Role of grain orientation distribution in the ferroelectric and ferroelastic domain switching of ferroelectric polycrystals

The non-linear electromechanical behavior of ferroelectric polycrystals stems from polarization/domain switching, which are affected by the grain boundaries and grain orientations. The effects of grain orientation distribution on the domain switching and non-linear behavior of a two-dimensional ferroelectric polycrystal subjected to an electric or/and mechanical load are investigated by computer simulations with a real-space phase-field model. Phase-field simulations indicate that the macroscopic coercive field, remanent polarization and remanent strain in the polycrystal with a random distribution of grain orientation are correspondingly smaller than those in the polycrystal with a uniform distribution of grain orientation. However, the polycrystal with randomly distributed grain has a larger strain variation with the electric field than the polycrystal with uniformly distributed grains, which suggests that the random polycrystal has a better piezoelectric property than the uniform one. The different macroscopic non-linear behaviors of the ferroelectric polycrystals are attributed to different microscopic domain switching processes. For the polycrystal with randomly distributed grains, the domain switching takes place from the regions near the large angle grain boundaries, while new domains nucleate from the cross sections between the grain boundaries and the material surface in the polycrystal with uniform grain orientation.     

Effect of grain orientation and grain size on ferroelectric domain switching and evolution: Phase field simulations

Phase field simulations were conducted in order to understand the effect of grain orientation, grain boundary and grain size on ferroelectric domain switching, stress distribution and evolution behavior under an applied electric field. Tetragonal ferroelectric domains were considered. Hysteresis loops were obtained for a single crystal, a bi-crystal and a polycrystal and the differences in their coercive fields were examined. It was found that the magnitude of the coercive field was closely related to the domain structures at the maximum electric field. Nucleation of new domains at a grain boundary led to local high stress. The effect of a reduced ferroelectric transition temperature at the grain boundary on the polarization distribution, domain structure and switching was studied.     

Direct observation of asymmetric domain wall motion in a ferroelectric capacitor

We report in situ transmission electron microscopy observations of the 180° polarization switching process of a PbZr_0.2Ti_0.8O₃ (PZT) capacitor. The preferential, but asymmetric, nucleation and forward growth of switched c-domains were observed at the PZT/electrode interfaces, arising due to the built-in electric field induced at each interface. The subsequent sideways growth of the switched domains was inhibited by the depolarization field due to the imperfect charge compensation at the counter-electrode and also at the boundaries with preexisting a-domains, which contributed further to the asymmetric switching behavior. It was found that the preexisting a-domains split into fine a- and c-domains constituting a 90° stripe domain pattern during the 180° polarization switching process, revealing that these domains also actively participated in the out-of-plane polarization switching. The real-time observations uncovered the origin of the switching asymmetry and further clarified the importance of charged domain walls and the interfaces with electrodes in the ferroelectric switching processes.     

On the role of plastic deformation during the mild wear of alumina

Polycrystalline -alumina was worn against Mg-partially stabilized zirconia (Mg-PSZ), using water lubrication, a sliding speed of 0.24 m/s and a load of 10 N. Differential wear between grains (maximum 33 nm) and fine (0.3–1.9 μm diameter) abrasive grooves were found on the worn surface. TEM of back-thinned samples indicated widespread dislocation flow at the surface, heterogeneously distributed between grains, and largely associated with abrasive grooves. Those grains standing proud of the surface invariably contained extensive dislocation damage. The dominant slip system was pyramidal ( , , and ) although occasional basal slip was also found. No prism slip was observed. The pyramidal slip planes were concentrated at angles of 6–33° to the worn surface. Basal slip was frequently associated with basal twinning on planes at 72–73° to the worn surface. Dislocation pile-ups at grain boundaries often coincided with grain boundary cracking. The extent of damage from abrasive grooves varied from grain to grain and was dictated by crystallographic orientation more than the grain height. No evidence of mechanical damage was found in those grains that had suffered the highest wear, indicating that material removal had been controlled by tribochemical mechanisms. The origin of the differential wear between grains is considered and the implications of the experimental observations on the time-dependent transition to severe wear in aluminas are discussed.     

