Role of atomic migration in nanocrystalline stability: Grain size and thin film stress states

As the length scale of materials decreases to the nanometer regime, grain boundaries occupy a relatively larger volume fraction. Consequently, they play an important role in stabilizing nanocrystalline systems. This review looks at the role of solute segregation to grain boundaries in stabilizing such systems. In recent years, grain size stabilization from solute segregation has led to new types of thermodynamic stability maps as a materials design tool. We propose to extend and adapt these concepts of grain boundary solute segregation as a stabilizing effect to thin film stress states. A recent study on Fe–Pt alloy films, where one species enriched the boundaries, was shown to manipulate the stress from tensile-to-compressive as a function of composition. This suggests that intrinsic segregation can be used as a tunable variable to manipulate stress states, analogous to changing film processing parameters, such as deposition rate and pressure. The application of such solute segregation is at the precipice of new opportunities in materials design of thin films.     

An intrinsic correlation between driving force and energy barrier upon grain boundary migration

The migration of grain boundary (GB), which plays a key role in the microstructural evolution of polycrystalline materials, remains mysterious due to the unknown relationship between GB mobility associated with specific geometry and external conditions (e.g. temperature, stress, etc., hence the thermodynamic driving force). Combining the rate equation of GB migration with molecular dynamics simulations, an intrinsic correlation between driving force and energy barrier for the migration of various types of GBs (i.e. twist, symmetric tilt, asymmetric tilt, and mixed twist-tilt) is herein explored, showing the decrease of energy barrier with increasing thermodynamic driving force.     

Strain rate effects in the ultrafine grain and nanocrystalline regimes—influence on some constitutive responses

Mounting evidence is pointing to some emerging novel behaviors of metals with ultrafine-grain (UFG) and/or nanocrystalline (NC) microstructures. One such novel behavior is related to the thermodynamic and kinetic aspects of plastic response in the UFG/NC regime. Two inter-related parameters, viz., the strain rate sensitivity (SRS) and the activation volumes of plastic deformation, are used as fingerprints for the thermodynamics and kinetics of plastic deformation. Changes of these parameters with grain size may indicate transition of plastic deformation mechanisms. Therefore, investigations of these phenomena may bring out new strategies for ingenious design and synthesis of UFG/NC materials with desirable properties. In this article, we present a critical review on the experimental results and theories associated with the SRS of UFG/NC metals with different lattice structures, and the influences on some constitutive responses.     

On the background damping in the vicinity of the grain-boundary damping peak in zinc

The background damping in the vicinity of the grain-boundary damping decreases with increasing grain size. An analysis of the strain amplitude dependence of this damping shows that as the grain size increases the distance between solute pinning atoms on dislocation decreases. This can be explained in terms of a movement of solute away from grain boundaries and to dislocations. Thus as the grain size increases the total number of solute atoms at grain boundaries decreases and is rejected both into the lattice and to dislocations. A mathematical model is used to explain this result. As a consequence an activation energy of 0.05 eV is obtained for the binding energy of the solute to the dislocation.     

Mechanism for the multi-stage precipitation of Fe-Ni based alloy

This work lays great emphasis on the distribution evolution of sigma(σ) phase in Fe-Ni based N08028 alloy during aging process,the result of which could provide new insights into phase change and precipitation mechanism.It is found that the σ phase,in any case,tends to separate out with a granular shape at grain boundaries(GBs) primarily,and with the increment of time,it is obliged to precipitate in grain interiors(GIs) with a lamellar structure.The mechanisms for the evolving volume fraction and morphology of σ phase are discussed,and a model appropriate for the multi-stage behavior transforming from intergranular to intragranular precipitation is derived,as well as revealing the thermodynamics and transformation kinetics corresponding to this process.The results reveal that the occurrence of multi-stage precipitation is correlated with the redistribution of solute atoms and with the difference in coupling effect of thermodynamic driving force and kinetic activation energy between the intergranular and intragranular precipitation.     

