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SYNTHESIS OF Sn3.5Ag0.5Cu NANOPARTICLE SOLDERS AND SOLDERING MECHANISM
Zhi JIANG,Yanhong TIAN,Su DING
Acta Metall Sin    2016, 52 (1): 105-112.   doi:10.11900/0412.1961.2015.00263
Accepted: 13 November 2015

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Solder has been long playing an important role in the assembly and interconnection of integrated circuit (IC) components on substrates, i.e., ceramic or organic printed circuit boards. The main function of solder is to provide electrical, thermal, and mechanical connections in electronic assemblies. Lead, a major component in Sn/Pb solder, has long been recognized as a health threat to human beings, which is the main reason for the requirement of environmental-friendly lead-free solder. A variety of lead-free solder alloys have been investigated as potential replacements for Sn/Pb solders, but there is still no perfect alternative. Three alloy families, Sn-Ag-Cu, Sn-Ag and Sn-Cu, seem to be of particular interest. However, concerns with this alloy family, including higher soldering temperature, poorer wettability due to their higher surface tension, and their compatibility with existing soldering technology and materials, have impeded their steps in completely replacing Sn/Pb solder. As the melting point can be dramatically decreased when the size of the particles is reduced to nanometer size, especially under 20 nm, and nanosolders have much better wettability at the same time. Furthermore, after heated and cooled, nanomaterials become bulk materials, which make them have the ability to endure a higher function temperature. Thus it is of great significance to conduct in-depth investigation on the synthesis of nanosolders and their soldering performance. In this work, Sn3.5Ag0.5Cu nanoparticles as a promising alternative of Sn/Pb solder was developed. The morphology, atomic structure, phase composition, and element composition of nanoparticles were characterized by SEM, TEM, XRD, and EDS, respectively. Size change of Sn3.5Ag0.5Cu nanoparticles under different sintering temperatures and sintering times was discussed. Microstructure of Cu/nanosolder/Cu sandwich structure under different soldering peak temperatures and soldering times was investigated. Shear strength and failure mode of the Cu/nanosolder/Cu sandwich structure under different pressure were also discussed. The results showed that the average diameter of nanoparticles was less than 10 nm with an agglomeration growth tendency. When sintering temperature was relatively low, the neck size increased steadily as temperature and time increased. In contrast, when sintering temperature was relatively high, the agglomeration mainly happened in the initial process and neck size changed little as the time increased. Thickness of intermetallics of Cu/nanosolder/Cu sandwich structure increased with the soldering temperature increased while the size and quantity of voids decreased. Shear strength of bonded sample increased with the increasing pressure, and got the maximum 14.2 MPa when the pressure reached 10 N.

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EFFECT OF VOID ON NANOINDENTATION PROCESS OF Ni-BASED SINGLE CRYSTAL ALLOY
Biao YANG,Bailin ZHENG,Xingjian HU,Pengfei HE,Zhufeng YUE
Acta Metall Sin    2016, 52 (2): 129-134.   doi:10.11900/0412.1961.2015.00193
Accepted: 06 September 2015

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Nanoindentation of Ni-based single crystal alloy which has a void defect is simulated by the molecular dynamics method. Three models with different voids which have a same radius but different depth (H=1.5 nm, 3.0 nm, 4.5 nm) are contrasted to the perfect model respectively. The influence of a void and misfit dislocation on nanoindentation process are analyzed using center symmetry parameter. Nucleation and growth of dislocation on various indentation depth are researched simultaneously. After relaxation, misfit dislocations occur in all models, which indicates that the void does not affect the generation of misfit dislocation in γ/γ' phase. The indentation load-depth curves show the shallow void (H=1.5 nm) has the greatest influence on nanoindentation. The results demonstrate that the void has two different ways to affect the nanoindentation process. Initially, the void softens the materials when the indentation depth is less than 0.375 nm. However, it will hinder the growth of dislocations because of a kind of surface force, which causes the increase of indentation load while the indentation depth is between 0.375 nm and 0.567 nm. The collapse of a void absorbs the strain energy, so the amount of stacking faults nucleation in γ phase in model with the shallow void is less than which in the perfect model. The indentation load-depth curves show that the indentation load in the H=1.5 nm model is larger than load in the perfect model at 1.263 nm indentation depth. But when the void collapses completely, dislocations tangle around the original location of the void and more stacking faults generate comparing to the perfect model at the same indentation depth h=1.743 nm. So the indentation load declines and becomes smaller than load in perfect model. If the void locates at the interface of γ/γ' phase (H=3.0 nm), it influence the nanoindentation process later than H=1.5 nm model. Dissociation of misfit dislocations is observed when the indentation depth arrives the maximum value 1.748 nm in H=3.0 nm model. Stairs form on the surface of γ phase because of the dissociation of misfit dislocations. There is almost no influence on the nanoindentation of Ni-based single crystal alloy when the void locates in the γ' phase (H=4.5 nm).

