A molecular dynamics study on mechanical performance and deformation mechanisms in nanotwinned NiCo-based alloys with nano-precipitates under high temperatures

Molecular dynamics simulations are performed to investigate the mechanical behavior of nanotwinned NiCo-based alloys containing coherent L1_(2) nano-precipitates at different temperatures,as well as the interactions between the dislocations and nano-precipitates within the nanotwins.The simulation results demonstrate that both the yield stress and flow stress in the nanotwinned NiCo-based alloys with nano-precipitates decrease as the temperature rises,because the higher temperatures lead to the generation of more defects during yielding and lower dislocation density during plastic deformation.Moreover,the coherent L1_(2) phase exhibits excellent thermal stability,which enables the hinderance of dislocation motion at elevated temperatures via the wrapping and cutting mechanisms of dislocations.The synergistic effect of nanotwins and nano-precipitates results in more significant strengthening behavior in the nanotwinned NiCo-based alloys under high temperatures.In addition,the high-temperature mechanical behavior of nanotwinned NiCo-based alloys with nano-precipitates is sensitive to the size and volume fraction of the microstructures.These findings could be helpful for the design of nanotwins and nano-precipitates to improve the high-temperature mechanical properties of NiCo-based alloys.

Interfacial enhancement of Ag and Cu particles sintering using(111)-oriented nanotwinned Cu as substrate for die-attachment

The electroplated(111)-oriented nanotwinned-Cu(nt-Cu) film was utilized as substrate for Ag and Cu sinter joining to improve the weak interface connection between the metal paste and the substrate.It was found that both Cu and Ag sinter joints using(111)-oriented nt-Cu film exhibited a higher bonding strength than that using traditional random-oriented Cu film.Especially,the joints sintered with Cu paste on(111)-oriented nt-Cu film possessed a higher bonding strength of 53.7 MPa at the sintering condition of 300 °C,0.4 MPa in N2 atmosphere,compared to that on random-oriented Cu film with a value of 31.3 MPa.The results show that as metal substrate layer,the(111)-oriented nt-Cu film can improve the connection performance of Ag and Cu sinter joints,which could further promote their application in dieattachment technology for the next-generation power semiconductors.

Molecular dynamics simulation on mechanisms of plastic anisotropy in nanotwinned polycrystalline copper with{111}texture during tensile deformation

Based on molecular dynamics(MD)simulation,the mechanisms of plastic anisotropy in nanotwinned polycrystalline copper with{111}texture during tensile deformation were systematically studied from the aspects of Schmid factor of the dominant slip system and the dislocation mechanism.The results show that the Schmid factor of dominated slip system is altered by changing the inclining angle of the twin boundaries(TBs),while the yield stress or flow stress does not strictly follow the Schmid law.There exist hard and soft orientations involving different dislocation mechanisms during the tensile deformation.The strengthening mechanism of hard orientation lies in the fact that there exist interactions between the dislocations and the TBs during plastic deformation,which leads to the dislocation blocking and reactions.The softening mechanism of soft orientation lies in the fact that there is no interaction between the dislocations and the TBs because only the slip systems parallel to the TBs are activated and the dislocations slip on the planes parallel to the TBs.It is concluded that the plastic anisotropy in the nanotwinned polycrystalline copper with{111}texture is aroused by the combination effect of the Schmid factor of dominated slip system and the dislocation mechanism.

Thermodynamic Properties of Nanograin Boundary and Thermal Stability of Nanograin Structure

The thermal features of the nanograin boundary were described by a developed thermodynamic model. Using the nanocrystalline Cu as an example, the pressure, the bulk modulus, and the volume thermal expansion coef- ficient were calculated to characterize the thermodynamic properties of the grain boundaries on the nanoscale. Based on the parabola-type relationship between the excess free energy and the excess volume of the nanograin boundary, the thermal stability, as well as its evolution characteristics, was analyzed. The experimental re- sults of the temperature-varying grain growth in the nanocrystalline Cu, which exhibited the discontinuous nanograin growth behavior, verified the thermodynamic predictions. In addition, the quantitative relationships correlating the excess volume and the lattice expansion with the nanograin size were discussed.

