基于整体结构的纳米晶体材料力学模型

为了系统地分析晶粒尺寸、应变速率和缺陷对纳米晶体材料的影响,提出了1个新的本构模型,运用能量法描述纳米晶体的变形机理、微结构演变和力学行为.在模型中,晶粒和晶界作为一个整体共同承担位错和晶界滑移.通过对纳米晶体Ni在不同应变速率和晶粒尺寸条件下的实验,验证改进模型的可行性.与实验数据对比发现,预测的模型可以用来描述纳米晶体材料的力学性能.     

金属纳米晶的界面热力学及热稳定性研究

以六方相Co为例，采用界面膨胀模型和普适状态方程，引用固体比热的Debye函数，模拟计算了金属纳米多晶体界面的热力学性质，描述了纳米晶界面过剩焓、过剩熵和过剩Gibbs自由能随过剩体积的变化规律.利用界面过剩体积和过剩自由能的非单调关系，预测了发生失稳快速晶粒长大的临界过剩体积和临界温度.实验研究中发现的α-Co纳米晶在宽温度范围内退火时，在较低温区内晶粒长大非常缓慢、而在773—873K温区内纳米晶突发快速晶粒长大的结果与理论预测符合良好.     

GaAs纳米晶体压致相变的尺寸模型

基于晶体相变压力和结合能的关系,提出了一种简单的热力学模型计算尺寸依赖的纳米晶体压致相变压力.根据该模型研究了GaAs纳米晶体压致相变的尺寸效应,结果表明GaAs纳米晶体稳定性的下降导致其相变压力随尺度的减小而降低.当尺寸进一步减小时,纳米晶体的相变压力随着尺寸减小急剧降低,显示了强烈的尺寸效应.该模型计算结果与分子动力学模拟结果显示了较好的一致性.     

球形金属纳米颗粒熔化温度、熔化熵及熔化焓的尺寸效应

根据结合能的键能模型,研究了球形金属纳米颗粒熔化相关热力学量的尺寸效应.在该模型中引入3个因子:晶体致密度k因子(晶胞中原子总体积与晶胞体积的比值);β因子(晶体中表面原子与内部原子结合能的比值)和qs因子(晶面被表面原子占据的比值).在考虑上述因子后,通过模型预测具有自由表面球形金属纳米颗粒的熔化温度、熔解熵、熔解焓等热力学量的尺寸依赖性,并将所得结果与相关实验数据、分子动力学模拟结果、液滴模型以及前人提出的模型进行了比较.结果显示,对于Au和Al纳米粒子熔化温度的尺寸效应,该模型相比之前的模型更接近实验结果,与Ag和Cu纳米粒子熔化熵与熔化焓的分子动力学结果吻合较好.     

纳米结构金属空位形成能的研究

本文在Tiwari and Patil（Scripta Metallurgica,1975,9：833）计算空位形成能的模型基础上,研究了纳米结构金属的空位形成能计算方法,通过引入形状因子,并考虑晶粒尺寸效应,计算了面心立方（fcc）、体心立方（bcc）和密排六方（hcp）结构纳米金属的空位形成能.结果表明,纳米结构金属的空位形成能随着晶粒尺寸的减小而下降 晶粒尺寸不变时,空位形成能随着晶粒形状因子的增大呈线性下降趋势.     

脉冲激光烧蚀制备纳米硅晶粒过程中稳定晶核大小研究

采用蒙特卡罗方法,模拟了脉冲激光烧蚀制备纳米硅晶粒的动力学过程.在模拟中采用了成核长大模型,对室温下的硅晶粒成核过程中稳定晶核所含硅原子数目进行了研究.经过大量的计算以及与实验数据的比较,发现当稳定晶核所含硅原子数目为6时,计算得出的纳米硅晶粒平均尺寸分布规律与实验数据符合较好.     

DP590级双相钢奥氏体晶粒长大模型

通过改变保温温度和保温时间研究了DP590级双相钢奥氏体晶粒长大行为,并探讨了加热速率对各温度下初始晶粒尺寸的影响规律.初始晶粒尺寸随着加热速率增加而不断降低,并最终趋于定值;奥氏体晶粒尺寸随保温时间的延长不断增大并最终趋于不变.采用Sellars晶粒长大模型对实验数据进行拟合,为避免处理方法不同造成的处理结果偏差,提出了一种新的实验数据处理方法,并建立了双相钢初始晶粒尺寸模型和晶粒长大模型.所得模型参数更加合理可靠,计算结果与实验结果吻合很好.     

