Modelling the influence of grain-size-dependent solute drag on the kinetics of grain growth in nanocrystalline materials

The large relative change in total grain-boundary area that accompanies grain growth in a nanocrystalline material has a potentially strong influence on the kinetics of grain growth whenever grain-boundary migration is controlled by solute (impurity) drag. As the grain-boundary area decreases, the concentration of solute or impurity atoms segregated to the boundaries is expected to increase rapidly, introducing a grain-size dependence to the retarding force on boundary migration. We have modified the Burke equation—which assumes the drag force to be independent of the average grain size—to take into account a linear dependence of grain-boundary pinning on grain size. The form of the resulting grain-growth curve is surprisingly similar to Burke's solution; in fact, a constant rescaling of the boundary mobility parameter is sufficient to map one solution approximately onto the other. The activation energies for grain-boundary motion calculated from the temperature dependence of the mobility parameter are therefore identical for both models. This fact provides an explanation for the success of Burke's solution in fitting grain-growth data obtained in systems, such as nanocrystalline materials, for which the assumption of grain-size-independent solute drag is incorrect.     

Microstructural and mechanical study of nanocrystalline (Ti0.8W0.2)C synthesized by high-energy ball milling

The microstructural characteristics of nanocrystalline (Ti_0.8W_0.2)C elaborated by mechanical alloying process were investigated by using both X-ray diffraction and electron microscopy. The diffraction crystallite size (DCS) and the microstrain of (Ti_0.8W_0.2)C ball milled powders have been determined according to the Rietveld refinement method. The results obtained showed that the (Ti_0.8W_0.2)C diffraction crystallite size decreases and its microstrain increases as the milling duration increases. A transition from grain-size hardening to grain-size softening was observed at DCS = 8 nm. It is demonstrated that, as the grain size decreases, the plastic deformation mechanism undergoes a transition from an intragranular deformation by dislocation sliding to an intergranular deformation by grain boundary sliding, as a result of increasing volume fraction (39%) of the grain boundary (GB) zone.     

Thermal Stability of Nanocrystalline AZ31 Magnesium Alloy Fabricated by Surface Mechanical Attrition Treatment

A nanocrystalline layer(NL) was fabricated on the surface of AZ31 magnesium(Mg) alloy sheet by surface mechanical attrition treatment(SMAT). The microstructure of the Mg alloy was characterized by optical microscopy,X-ray diffraction and microhardness test. The results showed that both the microstructure and microhardness of AZ31 Mg alloy sheet after SMAT revealed a gradient distribution along depth from surface to center. The thermal stability of the NL was investigated through characterizing the microstructure evolution during the post-isothermal annealing treatment within the temperature range from 150 to 250 ℃. The NL exhibits a certain degree of thermal stability below 150 ℃, while it disappears quickly when annealing at the temperature range of 200–250 ℃. The grain growth kinetics of the nanocrystalline of AZ31 Mg alloy induced by SMAT was investigated. The activation energy of nanocrystalline AZ31 Mg alloy was obtained with a value of 92.8 k J/mol.     

Atomistic origin of microstrain broadening in diffraction data of nanocrystalline solids

The origin of microstrain broadening in X-ray diffraction patterns of nanocrystalline metals is investigated by comparing data obtained from virtual diffractograms and from direct analysis of computer-generated samples. A new method is introduced that allows the local deformation gradient to be calculated for each lattice site in the microstructure from atomic coordinates obtained by molecular dynamics simulations. Our results reveal that microstrain broadening in undeformed samples cannot be attributed to lattice dislocations or strain fields near grain boundaries. The broadening arises, instead, from long-range correlated displacement fields that extend throughout the grains. The microstrain therefore provides a quantitative measure for distortions far from grain boundaries. This suggests that diffraction-based strategies for inferring the dislocation density in ultrafine-grained metals do not necessarily apply to nanocrystalline materials.     

Coefficients of thermal expansion of thin metal films investigated by non-ambient X-ray diffraction stress analysis

Cu, Ni and Pd layers were deposited by DC-magnetron sputtering on Si wafer substrates. The mechanical stresses and the coefficients of linear thermal expansion (CTEs) were investigated by non-ambient (in-situ) X-ray diffraction measurements employing the sin²ψ crystallite group method. It was found that, in the as-produced state, the CTEs were larger than expected on the basis of literature values for the Cu and Ni layers, but not for the Pd layer. Upon annealing of the layers grain growth and decrease of the crystalline imperfection, monitored by in-situ X-ray diffraction measurements of the diffraction line broadening, occurred. The increase of the crystallite size was paralleled by a decrease of the CTE values for the Cu and Ni films. The grain-size dependence of the CTE is discussed in terms of the coordination (state of bonding) of atoms at grain boundaries and surfaces.     

