Analysis of transformation superplastic deformation in pure iron

Summary Transformation superplastic deformation in pure iron is analysed. The transformation phenomenology is first studied when the material is exposed to the constant-rate cyclic temperature histories between 20°C and 1100°C. Transformation diagrams (T-T-A, T-T-T, C-H-T and C-C-T diagrams) are shown to be constructed by means of the transformation kinetics employed. Change in the fraction of austenite, or of ferrite, during the cyclic process is calculated. The deformation in thin-walled tubular specimen is then estimated when it is exposed under a constant tensile or torsional stress to the temperature cyclings. Accumulation of the superplastic deformation due to A ₃ transformation is shown to be well simulated by the theory presented here.

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Deformationsanalysis der Umwandlungsplastizität bei Eisen

Übersicht Die superplastische Deformation während der allotropen Umwandlung bei Eisen, A ₃ (Austenit Ferrit)-Umwandlung, wird analysiert. Die Umwandlungsphänomenologie wird theoretisch diskutiert, wenn das Material einer zyklischen Temperaturgeschichte zwischen 20°C und 1100°C ausgesetzt wird. Es wird gezeigt, daß man Umwandlungsdiagramme mit Hilfe der vorgeschlagenen Umwandlungskinetik gut konstruieren kann. Die Austenitbildung oder die Ferritbildung während des zyklischen Prozesses wird berechnet. Die Deformation von dünnwandigen, rohrförmigen Proben wird dann analysiert, wenn sie bei konstanter Zugspannung oder Torsionsspannung gleichzeitig durch ein zyklisches Temperaturfeld belastet werden. Die Ergebnisse zeigen, daß die akkumulierende Superplastizitätsdeformation von der Theorie gut beschrieben werden kann.

     

Superplastic characteristics of high purity yttria-stabilized zirconia polycrystals

The mechanical characteristics of superplastic yttria-stabilized zirconia polycrystals have been analyzed as a function of stress, temperature and grain size. The evolution of the stress exponent n with stress found in high purity materials is similar to that observed in superplastic metals. True creep parameters can be ascribed to the deformation mechanism at high stresses. By contrast, the creep parameters exhibit a continuous evolution with stress, temperature and grain size at low stresses. The threshold stress formalism used in conventional and high strain rate superplastic metals accounts for the mechanical characteristics observed in fine-grained zirconia polycrystals.     

A parametric-experimental study of void growth in superplastic deformation

Substantial void growth in metals constitutes a problem in many industrial operations that utilize superplastic deformation. This is because of the likelihood of material failure due to such growth. Hence, there is a need to study void growth mechanisms in an effort to understand the parameters governing it. In this work, numerical and experimental studies of void growth, and the parameters that affect it, in a superplastically deforming (SPD) metal have been performed. In the numerical studies, using the finite-element method, a 1×2 sized thin plate (i.e. plane stress conditions) of a viscoplastic material with pre-existing holes has been subjected to a constant extension rate. The experimental studies were performed under similar conditions to the numerical ones and provided for qualitative comparison. The parameters affecting void growth in SPD are: m (the strain-rate sensitivity), void size (i.e. diameter) and the number (density) of existing voids. The results showed that increased m values produced strengthening and decreased the rate of void growth. In addition, larger initial void size (or, equivalently, a larger initial void fraction) had the effect of weakening the specimen through causing accelerated void growth. Finally, multiple holes had the effect of increasing the metal ductility by reducing the extent of necking and its onset. This was realized through diffusing the plastic deformation at the different hole sites and reducing the stress concentration. The numerical results were in good qualitative agreement with the experiment and suggested the need to refine existing phenomenological void growth models to include the dependence on the void fraction.     

Constitutive modeling of deformation and damage in superplastic materials

The superplastic deformation and cavitation damage characteristics of a modified aluminum alloy are investigated at a temperature range from 500 to 550°C. The baseline alloy is AA5083. Nominally this alloy contains about 4.5% Mg, 0.8% Mn, 0.2% Cr, 0.037% Si, 0.08% Fe and 0.025% Ti by weight. The experimental program consists of uniaxial tension tests and digital image analysis for measuring cavitation. The experiments reveal that evolution of damage is due to both nucleation and growth of voids. A viscoplastic model for describing deformation and damage in this alloy is developed based on a continuum mechanics framework. The model includes the effect of strain hardening, strain rate sensitivity, dynamic and static recovery, and nucleation and growth of voids. The model predictions compare well with the experimental results.     

