FCC：BCC部分共格界面的计算机再研究

本文用计算机再次模拟了FCC:BCC部分共格界面结构。发现失配位错并不是FCC:BCC部分共格界面不可缺少的结构要素,界面上两个方向台阶存在可以使共格区增多和两相界面能降低,对R-A结构模型进行了补充和修正。     

Low-temperature shear modulus changes in solid 4He and connection to supersolidity

Superfluidity--liquid flow without friction--is familiar in helium. The first evidence for 'supersolidity', its analogue in quantum solids, came from torsional oscillator measurements involving 4He. At temperatures below 200 mK, the torsional oscillator frequencies increased, suggesting that some of the solid decoupled from the oscillator. This behaviour has been replicated by several groups, but solid 4He does not respond to pressure differences, and persistent currents and other signatures of superflow have not been seen. Both experiments and theory indicate that defects are involved; these should also affect the solid's mechanical behaviour. Here we report a measurement of the shear modulus of solid 4He at low frequencies and strains. We observe large increases below 200 mK, with the same dependence on measurement amplitude, 3He impurity concentration and annealing as the decoupling seen in the torsional oscillator experiments. We explain this unusual elastic behaviour in terms of a dislocation network that is pinned by 3He at the lowest temperatures but becomes mobile above 100 mK. The frequency changes in the torsional oscillator experiments appear to be related to the motion of these dislocations, perhaps by disrupting a possible supersolid state.     

高温有序金属间化合物研究的新进展

以铝化物为基的有序金属间化合物具有优异的抗高温氧化和热腐蚀的性能,作为高温下工作的结构材料是很有吸收力的。然而在许多情况下,脆性断裂和低的断裂抗力限制了它们作为有实用价值的工程材料。系统地研究了有序间化合物合金的脆性断裂行为,并弄清了脆性断裂的内在和外在的一些控制因素。第一原理计算和原子结构模拟的新近研究结果能进一步帮助了解金属间化合物的原子结合键性质、位错组态和合金元素的作用。对基本问题的更好了     

Pressure sensitivity of olivine slip systems and seismic anisotropy of Earth's upper mantle

The mineral olivine dominates the composition of the Earth's upper mantle and hence controls its mechanical behaviour and seismic anisotropy. Experiments at high temperature and moderate pressure, and extensive data on naturally deformed mantle rocks, have led to the conclusion that olivine at upper-mantle conditions deforms essentially by dislocation creep with dominant [100] slip. The resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth. The rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle. But new high-pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle, but with a different slip direction. Here we show that this high-pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy, consistent with seismological observations below 250 km depth. These results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below.     

Dislocation multi-junctions and strain hardening

At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects called dislocations. First proposed theoretically in 1934 (refs 1-3) to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening, a common phenomenon in which continued deformation increases a crystal's strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions that tie the dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed 'multi-junctions'. We first predict the existence of multi-junctions using dislocation dynamics and atomistic simulations and then confirm their existence by transmission electron microscopy experiments in single-crystal molybdenum. In large-scale dislocation dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication, thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in body-centred cubic crystals.     

超高速碰撞下体心立方纯铁的变形孪晶

采用非火药驱动二级轻气炮技术将2017Al合金弹丸以3km/s速度撞击纯铁靶板,通过光学显微镜、扫描电镜和透射电镜研究了纯铁材料的微观组织演化.根据微观组织的不同特征可将纯铁靶板分为细晶区、高密度孪晶区和低密度孪晶区.在低密度孪晶区的主要变形方式为{112}面的孪晶,而高密度孪晶区主要变形方式为{112}面的孪晶和{110}、{112}及{123}面的位错之间的交互作用.超高速碰撞下孪晶形成机制与普通变形条件下没有区别,但是孪晶形貌区别较大,高密度位错聚集在孪晶界附近造成孪晶界面模糊;螺位错在孪晶界上分解导致沿着孪晶界发生弯曲和凹痕甚至孪晶的断裂.     

位错和溶质原子交互作用的蒙特卡罗模拟

文章运用动态蒙特卡罗方法,模拟了二维条件下单个刃型位错和置换溶质原子间的相互作用.在假设位错始终静止的情况下,模拟得到位错中心形成密集的溶质气团,远离中心处形成弥散的溶质云层;动态结果反映,在低外加应力下位错受到溶质原子的有效钉扎,运动速度很小;高外加应力下位错运动几乎不受溶质原子的影响,速度较大;而在中间应力范围内,位错的运动状态具有歧义性,速度处于高低跳跃的状态.     

