多晶沉积薄膜生长过程中织构演变的模拟研究

将织构组态熵的概念应用于沉积多晶薄膜织构演变的模拟研究，考虑膜沉积过程中晶体表面能各向异性及应变能各向异性的变化，建立了沉积薄膜晶体择优生长的定量模型；模拟了Ａｌ多晶薄膜沉积过程中晶体的生长规律，分析了织构演变的主要微观物理因素。     

等轴应变作用下Cu,Al薄膜的取向择优生长

用分子动力学方法模拟了Cu双晶和Al双晶薄膜的（111）生长，模拟时假定在薄膜平面内保持恒定的等轴双向应变，薄膜中两晶粒的能量计算表明：不同晶粒的能量存在差异，能量较低的晶粒在沉积中择优生长，逐渐取代能量较高的晶粒，Cu膜择优生长速率显著高于Al膜：两种薄膜择优生长的机制完全不同，Cu膜中处于不利位向的晶粒通过孪晶过渡转变为择优取向，转变完成、晶界湮灭后薄膜中残留的缺陷为位错；而Al膜则是通过无序结构重结晶实现上述转变，转变完成、晶界湮灭后薄膜中残留的缺陷为间隙原子，文中对上述择优生长驱动力的来源、以及在纳米多晶中的重要性进行了讨论。     

多晶柱状Cu膜的表面粗化与织构

用磁控溅射工艺分别在Si和Al2O3衬底上沉积两种不同织构组分的多晶柱状Cu膜，基于动力学标度方法表征两种薄膜的表面粗化特征．结果表明，Cu（111）取向晶粒组分多的薄膜的生长指数较大、表面粗化速率较快．对于较低温度下沉积的多晶柱状薄膜，基于其晶粒几何形态和弱化的晶界限制的特点，提出了一种表面粗化机制，认为薄膜的表面粗化主要依赖于其晶粒表面的粗化过程，而薄膜织构决定了薄膜表面粗化速率．     

材料微结构三维建模及其晶体塑性有限元模拟

采用Voronoi图,生成具有随机晶粒形状的三维多晶集合体模型,并赋予每个晶粒相应的取向。基于率相关晶体塑性理论,开发了用户材料子程序,并将其嵌入有限元软件中模拟面心立方多晶集合体在单轴单向拉伸过程中的应力-应变响应,分析了网格细化及晶粒取向对模拟结果的影响。研究表明,随网格细化应力值有所降低,但变化不大,为保证结果的可靠度,平均每个晶粒离散的单元数目在5个以上;随多晶集合体中晶粒数目的增加,由于取向的随机性产生的应力-应变的差异逐渐减小,模拟时多晶集合体中晶粒的数目大于50个;模型较好的反映了材料的真应力随应变速率增加而增大的规律,且模拟结果和实验结果吻合良好,说明该模型具有较高的可靠性。     

材料微结构三维建模及其晶体塑性有限元模拟

采用Voronoi图,生成具有随机晶粒形状的三维多晶集合体模型,并赋予每个晶粒相应的取向。基于率相关晶体塑性理论,开发了用户材料子程序,并将其嵌入有限元软件中模拟面心立方多晶集合体在单轴单向拉伸过程中的应力-应变响应,分析了网格细化及晶粒取向对模拟结果的影响。研究表明,随网格细化应力值有所降低,但变化不大,为保证结果的可靠度,平均每个晶粒离散的单元数目在5个以上;随多晶集合体中晶粒数目的增加,由于取向的随机性产生的应力-应变的差异逐渐减小,模拟时多晶集合体中晶粒的数目大于50个;模型较好的反映了材料的真应力随应变速率增加而增大的规律,且模拟结果和实验结果吻合良好,说明该模型具有较高的可靠性。     

单晶高温合金定向凝固过程中晶体竞争生长观察

本文选用镍基单晶高温合金，用籽晶法控制各晶粒的晶体取向，进行了多晶组合的竞争生长实验，模拟了定向凝固中不同晶体取向晶粒间的生长过程与竞争规律，结果表明：晶粒的竞争生长过程取决于晶粒的相对取向关系及其与热流方向的夹角，择优生长方向与热流方向最接近的晶粒将淘汰其它晶粒并逐渐占据整个试样截面，当两个晶粒的择优生长方向都与热流方向不平行且大发散角方式组合时，两晶粒间可出现第三个晶粒并最最终扩展至整个试样截     

