ISSN 0412-1961
CN 21-1139/TG
Started in 1956

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The effect of aluminum on the solidification microstructure of M2 high speed steel, particularly the morphology and microstructure of eutectic carbides, has been investigated by OM, TEM, SEM, EBSD and XRD. The results show that the as-cast microstructure consists of dislocation martensite and M2C eu. . .
Acta Metall, 2014 Vol. 50 (7): 769-776    DOI: 10.3724/SP.J.1037.2013.00621
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Protection Mechanism Study of Enamel-based Composite Coatings under the Simulated Combusting Gas Shock PDF (1007KB)
Out-of-Plane Constraint Effect on the Fracture Toughness of Single Edge Notch Tension Specimens PDF (828KB)
Effect of Cold Rotary-swaging Deformation on Microstructure and Tensile Properties of TB9 Titanium Alloy PDF (1094KB)
Effect of manganese content on tensile deformation behavior of Fe-Mn-C TWIP steels PDF (968KB)
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      11 July 2018, Volume 54 Issue 7 Previous Issue   
    Suppression Mechanism and Method of Vortex During Steel Teeming Process in Ladle
    Qiang WANG, Lianyu WANG, Hongxia LI, Jiawei JIANG, Xiaowei ZHU, Zhancheng GUO, Jicheng HE
    Acta Metall. 2018, 54 (7): 959-968.   DOI: 10.11900/0412.1961.2017.00365
    Abstract   HTML   PDF (2302KB)

    With the developments of science and technology, the performance of steel is required strictly and the quality of steel needs to improve continuously. In continuous casting process, there is an important flow phenomenon when the molten steel flows from ladle to tundish. It is that the rapidly rotating free surface vortex will form as the liquid level descending continuously. The surface vortex can cause the slag entrainment. In order to suppress slag entrainment by vortex during the steel teeming process, and to improve the cleanliness and quality of steel, the movement process of vortex and variation of flow field are studied through both numerical simulation and water model experiments. Since the eccentricity (eccentricity is the ratio of the nozzle distance and the ladle radius) has a large effect on the vortex formation, the vortex movement and flow field variation at different eccentricities are analyzed in details. And the mechanism of vortex suppression is found. It is that disturbing the velocity distribution of vortex formation or/and decreasing the tangential velocity value can suppress the movement and development of vortex. Thus the critical height of vortex can be decrease and the vortex can be availably suppressed. And then a method of vortex suppression is proposed according to this mechanism of vortex suppression. The method by blowing gas at the bottom of ladle is proposed. And the optimal gas flow rate and gas nozzle position are obtained.

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    Study on Formation Mechanism of Necklace Structure in Discontinuous Dynamic Recrystallization of Incoloy 028
    Xiting ZHONG, Lei WANG, Feng LIU
    Acta Metall. 2018, 54 (7): 969-980.   DOI: 10.11900/0412.1961.2017.00461
    Abstract   HTML   PDF (17532KB)

    During hot deformation, discontinuous dynamic recrystallization (DDRX) taking place by nucleation and growth in materials with low to medium stacking fault energies (SFEs), plays a crucial role in grain refinement, especially for the material with coarse grains. In order to study the formation mechanism of typical microstructure (necklace structure) during DDRX, the behavior of Incoloy 028 alloy at temperature range of 1000~1150 ℃ and the strain rates of 0.001~1 s-1 was investigated by means of thermodynamic simulation, EBSD and TEM. The results show that with the decrease of deformation temperature or the increase of strain rate, the mechanism of DDRX is transformed from the traditional type nucleating at triple junctions, into necklace structure which dominated by the multilayer nucleation mechanism. The first strand of recrystallized grain is nucleated through the bulging of serrated grain boundaries which is assisted by twinning at the back of the fluctuation. With the increase of the true strain, the large strain gradient in the deformation band develops rapidly resulting in the transformation of the subgrain boundary into a high angle grain boundary, and then the second/followed layer nucleation occurs by the rotation of subboundaries accompanied with nucleation at triple junction. Twin boundaries are formed by strain-induced grain boundaries migration and disappeared after nucleation to enhance the recrystallization grain boundary mobility, and then formed again during growth to lower the interfacial energy of the system.