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HEAT STABILITY AND SILICONIZING BEHAVIOR OF SURFACE NANOSTRUCTURE OF SILICON STEEL INDUCED BY ASYMMETRIC ROLLING
Gang LIU, Chao LI, Ye MA, Ruijun ZHANG, Yongkai LIU, Yuhui SHA
Acta Metall Sin    2016, 52 (3): 307-312.   doi:10.11900/0412.1961.2015.00233
Abstract52)   HTML6)    PDF (4194KB)(599)      

Heat stability of nanostructure can be related to alloy element, in order to investigate the effect of external element diffusion, asymmetrical rolling was adopted to roll 3% non-oriented silicon steel to realize the surface nanocrystallization, heat-treatment with different parameters was carried out for the rolled sheet in vacuum and Si+1% (mass fraction) halide powder respectively, and different techniques were used to examine the microstructural evolution, phase transformation and Si distribution along the depth. Experimental results show that nanocrystallines about 10~20 nm in size with random orientations form in the top-surface layer after the asymmetrical rolling with the mismatch speed ratio 1.31 and rolling passes 20 for 91% reduction. In the heating process in vacuum, the recrystallization temperature of the nanocrystallines in the top surface layer of the rolled sheet was found to increase obviously comparing with that obtained after keeping at this temperature for a long duration. In the heating process in Si+1% halide powder, a further enhancement of the recrystallization temperature was observed for the nanocrystallines in the top surface layer of the rolled sheet due to the fastly diffusion of Si atoms along the defaults, then the larger volume fraction of grain boundaries can act as fast diffusion channel at higher temperature (750 ℃), that can accelerate the diffusion of Si atoms, therefore dense compound layer can be obtained within shorter duration and with lower fraction of halide (acts as activator).

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PRECIPITATION BEHAVIOR OF NANOMETER-SIZED CARBIDES IN Nb-Mo MICROALLOYED HIGH STRENGH STEEL AND ITS STRENGTHENING MECHANISM
Zhengyan ZHANG,Xinjun SUN,Qilong YONG,Zhaodong LI,Zhenqiang WANG,Guodong WANG
Acta Metall Sin    2016, 52 (4): 410-418.   doi:10.11900/0412.1961.2015.00482
Accepted: 26 January 2016

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Recently, increasing attention has been focused on the high strength low alloy (HSLA) steels mircoalloyed with multiple miroalloying elements, such as Nb-Ti, Nb-V and Ti-Mo, which can form synthetic carbide in steel, such as (Nb, Ti)C, (Nb, V)C and (Ti, Mo)C. Compared with the simplex carbide, such as NbC, TiC, those synthetic carbides with nanometer size exhibiting a superior thermal stability to exert their powerful influence mainly through their precipitation hardening in ferrite. It is reported that the precipitation hardening of approximate 300 MPa which can be obtained in Ti-Mo-bearing steel was developed by JFE steel, attributing to the synthetic (Ti, Mo)C particle precipitated in ferrite. However, as common microalloying elements, Nb and Mo are added synchronously in steel. The strengthening mechanism of Nb-Mo mircoalloyed as-rolled steel and the role of the carbide precipitated in Nb-Mo mircoalloyed as-rolled steel are rarely reported. Therefore, in the present study, the strengthening mechanism, microstructure and the precipitate characteristics of Nb and Nb-Mo microalloyed steels produced by thermo mechanical control process (TMCP) were comparatively investigated by means of SEM, EBSD, HRTEM and physical and chemical phase analysis, in order to systematically study the synergistic effect of Nb-Mo addition on the strength of as-rolled steel. The results shows that the microstructure is finer and the density of low-angle grain boundaries is higher in Nb-Mo microalloyed steel compared with that of in the Nb microalloyed steel. What's more, the Mo addition could increase the precipitation ratio of Nb, and the amount of the MC-type carbide with nanometer size in Nb-Mo microalloyed steel is evidently larger than that of in Nb microalloyed steel. Those MC-type carbide were identified as synthetic carbide (Nb, Mo)C, exhibiting low coarsening rate than that of NbC precipitated in Nb microalloyed steel, which thus contributed to a higher precipitation hardening. This is main reason of the difference in strength between Nb and Nb-Mo microalloyed steel.

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DETWINNING BEHAVIOR INDUCED BY LOCAL SHEAR STRAIN IN NANOTWINNED Cu
Jingsheng BAI,Qiuhong LU,Lei LU
Acta Metall Sin    2016, 52 (4): 491-496.   doi:10.11900/0412.1961.2015.00503
Accepted: 14 December 2015