Modeling the final sintering stage of doped ceramics: mutual interaction between grain growth and densification

Applying the thermodynamic extremal principle, a model for grain growth and densification in the final stage of sintering of doped ceramics was derived, with segregation-dependent interfacial energies and mobilities (or diffusivities). The model demonstrated an interdependence between the driving forces of grain growth and densification during sintering evolution, observed because the surface energy contributes positively to the driving force of grain growth while the GB energy negatively to the driving force of densification. The model was tested in alumina as a host system, and calculations demonstrate that dopants with more negative GB (or surface) segregation enthalpy or which cause lower GB diffusion coefficient can induce higher relative densities at a given grain size. Comparatively studying yttria- and lanthana-doped alumina, the lanthana doping showed significantly enhanced sintering attributed to the larger La³⁺ radius causing a more negative GB segregation energy. This present model is expected to help dopant designing to improve control over sintering.     

聚酰亚胺纸基材料的热老化研究

利用热失重分析了不同厚度聚酰亚胺(PI)纸基材料的热稳定性,采用Coats-Redfern法计算PI纸基材料的分解动力学参数,并预测其长期使用上限温度。实验结果表明:在氮气气氛中,PI纸基材料具有很好的热稳定性,且厚度越大,热稳定性越好。通过Coats-Redfern方法得知,PI纸基材料的热分解反应为二级反应;厚度较大的PI纸基材料具有较高的活化能E和使用上限温度,且不同厚度的PI纸基材料的耐热等级均为H级以上。最后,利用扫描电子显微镜和傅里叶变换红外光谱仪对PI纸基材料的热分解机理进行了分析。     

Adam-Gibbs Formulation of Enthalpy Relaxation Near the Glass Transition

The entropically based nonlinear Adam-Gibbs equation is discussed in the context of phenomenologies for nonlinear enthalpy relaxation within the glass transition temperature range. In many materials for which adequate data are available, the nonlinear Adam-Gibbs parameters are physically reasonable and agree with those obtained from linear relaxation data and thermodynamic extrapolations. Observed correlations between the traditional Tool-Narayanaswamy-Moynihan parameters are rationalized in terms of the Adam-Gibbs primary activation energy (Δμ) determining how close the kinetic glass transition temperature can get to the thermodynamic Kauzmann temperature. It is shown that increased nonlinearity in the glass transition temperature range is associated with greater fragility in the liquid/rubber state above T_g.     

The intercrystalline Gorsky effect

The intercrystalline Gorsky effect is a new relaxation mechanism due to the diffusion of interstitial solute atoms in the stress field of local elastic misfits in polycrystals. Theoretical expressions for the relaxation strength and relaxation time are given in analogy to the respective thermoelastic effect, but further details of the theory are still open. The experimental manifestation of the effect as a hydrogen damping peak is well understood with respect to grain size dependence, requirement of elastic anisotropy, and thermal activation parameters, whereas the shape of the peak is still under discussion. As concerns application, the effect might be interesting under the viewpoints of hydrogen diffusion, characterization of micro- to nanocrystalline materials, and damping properties.     

生物炭/二十烷复合定型相变材料制备及其光热、电热转换和储存性能

为解决单一有机相变材料二十烷(n-Eicosane)导热性差及在相变过程中易发生泄漏的问题,本实验选取玉米秸秆作为生物质原料,通过700℃高温热解、KOH刻蚀改性制备了具有多级孔道结构的生物炭(KBC)材料,再通过乙醇熔融、真空浸渍的方法将二十烷封装到生物炭内部孔道,得到了一种生物炭/二十烷(KBC/n-Eicosane)复合定型相变材料。通过SEM、XRD、FTIR等表征手段研究了复合材料的微观结构和形貌,同时利用TG及DSC测试了复合相变材料的热稳定性和储热性能,并探讨了复合相变材料中二十烷不同用量与焓值的关系。结果表明,复合相变材料的焓值与二十烷的用量成正比,当复合相变材料中生物炭与二十烷的质量比为1∶2时,复合相变材料未明显泄漏,定型效果良好,此时对应的熔融焓和凝固焓值分别为121.3 J·g^-1和117.6 J·g^-1,经过100次循环储热和放热性能测试后,未产生渗漏现象,相变焓值亦无明显变化,表明该复合相变材料的储热能力和稳定性较好。此外,还通过模拟太阳光辐射和接入直流电源的方式测试了复合相变材料的光热转换和电热转换能力,结果表明...     