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    Effect of High-Temperature Ageing on Microstructure and Mechanical Properties of Linear Friction Welded S31042 Steel Joint
    Yanmo LI, Chenxi LIU, Liming YU, Huijun LI, Zumin WANG, Yongchang LIU, Wenya LI
    Acta Metall. 2018, 54 (7): 981-990.   DOI: 10.11900/0412.1961.2017.00483
    Abstract   HTML   PDF (19737KB)

    S31042 steels with 25%Cr (mass fraction) and 20%Ni have been served as super-heaters and re-heaters in ultra-super critical (USC) plants, owing to their outstanding corrosion resistance and creep rupture strength. And the reliability of joints at high temperature has attracted much attention since the S31042 steels have been joined successfully by linear friction welding. In this work, the microstructures and mechanical properties of linear friction welded S31042 steel joint subjected to ageing treatment were investigated by using OM, SEM, TEM and mechanical test at 700 ℃. The recrystallized grains and nanoscale NbCrN particles have been stable during the high-temperature ageing, and the joint exhibited excellent performance due to the grain refinement strengthening and precipitation strengthening. The average size of M23C6 phase in weld zone, thermo-mechanically affected zone and heat affected zone increased with the ageing time. After ageing treatment at 700 ℃ for 500 h, σ phase precipitated at boundary junctions in thermo-mechanically affected zone. The average size of σ phase increased with the ageing time, as well as the volume fraction of the σ-phase. With the formation of σ phase, the fracture site of joints shifted from the parent material to the areas adjacent to the weld zone, and the high-temperature mechanical properties of joints were sharply decreased.

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    Cracking Behavior of Sol-Gel Derived YAlO3/Ti2AlC Composite Coatings During Drying and Pyrolysis
    Chao FAN, Qing JIA, Yuyou CUI, Rui YANG
    Acta Metall. 2018, 54 (7): 991-998.   DOI: 10.11900/0412.1961.2017.00325
    Abstract   HTML   PDF (6060KB)

    Sol-gel derived YAlO3/MAX composite coatings were designed as protective coatings for γ-TiAl base intermetallic compounds which exhibit insufficient oxidation resistance at temperatures above 800 ℃. However, at present, it's still a big challenge to achieve crack-free surfaces while preparing YAlO3/MAX composite coatings via sol-gel processing, especially during drying and low temperature heat treatment. Hence, cracking behavior of YAlO3/Ti2AlC composite coatings, which were derived from nanoparticles-gel system, was studied in this work by means of in situ techniques such as high-temperature optical microscopy (HTOM). According to this work, cracking of YAlO3/Ti2AlC composite coatings during drying and pyrolysis mainly occurred in stage 3, i.e., the pyrolysis stage of slurry, in which the maximum stress that coating system can tolerate decreased gradually as a result of pyrolysis of the gel network and was eventually exceeded by the increasing internal stresses generated owing to heating and volume change of coating system. Coating thickness, which varied in the plane of coatings and was affected by the difference of drying rate during stage 1, was a critical factor that determined the positions where cracks may be initiated. It was observed that cracks were more easily formed on those sites with thicker coatings, where often produced great stress concentration. Both crack width and spacing can be decreased by applying fast heating rate, since large-scale non-homogeneous distribution of internal stress concentration in coatings was reduced in this way and cracking behavior of coatings was consequently confined into very small region. In this work, a heating rate of 5 ℃/min was the best choice to obtain YAlO3/Ti2AlC composite coatings with acceptable surface quality.

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    Influence of Layer Thickness on Microstructure and Mechanical Properties of Selective Laser Melted Ti-5Al-2.5Sn Alloy
    Piao GAO, Kaiwen WEI, Hanchen YU, Jingjing YANG, Zemin WANG, Xiaoyan ZENG
    Acta Metall. 2018, 54 (7): 999-1009.   DOI: 10.11900/0412.1961.2017.00384
    Abstract   HTML   PDF (6969KB)

