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.     

Hardness of porous nanocrystalline Co-Ni electrodeposits

The Hall-Petch relationship can fail when the grain size is below a critical value of tens of nanometres. This occurs particularly for coatings having porous surfaces. In this study, electrodeposited nanostructured Co-Ni coatings from four different nickel electroplating baths having grain sizes in the range of 11–23 nm have been investigated. The finest grain size, approximately 11 nm, was obtained from a coating developed from the nickel sulphate bath. The Co-Ni coatings have a mixed face centred cubic and hexagonal close-packed structures with varying surface morphologies and different porosities. A cluster-pore mixture model has been proposed by considering no contribution from pores to the hardness. As the porosity effect was taken into consideration, the calculated pore-free hardness is in agreement with the ordinary Hall-Petch relationship even when the grain size is reduced to 11 nm for the Co-Ni coatings with 77±2 at% cobalt. The present model was applied to other porous nanocrystalline coatings, and the Hall-Petch relationship was maintained.     

A New Model for Inverse Hall-Petch Relation of Nanocrystalline Materials

In the present article, a new model for inverse Hall-Petch relation in nanocrystalline materials has been proposed. It is assumed that lattice distortion along grain boundaries can cause internal stresses and high internal stresses along grain boundaries can promote the grain boundary yielding. The designed model was then verified using the nanocrystalline-copper data. The minimum grain size for inverse Hall-Petch relation is determined to be about 11 nm for Cu.     

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。     

Microstructure evolution and mechanical properties of nanocrystalline FeAl obtained by mechanical alloying and cold consolidation

In the present study high energy mechanical milling followed by cold temperature pressing consolidation has been used to obtain bulk nanocrystalline FeAl alloy. Fully dense disks with homogenous microstructure were obtained and bulk material show grain size of 40 nm. Thermal stability of the bulk material is studied by XRD and DSC techniques. Subsequent annealing at a temperature up to 480 °C for 2 h of the consolidated samples enabled supersaturated Fe(Al) solid solution to precipitate out fine metastable Al₅Fe₂, Al₁₃Fe₄ and Fe₃Al intermetallic phases. Low temperature annealing is responsible for the relaxation of the disordered structure by removing defects initially introduced by severe plastic deformation. Microhardness shows an increase with grain size reduction, as expected from Hall-Petch relationship at least down to a grain size of 74 nm, then a decrease at smallest grain sizes. This could be an indication of some softening for finest nanocrystallites. The peak hardening for the bulk nanocrystalline FeAl is detected after isochronal ageing at 480 °C.     

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.     

Electrodeposition of sol-enhanced nanostructured Ni-TiO2 composite coatings

A novel electroplating method has been developed to produce nanocrystalline metal-matrix nano-structured composite coatings. A small amount of transparent TiO₂ sol was added into the traditional electroplating Ni solution, leading to the formation of nanocrystalline Ni-TiO₂ composite coatings. These coatings have a smooth surface. The Ni nodules changed from traditional pyramid-like shape to spherical shape. The grain size of Ni was also significantly reduced to the level of 50 nm. It was found that the amorphous anatase TiO₂ nano-particles (∼ 10 nm) were highly dispersed in the coating matrix. The microhardness was significantly increased from 320 HV₁₀₀ of the traditional Ni coating to 430 HV₁₀₀ of the novel composite coating with 3.26 wt.% TiO₂. Correspondingly, the wear resistance of the composite coating was improved by ∼ 50%.     