铜∑9〈110〉{221}对称倾斜晶界在纯剪切应力作用下结构转变行为的原子尺度模拟研究

采用原子尺度计算模拟方法并结合嵌入原子型原子间相互作用势,对含∑9〈110〉{221）对称倾斜晶界的铜双晶体在纯剪切载荷作用下的形变行为进行了研究．能量最小化弛豫分析表明,该晶界存在两个能量局部极小的优化结构,其能量低者为稳态结构,另一能量稍高者为亚稳态结构．对双晶体在室温下的剪切过程进行分子动力学模拟发现,在不同取向的剪切载荷作用下,该晶界的结构转变模式有如下三种方式：（1）纯粹的晶界滑移;（2）晶界局部原子重排同时伴随晶界位错发射;（3）晶界滑移与晶界迁移相结合的耦合运动．结构演化分析显示,纯粹的晶界滑移不改变晶界结构,晶界位错发射会引发孪晶的产生与面心立方结构向密排六方结构转化,而晶界耦合运动则随剪切方向的不同有两种不同的方式：其一为由晶界原子集体运动而引起的结构转变过程,其二为由晶界局部原子重排过程引起的结构转变,这些局部原子重排过程互不关联;其中,前者会使晶界结构在稳态与亚稳态之间转化,而后者会引起晶界小面化．分析表明,这些结构转变规律可采用晶界重位点阵理论给出较好的解释．     

径向资源集中配置BCC模型的投影机制

深入分析了Lozano and Villa(2004)提出的径向资源集中配置BCC模型(我们称之为LV模型)的投影机制.通过用经典的BCC模型研究LV模型的投影点,发现其达到了BCC有效.最后,把LV模型推广到评估部分决策单元总输出对总输入的效率.     

低温处理时钢铁中碳原子的扩散

讨论在各种温度下钢铁样品中碳原子的扩散行为．讨论了晶界、位错和点缺陷对碳原子扩散过程的影响，提出了钢铁材料在深冷处理中碳化物析出的可能机理．     

对灰铸铁压缩试件的变形及倾斜断口形成过程的探讨

本文在[1]的基础上,通过对断口特征的分区域研究及试件两端三向受压区的错位楔入假设,对试件主要的变形和破坏过程进行了定性的描述和解释。     

缺陷预熔化现象的晶体相场模拟

【目的】研究外应变作用下BCC晶体中晶界和位错的预熔化现象,揭示晶界预熔化的机理。【方法】构建包含外力场与晶格原子密度耦合作用项的三维体系自由能函数,并建立以温度为主要参数的晶体相场模型,模拟三维情况下某一平面的晶界和位错的预熔现象。【结果】随着温度的升高,位错熔解的区域不断扩大,在接近熔点温度时晶界处的位错首先诱发晶界出现预熔化现象;当熔解趋于稳定后,在应力作用下,开始出现位错滑移现象,同一水平直线上的一组位错对相互靠近,最终湮没变为一个完整晶界。【结论】体系的温度影响并决定位错的熔解。体系温度越高,位错处的熔解就越容易进行。     

分子动力学模拟裂纹扩展及相关尺寸行为

运用分子动力学模拟方法研究原子弛豫对于裂尖场的影响,发现弛豫后裂尖离散非线性区半径约为150(A);如果原子区大于这一尺寸,连续介质力学的弹性场可以通过边界条件,有效影响原子尺度的裂尖行为.进一步研究表明:体心立方铁中,张开型裂纹在低温时为脆性解理扩展,并伴随裂尖层错和孪晶的形成;随着温度升高,裂尖层错和孪晶的形成逐渐减弱,在250 K左右发生脆韧转变;同时观察到裂尖位错发射.     

含晶界孔洞粗晶工业纯铁的循环变形及损伤特征

为了更好地了解体心立方(bcc)结构金属的循环变形机制,在恒总应变幅控制的条件下研究了含晶界孔洞粗晶工业纯铁的疲劳变形特征.结果表明,含晶界孔洞的粗晶工业纯铁在不同总应变幅"εt/2下均发生不同程度的循环硬化现象,无循环饱和阶段出现.疲劳寿命与塑性应变幅的关系基本符合Coffin-Manson法则.循环变形的表面变形特征与外加总应变幅具有一定的相关性:随着总应变幅的增加,滑移变形及其导致的挤出侵入现象更为严重,滑移开裂更趋显著;越来越多的原来位于晶界上的微观孔洞发生扭曲变形、聚合、长大而产生裂纹,甚至导致沿晶开裂.在相对较高的总应变幅下,在表面还观察到了滑移扭折现象以及沿晶裂纹扩展进入晶粒内部的现象.在低应变幅"εt/2=1.0×10-3下循环变形后发现了类驻留滑移带(PSB)楼梯位错结构,随着应变幅的增加,位错胞结构发展成为主要结构特征,其平均尺寸逐渐减小.     

