Showing papers in "Acta Metallurgica Sinica (english Letters) in 2014"
TL;DR: In this paper, a review of carbon materials reinforced aluminum (C/Al) composites is presented, focusing on the categories, fabrication processes, existing problems and solutions, coatings and interfaces, challenges and opportunities of C/Al composites so as to provide a useful reference for future research.
Abstract: Carbon materials, including carbon fibers, graphite, diamond, carbon foams, carbon nanotubes, and graphene, are attractive reinforcements for aluminum matrix composites due to their excellent mechanical and/or physical properties as well as light weight. Carbon materials reinforced aluminum (C/Al) composites are promising materials in many areas such as aerospace, thermal management, and automobile. However, there are still some challenging problems that need to be resolved, such as interfacial reactions, low wettability, and anisotropic properties. These problems have limited the use of these composites. This review mainly focuses on the categories, fabrication processes, existing problems and solutions, coatings and interfaces, challenges and opportunities of C/Al composites so as to provide a useful reference for future research.
94 citations
TL;DR: An ω phase with a primitive hexagonal crystal structure has been found to be a common metastable phase in body-centered cubic (bcc) metals and alloys as discussed by the authors.
Abstract: An ω phase with a primitive hexagonal crystal structure has been found to be a common metastable phase in body-centered cubic (bcc) metals and alloys. In general, ω phase precipitates out as a high density of nanoscale particles and can obviously strengthen the alloys; however, coarsening of the ω particles significantly reduces the alloy ductility. The ω phase has coherent interfacial structure with its bcc matrix phase, and its lattice parameters are $$ {a_{{\omega }}} = \sqrt 2 \times {a_{\text{bcc}}} $$
and $$ {c_{{\omega }}} = \sqrt 3 /2 \times {a_{\text{bcc}}} $$
. The common {112}〈111〉-type twinning in bcc metals and alloys can be treated as the product of the ω → bcc phase transition, also known as the ω-lattice mechanism. The ω phase’s behavior in metastable β-type Ti alloys will be briefly reviewed first since the ω phase was first found in the alloy system, and then the existence of the ω phase in carbon steels will be discussed. Carbon plays a crucial role in promoting the ω formation in steel, and the ω phase can form a solid solution with various carbon contents. Hence, the martensitic substructure can be treated as an α-Fe matrix embedded with a high density of nanoscale ω-Fe particles enriched with carbon. The recognition of the ω phase in steel is expected to advance the understanding of the relationship between the microstructure and mechanical properties in bcc steels, as well as the behavior of martensitic transformations, twinning formation, and martensitic substructure.
82 citations
TL;DR: In this paper, the authors reviewed the ex situ and in situ manufacturing techniques for nano-sized particle-reinforced metal matrix composites and discussed the distinction between the two methods in some detail.
Abstract: Compared to the micro-sized particle-reinforced metal matrix composites, the nano-sized particle-reinforced metal matrix
composites possess superior strength, ductility, and wear resistance, and they also exhibit good elevated temperature properties. Therefore, the nano-sized particle-reinforced metal matrix composites are the new potential material which could be applied in many industry fields. At present, the nano-sized particle-reinforced metal matrix composites could be manufactured by many methods. Different kinds of metals, predominantly Al, Mg, and Cu, have been employed for the production of composites reinforced by nano-sized ceramic particles such as carbides, nitrides, and oxides. The main drawbacks of these synthesis methods are the agglomeration of the nano-sized particles and the poor interface between the particles and the metal matrix. This work is aimed at reviewing the ex situ and in situ manufacturing techniques. Moreover, the distinction between the two methods is discussed in some detail. It was agreed that the in situ manufacturing technique is a promising method to fabricate the nano-sized particle-reinforced metal matrix composites.
71 citations
TL;DR: In this paper, a review on the description of this processing technique in association with the means to obtain good wettability, the prerequisite for this kind of processing method, is presented, and the interfacial feature was also reviewed for the in-situ reaction synthesis.
Abstract: In the last two decades, light-weight magnesium matrix composites have been the hot issue of material field due to their excellent mechanical and physical properties, e.g., high-specific strength and modulus, good wear resistance, and damping capacity. As compared with aluminum matrix composites, magnesium matrix composites have merit in their specific weight and have wide applications in aerospace and aeronautical fields. Generally, the processing techniques for magnesium matrix composites can be categorized as conventional and special processing routes. In recent years, as a special processing route, metal melt infiltration into porous ceramic preform featured by its low cost and availability of high-volume fraction of reinforced ceramics have been receiving much attention. Thus, in this review, one emphasis was put on the description of this processing technique in association with the means to obtain good wettability, the prerequisite for this kind of processing method. Based on the recognized fact that there exist clean interface and bonding ability between ceramics and matrix metal, in-situ reaction synthesis is usually utilized to fabricate magnesium matrix composites. Therefore, the interfacial feature was also reviewed for the in-situ reaction synthesis. Characterizations of microstructures and various mechanical–physical properties were finally summarized for magnesium matrix composites including tensile response, wear resistance, creep behavior, and damping capacity.