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Nanotwinned materials have attracted widespread attention due to their superior mechanical properties, such as high strength, good ductility and work hardening. Experimental and molecular dynamics (MD) simulation results had indicated that there are three distinctly different dislocation-mediated deformation mechanisms in nanotwinned metals, namely dislocation pile-up against and slip transfer across twin boundaries (TBs), Shockley partials gliding on twin boundaries leading to twin boundary migration, and threading dislocations slip confined by neighboring twin boundaries. However, most of the previous studies are focused on the homogenous plastic deformation under tension and compression tests, the non-homogenous deformation and its deformation mechanism, especially under low strain and complex stress condition/confined condition, of nanotwinned metals are still not explored so far. In this study, the electrodeposited bulk Cu samples with preferentially oriented nanotwins were cold rolled with the normal of the rolling plane parallel to the growth direction (ND//GD) to strain of 15% at room temperature. The microstructure features of as-rolled Cu were investigated by SEM and TEM. Microstructure evolution indicates that many detwinning bands appeared in the direction about 30°~45° with respect to the rolling direction, which is the direction with the largest shearing stress. The twin lamellae in the detwinning bands coarsened obviously. Based on calculation of the local shear strain and strain gradient of TBs in a selected detwinning band, it indicates that the maximum shear strain occurs in the middle of the deformation bands, and its detwinning mechanism is directly related the localized shear strains (γ). The twin lamellae in the detwinning bands were coarsened obviously. When 0.3<γ<0.8, the detwinning process via producing amount of Shockley dislocations on twin boundaries dominates the deformation. After detwinning, Shockley partial dislocations stored at the area with the maximum strain gradient and formed incoherent twin boundaries (ITBs). The present investigation indicates detwinning process dominates the plastic deformation and sustains the local shearing strain in nanotwinned Cu at small strains under cold rolling.

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MAGNETIC VISCOSITY BEHAVIOR AND EXCHANGE INTERACTION FOR Nd2Fe14B/α-Fe NANOCOMPOSITE PERMANENT ALLOYS
Weidan LI,Xiaohua TAN,Kezhi REN,Jie LIU,Hui XU
Acta Metall Sin    2016, 52 (5): 561-566.   doi:10.11900/0412.1961.2015.00517
Accepted: 29 February 2016

Abstract52)   HTML1)    PDF (747KB)(656)      

A well known feature of ferromagnetic materials is the time dependent behavior of the magnetic polarization, i.e. magnetic viscosity, which arises from thermal activation over energy barriers. It is found that magnetic parameters, such as the fluctuation field (Hf) and the exchange interaction length (lex), have a close relationship with the microstructure of the materials. Therefore, investigation on magnetic viscosity is helpful to understand the coercivity mechanism of ferromagnetic materials. In this work, ingots with nominal composition Nd8.5Fe76Co5Zr3B6.5Dy1, Nd9.5Fe75Co5Zr3B6.5Nb1 and Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 were prepared by arc-melting pure metals Nd, Fe, Co, Zr, Dy, Nb, Ga and Fe-B alloy in an argon atmosphere. A small portion of an ingot weighing about 5 g was re-melted in a quartz nozzle and ejected onto a rotating copper wheel in a range of 10~30 m/s. The annealing treatment was carried out at 690~710 ℃ for 4~5 min. Vibrating sample magnetometer (VSM), XRD and TEM were used to study magnetic viscosity behavior and exchange interaction for Nd2Fe14B/α-Fe nanocomposite permanent alloys. Furthermore, the relationship among exchange interaction, microstructure and magnetic property was discussed. For the nanocomposite Nd8.5Fe76Co5Zr3B6.5Dy1, Nd9.5Fe75Co5Zr3B6.5Nb1 and Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 alloys, Hf and lex were obtaind by sweep rate measurement. The Hf were 4.80, 4.87 and 5.09 kA/m, and lex were 4.53, 4.41 and 4.20 nm for permanent Nd8.5Fe76Co5Zr3B6.5Dy1, Nd9.5Fe75Co5Zr3B6.5Nb1 and Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 alloys, respectively. It suggested that the lex had a minor change. The Nd9.5Fe75Co5Zr3B6.5Nb1 alloy had the strongest exchange interaction among three alloys in this work. It is due to a refined microstructure and uniform distribution of grains. Furthermore, the behavior of the irreversible susceptibility (χirr) as a function of applied magnetic field (H) was investigated. A single sharp peak could be seen near coercive field in the χirr-H curve in three alloys, suggesting that the magnetization reversal was a uniform reversal process. The Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 alloy exhibited a sharper and narrower peak, indicating a more rapid change in magnetization and a strong interaction between adjacent magnetic phases. Since exchange interaction of neighboring grains favors the nucleation of reversed domains, remanence enhancement is generally achieved at the expense of coercivity. Among three alloys, Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 alloy showed the optimum magnetic properties, that is, the coercivity Hc=687.56 kA/m, the remanence Br=0.92 T, the maximum magnetic energy product (BH)max=120.88 kJ/m3. It was mainly due to consisting of well-coupled grains with near perfect alignment of the easy magnetization direction, which improved the remanence and maximum energy product.

