12Ni3CrMoVA钢热激活塑性形变特征的研究SCIEI

本文研究了12Ni3CrMoVA合金钢流变应力随温度及应变速率的变化规律,以及组织因素的影响,测算了该材料激活能及激活体积等热激活参数。根据P-N机制由螺型位错芯结构分析了bcc金属的塑性形变规津,该规律同试验结果吻合,说明有效应力与温度关系曲线中的拐点是由于Peierls势能呈双驼峰分布所致,相应的位错线激活态形状呈双扭折对分布。显微组织仅影响位错的长程障碍阻力,而对位错热激活运动没有影响。     

纯镍挤压制造过程中的磨损行为和显微组织演变（英文）

挤压棒材表面形貌和显微组织是决定其力学性能的主要因素。本文作者采用现场挤压和有限元模拟相结合的方法，研究挤压工件的表面磨损行为及显微组织演化机制。结果表明，挤压温度对合金的表面形貌和显微组织有很大的影响。随着挤压温度的升高，磨损机制由磨粒磨损(脆性损伤)转变为粘着磨损(塑性损伤)，并且表面粗糙度逐渐增大。挤压过程中坯料的温度和应力呈梯度分布，变形晶粒中的位错墙在激活的滑移系上运动，这导致层状结构产生变形。随着坯料梯度分布幅度和界面摩擦力的增大，层状结构变窄。此外，变形晶粒内几何必要位错的取向梯度也主导5种强理想织构。这些结果为纯镍及镍合金的精确挤压提供了理论依据。     

γ—TiAl合金拉伸形变的热激活参量

采用拉伸变形方式，在２８５－１２７３Ｋ范围内测定了具有近全片层组织的γ－ＴｉＡｌ合金（Ｔｉ－４７Ａｌ－２Ｍｎ－２Ｎｂ－０．８ＴｉＢ２）在屈服点的热激活能量，激活体积Ｖ，激活焓△Ｈ，激活自由焓△Ｇ和激活灿△Ｓ，据此推断控制γ－ＴｉＡｌ合金拉伸形变的微观位错机制，发现在实验温度范围内，存在着三个温度区间，分别对应三个不同的可能热辅助位错运动机制；在低温温区（２８５－３９８Ｋ），位错运动阻力主要是Ｐｅｉ     

热变形温度对Cu-Sn-P合金微观组织的影响

通过EBSD,TEM等方法对Cu-Sn-P在合金200～500℃的热变形组织进行分析。研究表明:经热变形后的晶粒组织垂直于受力方向被拉长,大部分为变形晶粒,应变硬化效果明显,基体内部存在较大的形变储能。再结晶主要在位错密度较大区域形核,软化作用比较微弱。热变形组织内部亚晶组织及位错聚集区密集分布,发现了刃型位错的交割以及位错列的滑移作用。当变形温度为500℃时,在再结晶晶粒内部会出现台阶状的退火孪晶。     

不同时效态7050铝合金时效成形中析出相对应力松弛行为的影响

铝合金时效成形方法结合了合金的蠕变松弛和析出强化作用,作为一种先进的整体壁板制造技术倍受航空制造业青睐。7xxx系铝合金在时效成形过程中的应力松弛行为受到合金内析出相与位错蠕变交互作用的影响从而制约着成形后零件质量与性能。本文采用设计的应力松弛试验研究了不同时效态(固溶态,欠时效态和峰时效态)7050铝合金内析出相对时效成形过程中应力松弛行为的影响,并通过位错热激活动力学参数计算和显微组织表征分析析出相与位错运动的交互作用。结果表明时效成形过程中析出相对位错热激活运动有明显地阻碍作用,因此含有不同尺度析出相铝合金的应力松弛行为表现不同,随着析出相尺度的增加合金应力松弛速率减缓,应力松弛极限增大。不同时效态7050铝合金位错激活体积计算和显微组织表征结果都证明了应力松弛过程中析出相增大对位错运动的阻碍作用也越显著。峰时效态7050铝合金的位错激活体积最大,时效成形后塑性应变的转化率最低。此外,时效成形过程中,7050铝合金内析出相对位错热激活的阻碍作用引起了槛应力现象,且随着析出相的增大槛应力也逐渐增大。     

