纳米孪晶铜尺度效应的数值模拟

为了理解电解沉积纳米孪晶铜的拉伸变形行为,采用基于机制的应变梯度塑性理论对其拉伸变形进行数值模拟研究;提出孪晶薄层强化带的概念,并采用黏聚力界面模型模拟晶界的滑移和分离现象。采用的计算模型包含晶粒尺寸、弹性模量、塑性硬化指数、初始屈服应力和孪晶薄层分布等和尺度效应相关的一系列参数。计算结果有助于理解纳米孪晶铜的力学行为。     

梯度结构特征对梯度纳米晶Cu力学行为影响的有限元模拟

采用晶体塑性有限元方法，对具有不同晶粒尺寸梯度结构特征的梯度纳米晶Cu的力学行为、应变场和应力场进行了计算分析。结果表明，当晶粒尺寸分布的梯度率n=1时，即晶粒尺寸分布梯度满足线性关系，平衡了强度和塑性两个关键的力学性能指标，梯度纳米晶Cu具有最优的强塑性匹配。梯度纳米晶Cu在变形过程中，粗晶承担了较大的应变，而细晶粒承载了更大的应力。此外，当梯度率n=1时，梯度纳米晶Cu在塑性变形中具有最大的应变和应力梯度，而且体系达到稳定的应变和应力梯度较晚。模拟结果与理论分析和实验结果一致。     

高锰钢加工硬化

利用传统的喷丸技术对高锰钢表面喷丸处理,研究材料表层的组织结构特征.结果表明,纳米晶的演化,通过奥氏体粗晶内部位错增殖、湮灭和重组、位错缠结逐渐向位错胞过渡;应变量和应变速率的增加,诱发机械孪生,单系孪晶逐渐向多系孪晶过渡;同时多系孪晶之间的交割作用使晶粒尺寸不断细化;晶粒在位错运动和机械孪生的重复作用下,最终形成等轴状、取向呈随机分布的纳米晶组织.喷丸处理高锰钢表层明显强化.随层深减小,硬度急剧增加.高锰钢表层的加工硬化主要是由于晶粒细化、位错硬化和孪晶硬化,而与相变硬化无关.     

梯度纳米晶体材料的多尺度力学行为研究

建立晶粒尺寸梯度分布的表面纳米晶材料本构模型。运用分子动力学模拟软件,得到了梯度单晶铜试样的拉伸应力-应变曲线以及原子构型图。结果表明:计算得到的应力-应变曲线与实验数据大体一致。这种梯度结构使得材料的整体流变应力增加了10%以上。随着应变的增大,曲线会有一段隆起的部分,这与位错的形核以及晶界的限制紧密相关。当应变增加到3%、7%时,小晶粒区域先有位错运动,然后产生堆积缠绕,起到了强化作用。并且,大晶粒使得位错更易从小晶粒向大晶粒区域运动,从而抑制了小晶粒区域裂纹的形成,提高了材料的延展性。     

纳米面心立方金属的变形机制及影响其力学性能的因素

综述了纳米面心立方金属的变形机制随晶粒尺寸的减小而发生的变化,即变形机制由晶界处发射不全位错、形成孪晶转变为晶界滑移、晶粒转动.当变形机制为晶界处发射不全位错、形成孪晶时,存在最佳孪晶形成晶粒尺寸范围,此时的孪晶形核应力最小.另一方面,随着晶粒尺寸的减小,在变形机制发生转变的临界晶粒尺寸附近存在韧-脆断裂方式的转变.提高孪晶密度、在纳米晶材料中加入微米晶相形成双峰晶粒材料可以提高纳米晶材料的塑性,得到更好的综合机械性能.     

应变梯度塑性理论模拟晶粒尺寸对铝多晶体强度的影响

采用经典塑性理论与基于微观机制的应变梯度（MSG）塑性理论对不同晶粒尺寸铝多晶的应力-应变关系进行了模拟分析,结果表明：基于微观机制的应变梯度本构方程所得的应力-应变曲线中,随着晶粒尺寸的减小,塑性应变功明显增加．晶粒直径为20μm的应力-应变曲线稍高于经典塑性理论得到的曲线,这进一步说明随着晶粒尺寸的增大,应变梯度的贡献逐渐减小,同时计算所得的屈服强度与晶粒尺度关系拟合的直线与铝晶体的Hall-Petch直线比较吻合。     