Phase-field simulation of the coupled evolutions of grain and twin boundaries in nanotwinned polycrystals

Nanotwinned polycrystals exhibit an excellent strength-ductility combination due to nanoscale twins and grains. However, nanotwin-assisted grain coarsening under mechanical loading reported in recent experiments may result in strength drop based on the Hall-Petch law. In this paper, a phase-field model is developed to investigate the effect of coupled evolutions of twin and grain boundaries on nanotwin-assisted grain growth. The simulation result demonstrates that there are three pathways for coupled motions of twin and grain boundaries in a bicrystal under the applied loading, dependent on the amplitude of applied loading and misorientation of the bicrystal. It reveals that a large misorientation angle and a large applied stress promote the twinning-driven grain boundary migration. The resultant twin-assisted grain coarsening is confirmed in the simulations for the microstructural evolutions in twinned and un-twinned polycrystals under a high applied stress.

High-dielectric-constant nanograin BaTiO_(3)-based ceramics for ultra-thin layer multilayer ceramic capacitors via grain grading engineering

Four BT-based ceramic samples were prepared using a grain grading approach.The bigger-grained(~100 nm)and smaller-grained(~70 nm)BaTiO_(3)(BT)powders were mixed.The smaller-grained BT powder controlled the average grain size and guaranteed the reliability,while the bigger-grained powder enhanced the dielectric constant.Various percentages of bigger-grained BT powder were introduced to balance the average grain size and the dielectric constant.As the proportion of bigger grains increased,the dielectric constant(εr)improved significantly.The room-temperatureεr of 25%bigger-grain mixed BT(2623)was~50%higher than that of the sample with a similar average grain size without grain grading.The ceramic mixed with 15%bigger-grained BT showed comprehensive dielectric performance,which met the EIA X5R standard and provided a considerableεr of 1841 along with a low dielectric loss of 0.78%.Notably,the average grain size was 90 nm,which favors the applications in ultra-thin multilayer ceramic capacitors.

Influences of nanotwin volume fraction on the ballistic performance of coarse-grained metals

Coarse-grained（CG） metals strengthened by nanotwinned（NT） regions possess high strength and good ductility. As such, they are very suitable for applications in bullet-proof targets. Here, a numerical model based on the conventional theory of strain gradient plasticity and the Johnson–Cook failure criterion is employed to study the influences of volume fraction of NT regions on their ballistic performance.The results show that in general a relatively small twin spacing（4–10 nm） and a moderate volume fraction（7%–20%） will lead to excellent limit velocity and that the influences of volume fraction on limit displacement change with the category of impact processes.

Strengthening and softening in gradient nanotwinned FCC metallic multilayers

Plastic-deformation behaviors of gradient nanotwinned(GNT)metallic multilayers are investigated in nanoscale via molecular dynamics simulation.The evolution law of deformation behaviors of GNT metallic multilayers with different stacking fault energies(SFEs)during nanoindentation is revealed.The deformation behavior transforms from the dislocation dynamics to the twinning/detwinning in the GNT Ag,Cu,to Al with SFE increasing.In addition,it is found that the GNT Ag and GNT Cu strengthen in the case of a larger twin gradient based on more significant twin boundary(TB)strengthening and dislocation strengthening,while the GNT Al softens due to more TB migration and dislocation nucleation from TB at a larger twin gradient.The softening mechanism is further analyzed theoretically.These results not only provide an atomic insight into the plastic-deformation behaviors of certain GNT metallic multilayers with different SFEs,but also give a guideline to design the GNT metallic multilayers with required mechanical properties.

The wrinkling and buckling of graphene induced by nanotwinned copper matrix:A molecular dynamics study

The three-dimensional(3D)graphene-based materials have raised significant interest due to excellent catalytic performance and unique electronic properties,while the preparation of uniform and stable 3D graphene structures remains a challenge.In this paper,using molecular dynamics simulations,we found that the nanotwinned copper(nt-Cu)matrix with small twin spacing can induce the wave-shaped wrinkling and sawtooth-shaped buckling graphene structures under uniaxial compression.The nt-Cu matrix possesses a symmetrical lattice structure for the lattice rotation with the dislocation annihilation,resulting in the transition of sandwiched graphene from 2D to 3D structures with good uniformity.The newly formed twin boundaries(TBs)in the nt-Cu matrix improve the resistance of graphene against the out-of-plane deformation so that graphene can maintain a stable wrinkling or buckling morphology in a wide strain range.These 3D texturing structures show great flexibility and their micro parameters can be controlled by applying different compressive strains.Furthermore,we propose a simple sliding method for decoupling graphene from the nt-Cu matrix without any damage.This work provides a novel strategy to induce and transfer the uniform wrinkling and buckling of graphene,which may expand the application of graphene in energy storage and catalysts.