温度和压力对1Cr12Mo钢汽轮机叶片微观组织的影响

为了研究服役条件下的汽轮机叶片的微观组织演化行为,利用X射线衍射仪、光学显微镜、扫描电镜和透射电镜等手段详细表征了退役1Cr12Mo钢高压动叶片的微观组织结构,进而分析了不同温度和压力对微观组织的影响规律。试验结果表明,适当的应力能促进板条内部尤其是边界析出相弥散分布,维持细小组织特征;细小弥散的析出相能够延缓位错束集和位错胞亚晶的形成。位错胞亚晶的形成显著地降低位错密度;温度对马氏体及位错结构影响不大,但显著提高碳化物的长大速度。     

2099合金热变形过程中的动态软化机制

采用等温热压缩实验,通过计算和对比热激活参数,并利用EBSD和TEM分析技术,研究了2099合金在热变形过程中的动态软化机制.基于Zener-Hollomon参数(Z)和变形温度(T)并结合对热激活参数与微观组织的分析,给出了2099合金在热变形中的软化机制.在lnZ≥35.5和T≤380℃范围内,变形以位错的交滑移为主.在lnZ≤37.4和T≥340℃范围内,由位错的交滑移、攀移以及三维位错网脱缠等变形机制共同控制.在lnZ≤35.1和T≥420℃范围内,发生了动态再结晶,此时以位错的交滑移、攀移、动态再结晶以及位错的脱钉为主要软化机制.动态再结晶形核机制以晶界弓出和亚晶合并共存,并随着变形温度的升高和应变速率的降低,亚晶合并形核得到强化.     

AA7005铝合金的热加工变形特性

研究了AA7005合金高温压缩变形时的流变应力、动态回复与再结晶以变形组织变化特征。合金稳态变形时,应变速度、温度和流变应力之间满足包含热激活材料常数的Arrhenius项的双曲正弦关系,变形过程为受位错增殖和相互销毁速率控制的热激活过程,螺型位错的交滑移和刃型位错的攀移为主要动态回复机制。动态回复时,形成典型的变形亚晶组织,亚晶尺寸随1nZ的减小而增大。高温低速变形条件下,合金发生局部几何动态再结晶,流变曲线呈现连续下降的特征,形成与原始纤维组织不同的细小等轴大角度再结晶晶粒。     

轧制态6082-T6铝合金的热压缩力学行为及微观组织分析

采用热模拟试验机对轧制态6082-T6铝合金进行热压缩试验,分析了合金在变形温度100~400 ℃,应变速率0.01 s^-1条件下的流变应力,对不同温度热变形的微观组织进行了表征。结果表明,轧制态6082铝合金的力学性能受变形温度和轧制方向的影响。变形过程中应力呈现负的温度敏感性,即随着变形温度升高,应力不断下降。合金表现出明显的力学性能各向异性,压缩强度在与轧制方向呈0°和90°较高,45°方向强度较低。经过热压缩变形后,与轧向呈不同方向的6082-T6铝合金的晶粒组织均沿着剪切力方向发生扭曲,同时,变形温度对晶粒组织的演变影响不大。随着变形温度的升高,合金基体内的位错密度明显下降,析出相发生粗化。     

浇注与铸型温度对6082铝合金热裂倾向的影响

研究了浇注温度和铸型温度对6082铝合金热裂倾向的影响。使用金相显微镜观察不同浇注条件下合金的组织发展,较高浇注温度时组织呈树枝状,较低浇注温度时组织呈蔷薇状枝晶。应用ADSTENFAN模拟软件对合金充型、凝固以及应力分布进行联动模拟。试验结果表明,通过采用降低浇注温度,提高合金凝固速率的方法可减少合金热裂倾向,且通过计算机模拟可准确认识热裂倾向大小及裂纹产生位置,为改善工艺参数,减少热裂倾向打下良好基础。     