Crystallization of amorphous ceria solid solutions

Next-generation micro-solid oxide fuel cells for portable devices require nanocrystalline thin film electrolytes in order to allow fuel cell fabrication on chips at low operating temperatures and with high fuel cell power outputs. In this study amorphous gadolinia-doped ceria (Ce_0.8Gd_0.2O_1.9−x) thin film electrolytes were fabricated by spray pyrolysis and their crystallization to nanocrystalline microstructures was investigated by means of X-ray diffraction and transmission electron microscopy. At temperatures higher than 500 °C the amorphous films crystallize to a biphasic ceramic that is amorphous and nanocrystalline. The driving force for the crystallization is the reduction of the free enthalpy resulting from the transformation of amorphous into crystalline material. Self-limited grain growth kinetics prevail for the nanocrystalline grains where stable microstructures are established after short dwell times. A transition to classical curvature-driven grain growth kinetics occurs when the fully crystalline state is reached for average grain sizes larger than 140 nm and annealing temperatures higher than 1100 °C.     

Effect of nanocrystalline grain size on the electrochemical and corrosion behavior of nickel

Nanocrystalline nickel of different grain sizes (8–28 nm) was produced by electrodeposition using Watt's bath. Saccharine addition to the bath and pulsed current deposition were effective in lowering the grain size of the deposits. The grain size and microstrain of deposits was determined by X-ray diffraction analysis. The microhardness of nanocrystalline Ni ranged between 572 and 724 kg/mm². The electrochemical behavior of nanocrystalline Ni was evaluated in 1 mol/l H₂SO₄ and compared with that of coarse-grained nickel. All the nickel samples exhibited active–passive potentiodynamic polarization behavior. The zero current potential, passive current density and breakdown potential generally increased with decrease in grain size. The increased passive current density for nanocrystalline nickel confirmed the defective nature of passive film that forms on nanocrystalline nickel. The tendency for localized corrosion was lower in case of nanocrystalline nickel as indicated by increased breakdown potential. Tafel and linear polarization tests revealed that the corrosion rate of freshly exposed surfaces of Ni decreased with grain size, thereby indicating greater hindrance to anodic dissolution in nanocrystalline Ni. The magnitude of compressive microstrain in the Ni deposits increased with decrease in grain size.     

Microstructure evolution during cold rolling in a nanocrystalline Ni–Fe alloy determined by synchrotron X-ray diffraction

Stress softening after cold rolling is observed in an electrodeposited nanocrystalline Ni–Fe alloy. The grain-size distribution becomes much broader after the cold rolling. Microstructure changes, though moderate, such as simultaneously decreased dislocation and twin densities with grain growth during cold rolling, are systematically proved by synchrotron high-energy X-ray diffraction, transmission electron microscopy and differential scanning calorimetry (DSC). The amorphous fractions in the form of grain boundaries are evidenced by the diffuse-background scatterings and large DSC values. Partial dislocation separation calculation, a dislocation mean free path and annihilation model, and texture development together reveal that the current nanocrystalline Ni–Fe alloy exhibits the combined behavior of perfect dislocation slip and grain-boundary mediated deformation.     

TiB2/6351Al复合材料喷丸层再结晶过程的X射线衍射分析

利用X射线衍射线形分析方法,研究了TiB2/6351Al和6351Al喷丸形变层的再结晶行为,并得到了两种材料不同温度下等温退火晶块尺寸和显微畸变的变化.通过回归分析,获得了晶块长大激活能和显微畸变松弛激活能.结果表明,两种材料的再结晶激活能均大于纯Al的白扩散激活能,并且复合材料的再结晶激活能略大于铝合金基体的再结晶激活能.增强体阻碍加热过程中晶界和亚晶界的运动影响了复合材料再结晶激活能的提高.     