定向凝固条件下Ti_(3)Al合金中梯度纳米结构的形成机制与力学行为——原子尺度研究

通过分子动力学(MD)模拟方法对Ti_(3)Al合金在定向凝固条件下的生长机理进行系统研究,并通过对比纳米晶(NG)、粗晶(CG)和梯度纳米晶(GNG)合金,研究Ti_(3)Al合金的结构力学性能。结果表明,在凝固过程中Ti_(3)Al相优先生长为等轴晶组织,随后逐渐演变为柱状粒,并最终形成梯度纳米晶结构。此时,晶粒在与凝固方向平行的方向优先生长。此外,研究还发现,相较于NG和CG结构,GNG结构定向凝固合金具有更高的抗拉强度和更好的延展性。GNG结构不仅有效抑制了小晶粒区域的应变局域化和晶粒生长,而且促进了较大晶粒区域的位错形成,从而获得更好的力学性能。

碳纳米管-铜复合薄膜材料的抗辐照损伤性能研究

利用磁控溅射沉积技术获得的碳纳米管-金属复合材料是一种新型的纳米多孔材料,涂覆碳纳米管-金属复合材料的结构组件在新型核反应堆中具有广阔的应用前景。但是目前的报道中对碳纳米管-金属复合材料抗辐照损伤性能的研究还很少。因此,本工作通过磁控溅射沉积技术,制备了两种不同厚度的碳纳米管-铜复合薄膜,结合室温下的He离子辐照对石蜡基矿物油有机蒸汽的脱氢效应在复合薄膜表面包覆碳层,研究了碳层包覆前后碳纳米管-铜复合薄膜的抗辐照损伤性能。结果表明,随着辐照剂量和辐照温度升高,样品内逐渐产生孔洞等缺陷,且孔洞尺寸较小,这与碳纳米管-铜复合薄膜较高的晶界体积分数和比表面积以及降低的缺陷迁移速率有关。与平面基底上沉积的铜薄膜相比,复合薄膜在室温和高温辐照后,晶粒尺寸明显减小,这与一维碳纳米管基底可显著限制碳纳米管轴向垂直方向上的晶界迁移有关。碳层包覆后的碳纳米管-铜复合薄膜的高温结构稳定性和抗辐照损伤性能明显增强,这得益于碳包覆碳纳米管-铜复合薄膜较高的晶界体积分数和比表面积以及稳定的纳米晶粒。

水热-烧结法制备Cr^(2+):ZnSe/ZnSe核壳结构纳米孪晶

Cr^(2+)掺杂ZnSe纳米晶是一种重要的中红外材料,核壳结构的ZnSe基半导体纳米晶表现出优异的光学、电学与催化性能。纳米晶的缺陷可以影响其性能,如具有孪晶结构的纳米晶拥有更高的强度和硬度。为了提高掺杂纳米晶的综合性能,本工作以可溶性Zn盐为Zn源,以新制NaHSe溶液为Se源,以Cr(AC)_(2)为掺杂源,通过两次水热过程制备了核壳结构Cr^(2+):ZnSe/ZnSe,在氩气保护或高真空下分别于400和800℃烧结获得了室温下化学性质稳定的纳米晶。结构和形貌表征结果显示,纳米晶尺寸主要集中在20~30 nm之间,壳体厚度约为2.6 nm,纳米晶具有层错缺陷,并由此发展成为孪晶。分析可知孪晶面为(111),相邻两晶面夹角为70.02°,误差在±0.5°以内。随着样品结晶度提高,孪晶密度增大,表明释放晶格畸变能为层错和孪晶的形成提供驱动力,孪晶的形核与长大符合位错诱导机制。XPS分析表明,Cr元素以+2价存在于纳米晶中;反射光谱测试结果分析发现,烧结的纳米晶在1775 nm附近存在吸收带,表明所制纳米孪晶具有潜在的中红外发光性能。

纳米晶粒多晶Si薄膜的低压化学气相沉积

利用低压化学气相沉积(LPCVD)方法,以充Ar的S iH4作为反应气体源,在覆盖有热生长S iO2层的p-(100)S i衬底上制备了具有均匀分布的纳米晶粒多晶S i膜(nc-poly-S i)。采用扫描电子显微镜(SEM)、原子力显微镜(AFM)和拉曼谱等检测手段,测量和分析了沉积膜层的表面形貌、晶粒尺寸与密度分布等结构特征。结果表明,nc-poly-S i膜中S i晶粒的尺寸大小和密度分布强烈依赖于衬底温度、S iH4浓度与反应气压等工艺参数。典型实验条件下生长的S i纳米晶粒形状为半球状,晶粒尺寸约为40nm,密度分布约为4.0×1010cm-2和膜层厚度约为200nm。膜层的沉积机理分析指出,衬底表面上S i原子基团的吸附、迁移、成核与融合等热力学过程支配着nc-poly-S i膜的生长。

