase that directly strengthens the material. The high tensile ductility of the material is attributed to the weak extrusion texture caused by the presence of interdendritic LPSO. In addition, small LPSO plates inside the Mg phase can serve as dislocation nucleation sites, which leads to a high work hardening rate in the material.  相似文献   

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This work is concerned with the theoretical determination of the critical radius of a rigid inclusion at the moment of its decohesion from the material of a porous matrix. The criterion of V. V. Skorokhod served as the basis for analysis of the limiting state. Change of the porosity and accumulated strain of the solid phase in the process of plastic deformation were taken into account in the calculations. It is shown that the values of investigated properties depend on the concentration of inclusions and the porosity of the ductile layer. The dependence of accumulated strain in the solid phase on porosity was obtained.  相似文献   

15.
 In the present paper, tensile tests of Fe-30Mn-5Si -2Al steel were carried out for different strains of 0.05, 0.14, 0.26, and up to the strain-to-failure in order to observe the evolution of microstructure during deformation and investigate the strain hardening behavior. Three-stage strain hardening behavior was observed in this steel during tensile test. In stage I, planar dislocation structure was observed by TEM to be the main deformation mechanism, and low strain hardening rate exponent was exhibited. Primary deformation twinning occurred in stage II, and the strain hardening rate exponent increased due to the blockage of dislocations’ motion by twin boundaries. In stage III, the strain hardening rate exponent had increased to be higher than 0.5. The obstacle effect of twin boundaries and twin-twin interaction had been observed by TEM, and the interactions between primary and secondary twins were found to cause the additional hardening in addition to the obstacle effect on dislocations’ motion, which led to the twinning induced plasticity effect in the later stage of deformation.  相似文献   

16.
《粉末冶金学》2013,56(2):167-172
Abstract

Sintered steels in the density range 6.7-7 g cm- 3 and with a wide range of matrix hardnesses were produced with the main objective of understanding the interplay between porosity and matrix strength in determining their tensile and impact behaviour. The materials can be subdivided into two groups, characterised by different deformation and fracture mechanisms. The first group pertains to the as sintered materials (with a microhardness lower than 350HV0.1).They are characterised by a strain hardening stage before fracture, and both tensile strength and impact fracture load increase as density and matrix strength are increased. The second group pertain to the heat treated materials with a higher matrix hardness. These materials are characterised by a macroscopically brittle behaviour, since fracture takes place before general yielding. Because of the high matrix hardness, the peak stresses at the pore edges are not relieved by plastic deformation, and the pores behave as internal cracks. Fracture is then attained once the local applied stress intensity factor becomes equal to the matrix fracture toughness.  相似文献   

17.
Compaction bands, a form of localized deformation found in field and laboratory specimens of high porosity rock, consist of planar zones of pure compressional deformation that form perpendicular to maximum compression. Experimentalists report compaction bands and/or shear bands (angled to maximum compression) in high porosity sandstone during a transitional loading regime with multiple active deformation mechanisms. Conditions for localized deformation are determined using a two-yield surface constitutive model and bifurcation theory. The shear yield surface corresponds to a dilatant, frictional mechanism while the cap corresponds to a compactant mechanism. Unlike a single yield surface model, the two-yield surface model predicts both experimentally observed band types for reported values of key material parameters. Observed and predicted shear band angles generally agree. Theory suggests that shear band formation may coincide with activation of the shear yield surface by a previously active cap. If the bulk hardening modulus, k, equals zero (corresponding to localization on the peak or plateau of the mean stress–volume strain curve) compaction band conditions are more favorable than for small positive values of k.  相似文献   