    As an additive manufacturing technology, selective laser melting (SLM) process can solve the manufacturing difficulty of Ti-5Al-2.5Sn (TA7) easily. But the low building efficiency of SLM retards its wide applications in aviation, petrochemical and other fields. In order to solve the above problem, the influence of layer thickness on relative density, microstructure and mechanical properties of SLMed TA7 samples were studied in this work. The results show that when the laser power and hatching space are constant, the relative density gradually increases with the decrease of the laser volume energy density under the layer thicknesses less than or equal to 40 μm, whereas first increases and then declines with the decrease of the laser volume energy density under the layer thicknesses larger than 40 μm. At the same time, with the increase of layer thickness and the decrease of scanning velocity, the cooling rate gradually decreases during the SLM processing, when the cooling rate is lower than 6.8×107 K/s, the microstructure will gradually transform from acicular martensite α' to massive αm. Through the optimization of SLM parameters, the dense TA7 bulk specimens with higher microhardnesses, yield strengths and ultimate strengths in comparison to the as-cast and deformed TA7 alloys can be obtained under all layer thicknesses (20~60 μm). While when the layer thicknesses are not larger than 40 μm, the ductility of the SLMed TA7 is also superior to that of the as-cast TA7 and comparable to that of the deformed TA7. Finally, the optimal layer thickness and combination of SLM process parameters are successfully determined to balance the building efficiency, metallurgical quality and mechanical properties of the TA7 alloy parts.

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    Effect of Crevice Corrosion on the Degradation Rate ofFe-30Mn-1C Alloy
    Zheng MA, Xi LU, Ming GAO, Lili TAN, Ke YANG
    Acta Metall. 2018, 54 (7): 1010-1018.   DOI: 10.11900/0412.1961.2017.00451
    Abstract   HTML   PDF (10067KB)

    Fe-30Mn-1C alloy has great potential to become degradable cardiovascular stent material due to its degradability, excellent comprehensive mechanical properties and biocompatibility. In this work, in order to improve the degradation rate of Fe-30Mn-1C alloy, laser technology was used to process pores with different pore diameters on the samples, and the design of the scaffold was combined with crevice corrosion. The degradation behavior of the alloy was studied through in vitro soaking weight loss experiments and electrochemical tests. The results showed that crevice corrosion can increase the degradation rate of Fe-30Mn-1C alloy significantly.

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    Corrosion Behavior of Fe-Based Laser Cladding Coating in Hydrochloric Acid Solutions
    Li FAN, Haiyan CHEN, Yaohua DONG, Xueying LI, Lihua DONG, Yansheng YIN
    Acta Metall. 2018, 54 (7): 1019-1030.   DOI: 10.11900/0412.1961.2017.00437
    Abstract   HTML   PDF (9560KB)

    30CrMo alloy steel has a wide range of applications in the petrochemical industry such as the valve bodies and valve covers of subsea Christmas tree, and oil drilling pipes that working in strong acid environment. Therefore, the methods to improve the corrosion resistance of 30CrMo steel by surface modification techniques have become a hot topic of research. Laser cladding Fe-based coatings are regarded as promising materials, because of their high bonding strength, good hardness and excellent wear and corrosion resistance, and they might replace more expensive Co-based or Ni-based alloys. Additions of Cr, Mo, Y, Co and Ni are benefit to improve the corrosion resistance of Fe-based coatings. However, Cr, Y, Co and Mo are expensive. With consideration of reducing the materials cost, and at the same time maintaining the excellent corrosion resistance, a novel Fe-based alloy without, Y, Co and minor Mo content is synthesized. Therefore, in this study, to improve the corrosion resistance of 30CrMo alloy, the novel synthesized Fe-based powder was prepared on the surface by laser cladding. The microstructure, chemical and phase compositions of the fabricated coating were measured systemically by using a SEM equipment with EDS spectrometer, and XRD. The corrosion behavior of this Fe-based coating in 0.5 mol/L HCl solution were studied by polarization curve and EIS measurements, combined with immersion tests. The passive film formed on the surface of the alloy after immersion in the 0.5 mol/L HCl solution for 3 d was analyzed by XPS. The microstructure is mainly composed of dendrites and interdendritic phases, which are confirmed as austenite γ-Fe phase and the eutectics γ-Fe/M23C6. Similar to 304 stainless steel, the Fe-based alloy coating with a very broad passive region, shows positive corrosion potential and less corrosion current density than that of 30CrMo alloy steel. This indicates that the corrosion resistance of the Fe-based coating is superior to 30CrMo alloy steel, and almost the same as 304 stainless steel. The immersion tests show that the corrosion mechanisms of the coating are the combination of anodic dissolution and passive film protection. As for the eutectic region rich in Cr and Mo, the destruction and corrosion of this area in HCl solution are slowed down due to the passivation of Cr and Mo. The passive film is mainly composed of Cr2O3, FeCr2O4 and MoO3. The main reason for the excellent corrosion resistance of the coating is the mechanical barrier effect of the passivation effect of the high density composite oxide film.