TWIP钢中晶粒尺寸对TWIP效应的影响

冷轧TWIP钢经1073,1173,1273和1373K固溶处理10min后,得到了晶粒尺寸分别为7,13,30和63μm的奥氏体组织.拉伸实验表明,随着晶粒尺寸的增加,加工硬化速率(dσ/dε)与真应变(ε)的变化关系由2阶段变为3阶段.当晶粒尺寸大于30μm时,加工硬化速率与真应变关系中的第2阶段对应的应变长度随着晶粒尺寸的增加而迅速增加.当真应变为0—0.2时,加工硬化指数随真应变的增加而迅速增加;在随后的变形中,与上述4个晶粒尺寸对应的试样的加工硬化指数分别稳定在0.47,0.53,0.56和0.68.OM和TEM观察显示,随晶粒尺寸的增大,变形过程中形变孪晶数量增多.对于较大晶粒尺寸的试样,形变孪晶在拉伸变形过程中形核的临界应力较低,随变形量增加,形变孪晶可持续形成,使其加工硬化能力增加,从而增大了TWIP效应;相反,晶粒尺寸减小使变形过程中的形变孪晶形核临界应力增大,抑制形变孪晶的产生,从而减小了TWIP效应.     

Cu极薄带轧制中滑移与变形的晶体塑性有限元模拟

为了定量描述晶粒取向和结构对极薄带轧制微观塑性变形非均匀性的影响,采用晶体塑性有限元方法(CPFEM)和Voronoi图的多晶模型,考虑试样尺寸、晶粒尺寸、晶体取向及其分布,模拟了不同厚度Cu极薄带在相同压下率条件下的滑移与变形行为,得到了介观尺度上Cu极薄带的微观应力-应变和启动滑移系分布.模拟获得的应力-应变曲线和实验测得的曲线基本一致,验证了晶体塑性有限元模型的准确性.通过对40%压下率Cu极薄带轧制变形的研究表明,无论是在晶粒内部还是在晶粒间,材料内部的变形都非常不均匀,这种不均匀性主要是由初始晶粒取向和结构不同、近邻晶粒取向差以及变形时滑移系的运动特性和晶粒旋转不同引起的.滑移系首先在自由表面和晶界处被激活,而后引起晶粒内部滑移系的启动与运动.     

表面机械研磨诱导AISI 304不锈钢表层纳米化Ⅱ.晶粒细化机理

采用表面机械研磨处理(SMAT)在AISI 304不锈钢上制备出纳米结构表层，用透射电镜(TEM)研究组织演变过程．晶粒细化机理可归纳如下：位错在{111}面上滑移并相互交割形成网格结构；单系孪晶形成并逐渐过渡到多系孪晶；多系孪晶相互交割使晶粒尺寸不断减小，并在孪晶交叉处形成了马氏体相；孪晶系增多与孪晶重复交割强度加大使得细化晶粒的尺寸进一步减小；最终在大应变量、高应变速率和多方向重复载荷的作用下，形成等轴状、取向呈随机分布的马氏体相纳米晶组织．     

晶粒尺寸效应对铜极薄带轧制变形机制影响的模拟研究

采用退火态轧制铜箔为原料,进行晶粒尺寸效应的箔轧实验和晶体塑性有限元模拟。基于率相关晶体塑性理论,开发用户材料子程序(UMAT),建立轧制铜极薄带的晶体塑性有限元模型,改进Voronoi图种子生成的随机性,建立反映晶粒形貌、晶界不规则性的多晶极薄带几何模型,并编写赋予多晶取向的算法,用以控制多晶取向及织构分布,研究晶粒尺寸效应对其变形机制的影响。结果表明:在铜极薄带中尺寸较小晶粒中产生的剪切带相对于尺寸较大晶粒中产生的要均匀,可较好地减小变形局部化;不同晶粒尺寸铜极薄带的滑移系启动和累积滑移存在显著差异,启动的滑移系随晶粒尺寸的减小而增多;表层晶粒和内部晶粒的约束差异导致变形后晶粒取向主要绕横向(TD)进行旋转,旋转角度和极点分散度随晶粒平均尺寸的减小而减小。箔轧实验和模拟得到的轧制力-晶粒尺寸曲线基本一致,即晶粒取向对轧制力的影响随晶粒平均尺寸的减小而减弱。     

全片层TiAl基合金的屈服强度与显微组织关系

通过特定的热处理工艺,分离出全片层组织的晶粒尺度和片层厚度两个主要的显微组织参数,研究了晶粒尺度和片层间距对全片层组织合金的强化效果,研究结果表明：合金的屈服强度随着晶粒尺度和片层厚度的减小而增加,符合Hall-Petch强化关系,同时用屈服强度模型解释了晶粒尺度和片层厚度的强化效果。     