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EFFECT OF ELECTROLYTE ADDITIVE CONCEN-TRATION ON MICROSTRUCTURE OF DIRECT-CURRENT ELECTRODEPOSITED NANOTWINNED Cu
Shuai JIN,Zhao CHENG,Qingsong PAN,Lei LU
Acta Metall Sin    2016, 52 (8): 973-979.   doi:10.11900/0412.1961.2015.00536
Accepted: 25 April 2016

Abstract71)   HTML0)    PDF (1218KB)(656)      

Nanotwinned metals have attracted widespread attentions recently, due to their unique overall properties, such as high strength, considerable ductility, enhanced work hardening and high electrical conductivity. The method of synthesized nanotwinned metals is an essential factor for influencing its application. To date, the direct-current electrodeposition technique is successfully used to fabricate bulk nanotwinned Cu samples. However, many parameters, such as the density of current, additive, the concentration of Cu2+, pH and temperature, influence the formation of nanoscale twins during electrodeposited process. To understand the effect of electrolyte additive on the formation of twins, in this work, gelatin with different concentrations was added into the electrolyte while other parameters are kept invariant. Bulk Cu with preferentially oriented nanoscale twins was synthesized in CuSO4 electrolyte with different concentrations of gelatin. The nanotwinned Cu sample is composed of columnar grains with high density nanoscale coherent twin boundaries, most of them are parallel to the growth surface. It is found that the concentration of the electrolyte addition plays an important role in the twin lamellar spacing of the nanotwinned Cu samples, but has little effect on grain size. No twins or twins with micro-sized spacing are detected in electrodeposited Cu without the electrolyte addition. With the concentration of gelatin increasing from 0.5 mg/L to 5 mg/L, the average twin lamellar thickness of the bulk nanotwinned Cu samples decreased from 150 nm to 30 nm. Twin boundaries also grow longer in grains with the increase of gelatin. This is because that with the increase of the concentration of gelatin, the overpotential of cathode increases and nucleation of twins becomes easier, resulting in the reduction of twin spacing.

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CHARACTERIZATION OF NANOSIZED PRECIPITATES IN 9Cr-ODS STEELS BY SAXS AND TEM
Rui XIE,Zheng LU,Chenyang LU,Zhengyuan LI,Xueyong DING,Chunming LIU
Acta Metall Sin    2016, 52 (9): 1053-1062.   doi:10.11900/0412.1961.2016.00164
Accepted: 08 July 2016

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Oxide dispersion strengthened (ODS) steels are the leading candidate structural materials for fast reactor and fusion reactor application due to excellent radiation tolerance and high temperature creep strength. High number density nanoscale oxides play a key role in controlling microstructure and properties. Atomized alloy powders with different ball-milling times were employed to produce 9Cr-ODS steels by hot isostatic pressing (HIP). Nanosized precipitates in 9Cr-ODS steels with different ball-milling times were characterized by synchrotron small angle X-ray scattering (SAXS) together with high resolution transmission electron microscopy (HRTEM). Grain morphology and size were observed by electron backscatter diffraction (EBSD). The effects of nanosized precipitates on grain size and mechanical properties were analyzed. SAXS and TEM results indicated that the size of Y-Ti-O-rich nano-clusters in 9Cr-ODS steels decreases with the increasing milling time, while the distribution density increases. The maximum value of distribution density is about 2.93×1023 m-3 in 9Cr-ODS steel ball milled for 20 h. The maximum value of distribution density of pyrochlore structure Y2Ti2O7 is the highest (1.03×1022 m-3) in 9Cr-ODS steel ball milled for 8 h. Some large-scale Ti-Al-O-rich precipitates are observed and show core/shell structure. Their distribution density increases with ball milling time. With increasing ball milling time, the grain size decreases and the yield strength increases. The contribution of Y-Ti-O-rich nanosized precipitates to yield strength is dominated.

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SIZE EFFECTS ON THE DEFORMATION AND DAMAGEOF Cu-BASED METALLIC NANOLAYEREDMICRO-PILLARS
Jun SUN, Jinyu ZHANG, Kai WU, Gang LIU
Acta Metall Sin    2016, 52 (10): 1249-1258.   doi:10.11900/0412.1961.2016.00315
Accepted: 25 August 2016

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The nanostructured metallic multilayers (NMMs) are widely used as essential components of high performance microelectronics and interconnect structures. The deformation and damage of NMMs is the essential factor leading to the structural failure of these systems. In this paper, based on these experimental results achieved by the authors, as well as the state-of-the-art and progress at home and abroad in the plastic deformation behavior of micropillars of Cu-based NMMs, the correlation of microstructure-size constraint-mechanical performance in the Cu-based nanolayered micropillars is illustrated. The universality of their deformation modes and internal damage mechanisms are revealed, and the work hardening /softening behaviors of two types of nanolaminates, including crystalline/crystalline and crystalline/amorphous NMMs, are summarized. Finally, a brief prospect on the studies of NMMs in future is suggested.