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    Hydrogen Embrittlement of Intercritically AnnealedCold-Rolled 0.1C-5Mn Steel
    Xiaoli ZHAO, Yongjian ZHANG, Chengwei SHAO, Weijun HUI, Han DONG
    Acta Metall. 2018, 54 (7): 1031-1041.   DOI: 10.11900/0412.1961.2017.00435
    Abstract   HTML   PDF (8981KB)

    Medium-Mn steel typically alloyed with (3%~10%)Mn (mass fraction) has recently regained significant interest as one of the most promising candidates for the third-generation automobile steel due to its excellent combination of ultra-high strength and ductility as well as relatively low material cost and industrial feasibility. Considering the ever increasing strength level as well as the comparatively high amount of reverted austenite (RA) of medium-Mn steel, special attention began to be given to its hydrogen embrittlement (HE) behavior for ensuring the safety service of components made of this kind of steel. However, the effect of RA on HE of medium-Mn steel has not been fully understood. For this purpose, the susceptibility to HE of a cold-rolled medium-Mn steel 0.1C-5Mn intercritically annealed at 650 ℃ for different time to obtain different amounts of RA was investigated by using electrochemical hydrogen charging, thermal desorption spectrometry (TDS), slow strain rate test (SSRT) and SEM. The results show that the annealed samples exhibit a dual-phase microstructure of reverted globular shaped RA and ferrite. The ultimate tensile strength (σb) increases while the yield strength decreases with increasing annealing time, and both the total elongation (δ) and the product of σb to δ (σb×δ) initially increase and then decrease with increasing annealing time. That is to say, an excellent combination of strength and ductility could be obtained when the tested steel was annealed at 650 ℃ for 10 min. However, the results of TDS and SSRT show that both the absorbed diffusible hydrogen concentration and the susceptibility to HE increase with increasing annealing time, and the latter is more significant. SEM analysis of the fracture surfaces of fractured samples revealed that the hydrogen-charged annealed sample was fractured to leave both dimples filled with grains and empty dimples while the uncharged annealed specimen was ductile fractured to leave only empty dimples. The dimples filled with grains were basically a brittle intergranular cracking occurring along the boundaries of RA and/or martensite (formerly RA) grains by the hydrogen-assisted cracking mechanism. It is thus concluded that the HE behavior of intercritically annealed cold-rolled medium-Mn steel is primarily controlled by both the amount and mechanical stability of RA.

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    Investigation on the Change of Thermoelectric Power of Al-Fe Hypoeutectic Alloy Melt Caused by AC Magnetic Field
    Jianfeng ZHANG, Qing LAN, Qichi LE
    Acta Metall. 2018, 54 (7): 1042-1050.   DOI: 10.11900/0412.1961.2017.00421
    Abstract   HTML   PDF (4778KB)

    A lot of studies have shown that electromagnetic field can significantly refine the metal solidification structure, thus improve the deformation properties and functional performance of metallic materials. However, the mechanism of how electromagnetic field affects melt structure remains unclear, so an intensive study of the effects of electromagnetic field on melt structure is very important for an in-depth understanding of the essence of melt solidification under external electromagnetic field. The effect of alternating current (AC) magnetic field with different exciting currents, magnetic frequencies and loading time on the thermoelectric potential difference (U) and melt microstructure of Al-0.99%Fe (mass fraction) hypoeutectic alloy at different temperatures was investigated in this work. The results showed that AC magnetic field would lead to a decrease in U of liquid Al-0.99%Fe hypoeutectic alloy. When the magnetic field was removed, the decreased thermoelectric potential difference experienced a rapid recovery process and a poor recovery process to increase to the initial value. The influence of AC magnetic field on U was different at different temperatures. With the increase of the magnetic frequency, the influence of AC magnetic field on U decreased. And the influence of AC magnetic field on U increased with the increase of the exciting current and loading time, however, there was a saturated loading time. There was a correlation between the change of U of Al-0.99%Fe hypoeutectic alloy and the change of size of the primary α-Al phase caused by AC magnetic field, therefore, the change of thermoelectric potential difference could be used to characterize the effect of AC magnetic field on the microstructure of the alloy melt of Al-0.99%Fe hypoeutectic 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. 2018, 54 (7): 1051-1058.   DOI: 10.11900/0412.1961.2017.00411
    Abstract   HTML   PDF (5128KB)