An optimization-based computational model for domain evolution in polycrystalline ferroelastics

An optimization-based computational model is proposed to study domain evolution in polycrystalline ferroelastics composed of numerous randomly oriented grains, each of which consists of multiple types of domains. Under any prescribed loading, the volume fraction of each domain in a grain is obtained by minimizing the free energy of the said grain using an optimization method. The mechanical constraint from the neighboring grains is considered using Eshelby inclusion approach. This model has the similar superiority as the phase field model, which does not require imposition of any priori domain-switching criterion. The computational efficiency of this model is fairly high and it is feasible to study three-dimensional cases using numerous grains. Furthermore, this model can reproduce Taylor’s rule of plasticity very well. Simulation results for tetragonal, rhombohedral and morphotropic PZT ceramics are employed to validate the superiority and efficiency of this model. The domain texture evolution process can also be calculated.     

Evolution of grain structure in AA2195 A1-Li alloy plate during recrystallization

The evolution of the grain structures in AA2195 Al-Li alloy plate warm-rolled by 80% reduction during recrystallization annealing at 500℃ was investigated by electron backscatter diffraction, scanning electron microscopy and transmission electron microscopy. It is found that the elongated grain structures are caused by the lamellar distribution of recrystaUization nucleation sites, being lack of large second phase particles （〉 1μm）, and dispersive coherent particles （such as δ′ and β′concentrated in planar bands. The recrystallization process may be separated into three stages： firstly, recrystallization nucleation occurs heterogeneously, and the nuclei are concentrated in some planar zones parallel to rolling plane. Secondly, the grain boundaries interacted with small particles concentrate in planar bands, which is able to result in the elongated grain structures. The rate of the grain growth is controlled by the dissolution of these small particles. Thirdly, after most of small particles are dissolved, their hindrance to migration of the grain boundaries fades away, and the unrecrystallized zones are consumed by adjacent recrystallized grains. The migration of high angle grain boundaries along normal direction leads a gradual transformation from the elongated grains to the nearly equiaxed, which is driven by the tension of the grain boundaries.     

THREE DIMENSIONS PHASE FIELD SIMULATION OF MECHATRONIC COUPLE FOR FERROELECTRIC MATERIALS

利用三维相场理论模拟了铁电材料的自发极化、电滞回线以及力电耦合对畴变的影响. 结果表明, 外场下的畴变是通过新畴形核及畴壁移动所引起的长大实现的. 逐渐改变外场时, 在矫顽场附近各类电畴均发生90°畴变, 从而导致极化强度突变. 沿垂直电场方向加 拉、压应变能阻止或促进沿电场方向的畴变.     

Influence of deformation temperature on the ferrite grain refinement in a low carbon Nb–Ti microalloyed steel

Grain refinement is one of the effective methods to develop new generation low carbon microalloyed steels possessing excellent combination of mechanical properties. In the present work, the microstructural evolution and ferrite grain refinement at various deformation temperatures were investigated using single pass isothermal hot compression experiments for a low carbon Nb–Ti microalloyed steel. The physical processes that occurred during deformation were discussed by observing the optical microstructure and analyzing the stress–strain responses. The results show that there is a close relation between the microstructural evolution and true stress–true strain responses during the deformation. Microstructural observation indicates that very fine ferrite grains of about 1.8–3 μm are obtained by deformation at 830–845 °C, about Ar₃ ± 10 °C. The obtained stress–strain curves suggest the occurrence of strain-induced dynamic transformation (SIDT) of γ to at this deformation temperature range.     