共沉淀法制备的铁酸锌纳米材料的晶化与晶粒生长行为

采用共沉淀法合成了纳米铁酸锌粉体,并对所制粉体在100~1000℃不同温度下进行热处理,用TEM、XRD等手段对所制备的纳米铁酸锌晶体进行了表征,并对纳米铁酸锌晶体的晶粒生长进行了探讨.结果表明,具有尖晶石结构的铁酸锌纳米晶在共沉淀反应时就已形成,但晶粒的尺寸较小;随着热处理温度的进一步升高,晶粒迅速长大,粒径的尺寸分布在5~30 nm;当热处理温度升至1000℃左右时,晶粒尺寸已达微米数量级.对照TEM和XRD测量结果,表明所制备的铁酸锌纳米粉颗粒为单晶颗粒.     

DP590级双相钢奥氏体晶粒长大模型

通过改变保温温度和保温时间研究了DP590级双相钢奥氏体晶粒长大行为,并探讨了加热速率对各温度下初始晶粒尺寸的影响规律.初始晶粒尺寸随着加热速率增加而不断降低,并最终趋于定值;奥氏体晶粒尺寸随保温时间的延长不断增大并最终趋于不变.采用Se]lars晶粒长大模型对实验数据进行拟合,为避免处理方法不同造成的处理结果偏差,提出了一种新的实验数据处理方法,并建立了双相钢初始晶粒尺寸模型和晶粒长大模型.所得模型参数更加合理可靠,计算结果与实验结果吻合很好.     

析出粒子对钛微合金化高强钢奥氏体晶粒长大的影响

通过析出粒子与奥氏体晶粒尺寸的定量关系,建立奥氏体晶粒长大模型,计算TiN和TiC析出粒子共同作用下钛微合金化钢奥氏体晶粒尺寸. 根据析出相质点理论计算结果表明:随着加热温度的升高,析出粒子体积分数逐渐减少,粒子半径逐渐增大,TiC粒子强烈阻止奥氏体晶粒长大,TiN粒子对奥氏体晶粒长大钉扎效果一般. 采用实验测试手段测量不同加热温度下保温30 min后实验钢的奥氏体晶粒尺寸,与理论计算结果吻合较好.     

Formation and characterization of metallic iron grains in coal-based reduction of oolitic iron ore

To reveal the formation and characteristics of metallic iron grains in coal-based reduction, oolitic iron ore was isothermally reduced in various reduction times at various reduction temperatures. The microstructure and size of the metallic iron phase were investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and a Bgrimm process mineralogy analyzer. In the results, the reduced Fe separates from the ore and forms metallic iron protuberances, and then the subsequent reduced Fe diffuses to the protuberances and grows into metallic iron grains. Most of the metallic iron grains exist in the quasi-spherical shape and inlaid in the slag matrix. The cumulative frequency of metallic iron grain size is markedly influenced by both reduction time and temperature. With increasing reduction temperature and time, the grain size of metallic iron obviously increases. According to the classical grain growth equation, the growth kinetic parameters, i.e., time exponent, growth activation energy, and pre-exponential constant, are estimated to be 1.3759 ±0.0374, 103.18 kJ·mol-1, and 922.05, respectively. Using these calculated parameters, a growth model is established to describe the growth behavior of metallic iron grains.     

中温含铜取向硅钢的形变和再结晶织构特征

通过对比中温含铜取向硅钢与普通取向硅钢和高磁感取向硅钢的组织和织构特征,分析中温含铜取向硅钢独特的织构演变规律及其对二次再结晶行为的影响。结果表明,为了获得有利于高斯晶粒长大的强γ取向线织构,中温含铜钢需经过回复退火处理和高温退火阶段慢速升温。回复过程中γ取向线晶粒储能降低,同时慢速升温有利于γ取向线晶粒的形核和再结晶。中温含铜钢的二次再结晶开始温度超过1000益,由于初次再结晶晶粒组织以γ织构为主且非γ取向线晶粒较少,导致最终二次晶粒尺寸超大且晶界圆滑,二次再结晶机理以择优长大为主导,超大的二次晶粒尺寸导致最终成品的铁损升高,但通过激光刻痕处理后,整体铁损的降低效果比二次晶粒较小的高磁感取向硅钢更加显著。     

Structural study of sputtered nanocrystalline Ti and Zr by X-ray diffraction

Conclusion The structure of sputtered Ti and Zr deposited on substrate cooled by liquid nitrogen was investigated by X-ray diffraction technique. Results show that deposited Ti and Zr are composed of nanocrystalline particles, and the average grain sizes are respectively 7.2 and 6.3 nm. These nanocrystalline particles have different structures with common metal under the condition of room temperature and atmospheric pressure. The sputtered nanocrystalline Ti has b. c. c. structure, and the sputtered nanocrystalline Zr the ω-phase structure. Forming of the abnormal structure depends on the energy of sputtered particle and small grain size. The energy carried by sputtered particles could have met the kinetic requirement for forming of abnormal structure, while the nanometer grain size could make the abnormal structure have lower energy than normal structure.     