Ballistic performance of nanocrystalline and nanotwinned ultrafine crystal steel

Because of their remarkable mechanical properties, nanocrystalline metals have been the focus of much research in recent years. Refining their grain size to the nanometer range (<100 nm) effectively reduces their dislocation mobility, thus achieving very high yield strength and surface hardness—as predicted by the Hall-Petch relation—as well as higher strain-rate sensitivity. Recent works have additionally suggested that nanocrystalline metals exhibit an even higher compressive strength under shock loading. However, the increase in strength of these materials is generally accompanied by an important reduction in ductility. As an alternative, efforts have been focused on ultrafine crystals, i.e. polycrystals with a grain size in the range of 500 nm to 1 μm, in which “growth twins” (twins introduced inside the grain before deformation) act as barriers against dislocation movement, thus increasing the strength in a similar way as nanocrystals but without significant loss of ductility. Due to their outstanding mechanical properties, both nanocrystalline and nanotwinned ultrafine crystalline steels appear to be relevant candidates for ballistic protection. The aim of the present work is to compare their ballistic performance against coarse-grained steel, as well as to identify the effect of the hybridization with a carbon fiber-epoxy composite layer. Hybridization is proposed as a way to improve the nanocrystalline brittle properties in a similar way as is done with ceramics in other protection systems. The experimental campaign is finally complemented by numerical simulations to help identify some of the intrinsic deformation mechanisms not observable experimentally. As a conclusion, nanocrystalline and nanotwinned ultrafine crystals show a lower energy absorption than coarse-grained steel under ballistic loading, but under equal impact conditions with no penetration, deformation in the impact direction is smaller by nearly 40%. This a priori surprising difference in the energy absorption is rationalized by the more important local contribution of the deviatoric stress vs. volumetric stress under impact than under uniaxial deformation. Ultimately, the deformation advantage could be exploited in the future for personal protection systems where a small deformation under impact is of key importance.     

Effect of grain size on corrosion of nanocrystalline copper in NaOH solution

Effect of grain size on corrosion of bulk nanocrystalline copper was investigated using potentiodynamic polarization measurements in 0.1 M NaOH solution. Bulk nanocrystalline copper was prepared by inert gas condensation and in situ warm compress (IGCWC) method. The grain sizes of all bulk nanocrystalline samples were determined to be 48, 68 and 92 nm using X-ray diffraction (XRD). Results showed that bulk coppers displayed an active-passive-transpassive behaviour with duplex passive films. From polycrystalline to nanocrystalline, grain size variation showed little effect on the overall corrosion resistance of copper samples.     

Ion beam assisted deposition and characterization of Zr-based multilayered coatings

ZrN/W multilayered coatings with different nanoscale modulation periods have been synthesized at different deposition time using ion beam assisted deposition. XRD, AES, Nanoindenter and profiler were employed to investigate the influence of modulation period on microstructure and mechanical properties of the coatings. The results showed that all superlattice coatings almost revealed higher mechanical property than the monolithic ZrN and W coatings. At modulation period of 8.6 nm, XRD pattern showed a significant mixture of strong ZrN (111), W (110), as well as weak ZrN (220) textures. It possessed the highest hardness (∼ 26 GPa), elastic modulus (∼ 310 GPa), and fracture resistance (∼ 80 mN), compared with the ones with other modulation period.     

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.     

Ion beam assisted deposition of TiN-Ni nanocomposite coatings

Hard and tough nanocomposite coatings consisting of hard TiN nanograins embedded in a soft metallic intergranular phase of Ni have been produced using ion beam assisted deposition. The chemical composition has been obtained by Rutherford Backscattering and the microstructural properties: phases, grain size, and texture of the coatings have been investigated by X-Ray Diffraction. In the composition range 0-22.5 at.% Ni, δ-TiN is the only crystalline phase and Ni appears as an X Ray amorphous phase. The hardness increases up to a maximum of 41 GPa at ~ 7 at.% Ni which corresponds to a TiN crystallite size of ~ 8 nm and a Ni intergranular phase thickness of roughly 1 monolayer. It is shown that the hardness enhancement in TiN-Ni nanocomposite coatings is not correlated with residual stresses, but rather with the intrinsic properties of the nanostructure. An important improvement in wear resistance is obtained for the coatings exhibiting the highest toughness and not the highest hardness. These results show that ion assisted processing is an effective tool for producing dense TiN-Ni nanocomposite coatings and tailoring their structure and mechanical properties.     

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.     