18.
张宁飞  崔志强  王婕  侯清宇  黄贞益 《钢铁》2022,57(10):170-177
 为了分析硅镍合金化奥氏体基低密度钢在中温环境下的拉伸变形行为,采用Instron电子拉力试验机对Fe-28.64Mn-8.99Al-1.68Si-1.39Ni-1.0C(Mn29Al9Si2Ni,质量分数/%)低密度钢在23~300 ℃下进行了温拉伸试验,研究了该钢的温拉伸力学行为,并采用SEM、TEM和热力学计算对该钢的强韧化机制进行了研究。结果表明,随着应变的增加,温拉伸应力-应变曲线主要包括弹性变形、均匀塑性变形和断裂等几个过程,没有明显的屈服现象。随着温度的提高,该钢的强度逐渐降低,塑性(断后伸长率)先增加后减小再升高,于200 ℃时出现塑性低谷,此时该钢的应力-应变曲线和应变硬化率曲线均具有明显的锯齿状特征,应变硬化率随应变的增加变化不大。而该钢在其他温度下的应力-应变曲线和应变硬化率曲线没有发现明显的“锯齿状”特征,应变硬化率随应变的增加而平缓下降。试验钢在23~300 ℃下的主要强韧化机制为κ-碳化物强化、应变强化、孪生诱发塑性和动态应变时效强化。较低温度下位错可动性较差对孪生诱发的促进作用、镍元素和硅元素对孪生的抑制作用、较高温度下孪生现象的减弱和温度对动态应变时效的促进或抑制作用等使得试验钢在23、100和300 ℃时存在明显的孪生诱发塑性,而在200 ℃时存在明显的动态应变时效强化的主要原因。动态应变时效强化是该钢在200 ℃时出现塑性低谷的主要原因。  相似文献   

19.
On chromonickel γ-iron single crystals with $\left[ {\bar 111} \right]$ orientation and low packing-defect density, the plastic-flow localization during electrolytic saturation with hydrogen within a three-electrode electrochemical cell is investigated, with constant controllable cathode potential. On the plastic-flow curve for the extension of single crystals in the initial state (without hydrogen), beyond the transition from elasticity to developed plastic flow, linear strain hardening and then parabolic (Taylor) strain hardening may be observed. The plastic-flow curve for single crystals of austenitic steel saturated with hydrogen includes a small projection and a flow trough, stages of linear strain hardening and parabolic strain hardening, and a prefailure stage. Saturation of $\left[ {\bar 111} \right]$ single crystals with hydrogen reduces the yield point, increases the plasticity to failure by a factor of 1.3, and suppresses necking in crystals oriented for multiple slip. By double-exposure speckle photography, the basic types of plastic flow location at different stages of strain hardening may be identified, in the presence and absence of hydrogen, and the corresponding parameters may be determined. Hydrogenation of chromonickel γ-iron single crystals intensifies the localization of deformation and leads to considerable changes in the characteristic distances between the plastic-shear bands and the localized-strain zones.  相似文献   

20.
Minimising a metallic component's weight can be achieved by either using lightweight alloys or by improving the component's properties. In both cases, the material formability affects the utilisability for mass production processes. Most of the high‐strength materials show a material‐restricted formability and are difficult to forge. The formability of a material is described by its maximum forming limit. Large plastic strains can lead to mechanical damage within the material. A promising approach of handling low ductile, high‐strength alloys in a forming process is deformation under superimposed hydrostatic pressure by active media. In the present study, the influence of superimposed hydrostatic pressure on the flow stress is analysed as well as the forming ability for different sample geometries at different hydrostatic pressure and temperature levels. The experimental results show that the superimposed pressure has no influence on the plastic deformation, nor does a pressure dependent near‐surface material hardening occur. Nevertheless, the formability is improved with increasing hydrostatic pressure. The relative gain at room temperature and increase in the superimposed pressure from 0 bar to 600 bar for tested materials was at least 140 % and max. 220 %. Therefore, a cold forming process under superimposed pressure is developed to produce structure components with selective properties. For example, the gain in formability will be used to enlarge local plastic strains to higher limits resulting in higher local strain hardening and hardness. This offers new design possibilities with selectively adjusted local structure or structure component properties, especially adapted to their technical application. Additionally, by applying damage models, finite‐element analysis is used in order to predict damage occurring in the cold forming process under superimposed hydrostatic pressure for various sample geometries.  相似文献   

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