CREEP OF POLYCRYSTALLINE NEAR γ-TiAl WITH A LAMELLAR MICROSTRUCTURE

Creep of a polycrystalline near γ-TiAl alloy in two fully lamellar conditions is presented. A lamellar structure with fine interface spacing and planar grain boundaries provides improved creep resistance. The lamellar structure with wide interface spacing and interlocked grain boundaries has <1/2 the creep life, five times the minimum strain rate and greater tertiary strain.Creep strain is accommodated by dislocation motion in soft grains, but the strain rate is controlled by hard grains. The resistance to fracture is controlled by the grain boundary morphology, with planar boundaries causing intergranular fracture.To maximize the creep resistance of near γ-TiAl with a lamellar microstructure requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.     

Effect of fully lamellar morphology on creep of a near γ-TiAl intermetallic

Creep properties of a polycrystalline binary near γ-TiAl intermetallic in two fully lamellar microstructural conditions are presented. Creep tests (760°C/240 MPa) indicate that a lamellar structure with fine interface spacing and planar grain boundaries improves creep resistance. A lamellar structure with wide lamellar interface spacing and interlocked grain boundaries has less than half the creep life, five times higher minimum creep strain rate and a greater tertiary creep strain. The deformation substructures are presented in terms of the lamellar orientation to the stress axis and indicate that creep strain is accommodated by dislocation motion in soft oriented grains, but the creep strain rate is controlled by hard oriented grains. The extent of tertiary creep is controlled by the grain boundary morphology, with planar grain boundaries susceptible to intergranular cracking. The results suggest that to maximize the creep resistance of near γ-TiAl intermetallics with lamellar microstructures requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.     

晶粒尺寸和孪晶界间距对纳米多晶铝合金塑性变形影响的分子动力学模拟研究

采用分子动力学模拟方法,分别研究了晶粒尺寸和孪晶密度对纳米多晶铝合金塑性变形的影响。模拟结果表明,弛豫后的位错密度对纳米多晶Al的微观结构演变和逆Hall-Petch关系产生了重要影响。变形受晶粒大小限制,在细晶中可形成层错四面体和复杂层错结构,从而激活了晶界的辅助变形。当孪晶界间距（TBS）较大时,Shockley分位错在晶界处形核并增殖。然而,随着TBS的减小,孪晶界成为Shockley分位错的来源。孪晶界上大量的分位错形核会导致孪晶界迁移甚至消失。在塑性变形过程中还观察到形变纳米孪晶。研究结果为开发具有可调节力学性能的先进纳米多晶Al提供了理论基础。     

Thermal stability of lamellar structure of PST crystal and lamellar boundary design for creep resistant TiAl alloy

The stability of lamellar structure is crucial for the creep resistance of TiAl alloys, but degradation of the lamellar structure is unavoidable at high temperatures. The degradation of the lamellar structure in PST crystals of Ti-48mol.%Al was studied during high temperature exposure (annealing and creep testing) to examine how to make a stable lamellar structure with high creep deformation resistance. Since the six orientation variants of γ lamellae are nucleated independently of the adjoining lamellae, pseudo twin and 120° rotational fault boundaries are most frequently observed at the initial stage of lamellar formation. The preferential removal of high energy (pseudo twin and 120° rotational fault) boundaries during the evolution of lamellar structure results in the highly probable appearance of a true twin boundary at a later stage of lamellar evolution. The coarsening of lamellar spacing and the spheroidization of the lamellae are the major degradation events occurring during creep deformation, and the migration of the lamellar boundaries brings both of them about. The lamellar structures of TiAl alloy contain four types of lamellar boundaries. The stability of the four types of boundaries decreases in the following order: γ/α₂ > true twin > pseudo twin > or=120° rotational fault boundaries. The γ/α₂ boundary has the highest stability (lowest mobility), and the high density of γ/α₂ boundaries is proposed to make a stable lamellar structure with good creep resistance. A material having the high density of γ/α₂ boundaries was produced through the heat treatment of a PST crystal in the α+γ two-phase regime. The excellent creep properties of the material were proven through creep tests of hard oriented PST crystals made of the material. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

Electron Microscopic Study of the Structure of Tetragonal Martensite in In–4.5% Cd Alloy

In this work, the formation of a packet structure composed of colonies of lamellar plates separated by twin boundary {101}_fct in In–4.5 wt % Cd alloy upon cooling below the fcc → fct martensitic transition temperature has been shown using the methods of metallography, X-ray diffraction, transmission electron microscopy, and EBSD analysis. Two neighboring lamellae differ from each other by the direction of their tetragonality axes. Using ЕВSD analysis, it has been established that neighboring packets always contain three types of tetragonal martensite lamellae, which are in twin positions and differ from each other by the direction of their tetragonality axes. In turn, each martensite lamella consists of a set of smaller lamellae, which are in twin positions. After the cycle of fct → fcc → fct transitions, the alloy recrystallizes with a decrease in the grain size by several times compared with the initial structure such that the size of packets and the length and width of martensitic lamellae in a packet correlate with a change in the size of an alloy grain.     