    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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    Formation of Second-Phases in a Direct-Chill Casting Al-12Si-0.65Mg-xMn Alloy
    Guangdong WANG, Ni TIAN, Changshu HE, Gang ZHAO, Liang ZUO
    Acta Metall. 2018, 54 (7): 1059-1067.   DOI: 10.11900/0412.1961.2017.00475
    Abstract   HTML   PDF (13152KB)

    Mg-containing high Si aluminum alloy that can be heat treatment enhanced is widely used in the fields of engine, vehicle industry and aerospace, because of its high specific strength, high wear resistance, corrosion resistance and low thermal expansion coefficient. At present, the alloying to improve the microstructure of Mg-containing high Si aluminum alloy and improve its mechanical properties is an important research hotspot of this kind of alloy. As an important alloying element in aluminum alloy, Manganese is of great significance to study the type and formation process of Mn-containing second phase in Mg-containing high Si aluminum alloy. The second phases and their formation in a direct-chill casting Al-12Si-0.65Mg-(0~2.27)Mn (mass fraction, %) alloy were investigated by LSCM, XRD, SEM/EDS and TEM/EDS, combined with phase graph analysis. The results show that there are eutectic silicon, Mg2Si and π-(Al8Mg3FeSi6) besides matrix α-Al in the Mn-free Al-12Si-0.65Mg (mass fraction, %) alloy ingot, which are formed by the reactions of L+Al5FeSi→α-Al+Si+Al8Mg3FeSi6, L→α-Al+Si+Mg2Si and L→α-Al+Si+Mg2Si+Al8Mg3FeSi6 at 567, 555 and 550~554 ℃, respectively. The α-Al dendrites are obviously refined, and α-Al(FeMn)Si phase can be observed with the addition of Mn to Al-12Si-0.65Mg-(0.10~2.27)Mn (mass fraction, %) alloy ingot. With the Mn content increasing from 0.10% to 2.27%, the morphology of α-Al dendrites has no obvious change, and the number of α-Al(FeMn)Si increases gradually whereas the size of α-Al(FeMn)Si doesn't change much. There are some Al9(FeMn)4Si3 with the size of about 80 μm in the Al-12Si-0.65Mg-(1.07~2.27)Mn (mass fraction, %) alloy ingot with the Mn content over 1.07%, which are formed by the reaction of L+Al6Mn→α-Al+Al9Mn4Si3 at 647 ℃, and Al9Mn4Si3 turns into Al9(FeMn)4Si3 with Fe dissolved into it. The number of Al9(FeMn)4Si3 increases with the Mn content increasing from 1.07% to 2.27%, whereas the size of Al9(FeMn)4Si3 has no obvious change. Mg2Si entirely dissolves into the matrix. Eutectic silicon, π-(Al8Mg3FeSi6) and α-Al(FeMn)Si spheroidize into granules, whereas the size, the morphology and the number of Al9(FeMn)4Si3 remain unchanged after the Al-12Si-0.65Mg-xMn (mass fraction, %) alloy ingots were homogenized at 550 ℃. Simultaneously, there are many Al9(MnFe)2Si3 at hundreds of nanometer size precipitated out from the Al-12Si-0.65Mg-(0.10~2.27)Mn (mass fraction, %) alloy matrix after homogenization treatment, and the number of them increases with the increasing of Mn content.