Unraveling the nature of room temperature grain growth in nanocrystalline materials

We report on the observation of real-time-resolved room temperature grain growth in nanocrystalline metals. We find that neither the time evolution of size can be modeled by standard growth theories nor are there any other systems aware to us that manifest a similar growth behaviour. We detect a transition from an initially self-similar slow growth to abnormal grain growth. Its onset seems to be associated with the simultaneous decrease of microstrain with increasing grain size. Abnormal grain growth is considered as a generic feature of nanocrystallinity but is a transient state since we observed in the late stage of coarsening, using orientational imaging microscopy, a monomodal grain size distribution. We empirically find a nonlinear-response-type of growth law which is in agreement with the observed coarsening kinetics.     

Simulation of the influence of particles on grain structure evolution in two-dimensional systems and thin films

A two-dimensional front-tracking simulation of grain growth has been extended to treat the effects of particles on the evolution of grain structures during annealing. When grain boundaries come into contact with particles, boundary motion is assumed to be pinned. It is found that even a small volume fraction of particles retards grain growth, lowers the ultimate average grain size, and leads to significant changes in the grain-size and number-of-sides distributions. These changes differ in detail from the changes in the grain-size distribution predicted using the Potts model. The changes in the nature of the grain-size distribution are explained by considering the topology of the evolving and of the stagnant grain structures. The average grain size in the stagnant structure scales with the number of particles in a way consistent with a scaling with area fraction of the particles to the power 0.46, in near agreement with the expected dependence from a Zener-pinning analysis in two dimensions. Particle pinning is also simulated in conjunction with the effects of other mechanisms impeding grain growth such as solute drag or grain boundary grooving. In this case it is found that the stagnant grain-size distribution is determined by the competing stagnation forces, and that the Zener-pinning analysis is not obeyed and must be modified.     

Continuous recrystallization in austenitic stainless steel after large strain deformation

Static restoration mechanisms operating during annealing were studied in a 304 steel with strain-induced submicron grain structures. The initial microstructure with an average grain size of about 0.3 μm was developed by large strain deformation at 873 K. Early annealing leads to a full relaxation of high internal stresses associated with non-equilibrium strain-induced grain boundaries, while their boundary misorientations and the average grain size barely change. Further annealing results in a transient recrystallization followed by a normal grain growth. The average grain boundary misorientation increases up to around 45° in the former and becomes constant in the latter. This is associated with the change in the grain boundary misorientation distribution from a characteristic strain-induced one to a near random distribution corresponding to a fully recrystallized state. The annealing processes operating in the strain-induced fine grains take place homogeneously in the whole matrix and can be called continuous recrystallization.     

Formation of the Nanocrystalline Structure in an Equiatomic NiTi Shape-Memory Alloy by Thermomechanical Processing

The microstructural evolution during cold rolling followed by annealing of an equiatomic NiTi shape-memory alloy was investigated. The high purity Ni₅₀Ti₅₀ alloy was cast by a copper boat vacuum induction-melting technique. The as-cast ingots were then homogenized, hot rolled, and annealed to prepare the suitable initial microstructure. Thereafter, annealed specimens were cold rolled up to 70 % thickness reduction at room temperature. Post-deformation annealing was conducted at 400 °C for 1 h. The microstructure was characterized using scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and differential scanning calorimetry techniques. The initial microstructure was free from segregation and Ti- or Ni-rich precipitates and was composed of coarse grains with an average size of 50 μm. The cold rolling of NiTi alloy resulted in a partial amorphization and the deformation-induced grain refinement. A nanocrystalline structure with the grain size of about 20-70 nm was formed during the post-deformation annealing.     

Thermal stability of nanocrystalline in surface layer of magnesium alloy AZ91D

Isothermal and isochronal annealing was conducted to study the thermal stability of the nanocrystalline in the surface layer of Mg alloy AZ91D induced by high-energy shot peening(HESP) .Field emission scanning electron microscope(FESEM) and X-ray diffractometer were used to characterize the microstructure.Results showed that nanocrystalline produced by HESP on the surface layer of the magnesium alloy AZ91D was 60-70 nm on average.The nanocrystalline could remain stable at about 100℃,and grew up slowly between 100℃ and 200℃.When the annealing temperature reached 300℃,the growth rate of the nanocrystalline increased significantly.The kinetic coefficient n of the nanocrystalline growth was calculated to be 2-3 and the grain growth activation energy Q=39.7 kJ/mol,far less than the self-diffusion activation energy of magnesium atoms in the coarse polycrystalline material.     