Influence of substrate bias, deposition temperature and post-deposition annealing on the structure and properties of multi-principal-component (AlCrMoSiTi)N coatings

Nitride films are deposited from a single equiatomic AlCrMoSiTi target by reactive DC magnetron sputtering. The influence of the substrate bias and deposition temperature on the coating structure and properties are investigated. The bias is varied from 0 to − 200 V while maintaining a substrate temperature of 573 K. And the temperature is changed from 300 to 773 K whilst maintaining a substrate bias of − 100 V. From X-ray diffraction analysis, it is found that all the as-deposited coatings are of a single phase with NaCl-type FCC structure. This is attributed to the high mixing entropy of AlN, CrN, MoN, SiN, and TiN, and the limited diffusion kinetics during coating growth. Specific aspects of the coating, namely the grain size, lattice constant and compressive stress, are seen to be influenced more by substrate bias than deposition temperature. In fact, it is possible to classify the deposited films as large grained (~ 15 nm) with a reduced lattice constant (~ 4.15 Å) and low compressive residual stresses for lower applied substrate biases, and as small grained (~ 4 nm) with an increased lattice constant (~ 4.25 Å) and high compressive residual stresses for applied biases of − 100 V or more. A good correlation between the residual stress and lattice constant under various deposition conditions is found. For the coatings deposited at − 100 V, and at temperatures above 573 K, the hardness could attain to the range of 32 to 35 GPa.Even after annealing in vacuum at 1173 K for 5 h, there is no notable change in the as-deposited phase, grain size or lattice constant of the coatings but an increase in hardness. The thermal stability of microstructure is considered to be a result of the high mixing entropy and sluggish diffusion of these multi-component coatings. For the anneal hardening it is proposed that the overall bonding between target elements and nitrogen is enhanced by thermal energy during annealing.     

Bulk Mg-3Al-Zn alloy with ultrafine grain size produced by powder metallurgy

Ultrafine-grained Mg-3Al-Zn alloys with an average grain size of 180 nm have been made by powder metallurgy. First, the nanocrystalline powders with mean grain size of 45 nm were produced by ball milling under argon atmosphere, and then through vacuum hot pressing at 633 K for 40 min and warm extrusion at 373 K, bulk solid samples were compacted successfully from the mechanically milled powders, and the relative density of the samples was about 98.87% (1.8003 g/cm³). XRD, SEM and TEM analysis showed that the microstructure of the samples consists of homogeneous equiaxed grains and grain growth has taken place during the consolidation process.     

Spark plasma sintering consolidation of nanostructured TiC prepared by mechanical alloying

Spark plasma sintering technique was used for the consolidation of nanostructured titanium carbide synthesized by mechanical alloying in order to avoid any important grain growth of the compact materials. The TiC phase was obtained after about 2 h of mechanical alloying. Towards the end of the milling process (20 h), the nanocrystalline powders reached a critical size value of less than 5 nm. Some physical and mechanical properties of the consolidated carbide were reported as a function of the starting grain size powders obtained after different mechanical alloying durations. The crystalline grain size of the bulk samples was found to be increased to a maximum of 120 nm and 91 nm for carbides mechanically alloyed for 2 h and 20 h respectively. The Vickers hardness showed to be improved to about 2700 Hv for a maximum density of 95.1% of the bulk material.     

Pitting behavior of a bulk Ni-18 wt.% Fe nanocrystalline alloy

Three electrodeposited Ni-18 wt.% Fe samples were annealed at 400 °C for 3 h (hrs), 8 h, and 24 h to study the effects of grain size on the electrochemical properties of bulk Ni-18 wt.% Fe in 3.5 wt.% NaCl. The electrochemical results from the annealed samples are compared with those measured for the as-received Ni-18 wt.% Fe material consisting of an average grain size of 23 nanometers (nm). Of the four materials studied, the as-received nanocrystalline alloy less sensitive to localized corrosion.     