Maximum energy release theory for recrystallization textures

In the maximum energy release theory for the interpretation of recrystallization textures, the most important driving force for recrystallization is assumed to be the stored energy due to dislocations. The arrangement of dislocations can be approximated by the edge dislocation arrangement which is determined from the deformation mode and texture, and in turn enable us to determine the absolute maximum principal stress direction. The maximum stress direction becomes parallel to the minimum elastic modulus direction on recrystallization, whereby energy release of the system can be maximized. The theory can explain recrystallization textures from electrodeposits, axisymmetrically deformed fcc metals, plane strain compressed fcc metals and aluminum single crystal, and cold rolled bcc iron alloy sheets.     

Multiscale modeling of plastic deformation of molybdenum and tungsten: I. Atomistic studies of the core structure and glide of 1/2〈1 1 1〉 screw dislocations at 0 K

Owing to their non-planar cores, 1/2〈1 1 1〉 screw dislocations govern the plastic deformation of body-centered cubic (bcc) metals. Atomistic studies of the glide of these dislocations at 0 K have been performed using Bond Order Potentials for molybdenum and tungsten that account for the mixed metallic and covalent bonding in transition metals. When applying pure shear stress in the slip direction significant twinning–antitwinning asymmetry is displayed for molybdenum but not for tungsten. However, for tensile/compressive loading the Schmid law breaks down in both metals, principally due to the effect of shear stresses perpendicular to the slip direction that alter the dislocation core. Recognition of this phenomenon forms a basis for the development of physically based yield criteria that capture the breakdown of the Schmid law in bcc metals. Moreover, dislocation glide may be preferred on {1 1 0} planes other than the most highly stressed one, which is reminiscent of the anomalous slip observed in many bcc metals.     

Ductile tensile failure in metals through initiation and growth of nanosized voids

We here reveal the initiation of ductile failure in metals at the nanometer scale by molecular dynamics simulations coupled with a novel analytical model. This proceeds by the emission of a special type of dislocation shear loop, which can expand as a partial or perfect dislocation, evolve into a prismatic loop through reaction, or develop into twins. Molecular dynamics (MD) simulations predict a strong dependence of the stress required for the initiation of plastic flow at the surface of the void for both Cu (a model fcc metal) and Ta (a model bcc metal). The decrease in stress with increasing void size is also analyzed in terms of a new analytical approach based on the energetics of dislocation loop emission. For both fcc (copper) and bcc (tantalum) metals initiation of plastic flow in MD simulations takes place at voids as small as a tri-vacancy (radius R ≈ 0.1 nm). Extensive calculations for tantalum combined with the analytical model, which tracks the simulations, enable extrapolation to R ≈ 300 nm, in the realm of second phase particles and inclusions. Thus we conclude that this is a general mechanism of tensile failure in pure monocrystalline metals where other initiation sites are absent.     

Bordoni relaxation in kink-chains

The Bordoni relaxation in fcc metals is generally assumed to result from thermally activated kink-pair formation in dislocation segments aligned along Peierls valleys in crystallographic close-packed directions. Objections against this interpretation have been raised by pointing out, that, at the small stresses applied in the experiments, the Paré condition is not satisfied and that the Peierls stress derived from the experiments by far exceeds the experimental values for the flow stress extrapolated to 0 K. It is shown that, when the bow-out configuration of a dislocation segment is treated as a kink-chain, and when the splitting of dislocation into partials is accounted for, both objections cannot be maintained. Under the assumption that the kink mobility is high and using analytical expressions for the kink-chain configuration and the kink-chain enthalpy under stress an expression for the bow-out rate of dislocation segments is obtained. Trivial numerical integration, under the action of a periodic applied stress, leads to a phase-lag between strain and stress and hence to energy dissipation. The dependence of this energy loss on segment lengths, temperature and internal stress is derived explicitly.     