High tensile ductility in a nanostructured metal

Nanocrystalline metals--with grain sizes of less than 100 nm--have strengths exceeding those of coarse-grained and even alloyed metals, and are thus expected to have many applications. For example, pure nanocrystalline Cu (refs 1-7) has a yield strength in excess of 400 MPa, which is six times higher than that of coarse-grained Cu. But nanocrystalline materials often exhibit low tensile ductility at room temperature, which limits their practical utility. The elongation to failure is typically less than a few per cent; the regime of uniform deformation is even smaller. Here we describe a thermomechanical treatment of Cu that results in a bimodal grain size distribution, with micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, as expected from an extrapolation of the Hall-Petch relationship. Meanwhile, the inhomogeneous microstructure induces strain hardening mechanisms that stabilize the tensile deformation, leading to a high tensile ductility--65% elongation to failure, and 30% uniform elongation. We expect that these results will have implications in the development of tough nanostructured metals for forming operations and high-performance structural applications including microelectromechanical and biomedical systems.     

高能球磨纳米镍粉制备块体材料的研究

采用高能球磨法制备了纳米晶Ni粉末,对纳米晶粉末进行预压烧结,获得纳米晶镍块体材料.采用显微分析方法研究了纳米晶粉末和块体材料的显微组织结构.试验结果表明,高能球磨所得镍粉平均晶粒尺寸为10 nm;预压烧结块体的平均晶粒尺寸在100 nm以下;块体相对致密度在烧结温度为0.6Tm时达到最大值.     

In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel

In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100℃ for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite grain size increases with austenitizing temperature and holding time. When the austenitizing temperature is above 1100℃, the austenite grains grow rapidly, and abnormal austenite grains occur. In addition, the effect of heating rate on austenite grain growth was investigated, and the relation between austenite grains and bainite morphology after bainitic transformations was also discussed.     

Austenite grain growth behavior of Q1030 high strength welded steel

The austenite grain growth behavior of Q1030 steel was studied under different heating conditions. The austenite grain size increases with the heating temperature and holding time increasing. Austenite grains grow in an exponential manner with rising heating temperature and in a parabolic manner with prolonging holding time. A mathematical model for describing the austenite grain growth behavior of Q1030 steel was obtained on the basis of experimental results using regression analysis. When the heating temperatures lie between 1000 and 1100℃ at a certain holding time, abnormal grain growth appears, which causes mixed grains in Q1030 steel.     

Al-Mn-Si-X合金的再结晶行为研究

对Al-Mn-Si-X合金进行不同温度和不同时间的退火,采用金相显微镜（OM）、扫描电子显微（SEM）和电导率仪对不同状态的合金组织进行观察、能谱（EDS）分析和电导率测试,研究Al-Mn-Si-X合金的再结晶行为.结果表明：随着退火温度的升高,再结晶晶粒尺寸越来越小.Al-Mn-Si-X合金的再结晶晶粒大小主要受再结晶形核数量的影响,再结晶后期的晶粒长大现象不明显.Al-Mn-Si-X合金存在着大量细小弥散的AlMnSi/AlMnSiFe析出相,这些析出相强烈抑制了再结晶形核和再结晶后期的晶粒长大.当退火温度低时,形核激活能较大,形核率低,再结晶晶粒粗大;当退火温度高时,形核激活能较小,形核率增加,再结晶晶粒细小.     

合金凝固过程中晶粒尺寸和形貌演变的预测

利用合金凝固热力学原理,考虑形核和长大因素,建立了预测晶粒尺寸的理论模型,利用该模型对近液相线半连续铸造6061铝合金的晶粒大小进行了计算,所得结果与实验吻合.在所计算的几个条件中,最大误差小于7μm.通过对凝固过程中液固界面法向生长的扰动分析,得出球形晶粒周边主导波长及对应于一定波数的临界晶粒直径表达式.利用此关系可以很好地解释球晶生长及其向一定形貌的枝晶转变的条件及机理;对于分析凝固组织中不同形貌晶粒的形成及演化具有参考价值;对于半固态合金组织性能预测及其制备工艺优化具有实际意义.     

材料凝固时晶粒生长-粗化的二维元胞自动机模型

建立了模拟凝固进行时晶粒成核、生长与粗化过程以及凝固完成后晶粒粗化过程的二维元胞自动机模型,模型中将原子在固/液界面、晶界面的迁移过程以概率的方式表现在元胞网格演化的规则中.模拟结果表明,所建立的模型可以合理地描述晶粒在凝固过程中的生长-粗化过程.应用本模型分析了冷却率、结晶取向数、温度等因素对于晶粒微观结构演化的影响.研究结果表明:大冷却率有助于得到细小的晶粒;当结晶取向数大时晶粒较小;保温温度低会抑制晶粒粗化.