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EFFECT OF HYBRID SURFACE NANOCRYSTALLI-ZATION ON THE ELECTROCHEMICAL CORROSION BEHAVIOR IN 2A14 ALUMINUM ALLOY
Jianhai YANG,Yuxiang ZHANG,Liling GE,Jiazhao CHEN,Xin ZHANG
Acta Metall Sin    2016, 52 (11): 1413-1422.   doi:10.11900/0412.1961.2016.00102
Accepted: 17 August 2016

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In recent years, the surface nanocrystallization (SNC) technology has received extensive attentions in the field of metal material. The shot peening and surface mechanical rolling processing technology can form the gradient nanostructured (GNS) layer on the surface of metal. The material surface roughness is large generally. Therefore, the problem how to form the thick, smooth, flawless GNS layer is need to solve urgently. By means of the hybrid surface nanocrystallization (HSNC) method of both supersonic fine particles bombarding (SFPB) and surface mechanical rolling treatment (SMRT), a gradient nanostructured surface layer was formed on 2A14 aluminum alloy plate. The electrochemical corrosion behavior of the HSNC sample at the air of room temperature and low temperature liquid nitrogen was compared with that of the original sample in aqueous solution of 3.5%NaCl. The results showed that grain size increases from about 30 nm at the surface layer gradually to coarse grain size of the matrix when the sample was processed by HSNC. The total thickness of the plastic deformation layer is about 130 μm. The surface roughness Ra is about 0.6 μm with the surface microcrack disappeared. Compared to the original sample, the pitting corrosion resistance of the SFPB samples was not improved and the pitting corrosion resistance of the HSNC samples was improved. The self-corrosion potential and pitting corrosion potential increase respectively from -1.01228 and -0.29666 V in the original sample to -0.67445 and 0.026760 V at the air room temperature of the HSNC sample. The pitting corrosion resistance of the HSNC sample at the air of room temperature was the biggest. The analysis showed that the surface GNS grain, significant increase of the nanocrystal boundaries, the introduction of compressive residual stress and the decrease of surface roughness were beneficial to improve the pitting corrosion resistance.

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Strain Rate Sensitivity of Cu/Ni and Cu/NbNanoscale Multilayers
Yao WANG,Xiaoying ZHU,Guimin LIU,Jun DU
Acta Metall Sin    2017, 53 (2): 183-191.   doi:10.11900/0412.1961.2016.00358
Accepted: 08 December 2016

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Different from monolayers of same components, nanoscale multilayers have different mechanical properties owing to their relatively high interfacial density, such as extremely high yield strength, high ductility and outstanding wear resistance. Furthermore, their precise modulation period and unique interfacial structures contribute to investigate the plastic deformation mechanism of metal materials. As the plastic deformation behaviors of nanoscale multilayers were reflected in a thermal activation process, strain rate sensitivity index m can be used to characterize the tendency of material strengthening as the strain rate increases. To investigate the impacts of modulation period and interfacial structures upon strain rate sensitivity of nanoscale multilayers, Cu/Ni nanoscale multilayers with different periods (Λ=4 nm, 12 nm, 20 nm) were prepared on Si substrate with e-beam evaporation technologies, while Cu/Nb nanoscale multilayers with different periods (Λ=5 nm, 10 nm, 20 nm) were prepared on Si substrate with magnetron sputtering technologies. Under vacuum conditions, the Cu/Ni nanoscale multilayers of different periods were annealed at 200 and 400 ℃ for 4 h respectively, and the Cu/Nb nanoscale multilayers of different periods were annealed at 200, 400 ℃ and 600 ℃ for 4 h respectively. Microstructures of Cu/Ni and Cu/Nb nanoscale multilayers were characterized with XRD and TEM. Besides, the hardness of nanoscale multilayers was measured by nano-indentation techniques under different loading strain rates (including 0.005, 0.01, 0.05 and 0.2 s-1). The results suggested that strain rate sensitivity was impacted by interfacial structures and grain size. Both increased density of incoherent interfaces and grain size could result in weaker strain rate sensitivity. As the period increases, the density of incoherent interfaces and the grain size of Cu/Ni nanoscale multilayers increased, leading to a decline in the strain rate sensitivity. While for Cu/Nb nano scale multilayers, the density of incoherent interfaces decreased and their grain size was enlarged with longer period, the m value kept unchanged as a result. As the annealing temperature increasing, the strain rate sensitivity of Cu/Ni and Cu/Nb nanoscale multilayers generally tended to decline, which should be ascribed to increased density of incoherent interfaces and grain size in the course of annealing.

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Nanoindentation Creep Behavior of U65Fe30Al5 Amorphous Alloy
Hongyang XU,Haibo KE,Huogen HUANG,Pei ZHANG,Pengguo ZHANG,Tianwei LIU
Acta Metall Sin    2017, 53 (7): 817-823.   doi:10.11900/0412.1961.2016.00322
Accepted: 25 April 2017

Abstract72)   HTML4)    PDF (2482KB)(436)      

Uranium is a valuable nuclear fuel material, but this application is unavoidably handicapped by the easy creep behavior of the metal caused by the combination of stress and irradiation in nuclear reactor. Uranium-based amorphous alloys, as a kind of potential new materials in the nuclear industry, would be challenged by this issue when used in such situation. However, creep properties of these materials have not been reported in the previous studies. In order to preliminarily investigate the creep phenomenon derived from stress function, this work is performed to study the ambient creep behavior of a new amorphous alloy U65Fe30Al5. This alloy was tested by using a nanoindentation technique under different peak loads and loading rates. The results indicate that the creep displacement gradually increases with either the peak load or the loading rate in equal creeping time, but this tendency vanishes when exceeding a critical loading rate. The fitting based on an empirical creep equation reveals that the stress exponent of the alloy ascends when raising the peak load, and firstly declines with the loading rate and then keeps constant above the critical rate. Compared with conventional crystalline alloys, the U-Co-Al alloy shows a larger stress exponent, reflecting the possible existence of rich free volume in the amorphous alloy.