Combinatorial approach to the growth of α-(Al1 − x,Crx)2O3 solid solution strengthened thin films by reactive r.f. magnetron sputtering

The development of new coatings with superior functionalities for high performance cutting tools is a key challenge in manufacturing. In this context, the synthesis of aluminium oxide and derivative oxide thin films is attracting large scientific and technical interests. The present paper addresses fundamental materials science-based aspects of the physical vapour deposition (PVD) growth of Al-Cr-O thin films at a substrate temperature of 500 °C. A combinatorial experimental approach was chosen to describe the growth and microstructure evolution of Al-Cr-O thin films by means of reactive r.f. magnetron sputtering. A segmented target consisting of two half plates of Al and Cr was used for the deposition carried out under stationary conditions in a laboratory-scale PVD coater. Opposite to the cathode five substrate samples were placed in a line. The r.f. cathode power was set to 500 W and the r.f. substrate bias was set to − 100 V. The total gas pressure was kept constant at 0.4 Pa for all experiments with a fixed ratio of oxygen to argon gas flow. Detailed results on the coatings composition, constitution, microstructure and properties as a function of the elemental composition are presented. X-Ray Diffraction (XRD), X-Ray Reflection (XRR), Transmission Electron Microscopy (TEM) and Electron Probe Microanalysis (EPMA) studies prove the growth of nanocrystalline, stoichiometric, metastable corundum-like solid solution strengthened α-(Al_1 − x,Cr_x)₂O₃ thin films with a high degree of crystallinity, grain sizes between 27 ± 6 nm (in the case of Al-rich coatings) and 44 ± 17 nm (in the case of Cr-rich coatings), Vickers micro hardness values up to 2620 ± 80 HV0.05 and thin film densities between 4.00 g/cm³ (in the case of Al-rich coatings) and 4.86 g/cm³ (in the case of Cr-rich coatings).     

Structural and Mechanical Properties of Zr-Si-N Coatings Deposited by Arc Evaporation at Different Substrate Bias Voltages

ZrN and Zr-Si-N coatings were formed using vacuum-arc plasma fluxes deposition system at the substrate bias voltage (U_B) ranged from ??50 to ??220 V on HS6-5-2 steel substrates. The structural, mechanical and tribological properties were characterized using x-ray diffraction, atomic force microscopy, scanning electron microscopy, optical microscopy, nanoindentation and ball-on-disk test. The surface roughness parameter Ra of ZrN coatings is lower than Zr-Si-N coatings. Both roughness Ra of Zr-Si-N coatings and the number of surface defects with mainly small dimensions to 1 µm decrease with increasing negative substrate bias voltage. The addition of silicon to ZrN significantly reduces the crystallite size, from about 18.3 nm for ZrN coating to 6.4 nm for Zr-Si-N coating both deposited at the same U_B?=???100 V and 7.8 nm for U_B?=???150 V. The hardness of Zr-Si-N coatings increases to about 30 GPa with the increase in negative substrate bias voltage (U_B?=???220 V). Adhesion of the coatings tested is high, and critical load is above 80 N and reduces with U_B increase. Coefficient of friction determined using AFM shows similar trend as surface roughness in microscale.     

Characterization of nitride coatings on high density graphite deposited by magnetron sputtering

Cathode deposit consolidation operation after electrorefining of spent metallic fuels of fast breeder reactors involves melting of U and Pu at 1300 °C after distillation of occluded chloride salt and Cd, in graphite crucibles. Nitride coatings possessing high hardness, melting point and thermodynamic stability against reactive materials and molten LiCl-KCl salts have greater potential for coating the graphite crucibles. In the present study nanocrystalline TiN, ZrN and Ti-Si-N coatings were deposited on high density graphite disc and crucible samples by DC/RF magnetron sputtering. The coated samples were characterized by SEM, GIXRD and AFM. The results indicated that coatings with uniform thickness of 3 to 6 µm were deposited on high density graphite which adheres well to the substrate. The surface morphology of TiN, ZrN and Ti-Si-N coatings examined by SEM and AFM showed the presence of spherical nanoparticles of nitrides getting agglomerated into clusters. Characterisation of nitride coated crucibles was carried out before and after uranium melting by induction heating to simulate cathode processor crucible conditions. TiN and Ti-Si-N coating appears to offer better stability, ease of ingot release and coating adhesion. The paper highlights the results of the present investigation.