Effects of discontinuour coarsening of lamellae on creep strength of fully lamellar TiAl alloys

High temperature creep of a binary Ti-42mol%Al alloy with fully lamellar structure was studied to examine effects of lamellar spacing on creep strength. Strain hardening is more significant in a finer lamellar material, resulting in higher creep strength at high stresses. Discontinuous coarsening of lamellae takes place during creep, and is more substantial in the finer lamellar material at low stresses. Because of the microstructural degradation, the strengthening by fine lamellae diminishes at low stresses. Some specimens were annealed at high temperatures to finish the discontinuous coarsening prior to creep testing. In these specimens, the strengthening by fine lamellae becomes effective even at low stresses.     

The relationships of microstructure and properties of a fully lamellar TiAl alloy

The relationship between the yield strength and microstructure parameters of a fully lamellar TiAl alloy has been studied systematically. The grain size and the lamellar spacing were chosen as microstructure parameters. The experimental results showed that the yield strength increases with the decrease of grain size and more obviously with the decrease of the lamellar spacing. The relationship between yield strength and grain size and lamellar spacing can be approximately described by Hall–Petch relation.     

C含量对铸造TiAl合金组织和力学性能的影响

在Ti-47.5Al-3.7(Cr,V,Zr)合金中添加0.05%~0.2%C(原子分数,下同),采用冷坩埚悬浮熔炼方法制备出了层片组织TiAl合金铸棒,通过组织观察、室温拉伸和蠕变性能测试研究了C含量对TiAl合金组织和力学性能的影响。结果表明,添加0.05%~0.2%C后,合金仍可获得择优取向层片组织。随C含量增加α2层片体积分数略有增加,层片间距呈细化趋势。当C含量超过0.1%时,在α2和γ层片内和层片界面上有细小的Ti2AlC型碳化物析出,碳化物析出相的尺寸和数量随C含量增加有所增加。添加0.05%~0.2%C后提高了合金室温的抗拉强度和屈服强度,且随C含量增加提升幅度逐渐增大,当C含量为0.2%时,分别将抗拉强度和屈服强度提升了101和123 MPa。添加C元素后显著改善了合金的蠕变性能,当C含量为0.1%时蠕变性能最佳,与不含C的合金相比,其塑性蠕变应变降低了一半、相同应变时的蠕变速率降低了1个数量级以上。添加0.1%C提升合金蠕变抗力的机制主要是通过抑制合金在蠕变初期的位错萌生和增殖过程;在γ层片中形成割阶和位错碎片阻碍位错继续运动,使得合金在蠕变第一阶段的应变硬化程度迅速增加;此外,析出的Ti2AlC型碳化物进一步强化层片界面和基体,与层片间距细化共同提高了穿层片滑移位错的运动阻力。     

纯度对ECAP制备超细晶铜疲劳性能的影响

采用应力比R=–1的对称加载疲劳试验,研究了ECAP制备的超细晶高纯铜(HPCu)、低纯铜(LPCu)的疲劳行为,分析了循环应力-应变响应、疲劳寿命和疲劳前后晶粒取向分布,讨论了纯度与超细晶材料疲劳稳定性的关系。结果表明:在任何应力幅下,获得的超细晶低纯铜的寿命都大于ECAP变形前的粗晶铜;在相同应力幅下,循环周次提高1.6~2.0倍。而超细晶高纯铜的疲劳曲线,表现出不同的特性,在高应力幅下,超细晶高纯铜具有较高的疲劳寿命,但在低应力幅下,超细晶高纯铜循环周次下降,疲劳寿命低。在应力控制条件下,随应力幅的降低,超细晶纯铜的循环应力-应变响应从循环软化逐渐过渡为循环硬化。杂质的存在能有效阻止疲劳过程中晶粒的转动和位错的运动,降低其回复软化,减小相邻晶粒间取向差变化,使超细晶低纯铜与超细晶高纯铜相比有较大的循环硬化指数n和循环硬化系数K,具有较好的疲劳稳定性。