Grain growth and stabilisation of nanostructured aluminium at high temperatures: review

Nanostructured (NS) materials have a large stored energy due to their large grain boundary area and thus tend to be unstable with respect to grain growth during high temperature annealing or deformation. This problem can limit the application of NS materials at high temperatures (>0·5T_m, absolute melting temperature), especially Al alloys owing to their low melting points. Restoration processes and grain growth in NS Al based materials are critically reviewed, with emphasis on nanostructure grain stabilisation at high temperatures. The mechanisms of normal and abnormal grain growth during isothermal annealing are presented, followed by consideration of thermal stabilisation by the addition of solute atoms/impurities and/or dispersion of second phase particles. Grain growth is significantly facilitated by applying deformation at elevated temperatures during preparation or further processing of semifinished NS materials. The dynamic restoration processes, dynamic grain growth and dynamic particle coarsening are addressed in NS Al. Finally, grain growth during consolidation of nanocrystalline powders (one of the principal methods to fabricate bulk NS Al) is presented, and the effects of processing parameters on grain size stabilisation are discussed.     

The spectrum of atomic excess free volume in grain boundaries

The grain boundary (GB) excess volume is an important structural factor that is strongly correlated with various thermodynamic and kinetic properties of GBs such as GB energy, GB mobility, GB diffusivity, and GB segregation energy, etc. However, the excess volume is usually reported as an average value of the entire GB. Such simplification does not consider the spectral nature of the excess volume in a GB, which cannot be used to describe the atomic mechanisms of some kinetic process, such as GB migration, that involves only a few atoms at a time. Here, we explore the spectrum of atomic excess volume in representative nanocrystalline Ni and Al samples as well as 388 Ni bicrystals based on the Olmsted dataset by using atomistic simulations. It is found that the nanocrystalline Ni and Al models show a skew-normal distribution in the spectrum of both the atomic excess volume and the atomic excess energy in the GBs, which show a weak inverse correlation between them. This is in stark contrast to the widely reported positive correlation between GB energy and excess volume based on the average value. We further show based on the statistical analysis that the correlation between the atomic excess volume and excess energy strongly depends on the GB type and a universal trend between them does not exist. While low ∑ Ni GBs generally shows a strong inverse linear correlation between these two properties, such correlation is weak for high ∑ Ni GBs. Moreover, we find that the spectrum of the excess volume shows characteristics distribution in some special Ni GBs. For example, twist GBs generally show a symmetrical unimodal distribution while most surveyed ∑3 Ni GBs with anti-thermal behavior show an apparent bimodal distribution. Nevertheless, a strong correlation is found between the atomic excess volume and the segregation energy based on the nanocrystalline Al model with Mg impurity, which implies a possible universal trend between the two properties. The current study thus shows that the excess volume provides useful insights in revealing the elemental structure–property correlations in GBs, which may be used as a structural variable in future thermodynamic modeling of GBs.

     

Influence of Al2O3 particle pinning on thermal stability of nanocrystalline Fe

Second-phase particle pinning has been well known as a mechanism impeding grain boundary (GB) migration, and thus, is documented as an efficient approach for stabilizing nanocrystalline (NC) materials at elevated temperatures. The pinning force exerted by interaction between small dispersed particles and GBs strongly depends on size and volume fraction of the particles. Since metallic oxides, e.g. Al₂O₃, exhibit great structural stability and high resistance against coarsening at high temperatures, they are expected as effective stabilizers for NC materials. In this work, NC composites consisting of NC Fe and Al₂O₃ nanoparticles with different amounts and sizes were prepared by high energy ball milling and annealed at various temperatures (T_ann) for different time periods (t_ann). Microstructures of the ball milled and annealed samples were examined by X-ray diffraction and transmission electron microscopy. The results show that the addition of Al₂O₃ nanoparticles not only enhances the thermal stability of NC Fe grains but also reduces their coarsening rate at elevated temperatures, and reducing the particle size and/or increasing its amount enhance the stabilizing effect of the Al₂O₃ particles on the NC Fe grains.     