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Dual-Cluster Characteristic and Composition Optimization of Finemet Soft Magnetic Nanocrystalline Alloys
Yaoxiang GENG,Xin LIN,Jianbing QIANG,Yingmin WANG,Chuang DONG
Acta Metall Sin    2017, 53 (7): 833-841.   doi:10.11900/0412.1961.2016.00546
Accepted: 25 April 2017

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The development of nanocrystalline Fe-Si-B-Nb-Cu alloys, commercially known as Finemet, has established a new approach to obtain soft-magnetic materials with high magnetic flux density. The material consists of α-Fe(Si) nanocrystals embedded in an amorphous matrix, which is made by means of partial crystallization. The composition and local structure of the precursor amorphous alloys are crucial for the formation of the unique nanocrystalline structure. The present study is devoted to understanding the composition characteristics and developing new compositions of Finemet alloys. Using the “cluster-plus-glue-atom” model and noticing the crystallization characteristic of Finemet alloy, a “dual-cluster” amorphous structure model is proposed. In this model, the precursor amorphous structure of Finemet alloy is considered to contain a mixture of the [(Si, B)-B2(Fe, Nb)8]Fe cluster derived from the Fe-B-Si-Nb bulk glassy alloys, and the [Si-Fe14](Cu1/13Si12/13)3 cluster from Fe3Si phase. A series of new Finemet nanocrystalline alloy compositions are designed by mixing [(Si, B)-B2(Fe, Nb)8]Fe and [Si-Fe14](Cu1/13Si12/13)3 cluster formulas with a ratio of 1∶1. Thermal analysis results show that [(Si0.8B0.2)-B2Fe7.2Nb0.8]Fe+[Si-Fe14](Cu1/13Si12/13)3 (alloy composition: Fe74B7.33Si15.23Nb2.67Cu0.77) amorphous alloy exhibits a maximal temperature interval of about 192 K between the first and second crystallization peaks. Magnetic measurement results show that the Fe74B7.33Si15.23Nb2.67Cu0.77 nanocrystalline alloy exhibits optimal soft magnetic properties with a saturation magnetization Bs about 1.26 T, a coercive force Hc about 0.5 A/m and an effective permeability μe about 8.5×105 at 1 kHz after isothermal annealing at 813 K for 60 min. The soft magnetic properties of the new composition nanocrystalline alloys are better than that of the typical Finemet nanocrystalline alloy (Fe73.5Si13.5B9Cu1Nb3).

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Effect of Hybrid Surface Nanocrystallization Before Welding on Microstructure and Mechanical Properties of Friction Stir Welded 2A14 Aluminum Alloy Joints
Jianhai YANG,Yuxiang ZHANG,Liling GE,Xiao CHENG,Jiazhao CHEN,Yang GAO
Acta Metall Sin    2017, 53 (7): 842-850.   doi:10.11900/0412.1961.2016.00421
Accepted: 19 April 2017

Abstract67)   HTML0)    PDF (1423KB)(500)      

2A14 aluminum alloy is the important raw materials of aerospace, which belongs to the heat treatment aluminum alloy. Friction stir welding (FSW) can weld aluminum alloy with high quality, and can avoid the pores and cracks of fusion welding effectively. In order to obtain better mechanical properties of FSW joints, the surface nanocrystallization method is introduced into FSW technology. By means of the hybrid surface nanocrystallization (HSNC) method of both supersonic fine particles bombarding (SFPB) and surface mechanical rolling treatment (SMRT), a smooth gradient nanostructured (GNS) layer was formed on the surface of 2A14 aluminum alloy before FSW. The FSW joints microstructure and fracture morphology of the original and HSNC specimens were researched by OM, SEM and TEM. The results showed that nanostructure layer zone (NLZ) was formed when GNS with shape similar to the "S" line was distributed in the thermal-mechanical affected zone (TMAZ) and the nugget zone (NZ) of the HSNC specimen. The lowest micro-hardness and fracture position of the original specimen occurred on the TMAZ of advancing side (AS). The lowest micro-hardness and fracture position of the HSNC specimen occurred on the NZ. The tensile strength of HSNC specimen was 6.4% higher than the original sample. The elongation of HSNC specimen was 14.1% more than the original specimen. The fracture mode of both specimens was toughness fracture. The fracture morphology of the HSNC was isometric dimple when the fracture morphology of original specimen were non-isometric dimple and avulsion dimple. Analysis showed that the NLZ of the FSW joints was beneficial to improving the strength and the plastic deformation capability simultaneously.