Aermet100钢的高温变形本构关系与微观组织演变

在变形温度800～1200℃和应变速率0.01～50s-1下,利用Gleeble-3800热模拟试验机对Aermet100钢的高温变形本构关系与微观组织演变进行了研究。结果表明,增加应变速率和降低变形温度都能提高材料的流动应力,延迟动态再结晶发生,使变形材料表现出加工硬化和动态回复。运用位错理论研究了微观组织和流动应力曲线的变化规律并做出了合理的解释。在压缩实验的变形条件下变形激活能为489.10kJ/mol。确定了峰值应力、变形温度和应变速率之间的双曲正弦模型的本构关系。     

Thermally activated deformation of metals

The dynamic nature of the plastic deformation of metals and its relation to specific thermally activated dislocation mechanisms are discussed. Experimental techniques for evaluating the deformation parameters needed to identify the rate-controlling dislocation mechanism are described. The mechanisms that appear to be controlling in various temperature ranges are listed. Such fundamental lattice properties as the force-distant curve for a thermally activated process, the energy of a dislocation kink or of a jog, the stacking-fault energy, and the activation energy for self diffusion are indicated to be derivable from macroscopic mechanical property data.     

Microstructure evolution model based on deformation mechanism of titanium alloy in hot forming

The microstructure evolution in hot forming will affect the mechanical properties of the formed product. However, the microstructure is sensitive to the process variables in deformation process of metals and alloys. A microstructure evolution model of a titanium alloy in hot forming, which included dislocation density rate and primary a phase grain size, was presented according to the deformation mechanism and driving forces, in which the effect of the dislocation density rate on the grain growth was studied firstly. Applying the model to the high temperature deformation process of a TC6 alloy with deformation temperature of 1 133 - 1 223 K, strain rate of 0.01 -50 s^-1 and height reduction of 30%, 40% and 50%, the material constants in the present model were calculated by the genetic algorithm（GA） based objective optimization techniques. The calculated results of a TC6 alloy are in good agreement with the experimental ones.     

A dislocation dynamics study of the transition from homogeneous to heterogeneous deformation in irradiated body-centered cubic iron

Low temperature irradiation of crystalline materials is known to result in hardening and loss of ductility, which limits the usefulness of candidate materials in harsh nuclear environments. In body-centered cubic (bcc) metals, this mechanical property degradation is caused by the interaction of in-grown dislocations with irradiation defects, particularly small dislocation loops resulting from the microstructural evolution of displacement cascades. In this paper, we perform dislocation dynamics simulations of bcc Fe containing various concentrations of dislocation loops produced by irradiation in an attempt to gain insight into the processes that lead to hardening and embrittlement. We find that a transition from homogenous to highly localized deformation occurs at a critical loop density. Above it, plastic flow proceeds heterogeneously, creating defect-free channels in its wake. We find that channel initiation and size are mediated by loop coalescence resulting from elastic interactions with moving dislocations.     

Dislocation annihilation in plastic deformation: I. Multiscale irreversible thermodynamics

Irreversible thermodynamics is employed as a framework to describe plastic deformation in pure metals and alloys. Expressions to describe saturation stress in single crystals and nanocrystals are employed over wide ranges of temperature, strain rate and grain size. The importance of the roles played by vacancy self-diffusion in dislocation climb and in plasticity is shown. Equations to describe the stress–strain response of single crystals and ultrafine-grained metals are derived. The activation energy for dislocation annihilation plays a central role in the mechanical response of the systems. Succinct formulations for predicting hot deformation behaviour and relaxation of industrial alloys are presented; the influence of composition in the activation energy for dislocation annihilation is shown. All formulations describing stress–strain relationships can be reduced to Kocks–Mecking classical formulation, but incorporating grain size and compositional effects. The importance of the recovery term in such formulation is established, as well as the need to obtain it employing more fundamental approaches.