面心纯铝单向拉伸的晶体塑性学有限元模拟

基于率相关晶体塑性本构模型,实现了晶体塑性学有限元模拟过程。直接将电子背散射衍射(EBSD)获取的晶粒初始取向输入晶体塑性有限元模型,分别预测了单向拉伸面心1050纯铝过程中的力学响应与织构演化。应力应变响应数值模拟结果与实验结果有较好的一致性,同时也存在一定的偏差。两种多晶模型(Taylor模型和有限单元模型)分别模拟了单向拉伸真应变0.25和0.37时的织构演化。随着真应变的增加,两种丝织构(〈111〉织构和〈100〉织构)变得更加锋锐,模拟结果与EBSD实验测得的织构演化结果有较好的一致性。     

过量PbO对TGG法制备0.675PMN-0.325PT织构陶瓷的影响

以0.675PMN-0.325PT反应原料为基体，〈001〉取向的片状SrTiO3为模板，利用模板晶粒生长技术在较低的温度下制备出具有一定取向度的0.675PMN-0.325PT多晶织构陶瓷。研究了烧结过程中基体晶粒自身长大和模板外延生长情况以及PbO添加量对织构组织形成的影响。结果表明：增加PbO含量可加快基体晶粒溶解-析出过程，促进基体晶粒的长大；同时液相含量的增加还有利于模板的外延生长以及织构组织的形成；与以PMNT为基体相比，以反应原料为基体可大幅度降低烧结温度；添加3%过量PbO后，在1 000 ℃保温2 h后即可获得取向度为40%的PMN-PT多晶织构陶瓷。     

外应力对AZ31镁合金晶粒长大和织构影响的相场模拟

建立了一个模拟外应力作用下AZ31镁合金在高温退火过程中晶粒长大和织构演化的三维相场模型。通过3个欧拉角构成的欧拉空间表达晶体学取向,赋予有序化参数以晶体学取向的物理意义。由于镁合金晶体结构为密排六方结构,不同晶体学方向存在弹性各向异性,根据每个取向晶粒的(0001)面相对于外应力方向的角度旋转刚度矩阵,得到不同取向晶粒对应的刚度矩阵,从而计算出外应力对不同取向晶粒做的功。结果表明:将模拟结果与已有的实验结果进行了对比分析,织构模拟结果与实验观察到的织构相一致;外应力的增加会使晶粒长大速率加快,当外应力大于600 MPa时,可能会导致晶粒的异常长大;此外,当压应力大于400 MPa时,AZ31镁合金中会产生(0001)晶向平行于外应力方向的基面织构。     

纳米晶Al薄膜Bauschinger效应的分子动力学模拟

运用大规模的分子动力学模拟研究了厚度和晶粒取向对纳米晶Al薄膜Bauschinger效应的影响.模拟结果表明:晶粒取向的不均匀性对早期Bauschinger效应及相关塑性变形机制有显著的影响.相对于没有织构的薄膜试样而言,尽管晶粒尺寸、形状和厚度相同,具有(110)织构的薄膜表现出较轻微的Bauschinger效应.同时,分子动力学模拟也揭示早期Bauschinger效应起源于卸载过程中位错的反向运动和由于位错反应造成位错密度的降低.这些位错机制是由加载过程中产生的不均匀变形引起的内在残余应力所驱动的.     

衬底与缓冲层的晶格失配对CeO_2缓冲层外延生长的影响

采用金属有机沉积(MOD)技术在La Al O3(LAO)、Y稳定的氧化锆(YSZ)和Ni-W衬底上沉积了Ce O2缓冲层薄膜,并研究了衬底与缓冲层的晶格失配对其外延生长的影响。结果表明,随着衬底和缓冲层薄膜之间晶格失配的增大,缓冲层薄膜内部的压应变增加,晶界浓度增加,晶粒生长速率减小。衬底和缓冲层薄膜之间的晶格失配越小,越有利于薄膜织构度的增大。Ce O2薄膜的表面形貌及粗糙度的演化对衬底和缓冲层薄膜之间的晶格失配并没有明确的依赖关系。     