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Study on Nano-Crystallization Mechanism and Tribological Performance of Amorphous Carbon-Based Coatings
Dan DONG,Bailing JIANG,Meng GUO,Chao YANG
Acta Metall Sin    2017, 53 (7): 879-887.   doi:10.11900/0412.1961.2016.00436
Accepted: 17 April 2017

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Amorphous carbon coatings mainly composed of sp3 and sp2 bonds have a great potential to be widely used in modern industry for their attractive properties, such as high hardness, high wear resistance and low friction coefficient. However, the high internal stress and poor adhesion of amorphous carbon coatings limit the range of industrial applications. In order to reduce the internal stress and improve the tribological performance, a series of carbon-based coatings with different atomic fraction of Cr were prepared by magnetron sputtering. The microstructure of coatings was characterized by XRD, SEM, TEM, XPS and Raman spectra. The mechanical and tribological properties of coatings were analyzed. The results showed that with the increase of atomic fraction of Cr, the carbon-based coatings changed from amorphous structure to nano-crystalline/amorphous composite structure, the ratio of sp2 bond increased and the ratio of sp3 bond decreased gradually. Also, the hardness and the internal stress showed a decreasing trend with the increase of atomic fraction of Cr. In addition, a small amount of Cr doping could effectively reduce the friction coefficient and specific wear rates of coatings. Excessive Cr doping is beneficial to the increase of the ratio of sp2 bond, however, the dispersion distribution of the metal phase leads to the increase of the friction coefficient and specific wear rates, so that the tribological properties were deteriorated.

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Effect of Interfacial Energy on Distribution of Nanoparticle in the Melt During the Preparation of Fe-Based ODS Alloys by Thermite Reaction
Jianxue LIU, Wenjun XI, Neng LI, Shujie LI
Acta Metall Sin    2017, 53 (8): 1011-1017.   doi:10.11900/0412.1961.2016.00438
Accepted: 17 April 2017

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Fe-based oxide dispersion strengthened (ODS) alloys are conventionally manufactured through mechanical alloying. Such route even involves an expensive milling step but the oxide surface still could not avoid being contaminated. This work developed a new method by combination of thermite reaction and rapid solidification (RS) to prepare ODS alloys. Attributing to the optimization of thermite mixture composition, nanoparticle α-Al2O3 was synthetized in situ and the molten alloy was modulated by spinodal decomposition (SD) into Fe, Cr-rich and Ni, Al-rich regions. During the cooling of the melt, the low interfacial energy between α-Al2O3 and Ni, Al-rich region was also considered in the process for nanoparticles α-Al2O3 to assemble into NiAl, thus they could uniformly distribute in matrix. This work focuses on the thermodynamic analysis of SD in the melt alloy and the speed of the nanoparticles α-Al2O3 under the influence of interfacial energy and Brownian motion. Experiment results shows that the spherical NiAl segregated by SD has a mean diameter of about 50 nm, whose volume fraction reaches up to 50%; and nanoparticle α-Al2O3, formed during thermite reaction, has a diameter of 5 nm combined into NiAl under the influence of interfacial energy. Computation results indicate that, driven by interfacial energy and Brownian motion, nanoparticle α-Al2O3 could move fast enough into Ni, Al-rich region before solidification accomplishes during RS. Test results imply that the tensile strength of Fe-based ODS alloy is 602 MPa with ultimate elongation of 21% and its mass gain under 1000 ℃ in air for 100 h is 0.4 mg/cm2.

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Progress in Thermal Fatigue of Micro/Nano-ScaleMetal Conductors
Guangping ZHANG, Honglei CHEN, Xuemei LUO, Bin ZHANG
Acta Metall Sin    2018, 54 (3): 357-366.   doi:10.11900/0412.1961.2017.00371
Accepted: 22 December 2017

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The world has gradually entered the industrial 4.0 Era, which is dominated by the Internet of Things (IOT) and intelligent manufacturing. Especially, strong requirement for artificial intelligence and big data processing, the development and preparation of micro/nano electronic devices is becoming increasingly active, and much more concerns have been attracted to small-scale materials. Because of the constraint effect of geometric and microstructural dimensions of these materials, the thermal fatigue damage behavior is different from that of the bulk counterparts. At the same time, the change of the material scale from microns to nanometers also results in the transformation of the deformation mechanism, so that the materials exhibit different damage behaviors and significant size effects. In this paper, thermal fatigue testing methods, thermal fatigue damage and evolution, and the factors influencing thermal fatigue properties of metal film/line are reviewed, the corresponding mechanism of thermal fatigue and the size effect of the micro/nano-scale metals are discussed. The prospective research of this field in the future is addressed.