冷加工态Ti6Al4V合金的回复和再结晶行为研究

对冷拉拔变形量为60%的钛合金进行700～880℃,1～240min再结晶退火,利用金相显微镜、X射线衍射仪和透射电镜等手段分析不同状态下的组织演变、织构组成和位错组态。结果表明:冷变形后的Ti6Al4V合金经完全再结晶后α晶粒呈等轴状,β相在α相周围以条状沿α晶界析出或以小晶粒形式存在。计算表明,经60%冷变形量的钛合金再结晶激活能为107kJ/mol,较相同变形量的纯钛再结晶激活能高约50%。钛合金的再结晶分为回复、形核和晶核长大阶段,包括位错胞向亚晶转变、回复亚晶通过合并或长大形核、形核诱导高角度晶界形成而长大成新晶粒。经过冷拉拔后的丝材,存在着较强的100织构,而在再结晶过程中,沿100方向上产生的回复亚晶优先形核并长大形成新的晶粒。这导致在初始再结晶阶段,再结晶织构与冷变形织构取向一致,而在晶粒长大阶段,原先取向不利的晶粒吞并周围小晶粒长大,形成新的织构组元使原来的织构被弱化。     

凝固过程晶粒生长动力学及拓扑转变机制的相场研究

基于连续相场动力学模型,研究了凝固过程形核、长大及粗化阶段的组织形貌演化,动力学转变,以及粗化过程拓扑转变,分析了形核与长大过程的关系。研究结果表明,形核长大过程中,晶粒体积分数逐渐增大至平衡值,总表面积先增大后减小,体积自由能是形核的驱动力,表面能是形核的阻力。形核伴随着长大,两者是相互重叠相互竞争的两个过程。晶粒生长过程中,边数大于六的晶粒持续长大,而边数少于六的晶粒不断缩小。小晶粒消失机制有:临近切换机制;三边﹑四边及五边晶粒直接消失机制;四边交叉点分离并最终导致小晶粒消失机制;晶界直接消失机制。模拟结果与实验结果符合较好。     

Combined experimental and numerical study on the effective grain growth dynamics in highly anisotropic systems: Application to barium titanate

Abnormal grain growth is a commonly observed phenomenon in barium titanate. It is usually associated with grain boundaries of different mobility and energy present in the microstructure. The influence of interfaces with variable mobility and energy on grain growth is investigated by a combined experimental and numerical approach in a transition region where growth behaviour strongly deviates from Arrhenius behaviour. Abnormal growth occurs between 1275 and 1325 °C, with normal grain growth occurring above and below this temperature range. The overall grain growth rate of the small matrix grains in the transition region is found to increase nonlinearly with inverse temperature between the high- and low-temperature states. A similar behaviour is found in simulations using a 3-D mesoscale grain growth model under the assumption of fractions of grain boundaries being in the high- or low-temperature state. The transition at the grain boundary is in agreement with the complexion model. Additionally, the simulation is used to map the nucleation probability for abnormal grains in the transition region as a function of combined energy and mobility advantages. The energy advantage of the grain boundaries is found to be of greater importance for the nucleation of abnormal grains compared to results from mean field models.     

Deformation twinning in nanocrystalline Al by molecular-dynamics simulation

We use a recently developed, massively parallel molecular-dynamics code for the simulation of polycrystal plasticity to elucidate the intricate interplay between dislocation and GB processes during room-temperature plastic deformation of model nanocrystalline-Al microstructures. Our simulations reveal that under relatively high stresses (of 2.5 GPa) and large plastic strains (of ˜12%), extensive deformation twinning takes place, in addition to deformation by the conventional dislocation-slip mechanism. Both heterogeneous and homogeneous nucleation of deformation twins is observed. The heterogeneous mechanism involves the successive emission of Shockley partials from the grain boundaries onto neighboring slip planes. By contrast, the homogeneous process takes place in the grain interiors, by a nucleation mechanism involving the dynamical overlap of the stacking faults of intrinsically and/or extrinsically dissociated dislocations. Our simulations also reveal the mechanism for the formation of a new grain, via an intricate interplay between deformation twinning and dislocation nucleation from the grain boundaries during the deformation. The propensity for deformation twinning observed in our simulations is surprising, given that the process has never been observed in coarse-grained Al and that the well-known pole mechanism cannot operated for such a small grain size. It therefore appears that the basic models for deformation twinning should be extended with particular emphasis on the role of grain-boundary sources in nanocrystalline materials.     