Origins of microstructure and stress gradients in nanocrystalline thin films: The role of growth parameters and self-organization

The development of depth gradients of texture, morphology and stresses in thin nanocrystalline films was experimentally demonstrated for a nanocrystalline CrN film by means of position-resolved synchrotron X-ray nanodiffraction and explained by atomistic processes at the growing film surface and the effect of interfaces, both controlled by the deposition conditions. Controllable changes in the energy of incident particles adjusted by bias voltages ranging from ?40 to ?120 V affect the competitive growth of grains with different orientations, induce disruption of grain growth and thus give rise to structural variations across the film thickness. Subsequent changes in the volume fraction of grain boundaries and film texture were found to be responsible for changes in the residual stress state as defect generation proceeds to different extents in the interior of differently oriented grains and in the interfacial area. While the defect density predominantly affects the development of intrinsic stress, the variation in the number of weakly bonded atoms of grain boundaries determines the thermal stress component. The structural dependence of both stress components thus contributes to the characteristic development of stress gradients in thin nanocrystalline films.     

Effect of grain boundary energy on surface-energy induced abnormal grain growth in columnar-grained film

In columnar-grained film with anisotropy in the surface energy, abnormal grain growth occurs to minimize the overall energy. We studied the effect of grain boundary energy on the surface-energy driven abnormal grain growth in thin films using the Monte Carlo computer simulation. Our simulation results show that the growth speed of abnormal growth slows down when grain boundary energy increases. It is because the speed of normal growth driven by the grain boundary energy minimization gets faster with increasing grain boundary energy, while that of the abnormal growth induced by the fixed surface energy is consistent. The growth kinetics was monitored based on the growth velocity of the abnormal grains relative to the average growth velocity of all the grains. It was also found that the abnormal growth kinetics was retarded by impingements among abnormally growing grains when there was more than one abnormal grain.     

Tetragonal phase stability in ZrO2 film formed on zirconium alloys and its effects on corrosion resistance

A thermodynamic model is developed to understand the origin of variation in the microstructure of ZrO₂ film formed on zirconium alloys and its effects on corrosion resistance. The correlation among the tetragonal phase fraction, the stress (macroscopic and internal one), the ZrO₂ grain size and the microstructural change of oxide film is formulized, and then analyzed. The results show that many complicated factors simultaneously govern the microstructure of oxide film. The tetragonal phase content near the oxide/metal interface, the macroscopic compressive stress near the interface, the decline gradient of macroscopic compressive stress and the internal stress induced by the transformation from the tetragonal to the monoclinic phase have very important influences on the transition from columnar grains to equiaxed grains, the crack formation and the degradation of oxidation resistance. The presence of intermetallic precipitates in oxide film may effectively relax the internal stress caused by transformation strain, stabilize the columnar-grain structure and reduce the probability of crack formation. How to reduce the transformation stress in the oxide film is a key to improve the corrosion resistance of zirconium alloys.     

Abnormal growth of “giant” grains in silver thin films

Abnormal growth of “giant” grains in the millimeter range was observed in silver thin films with thicknesses of 2.0 and 2.4 μm. The effect depends on deposition temperature and deposition geometry. The microstructure and texture of as-deposited and annealed films have been characterized using X-ray, electron backscatter diffraction (EBSD) and focused ion beam (FIB) techniques. Abnormal grain growth is found whenever a special texture is formed during film deposition. Otherwise normal grain growth occurs. The texture type—and thus the grain growth mode—can be controlled by adjusting the process parameters. During abnormal grain growth, the initial 〈111〉 texture transforms completely into 〈001〉. Growth of 〈111〉-oriented grains stagnates at a size smaller than the film thickness with a non-columnar grain structure. This stagnation promotes orientation-selective growth of 〈001〉 grains.     

Deformation behavior of thin copper films on deformable substrates

The role of strain hardening for the deformation of thin Cu films was investigated quantitatively by conducting specialized tensile testing allowing the simultaneous characterization of the film stress and the dislocation density as a function of plastic strain. The stress–strain behavior was studied as a function of microstructural parameters of the films, such as film thickness (0.4–3.2 μm), grain size and texture. It was found that the stress–strain behavior can be divided into three regimes, i.e. elastic, plastic with strong strain hardening and plastic with weak hardening. The flow stresses and the hardening rate increase with decreasing film thickness and/or grain size, and are about two times higher in (111)-grains compared to the (100)-grains. These effects will be discussed in the light of existing models for plastic deformation of thin films or fine grained metals.     

