Influence of Temperature on the Inverse Hall-Petch Effect in Nanocrystalline Materials:Phase Field Crystal Simulation

The influence of temperature on the inverse Hall-Petch effect in nanocrystalline(NC) materials is investigated using phase field crystal simulation method.Simulated results indicate that the inverse Hall-Petch effect in NC materials becomes weakened at low temperature.The results also show that the change in microscopic deformation mechanism with temperature variation is the main reason for the weakening of the inverse Hall-Petch effect.At elevated temperature,grain rotation and grain boundary(GB) migration seriously reduce the yield stress so that the NC materials exhibit the inverse Hall-Petch effect.However,at low temperature,both grain rotation and GB migration occur with great difficulty,instead,the dislocations nucleated from the cusp of serrated GBs become active.The lack of grain rotation and GB migration during deformation is mainly responsible for the weakening of the inverse Hall-Petch effect.Furthermore,it is found that since small grain size is favorable for GB migration,the degree of weakening decreases with decreasing average grain size at low temperature.     

Effects of milling and subsequent consolidation treatment on the microstructural properties and hardness of the nanocrystalline chromium carbide powders

The influence of milling and subsequent consolidation treatments on the microstructural properties and hardness of the fabricated Cr₃C₂, Cr₇C₃ and Cr₂₃C₆ ceramic powders are investigated. For this reason, the elemental powders of Cr and C were mixed with proper ratio and then milled to the nanometer crystallite sizes (between 6 and 20 nm) and then were consolidated by using uniaxial cold press and subsequent heat treatment (at 1100 °C for 1 h) in Argon atmosphere. Microstructures of consolidated samples were characterized using X-ray diffraction (XRD) and microhardness measurements. A drastic increase in crystallite size of the samples was observed due to the effect of heat treatment. However, the as-consolidated samples still maintained their nanocrystalline characteristic with an average grain size of less than 100 nm. Besides, a very high hardness of 25 GPa was achieved for the Cr₃C₂ composition. This high hardness is attributed to the formation of carbide phases in the consolidated samples.     

Anomalies in Hall-Petch strengthening for nanocrystalline Au-Cu alloys below 10 nm grain size

The mechanical behavior of an electrodeposited nanocrystalline alloy is assessed with regards to the experimentally measured strain-rate sensitivity. Foils are characterized with grain sizes as small as 3 nm, a nano-scale regime that has previously gone without detailed experimental examination. It is found from micro-scratch measurements that hardness, hence strength, approaches ideal values as the grain size decreases to 7 nm. Below 7 nm, softening in strength and departure from Hall-Petch behavior is related to an increase in the activation volume for deformation as grain size decreases further.     

Deposition and corrosion resistance of HVOF sprayed nanocrystalline iron aluminide coatings

The microstructure and corrosion properties of nanocrystalline Fe–40Al coatings obtained by thermal spraying of milled powder were investigated. The coatings were sprayed under similar high-velocity oxy-fuel (HVOF) conditions and were varied by the size of the starting feedstock powder. The coatings have complex microstructure consisting essentially of a mixture of well-flattened splats and non-fully melted powder particles within which an equiaxed nanometer-scale structure is retained. Amorphous Al₂O₃ and nanocrystalline Fe-rich oxides together with Fe₃Al (resulting from Al depletion and reaction in the flame) were present at intersplat boundaries. The amount of these phases and porosity, as well as the presence of unmelted powder particles, has been quantified. It is shown that the feedstock powder size has a strong effect on the coating hardness by modifying the amount of hard unmelted powder particles. The electrochemical response of the coatings shows the same general type of active–passive–transpassive behaviour than the microcrystalline bulk Fe–40Al but with poorer corrosion resistance parameters. Analysis of corrosion damage shows a prevalent localized attack at intersplat boundaries or around unmelted powder particles, probably enhanced by galvanic phenomena, that is likely responsible for the poorer corrosion properties of the coatings. To a lesser extent, corrosion takes place by a more global form of attack within splats containing ultrafine grains. If the amount of unmelted powder particles controls the overall hardness of the coatings, it appears to have limited direct effect—if any—on the corrosion behaviour. Thus, the hardness/corrosion balance can be optimized by a good selection of powder size feedstock.     