Dynamical fracture instabilities due to local hyperelasticity at crack tips

As the speed of a crack propagating through a brittle material increases, a dynamical instability leads to an increased roughening of the fracture surface. Cracks moving at low speeds create atomically flat mirror-like surfaces; at higher speeds, rougher, less reflective ('mist') and finally very rough, irregularly faceted ('hackle') surfaces are formed. The behaviour is observed in many different brittle materials, but the underlying physical principles, though extensively debated, remain unresolved. Most existing theories of fracture assume a linear elastic stress-strain law. However, the relation between stress and strain in real solids is strongly nonlinear due to large deformations near a moving crack tip, a phenomenon referred to as hyperelasticity. Here we use massively parallel large-scale atomistic simulations--employing a simple atomistic material model that allows a systematic transition from linear elastic to strongly nonlinear behaviour--to show that hyperelasticity plays a governing role in the onset of the instability. We report a generalized model that describes the onset of instability as a competition between different mechanisms controlled by the local stress field and local energy flow near the crack tip. Our results indicate that such instabilities are intrinsic to dynamical fracture and they help to explain a range of controversial experimental and computational results.     

由色散材料构成的体心立方晶体的光子能带计算

计算了小球排列构成的体心立方结构的光子带隙性质，小球的介电函数可以是极化激元型或等离子型．虽然在通常的介电材料体心立方结构中并不容易形成光子带隙，但在我们考虑的这两类情况中均发现绝对带隙．本中讨论的光子能带结构是由非常有效的矢量波多重散射方法计算，该方法可以精确地处理介电函数在界面附近的突变．     

制造服务中服务协作链构建与优化研究

制造服务是制造与服务的融合,是面向制造的服务,并且可进行分解和重构;根据制造服务的特征,以制造企业外协加工制造服务为例,构建了制造服务服务协作链,定义了多元组的服务协作链,利用Petri网建立了五元组服务协作链模型,针对服务协作链优化这一N-P Hard问题,运用贪婪算法选取服务时间作为贪婪因子进行了优化,并进行了实例验证;通过算例验证了服务协作链优化的可行性,结果表明:构建制造服务中的服务协作链以及对服务协作链进行优化可实现制造服务中业务协作优化决策,为实现智能化制造系统奠定坚实基础。     

基于原子模型的非局域连续模型

在传统的连续模型中,本构关系,例如胡克定律、热传导的傅立叶定律是宏观现象的表征.在原子尺度下,传统连续模型具有很大的局限性,其中一个原因就是传统连续模型忽略了原子间相互作用的非局域性.本文在不采用任何经验的本构假设和局部均匀变形假设(如Cauchy-Born假设)的情况下,严格基于原子模型,构造了一个连续模型,将原子间相互作用力的非局域性和非线性性自然地嵌入到模型中.此模型可以有效表现原子尺度的非均匀变形.     

Hydrogen effect on the mechanical behaviour and microstructural features of a Fe-Mn-C twinning induced plasticity steel

The influences of hydrogen on the mechanical properties and the fracture behaviour of Fe-22Mn-0.6C twinning induced plasticity steel have been investigated by slow strain rate tests and fractographic analysis.The steel showed high susceptibility to hydrogen embrittlement,which led to 62.9%and 74.2%reduction in engineering strain with 3.1 and 14.4 ppm diffusive hydrogen,respectively.The fracture surfaces revealed a transition from ductile to brittle dominated fracture modes with the rising hydrogen contents.The underlying deformation and fracture mechanisms were further exploited by examining the hydrogen effects on the dislocation substructure,stacking fault probability,and twinning behaviour in pre-strained slow strain rate test specimens and notched tensile specimens using coupled electron channelling contrast imaging and electron backscatter diffraction techniques.The results reveal that the addition of hydrogen promotes planar dislocation structures,earlier nucleation of stacking faults,and deformation twinning within those grains which have tensile axis orientations close to<111>//rolling direction and<112>//rolling direction.The developed twin lamellae result in strain localization and micro-voids at grain boundaries and eventually lead to grain boundary decohesion.     

Dislocation creep in MgSiO3 perovskite at conditions of the Earth's uppermost lower mantle

Seismic anisotropy provides an important observational constraint on flow in the Earth's deep interior. The quantitative interpretation of anisotropy, however, requires knowledge of the slip geometry of the constitutive minerals that are responsible for producing rock fabrics. The Earth's lower mantle is mostly composed of (Mg, Fe)SiO3 perovskite, but as MgSiO3 perovskite is not stable at high temperature under ambient pressure, it has not been possible to investigate its mechanical behaviour with conventional laboratory deformation experiments. To overcome this limitation, several attempts were made to infer the mechanical properties of MgSiO3 perovskite on the basis of analogue materials. But perovskites do not constitute an analogue series for plastic deformation, and therefore the direct investigation of MgSiO3 perovskite is necessary. Here we have taken advantage of recent advances in experimental high-pressure rheology to perform deformation experiments on coarse-grained MgSiO3 polycrystals under pressure and temperature conditions of the uppermost lower mantle. We show that X-ray peak broadening measurements developed in metallurgy can be adapted to low-symmetry minerals to identify the elementary deformation mechanisms activated under these conditions. We conclude that, under uppermost lower-mantle conditions, MgSiO3 perovskite deforms by dislocation creep and may therefore contribute to producing seismic anisotropy in rocks at such depths.