TiO_2纳米晶的制备

本文采用一种新的制备工艺制备了二氧化钛纳米晶 ,研究了二氧化钛的晶型和晶粒尺寸与工艺条件的关系 。

基于热力学函数的纳米晶粒长大Cellular Automaton仿真研究

基于纳米晶热力学特性表征函数,将纳米晶热力学性质对晶界迁移的影响引入Cellular Automaton算法,对纳米晶粒长大行为进行了定量化和可视化的仿真研究.模拟结果表明,纳米晶粒长大的动力学与传统粗晶材料不同,在恒温条件下,纳米晶粒的长大指数n不是常数(传统粗晶材料的晶粒长大指数n=2为常数),随纳米晶粒长大过程的进行,n值从1.70至6.59发生变化.作为纳米晶粒长大的驱动力,纳米晶界的过剩自由能与纳米晶粒尺寸的变化直接相关.由于纳米晶材料强烈的小尺寸效应,纳米晶组织的热力学性质较大地影响纳米晶界的结构和能量状态,从而影响纳米晶粒长大的动力学特征.因此,只有结合纳米晶热力学特性的仿真研究才能获得对纳米晶粒长大行为本质性的认识.

纳米晶硬质合金进展

主要对纳米晶硬质合金研究中的几个热点领域 :纳米粉体制备 ,晶粒长大控制 ,低温烧结新型粘结剂等的发展状况进行了分析 ,并指出其中存在的主要难点问题 ,同时还对这些问题的解决途径和发展趋向进行了展望。

纳米晶W-Cu复合粉末烧结行为

研究了机械合金化制备的纳米晶W-xCu(x=15,20,25)复合粉末的烧结行为.结果表明,纳米晶W-Cu复合粉末烧结致密化强烈地依赖于烧结温度与烧结时间.当烧结温度从1 150℃提高到1 200℃时,烧结30min后的烧结体相对密度由91%～94%增加到97%～98%;当烧结温度超过1 300℃时,烧结体发生快速致密化,5 min内相对密度即可达到98%左右.研究还发现,W-Cu合金中W晶粒尺寸也强烈地依赖于烧结温度,即烧结温度愈高,W晶粒长大愈显著.当压坯在1 200～1 250℃烧结30 min后,所得到的晶粒度约为300～500 nm,其中经1 200℃烧结时的晶粒尺寸约为300～350 nm.另外,Cu含量增加有利于烧结致密化,并降低W晶粒长大的趋势.

芳纶纤维表面化学镀镍的研究

采用化学镀技术,实现了芳纶纤维表面镀镍金属化,利用SEM、EDS和XRD分别对芳纶纤维原始样品、粗化后及施镀后的表面形貌、原始纤维及镀层的成分和物相组成进行了分析比较,并用冷热循环法对镀镍芳纶纤维进行了结合力测试。结果表明:芳纶纤维化学镀镍后呈灰黑色,表面基本均匀光滑。镀层与基体结合力良好,镀层厚度可达0.375μm左右,镀层中镍含量在89.58%以上,且以单质镍为主,镀层晶粒细小,基本为纳米晶。

金属纳米晶热稳定性和晶粒长大行为的研究

推导出了金属纳米晶界的基本热力学函数,模拟计算了金属纳米晶界的吉布斯自由能随晶界过剩体积和温度的变化规律。以铜纳米晶材料为例,应用纳米晶热力学模型预测了纳米晶组织的热稳定性及纳米晶粒长大行为。将纳米晶界的热力学函数引入元胞自动机仿真算法,利用计算机模拟研究了金属纳米晶的变温晶粒长大过程。实验证实铜纳米晶粒长大的动力学特征符合纳米晶热力学模型的计算预测结果。

纳米晶W-25Cu复合粉末烧结行为的研究

研究了机械合金化工艺制备纳米晶W-25Cu复合粉末的烧结致密化和晶粒长大行为,并考察了一种新型晶粒长大抑制剂对抑制W晶粒长大的作用,探讨了其作用机理。结果表明,烧结致密化和晶粒长大强烈依赖于烧结温度和时间。经1200℃烧结30min后,烧结相对密度和W晶粒尺寸为97.7%和310nm。新型晶粒长大抑制剂抑制W晶粒长大效果明显。