The effect of current density on the grain size of electrodeposited nanocrystalline nickel coatings

The aim of this work was to investigate the effect of current density on the grain size of electrodeposited nickel coatings. For this purpose, nanocrystalline nickel coatings were deposited from a Watts bath containing 5 g/l sodium saccharin as an additive, by direct current electroplating at different current densities. X-ray diffraction analysis and modified Williamson–Hall relation were used to determine the average grains size of the coatings. The experimental results showed that the coating grains size decreased sharply by increasing the current density from 10 mA/cm² to 75 mA/cm². Nanocrystalline nickel coating with average grain size smaller than 30 nm can be achieved at the current densities higher than 50 mA/cm². Furthermore, a general and simple theoretical model based on atomistic theory of electrocrystallization has been made in order to find out the relationship between the grain size and current density. According to this model the variation of log (d) versus log (i) was linear which is in accordance with experimental results for the current densities lower than 75 mA/cm².     

Grain size softening in nanocrystalline TiN

The effect of grain size d on the hardness H of TiN is considered, with focus on the nanometer grain size range. H increases from 22 GPa to 32 GPa with decrease in d from single crystals to d = 50 nm. Three sets of data show that H decreases with further reduction in d below 50 nm, i.e., grain size softening occurs. The softening is not in complete or unequivocal accord with any of the three models normally proposed for such softening, namely Coble creep, grain boundary shear and dislocation line tension modification. Factors which could have contributed to the softening are: (a) changes in texture and in turn the corresponding value of the Taylor orientation factor M and (b) the presence of weakening imperfections produced during fabrication.     

A quantitative study of the hardness of a superhard nanocrystalline titanium nitride/silicon nitride coating

The hardness of nanocrystalline two-phase TiN/a-SiN_x coatings has been measured by nanoindentation. An optimum hardness is found for the coatings containing 2–5 at.% silicon. In this regime the microstructure combines mechanical confinement effects with optimum load transfer between the phases as illustrated by finite element simulations.     

Al-TiB2纳米晶薄膜中的反Hall-Petch效应

多种微结构因素作用的相互交织使纳米晶合金中是否存在与纯金属类似的反Hall-Petch现象难以得到实验证实。选用Al-TiB_2体系,采用二维结构纳米多层膜的方法,实现了对晶粒尺寸因素的孤立和使其独立地改变,研究了晶粒尺寸对薄膜力学性能的作用规律。结果表明:Al-TiB_2过饱和固溶纳米晶薄膜也与纳米晶纯金属Al一样,存在硬度随晶粒尺寸减小从遵从Hall-Petch关系提高转变为偏离Hall-Petch关系,并进一步出现反Hall-Petch效应的3个阶段,实验得到了偏离Hall-Petch关系为32 nm,产生反Hall-Petch现象的临界晶粒尺寸为8 nm,这2个临界晶粒尺寸与分子动力学方法对纳米晶纯金属A1计算的结果相当。     

The effect of saccharin addition and bath temperature on the grain size of nanocrystalline nickel coatings

Nanocrystalline nickel coating was synthesized by direct current electrodeposition from a Watts bath at the current density of 100 mA/cm² and pH = 4. The effect of saccharin addition (0-10 g/l) and bath temperature (45-65 °C) on the average grain size of the deposits was investigated by XRD technique. The results showed that the average grain size decreased from 426 nm to 25 nm as the saccharin concentration increased from 0 to 3 g/l, while further increase in saccharin concentration had no significant effect. Theoretical model also indicated a non-linear function for dependence of grain size on saccharin concentration, which was in accordance with experimental results. The experimental results showed that the increases in the bath temperature had no considerable effect on the average grain size of the deposits. A theoretical formula was also established for the temperature dependence of the grain size.     