Collective dynamics in nanostructured polycrystalline ferroelectric thin films using local time-resolved measurements and switching spectroscopy

Grain-to-grain long-range interactions and the ensuing collective dynamics in the domain behavior of nanostructured polycrystalline Pb(Zr,Ti)O₃ ferroelectric thin films have been investigated. To identify the key factors and interactions controlling local polarization dynamics we utilize a synergistic approach based on focused ion beam (FIB) milled damage-free nanostructures to isolate single grains and grain clusters, time-resolved piezoresponse force microscopy and switching spectroscopy PFM (SSPFM) (PFM) to address polarization dynamics within individual grains, and finite-element simulations to quantify the local ferroelectric interactions and hence assess the weight of several possible switching mechanisms. The experiments find that of the three possible switching mechanisms, namely direct electromechanical coupling, local built-in electric field and strain, and grain boundary electrostatic charges, the last one is the dominant mechanism. Although finite-element simulations find that direct electromechanical coupling and local built-in field-induced switching are possible, calculations confirm that for the utilized material properties, the aforementioned mechanisms are energetically unfavored.     

Three-dimensional analysis of microstructures in titanium

Samples of Ti–4.3Fe–6.7Mo–1.5Al were isothermally annealed in the temperature range of 730–780 °C for various times to study the β–α transformation. Serial sectioning in conjunction with both optical and EBSD analyses was applied to determine the three-dimensional (3-D) morphologies of primary α phase. The 3-D analysis proved to be essential for characterization of the complex morphologies of α grains and consequently for the identification of growth behavior. It showed that nucleation of α grains takes place at β–β grain boundaries and significant branching takes place after initial growth of α grains along β–β grain boundaries. Some branches grow inside the β grain interior. The branching behavior is shown to interact with β–β grain boundaries, leading to a zig-zag morphology. The presented 3-D analysis of α grains and their influence on β–β grain boundaries clearly show that 2-D observations of the microstructural morphologies are not sufficient to adequately represent the transformation characteristics.     

Analytical electron microscopy of C-free and C-containing Ti–15–3

Optical microscopy, analytical scanning and transmission electron microscopy have been used to interpret the influence of C on the ageing response of Ti–15–3 (Ti–15V–3Al–3Sn–3Cr (wt.%)). It has been found that the addition of carbon reduces the extent of oxygen segregation to grain boundaries and thus reduces the tendency for grain boundary alpha to form during ageing. The ageing response and the scale of precipitation at 600 °C have been found to depend on the heating rate used. The as-quenched microstructure is characterised by striations typical of pre-martensite-type contrast with a spacing of about 20–25 nm. Diffraction patterns in as-quenched samples show diffuse scattering in addition to the maxima associated with this large spacing. The striations and diffuse scattering anneal out at ageing temperatures above 400 °C. Contrary to earlier work no evidence has been obtained for omega in as-quenched or aged samples. The alpha precipitation is on a finer scale than can be accounted for by the carbides or by the dislocations punched out by the carbides. This conclusion, taken together with the absence of any evidence for omega, leads to the view that the presence of carbon in solution, rather than the carbides, limits diffusion of oxygen and provides additional nucleation sites for alpha – perhaps through vacancy–carbon–oxygen complexes.     

Advances in Characterization of Non-Rare-Earth Permanent Magnets: Exploring Commercial Alnico Grades 5–7 and 9

The magnetic domain structure of commercial alnico grades 5–7 and 9 was investigated using a magneto-optical Kerr effect (MOKE) to gain an understanding of their coercivity mechanisms at the micron to millimeter scale. In alnico 5–7, the magnetic domain structure exhibits stripes of alternating high and low induction. Magnetic domains easily cross grain boundaries if neighboring grains have a similar tilt and rotation of their crystallographic axes relative to the magnet body. In contrast for alnico 9, stripe-like magnetic domains are not observed regularly throughout the transverse section; rather, discrete localization of high- and low-induction stripe features are observed. In higher magnification MOKE experiments, i.e., ~100 μm, a zigzag-shaped magnetic domain structure was observed in both alnico 5–7 and 9. The zigzag features are four to five times smaller in size than an average grain of alnico 5–7, implying a pinning mechanism that is caused by structural elements within the grains. Discontinuous and reversible motion on a length scale of a few microns was observed for the zigzag-shaped domains for incremental changes in the applied field of ~10 Oe. Complimentary magnetic force microscopy measurements show that there are domain structures on an even smaller scale, i.e., 2 μm to 100 μm.     