SiCw/Al复合材料高温再结晶行为的X射线衍射分析

利用原位X射线衍射及线形分析方法，测量加热过程中冷轧SiCw／Al复合材料基体晶块尺寸及显微畸变的变化规律，探讨复合材料再结晶行为．结果表明，复合材料基体晶粒长大及高温回复激活能均与纯Al自扩散激活能接近，证实了再结晶后晶粒长大伴随着高温回复现象，并且晶须对复合材料基体晶粒长大及高温回复的影响不明显。     

Peculiarities of vacuum annealing of nanocrystalline WC powders

The effect of vacuum annealing temperature on the phase and chemical composition, particle size, and microstrains of nanocrystalline powders of tungsten carbide WC with particles from 20 to 60 nm in size has been studied using X-ray diffraction and electron microscopy methods. It is established that nanocrystalline WC powders stored in air, contain from 1 to 2 wt.% of impurity oxygen. It is found that vacuum annealing of WC nanopowders at a temperature up to 1400 °C is accompanied by appreciable decarburization and variation in the phase composition due to carbon desorption as a result of interaction with impurity oxygen. Annealing leads to coarsening of powder particles caused by intergrowth of aggregated nanoparticles and to decreasing microstrains.     

Microstructure,texture and grain boundaries character distribution evolution of ferritic stainless steel during rolling process

In order to better understand the texture, microstructure and grain boundaries character distribution evolution of ferritic stainless steel, the texture, microstructure and grain boundaries character distribution of ferritic stainless steel (hot rolled sheet, cold rolled sheet and annealing sheet) with 11 wt%Cr content were studied using X-ray diffraction and electron back scattered diffraction technique. The texture of the hot and cold rolled sheets has a through-thickness texture gradient. In the center layer of the hot and cold rolling sheet, α-fiber texture was observed which was attributed to ideal plane strain deformation. Close to the surface a Gross orientation was detected which was attributed to shear deformation. During annealing, the γ-fiber was formed attributed to recrystallization process. The microstructure of the hot and cold rolled sheets was non-homogeneous through the sheet thickness, while, the microstructure of annealing sheets was homogeneous through the sheet thickness. Grain boundaries character distribution results show that there are many low angle grain boundaries in hot and cold rolled sheets and many high angle grain boundaries and coincidence site lattice after annealing. The above results indicated that the changes in texture are closely related to the grain boundaries type.     

Annealing behaviour of Mg-3Al-1Zn alloy sheet obtained by a combination of high-temperature rolling and subsequent warm rolling

The recrystallization and grain growth behaviour of Mg-3Al-1Zn alloy sheets with a deformation microstructure, obtained by a combination of high-temperature rolling and subsequent warm rolling, was investigated at different stages of annealing. The basal texture was significantly weakened as a result of the formation of new grains with a largely altered c-axis orientation relative to the initial basal orientation owing to discontinuous static recrystallization during primary recrystallization. The new grains nucleated mostly at the pre-existing grain boundaries rather than at the intersections of twins or within the twins. Subsequent grain growth led to further progression of the texture weakening accompanied by an enhancement in the basal pole inclination.     

Uptake of iron,oxygen and nitrogen in molybdenum during ball milling

Molybdenum powder was ball milled in a steel vessel with hardened steel balls under Ar, O₂ and N₂ atmosphere, respectively. The milling products were examined by chemical analysis, X-ray diffraction and neutron diffraction. During milling under Ar or O₂ the iron as well as the oxygen content of the samples increases dependent on the number of interruptions of the milling process. The presence of dissolved oxygen seems to facilitate the solubility of iron in the molybdenum powder. Milling in a nitrogen gas atmosphere results in the formation of a nanocrystalline cubic γ-Mo₂N phase and a Mo-rich amorphous-like grain boundary phase. The microstructural imperfection was characterised by X-ray diffraction-line profile analysis. A method was proposed for the evaluation of the crystallite size and the microstrain in nanocrystalline materials, as well as the amount and structure of disordered grain boundaries from the reduced pair correlation function.     

纳米晶材料的晶粒长大

本文综述了纳米晶材料晶粒长大的研究进展,简述了纳米晶材料晶粒长大的等温动力学理论,讨论了溶质原子、孔洞、第二相粒子和微观应变对纳米晶材料晶粒长大的影响。