Evaluation of effect of grain size on mechanical and tribological properties of pulse electrodeposited nanocrystalline nickel using nanoindentation techniques

Abstract

Pulse electrodeposition of nanocrystalline nickel has been carried out on AA 6061 substrate from a modified Watt’s bath using saccharin as a grain refining additive. By varying the concentration of saccharin and other operating parameters, nanocrystalline nickel electrodeposits of varying average grain sizes (from 115 down to 17 nm) have been obtained. Nanoindentation was employed for studying the effect of average grain size on the mechanical and tribological properties of the electrodeposits, with emphasis on hardness, elastic modulus, wear resistance and coefficient of friction. The study confirms that the hardness of nanocrystalline nickel electrodeposits increases as the average grain size decreases and a value as high as 7·2 GPa is obtained for a coating having an average grain size of 17 nm. No inverse Hall–Petch relationship is observed for the entire range of grain sizes studied. The elastic modulus of the electrodeposits remained almost constant (between 150 and 160 GPa), irrespective of the average grain size and a coefficient of friction value of 0·25 has been obtained for a deposit having an average grain size of 17 nm.     

Nanostructures in thermal spray coatings

The nature of the nanograins formed by high velocity oxy-fuel thermal spraying of (FeAl) milled powder has been investigated using transmission electron microscopy on cross-sectional thin foils. Equiaxed 3D nanometer crystallites are formed by recrystallization in the unmelted powder particles while 2D nanometer columnar grains are produced by rapid solidification within the fully molten splats.     

Mechanical properties of nanocrystalline nickel films deposited by pulse plating

This paper reports the mechanical properties of Ni films fabricated by pulse electrodeposition. Transmission electron microscope revealed that the prepared films had an average grain size of 25 nm with a narrow size distribution and the absence of dislocations. Small grain size leads to an increasing hardness as high as 7.8 GPa while Young's moduli keep a constant bulk value of 215 GPa, resulting in an increasing ratio of hardness (H) to elastic modulus (E). Interestingly, the wear resistance was also improved significantly. Under a constant normal load of 500 μN, the penetration depths of indenter slightly increased from 25 nm to 30 nm and the coefficient of friction varied from 0.12 to 0.20, depending on sliding scans. Depth sensing instrumented indentation experiments performed at different loading rates on specimens revealed an increasing rate-sensitivity of hardness, which concerns with a significantly small activation volume for plastic flow.     

Hardness and elastic modulus of amorphous and nanocrystalline SiC and Si films

Hydrogen-free amorphous and nanocrystalline films were prepared by magnetron sputtering of the SiC or Si targets. Mechanical properties (hardness, elastic modulus, intrinsic stress) and film structures were investigated in dependence on the substrate bias and temperature. It was found that both hardness and elastic modulus of all amorphous a-SiC films prepared at different substrate temperatures and biases are always lower than those for bulk α-SiC single crystal while the hardness of partially crystalline SiC films is higher and the elastic modulus lower than those for α-SiC one. In contrast, both hardness and elastic modulus of all amorphous Si films are always lower than those for nanocrystalline Si films which show approximately the same value as the Si single crystal.     

Effect of sintering temperature on microstructure and properties of SiC coatings for carbon materials

SiC coating for the graphite materials was prepared by slurry-sintering technique in a vacuum. The phase, microstructure, thickness and resistance against irradiation of the SiC coatings prepared from 1500 to 1800 °C were investigated. Research results showed that, the porous β-SiC coating occurred at 1500 and 1600 °C, while compact β-SiC/Si coatings obtained at 1700 and 1800 °C. The thickness of the coatings was in most cases around 150 μm when the sintering was performed at 1500 and 1600 °C. However, the thickness was decreased, and the crystal size of SiC particles was increased when the sintering temperature was higher than 1600 °C. Thermal fatigue tests showed that, based on the surface morphology changes, the sintering temperature of 1700 and 1800 °C gave much improved irradiation resistance over that of coatings formed at 1500 and 1600 °C.     

Entropic effect on creep in nanocrystalline metals

We report a significant entropic effect on creep of nanocrystalline metal using molecular dynamics. Our simulations reveal that the activation entropy may contribute a multiplicative factor of many orders of magnitude to the steady-state creep rate. The relationship between activation entropy and enthalpy obeys an empirical Meyer–Neldel compensation rule. The activation volume is found to decrease with increasing temperature for dislocation nucleation creep, which agrees well with experimental results. The study opens up an avenue for quantitatively discussing the entropic effects on various thermally activated deformations in nanocrystals.     