Thermomechanics of elastoplastic and superplastic deformation of metals

Using the process theory of A. A. Il’yushin, we consider the problem of determining the thermomechanical parameters of a material element for specified deformation and temperature-variation processes with allowance for the elastic, plastic, and viscous properties of superplastic deformation. The relations obtained are applicable for the case of arbitrary stresses and finite strains. The strain and stress measures are decomposed into elastic, plastic, and viscous components by classifying the processes into reversible, irreversible equilibrium, and nonequilibrium processes. Tula State University, Tula 300600. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 5, pp. 164–172, September–October, 1999.     

Forming limit during superplastic deformation of sheet metals

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Study of transformation plasticity in tetragonal zirconia polycrystals by moiré interferometry

Transformation plasticity in ceria-stabilized tetragonal zirconia polycrystals due to the stress-induced tetragonal-to-monoclinic martensitic transformation under tension and bending is studied by moiré interferometry. The whole fringe patterns includingu fields andv fields are acquired. According to these patterns, the distributions of transformation plasticity in transformation zones are obtained, and the phenomenon of plastic flow localization for transformation is revealed. The above work provides a significant experimental foundation for establishing transformation constitutive relations The project supported by the National Natural Science Foundation of China     

Electrical double layer and rheological properties of yttria-stabilized zirconia suspensions in solutions of high molecular weight polyacrylic acid polymers

This work deals with the effect of the adsorption of two high molecular weight polyacrylic acid polymers (Carbopol) on the interfacial properties, and the rheology of aqueous zirconia suspensions. Since the Carbopol-covered particles can be thought of as soft colloids, Ohshimas theory was used to gain information on the surface potential and the charge density of the polymer layer (Ohshima H (1995) Electrophoretic mobility of soft particles. Colloids Surf A Physicochem Eng Aspects 103:249–255). The effect of the pH of the solution on the double layer characteristics is related to the different conformations of the adsorbed molecules provoked by the dissociation of the acrylic groups present in polymer molecules. The electrokinetic properties of the suspensions are studied for different pH and Carbopol concentrations in solution in order to investigate the possible stabilization of the suspensions by electrostatic repulsion between the particles. The rheological behavior of the suspensions was investigated in steady-state and dynamic conditions, and the corresponding yield stress and storage modulus were obtained in absence and presence of polymer in solution. The competition between bridging flocculation provoked by polymer adsorption and electrosteric stabilization determines the rheological properties of the suspensions. In the pH range investigated, bridging flocculation predominates at the neutral pH because of the graft of the uncoiled polymer to more than one particle, while at the extreme pH values (pH 3, pH 9) steric or electrosteric stabilization seems to be the predominant mechanism that explains the rheological results. These facts were confirmed by estimating the zirconia particle (or aggregate) diameter in the liquid medium by means of light scattering measurements.     

镁合金塑性机制研究综述

纯镁具有丰富的微观塑性机制,尤其是孪晶,导致其塑性变形错综复杂,力学性能也与常见的面心及体心立方金属有显著差异.由于现今学界对位错滑移与孪晶变形等塑性机制缺乏充分认识,镁合金性能调控效果尚不理想,与铝合金相比,镁合金的力学性能还有很大的提升空间.基于此背景,论文首先回顾了镁合金的发展历史与应用现状.然后介绍了镁中位错滑移与孪晶变形等塑性机制的研究进展,重点阐述位错、孪晶、晶界、析出相、溶质原子等重要的微结构,并简要介绍了计算机模拟方法,最后展望了强韧性能方面值得重视的若干研究方向.     

新型纳米结构金属材料的力学性能及变形机制

“纳米结构” 化是金属及其合金材料获得优异力学性能的有效途径.纳米结构金属材料表面或内部的缺陷, 包括晶界、位错、孪晶、孔洞、裂纹、第二相等, 其形核、演化及互相作用对材料的强度和韧性具有重要影响. 该文综述了与上述科学问题相关的新型纳米结构金属材料的微观组织结构表征及力学性能测试、强韧化机制计算模拟方面的研究进展. 并讨论了急需从微观尺度上就新型纳米结构金属材料的特征力学行为和关键变形机制开展深入、系统研究.      

Constitutive behavior of superplastic materials

Superplasticity is an intriguing inelastic process in solid materials with deformation upto several thousand percent. Forming sheet and bulk materials using superplastic forming has become an established manufacturing method in aerospace and lately in other industries. Developing the right constitutive behavior is important not only for modeling the process for manufacturing by engineering mechanicians but for choosing the right composition and processing for material scientists. Such an ideal constitutive equation has been eluding the analysts so far. This paper examines some of the fundamental misgivings about the origin of inelastic process in superplasticity compared to other well known deformation processes. Also an attempt is made to understand the basic characteristics of superplastic inelastic deformation at macroscopic, mesoscopic and atomic levels.     