Behavior of titanium in the interfacial region between cubic BN and active brazing alloy

In the present investigation, the diffusion behavior of the element titanium in the interfacial region between Ag-based active brazing alloy and cubic BN grain was investigated by scanning electron microscope and energy dispersion spectrometer, as well X-ray diffraction. Meanwhile, the growth mechanism and activation energy of the reaction layer was also deeply studied from the viewpoint of kinetics. The result showed that the active element Ti concentrated to and reacted with the grain crystal to form TiB₂ and TiN to join hard grains and tool substrate. Furthermore, the calculated activation energy value indicated that the width of the interfacial reaction layer was mainly depended on the TiN layer in the special growing process of brazing temperature 880–920 °C and dwell time 300–1200 s.     

Computer modeling of particle pushing and clustering during matrix crystallization

Two-dimensional cellular automaton computer simulations were carried out to model the geometric interaction between mobile, equiaxed particles and growing matrix grains, thus simulating crystallization (respectively, recrystallization, phase transformation or solidification) of a matrix material containing a mobile second phase (e.g. solid particles, liquid droplets or gas bubbles). The model allows the study of particle pushing by growing grains, which leads to particle accumulation and clustering at grain boundaries and triple points, and concomitant particle depletion within grains. Parameters explored are particle area fraction, particle settling speed, particle cluster mobility and grain nucleation rate under continuous nucleation conditions. These parameters are found to strongly affect the particle spatial distribution and clustering during and after crystallization. Conversely, the particles have no measurable effect on the grain shape or size. Finally, site-saturated nucleation at the boundaries of the simulation field is investigated, simulating e.g. solidification from crucible walls or recrystallization from sample edges. Pronounced clustering of particles takes place at grain boundaries and is further accentuated by particle settling.     

MAGNETIC DOMAIN STRUCTURE AND RELATED MAGNETIC PROPERTIES OF(NdPr)_(16)Fe_(76)B_8 PERMANENT MAGNET

The magnetic domain structure and related magnetic properties of (NdPr)_(16)Fe_(76)B_8 permanentmagnet have been studied by colloid-SEM method.In thermally demagnetized state,the ma-trix grains in the magnet generally exhibit multidomain structure,i.e.,180° plate-like indi-vidual domain and a few spike-like or maze-like domains.The average width of the domainmeasured was 1.5μm.Grain size of single-domain was observed to be generally about 1μmand only a few up to 3μm.The results calculated are:the domain wall energy γ=36MJ/m~2,the exchange constant A=4.5×10~(-11)J/m,the domain wall width δ=15.7nm,and the critical grain diameter of single-domain D_c=0.5μm.The dependence of high magnet-ic field gradient at grain boundaries in different magnetization directions in the adjacent mag-netic domains located on both sides of grain boundaries and the effect of microstructure on thevalues of K,A and δ were discussed.     

Haynes 230合金热变形组织演化规律研究

采用Thermecmastor-Z型热模拟机对Haynes230合金进行变形温度为950~1250 ℃,应变速率为0.001～10 s-1范围内的高温压缩试验,并利用OM和TEM分析研究了热变形组织演化特征和动态再结晶形核机制。结果表明：动态再结晶晶粒尺寸和体积分数随着变形温度的升高而增大和增多,随着应变速率的升高而变小和减少;晶界弓出是合金动态再结晶的主要形核机制,项链组织在热变形组织演化过程中起着重要作用;动态再结晶稳态晶粒尺寸Dss与Z参数之间符合幂函数关系