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.     

Al-Cu-Cr quasicrystalline coatings prepared by low power plasma spraying

Al-Cu-Cr quasicrystalline coatings were prepared by low power plasma spraying with axially-fed powder systerm. The Al₆₅Cu₂₀Cr₁₅ powders were deposited on AISI 1045 steel substrate at the power ranged from 4.0 to 6.0 kW. The effects of H₂/Ar flow ratio on the phase composition, microstructure and microhardness properties of the as-sprayed coatings were investigated. The XRD results showed that the original powders and as-sprayed coatings contained a predominant icosahedral quasicrystalline phase I-Al₆₅Cu₂₄Cr₁₁ and three minor crystalline phases, including a body-centered cubic α-Al₆₉Cu₁₈Cr₁₃, a monoclinic θ-Al₁₃Cr₂ (i.e. Al₈₃Cu₄Cr₁₃) and a hexagonal ε-Al₂Cu₃. A qualitative analysis on the XRD patterns indicated that the volume fraction of any crystalline phase (α, ε or θ) in the coatings increased, while the quasicrystalline I-phase decreased with increasing hydrogen content in the plasma gas. As H₂/Ar flow ratio increased from 4.8% to 18.8%, the coating hardness increased whilst its porosity decreased, and they reached the maximum (4.98 Gpa) and the minimum (8%) respectively. However, with increasing H₂ content further, the hardness of the coating slightly decreased and its porosity increased because of the excessive vaporization of aluminum at higher plasma energy.     

Adherent nanocrystalline diamond coatings deposited on Ti substrate at moderate temperatures

Microwave plasma enhanced CVD deposition of adherent nanocrystalline diamond coating on pure Ti substrate was studied at a moderate temperature and with a wide range of CH₄ concentrations. Under low CH₄ concentrations, the adhesion failure of diamond coatings is primarily observed at the titanium carbide–substrate interface. Under higher CH₄ concentrations, the diamond coating debonding occurs both at the diamond–carbide interface and carbide–substrate interface. On the whole, the nucleation density, nucleation rate and adhesion strength of diamond coatings grown on Ti substrate are enhanced with increasing CH₄ concentrations. Synchrotron X-ray Laue micro-beam diffraction characterization of the underlying Ti substrate reveals that a microstructure coarsening occurs after hydrogen plasma etching, whereas the hydrogen penetration is effectively mitigated under super high CH₄ concentrations.     

Influence of Temperature on in Nanocrystalline Materials： the Inverse Hall-Petch Effect Phase Field Crystal Simulation

The influence of temperature on the inverse Hall-Petch effect in nanocrystalline(NC) materials is investigated using phase field crystal simulation method.Simulated results indicate that the inverse Hall-Petch effect in NC materials becomes weakened at low temperature.The results also show that the change in microscopic deformation mechanism with temperature variation is the main reason for the weakening of the inverse Hall-Petch effect.At elevated temperature,grain rotation and grain boundary(GB) migration seriously reduce the yield stress so that the NC materials exhibit the inverse Hall-Petch effect.However,at low temperature,both grain rotation and GB migration occur with great difficulty,instead,the dislocations nucleated from the cusp of serrated GBs become active.The lack of grain rotation and GB migration during deformation is mainly responsible for the weakening of the inverse Hall-Petch effect.Furthermore,it is found that since small grain size is favorable for GB migration,the degree of weakening decreases with decreasing average grain size at low temperature.     

A comparative study of magnetron co-sputtered nanocrystalline titanium aluminium and chromium aluminium nitride coatings

A comparative study of magnetron co-sputtered (Ti,Al)N and (Cr,Al)N coatings was made. It was found that while both coatings followed similar development pattern with increasing nitrogen pressure, the (Cr,Al)N coatings achieved much higher deposition rate and hardness, suggesting the coatings had a great potential for many industrial applications.