Analysis of elastic-plastic deformation in TiAl polycrystals

A computational model is described for analyzing stress variations within polycrystals of γ-TiAl, including the effect of anisotropic yielding and small-scale plastic flow. Interlamellar (soft mode) slip behavior is controlled by a separate collection of slip systems whose properties are derived from measurements on polysynthetically twinned (PST) specimens. When used to represent several hundred randomly oriented material grains, the model provides distributions and statistical data about the local stress, strain, and plastic deformation resulting from a prescribed macroscopic loading.     

双相高强钢FeNiAlC的动态剪切行为及微结构机理

绝热剪切带是金属材料在高应变率载荷下常见的一种失效模式。利用霍普金森压杆装置,对双相钢Fe-24.86Ni-5.8Al-0.38C不同微结构的帽形样品施加冲击载荷,研究它的动态剪切变形行为及微结构机理。先通过对固熔处理得到的粗晶态样品进行大应变冷轧获得冷轧态样品,再使用透射电子显微镜和扫描电子显微镜表征两种样品冲击前后微结构的变化差异。结果表明,双相钢FeNiAlC拥有较优异的动态剪切性能,剪切强度达1.3 GPa,均匀剪切应变达1.5。变形前,材料由奥氏体相和马氏体相构成,马氏体体积分数约为20%。变形过程由位错滑移和孪生变形主导,但因应变速率较高致使马氏体相变被抑制。不同微结构样品内均形成绝热剪切带,带内发生动态再结晶,形成超细晶粒,平均晶粒尺寸约300 nm,且剪切带内不发生相变;冷轧态剪切带宽度的实验值（14.6 μm）与理论计算值(12.3 μm)较好吻合,而粗晶态剪切带宽度的实验值（14.6 μm）与理论计算值（30 μm）相差甚远,初步分析可能是因为粗晶态样品应变较大基本不满足完全绝热的理论条件。在变形过程中,粗晶态因塑性变形做功产生的绝热温升高达720 K,而冷轧态的只有190 K。通过实验结果与热塑模型分析,得出绝热温升不是形成绝热剪切带的唯一因素,而应考虑材料的微观结构和局部化变形等的共同影响。     

Analysis of prestressed mechanisms

A new theory is presented for the matrix analysis of prestressed structural mechanisms made from pin-jointed bars. The response of a prestressed mechanism to any external action is decomposed into two almost separate parts, which correspond to extensional and inextensional modes. A matrix algorithm which treats these two modes separately is developed and tested. It is shown that the equilibrium requirements for the assembly, in its initial configuration as well as in deformed configurations which are obtained through infinitesimal inextensional displacements, can be fully described by a square equilibrium matrix. It is also shown that any set of extensional nodal displacements has to satisfy some equilibrium conditions as well as standard compatibility equations, and that the resulting system of linear equations defines a square kinematic matrix. Theoretical as well as experimental evidence supporting this approach is given in the paper ; two simple experiments which were of crucial importance in arriving at the equilibrium conditions on the extensional displacements are described.The interaction between the two modes of action of a prestressed mechanism is discussed, together with a rapidly converging iterative procedure to handle it. A study of the non-linear effect by which the self-stress level in a statically indeterminate assembly rapidly increases if an inextensional mode is excited, supported by further experimental results, concludes the paper. This work is relevant to the analysis of most cable systems, pneumatic domes, fabric roofs, and “Tensegrity” frameworks.     

粗晶超塑性单轴拉伸颈缩模拟

利用粘塑性本构模型模拟粗晶超塑性单轴拉伸。数值结果表明,颈缩的位置及发展过程受拉伸应变速率的影响。不只一处分散不均匀变形相互牵制与协调,使材料得以在接近均匀的状态下经受大的变形,模拟得到的局部应变速率演化曲线,可以预测变形局部化发展的情况。     

Analysis of deformation process characteristics in amorphous-crystalline polymers

In the framework of a two-component medium, the phenomenological approach is used to develop a system of constitutive equations describing the thermomechanical behavior of amorphous-crystalline polymers. This model is designed to describe the stress-strain states in the temperature range comprising the intervals of phase and relaxation transitions. The results of numerical experiments demonstrating the possibility of describing the characteristic properties of deformation processes typical of amorphous-crystalline polymers are given.