Size effects on yield strength and strain hardening for ultra-thin Cu films with and without passivation: A study by synchrotron and bulge test techniques

We present a systematic study of the mechanical properties of different Cu, Ta/Cu and Ta/Cu/Ta films systems. By using a novel synchrotron-based tensile testing technique isothermal stress–strain curves for films as thin as 20 nm were obtained for the first time. In addition, freestanding Cu films with a minimum thickness of 80 nm were tested by a bulge testing technique. The effects of different surface and interface conditions, film thickness and grain size were investigated over a range of film thickness up to 1 μm. It is found that the plastic response scales strongly with film thickness but the effect of the interfacial structure is smaller than expected. By considering the complete grain size distribution and a change in deformation mechanism from full to partial dislocations in the smallest grains, the scaling behavior of all film systems can be described correctly by a modified dislocation source model. The nucleation of dissociated dislocations at the grain boundaries also explains the strongly reduced strain hardening for these films.     

Mechanical properties and microstructures of Al-Cu Thin films with various heat treatments

The relationship between microstructure and mechanical properties has been investigated in Al-Cu thin films. The Cu content in Al-Cu samples used in this study ranges from 0 to 2 wt.% and substrate curvature measurement was used to measure film stress. In thin films, the constraints on the film by the substrate influence the microstructure and mechanical properties. Al-Cu thin films cooled from high temperatures have a large density of dislocations due to the plastic deformation caused by the thermal mismatch between the film and substrate. The high density of dislocations in the thin film enables precipitates to form inside the grain even during a very rapid quenching. The presence of a large density of dislocations and precipitates will in turn cause precipitation hardening of the Al-Cu films. The precipitation hardening is dominant at lower temperatures, and solid solution hardening is observed at higher temperatures in the tensile regime. Pure Al films showed the same values of tensile and compressive yield stresses at a given temperature during stress-temperature cycling.     

A Predictive Model for Whisker Formation Based on Local Microstructure and Grain Boundary Properties

Whisker and hillock formation in thin films is well known as a highly local mechanism for stress relaxation, where in many cases, only a few whiskers form out of thousands of grains in a film. In this article, the microstructural characteristics for specific grains to form whiskers in β-Sn films are discussed in light of our recent whisker growth model, establishing a relationship among grain boundary sliding limited Coble creep, surface grain geometry, and film stress for different stress conditions, including for thermal cycling. Through our recent finite-element simulations of stresses induced by room-temperature aging and thermal cycling of textured microstructures, the role of elastic and thermoelastic anisotropy in creating preferred whisker formation sites and the general propensity of a film to form whiskers have been proposed for a range of β-Sn film textures. Taken together, these models suggest a strategy for identifying the effects of local microstructure and β-Sn anisotropy on whisker formation. If these predictions are accurate, then whisker growth risk may be effectively reduced by engineering film microstructures and textures for specific applications and stress conditions.     

靶基距对Cu/Si(100)薄膜结构和残余应力的影响

采用高功率调制脉冲磁控溅射(MPPMS)技术在 Si(100)基体上沉积 Cu 薄膜,SEM 观察薄膜厚度及生长特征、XRD 分析薄膜晶体结构、nanoindentor 测量薄膜纳米硬度和弹性模量、Stoney 公式计算薄膜残余应力,研究沉积过程靶基距对 Cu / Si(100)薄膜沉积速率、微结构及残余应力的影响。 随着靶基距的增大,薄膜沉积速率降低,薄膜的生长结构由致密 T 区向 I 区转变,Cu(111)择优生长的晶粒逐渐减小,薄膜纳米硬度和弹性模量也相应降低,残余拉应力约为 400 MPa。 较小靶基距时增加的沉积离子通量和能量,决定了薄膜晶粒合并长大体积收缩过程的主要生长形式,导致了 Cu / Si(100)薄膜具有的残余拉应力状态。 MPPMS 工艺的高沉积通量和粒子能量可实现对 Cu / Si(100)薄膜残余应力的调控。