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    Microstructure and Mechanical Property of the Welding Joint of TA1/Cu/ X65 Trimetallic Sheets
    Min ZHANG, Erlong MU, Xiaowei WANG, Ting HAN, Hailong LUO
    Acta Metall. 2018, 54 (7): 1068-1076.   DOI: 10.11900/0412.1961.2017.00423
    Abstract   HTML   PDF (7225KB)

    Titanium and its alloys with fine corrosion resistance and specific tenacity are widely used in the fields of astronautics, chemical industry and so on. While the pipeline steels with low price and good mechanical properties are always used in petroleum industry. For now, composite panels are widely used in petrochemical industry, aerospace engineering and other fields, which can combine the respective features of the dissimilar materials together so as to meet the special requirements and save a lot of rare and precious metals. Previous studies have showed that the joining of titanium and steel suffered from two major challenges: one was the emergence of continuous distributed intermetallics of TiFe and TiFe2 in the weld, which could cause brittle fracture with low strength; the other was the occurrence of residual stresses that were caused by the great differences in thermal properties between titanium and steel. This work is aimed to join the explosion-bonded TA1/Cu/X65 trimetallic sheets (titanium flyer plate with thickness 2 mm, copper intermediate plate 1 mm, and X65 base plate 12 mm) with Cu-based flux-cored wires by the tungsten inert gas (TIG) welding. The microstructure and mechanical properties of welded joint was characterized by using SEM, EDS, TEM, XRD and tensile and microhardness tests. The results indicated that the filler metals for each weld layer have obvious zoning by using solid solution phases and intermetallic compounds. There was about 150 μm width Ti-Cu reaction zone between the Ti weld and transition layer weld. The microstructures of Cu-Ag-Mo-Nb/ER50-6 transition interface were composed of Fe-based and Cu-based solid solution. The intermediate copper played an important role in reducing the high temperature residence time of welded joints so as to reduce the interdiffusion of Ti, Fe element. Consequently, the hard-brittle Ti-Fe intermetallic compounds were partly replaced by Cu-based solid solution and Ti-Cu, Ti-Ag intermetallic compounds with relatively good ductility and toughness. The average tensile strength of the butt joints is 507 MPa at room temperature, mainly of that of X65 was obtained. ERTi-1 weld metal exhibited higher hardness than Cu-Ag-Mo-Nb weld metal, and their microhardness values were 507 and 447 HV100, respectively. In addition, the microhardness in reaction zone presented a slightly drop. The lowest values occurred in ER50-6 weld metal.

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    Interaction Between β-Sn Grain Orientation and Electromigration Behavior in Flip-Chip Lead-Free Solder Bumps
    Mingliang HUANG, Hongyu SUN
    Acta Metall. 2018, 54 (7): 1077-1086.   DOI: 10.11900/0412.1961.2017.00426
    Abstract   HTML   PDF (8515KB)

    With the increasing demands for miniaturization, the electromigration (EM)-induced failure by diffusion anisotropy in β-Sn is expected to be more serious than that induced by local current crowding effect, especially with the downsizing of solder bumps. In this work, the effects of Sn grain orientation on intermetallic compound (IMC) precipitation, dissolution of Ni under bump metallurgy (UBM) at the cathode, EM failure mechanism as well as the EM-induced β-Sn grain rotation in Ni/Sn-3.0Ag-0.5Cu/Ni-P flip-chip interconnects undergoing solid-solid EM under a current density of 1.0×104 A/cm2 at 150 ℃ were in situ studied. (Ni, Cu)3Sn4-type IMCs precipitated in these β-Sn grains with a small angle θ (between the c-axis of Sn grain and electron flow direction), i.e., along the c-axis of β-Sn grains. Stress relaxation, squeezing β-Sn whiskers near the anode, was also observed during EM. A mathematical model on the relationship between the dissolution of Ni UBM and β-Sn grain orientation was proposed: when the c-axis of β-Sn grain is parallel to the electron flow direction, excessive dissolution of the cathode Ni UBM occurred due to the large diffusivity of Ni along the c-axis; when the c-axis of β-Sn grain is perpendicular to the electron flow direction, no evident dissolution of cathode Ni UBM occurred. The proposed model agreed well with the experimental results. EM-induced β-Sn grain rotation was attributed to the different vacancy fluxes caused by EM between adjacent grains of various grain orientation, when vacancies reached supersaturation and undersaturation at the interfaces of the anode and the cathode, respectively. Vacancy fluxes went through free surface along the interface, resulting in a normal vacancy concentration gradient. Accordingly, stress gradient produces a torque to rotate the β-Sn grain.

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