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Effect of Electrolyte Temperature on Microstructures of Direct-Current Electrodeposited Nanotwinned Cu
Zhao CHENG, Shuai JIN, Lei LU
Acta Metall Sin    2018, 54 (3): 428-434.   doi:10.11900/0412.1961.2017.00208
Accepted: 08 November 2017

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Nanotwinned (NT) metals are promising structural materials due to their excellent combination of strength and ductility. These superior properties are strongly dependent on the microstructures i.e. the twin length (grain size), the twin thickness and the twin orientation. Understanding the synthesis process and growth mechanism of NT metals is essential for their structure design. In this work, the effect of electrolyte temperature on the microstructures of highly oriented NT Cu samples, including twin thickness and twin length (grain size), are systematically studied. The NT Cu samples were prepared by means of the direct-current electrodeposition at 293, 298, 303, 308 and 313 K, respectively, while other deposition parameters such as current density, concentration of additive and pH value were kept constant. With decreasing the temperature from 313 K to 293 K, the average grain size decreases from 27.6 μm to 2.8 μm and the average twin thickness decreases from 111 nm to 28 nm, which results in an increment of hardness from 0.7 GPa to 1.5 GPa. This is because with decreasing the temperature, the overpotential of cathode for depositing metal elevates, leading to the nucleation rate of both the grain and twin enhanced.

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In Vitro Corrosion Resistance of Ta2N Nanocrystalline Coating in Simulated Body Fluids
Jiang XU, Xike BAO, Shuyun JIANG
Acta Metall Sin    2018, 54 (3): 443-456.   doi:10.11900/0412.1961.2017.00246
Accepted: 19 December 2017

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Due to its combination of outstanding characteristics, such as superior biocompatibility, excellent mechanical properties as well as good corrosion resistance, Ti-6Al-4V alloy has gained much attention as one of the most popular load-bearing biomedical metals in the area of orthopedic and dental. Unfortunately, Ti-6Al-4V alloy suffers from the localized corrosion damage in human body ?uids containing high chloride ion concentrations, which leads to the release of metal ions into the human body. The released ions (e.g., Al and V) are found to not only cause allergic and toxic reactions but also exhibit potential negative effects on osteoblast behavior. To improve the corrosion resistance of Ti-6Al-4V alloy in simulated body ?uids, a 40 μm thick Ta2N nanocrystalline coating with an average grain size of 12.8 nm was engineered onto a Ti-6Al-4V substrate using a double cathode glow discharge technique. The hardness and elastic modulus of the Ta2N coating were determined to be (32.1±1.6) GPa and (294.8±4.2) GPa, respectively, and the adhesion strength of the coating deposited on Ti-6Al-4V substrate was found to be 56 N. There is no evidence of crack formation within the coating under loads ranging from 0.49 N to 9.8 N, implying that the Ta2N nanocrystalline coating has a high contact damage resistance. Moreover, the corrosion resistance of the Ta2N nanocrystalline coating is significantly greater than that of Ti-6Al-4V alloy when tested in naturally aerated Ringer's solution at 37 ℃. This is due to that the passive film developed on the coating has superior compactness compared with that formed on the uncoated Ti-6Al-4V alloy. XPS analysis indicated that at a low polarized potential, the passive film consisted of TaOxNy, which would be converted to Ta2O5 at a higher polarized potential. The analysis of Mott-Schottky curves suggested that the passive film formed on the coating exhibits n-type semiconductor properties and, as such, the density and diffusivity of carrier for the coating was considerably lower than that for the uncoated Ti-6Al-4V alloy.

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Research of Surface Defects of Polycrystalline Copper Nanoindentation Based on Microstructures
Pengyue ZHAO, Yongbo GUO, Qingshun BAI, Feihu ZHANG
Acta Metall Sin    2018, 54 (7): 1051-1058.   doi:10.11900/0412.1961.2017.00411
Accepted: 08 January 2018

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In the present technology, the manufacture of micro-electro-mechanical system (MEMS) and nano-electro-mechanical system (NEMS) are limited by the lack of mechanism of material processing, especially the mechanism of the polycrystalline materials. In this work, based on the microstructures of polycrystalline copper, the evolution mechanism of dislocations on the polycrystalline copper nanoindentation surface is researched by the four types of microstructures in polycrystalline materials, including grain cell, grain boundary, triple junction and vertex points. In addition, the coordination number, internal stress and atomic potential energy of the dislocations defects are also considered. The results show that when the microstructures with high dimension number carry the compressive stress, the adjacent microstructures with low dimension number appear tensile stress and the microstructures with lower dimension number like vertex points is more likely to appear tensile stress. The dislocation atoms accumulate high internal stress and atomic potential energy during the dislocation nucleation. The internal stress of the imperfect dislocation atoms at the dislocation edge is higher than that of the stacking layer atoms inside the dislocations during the dislocation growth. The process of nucleation and growth, and the internal stress accumulation and release both have similar directionality. They both firstly extended to the microstructures with lower dimension number like vertex points and triple junction, and then expend to and stop at the grain boundary with high dimension number.

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