Zr65Al7.5Ni10Cu15Co2.5合金的玻璃转变温度

根据Ｚｒ６５Ａｌ７。５Ｎｉ１０Ｃｕ１５Ｃｏ２。５合金的纳米晶，晶体，液体和玻璃比热的测量结果，研究了合金的玻璃转变温度与全金的热力学函数，动力学参数以及加热速度的关系。结果表明，非晶态合金玻璃转变所需转变激活能很小，玻璃转变温度实际上是由于加热速度引起的不同状态的玻璃与液体的热力学平衡温度。     

Solid-solution hardening and softening in binary iron alloys

Six dilute (0.2, 0.5 and 1 at %) binary iron-base alloys with Co, Cr, Al, Si, Mn and Ni were prepared after scavenging inherent carbon with Ti. From tensile and stress relaxation tests in the temperature range of 77 to 450 K, stress-strain behaviours and thermal activation parameters were analysed as functions of solute content and temperature. In the four alloys containing Ni, Mn, Al and Si, solid-solution softening occurs below 250 K while solid-solution hardening occurs above 250 K. In the alloys containing Co or Cr, neither softening nor hardening due to solute additions occurs at any temperature. Detailed analysis of thermal activation parameters leads one to conclude that the solid-solution softening in the above mentioned four alloys is due to a decrease in kink energy with increasing solute content, while in the latter two alloys no change in kink energy occurs. On the other hand, there exists a strong solute concentration dependence of the athermal component, suggesting that the solid-solution hardening is due to the interaction of dislocations with groups of substitutional solute atoms that create lattice and modulus misfits.     

Thermal degradation kinetics and solid state,temperature dependent,electrical conductivity of charge-transfer complex of phenothiazine with chloranil and picric acid

Temperature dependent electrical conductivity and thermal degradation kinetics of charge-transfer (C-T) complexes of phenothiazine (PTZ) with p-chloranil (CHL) and picric acid (PA), are reported. These C-T complexes exhibited semiconducting behaviour. The activation energies for PTZ-CHL and PTZ-PA complexes are calculated based on their electrical conductivities measured over the temperature ranges 30–110°C and 30–90°C, respectively. And these energies for PTZ-CHL and PTZ-PA are 0·54 eV and 0·75 eV, respectively. The complexes are analysed for the kinetic parameters like the activation energy for decomposition and the Arrhenius pre-exponential factors in their pyrolysis region using Broido’s, Coats-Redfern as well as Horowitz-Metzger methods. Using standard equations, thermodynamic parameters such as enthalpy, entropy and free energies, are calculated.     

Phase transformations at interfaces: Observations from atomistic modeling

We review the recent progress in theoretical understanding and atomistic computer simulations of phase transformations in materials interfaces, focusing on grain boundaries (GBs) in metallic systems. Recently developed simulation approaches enable the search and structural characterization of GB phases in single-component metals and binary alloys, calculation of thermodynamic properties of individual GB phases, and modeling of the effect of the GB phase transformations on GB kinetics. Atomistic simulations demonstrate that the GB transformations can be induced by varying the temperature, loading the GB with point defects, or varying the amount of solute segregation. The atomic-level understanding obtained from such simulations can provide input for further development of thermodynamics theories and continuous models of interface phase transformations while simultaneously serving as a testing ground for validation of theories and models. They can also help interpret and guide experimental work in this field.     

乙二胺双马来酰胺酸根合镧(Ⅲ)-层状矿物复合热稳定剂对聚氯乙烯的热稳定作用

利用稀土配合物具有较强协同作用的优点,将自身对聚氯乙烯(PVC)具有一定热稳定作用的乙二胺双马来酰胺酸根合镧(Ⅲ)配合物(LaL)与层状矿物,如高岭土、蒙脱土、水滑石和水镁石进行复配,通过热重分析、刚果红试验和静态烘箱老化法研究了乙二胺双马来酰胺酸根合镧(Ⅲ)与高岭土、蒙脱土、水滑石和水镁石复合物对PVC热稳定性的影响。结果表明,乙二胺双马来酰胺酸根合镧(Ⅲ)与层状矿物具有良好的协同效应,且与水镁石效果最为显著,当二者质量比为1∶1时,PVC热稳定时间由2 min提升至70 min,且能有效地抑制单一稀土热稳定剂使用时的初期着色能力。复合热稳定剂的添加大幅度提高了PVC热降解的活化能,尤其是LaL-水镁石复合物(LaL-Brucite)的添加,N_2气氛下的活化能相对于纯PVC样品,由123.5 kJ/mol提升至234.7 kJ/mol,提高了111.2 kJ/mol;空气气氛下的活化能相对于纯PVC样品,由115.9 kJ/mol提升至133.0 kJ/mol,提高了17.1 kJ/mol;有效地提高了PVC的热稳定性。