69 citations
TL;DR: In this paper, the authors compared various solidification processing techniques available for the fabrication of functionally gradient metals and metal-ceramic composites and lists their properties and possible applications, and concluded that the ones based on solidification route are preferred for FGMs because of their economics and capability to make large size products.
Abstract: Functionally gradient/graded materials (FGMs), an emerging new class of materials, are the outcome of the recent innovative concepts in materials technology. FGMs are in their early stages of evolution and expected to have a strong impact on the design and development of new components and structures with better performance. FGMs exhibit gradual transitions in the microstructure and/or the composition in a specific direction, the presence of which leads to variation in the functional performance within a part. The presence of gradual transitions in material composition in FGMs can reduce or eliminate the deleterious stress concentrations and result in a wide gradation of physical and/or chemical properties within the material. Functionally graded metal–ceramic composites are also getting the attention of the researchers. Among the fabrication routes for FGMs such as chemical vapour deposition, physical vapour deposition, the sol–gel technique, plasma spraying, molten metal infiltration, self propagating high temperature synthesis, spray forming, centrifugal casting, etc., the ones based on solidification route are preferred for FGMs because of their economics and capability to make large size products. The present paper discusses and compares various solidification processing techniques available for the fabrication of functionally gradient metals and metal–ceramic composites and lists their properties and possible applications. The other processing methods are briefly described.
69 citations
TL;DR: In this paper, the effect of chemical composition and temperature on stacking fault energy (SFE) maps of high-Mn austenitic steels is investigated. And the control parameters of strain hardening behavior in these steels are described, and consequences for future developments are discussed.
Abstract: Austenitic high-Mn steels with Mn contents between approximately 15 and 30 wt% gain much interest because of their excellent mechanical properties and the option for adjusting strain hardening behavior due to different deformation mechanisms. 2D and 3D composition-dependent stacking fault energy (SFE) maps indicate the effect of chemical composition and temperature on SFE and consequently on the deformation mechanisms. Three steels with different chemical compositions and the same or different SFE are characterized in quasi-static tensile tests. The control parameters of strain hardening behavior in the high-Mn austenitic steels are described, and consequences for future developments are discussed.
63 citations
TL;DR: In this paper, the average hardness of ER2209 weldments was found to be greater than ERNiCrMo-3 weldments, and tensile fracture was observed at the weld zones for both the fillers.
Abstract: The dissimilar combinations of Inconel 625 and duplex stainless steel SAF 2205 obtained from manual GTA welding process employing ER2209 and ERNiCrMo-3 filler metals have been investigated. Formation of secondary phases at the HAZ of Inconel 625 and grain coarsening at the HAZ of SAF 2205 were witnessed while using these filler wires. The average hardness of ER2209 weldments was found to be greater than ERNiCrMo-3 weld. Tensile fracture was observed at the weld zones for both the fillers. Impact test trials showed brittle mode of fracture on employing ER2209 filler and mixed (ductile–brittle) mode of fracture while using ERNiCrMo-3 filler. Further optical microscopy and SEM/EDS analysis were carried out across the weldments to investigate the structure–property relationships.
63 citations
TL;DR: A novel powder metallurgy (PM) strategy called flake PM was developed to fabricate metal matrix composites (MMCs) with nano-laminated or hierarchical architectures as discussed by the authors.
Abstract: Powder metallurgy (PM) is one of the most applied processes in the fabrication of metal matrix composites (MMCs). Recently, a novel PM strategy called flake PM was developed to fabricate MMCs with nano-laminated or hierarchical architectures. The name “flake PM” was derived from the use of flake metal powders, which could benefit the uniform dispersion of reinforcements in the metal matrices and thus result in balanced strength and ductility. Flake PM has been proved to be successful in the dispersion of nano aluminum oxides, carbon nanotubes, graphene nano-sheets, and microsized B4C particles in aluminum or copper matrix. This paper reviews the technique and mechanism developments of flake PM in previous studies, and foresees the future develop of this new fabricating method.
49 citations
TL;DR: In this article, the current state of understanding and development of welding tool wearing and FSW parameters of AMCs are evaluated in detail and the factors affecting the microstructure and mechanical properties of the joints are evaluated.
Abstract: Friction stir welding (FSW) is considered a promising welding technique for joining the aluminum matrix composites (AMCs) to avoid the drawbacks of the fusion welding. High joint efficiencies of 60%–100% could be obtained in the FSW joints of AMCs. However, due to the existence of hard reinforcing particles in the AMCs, the wearing of welding tool during FSW is an unavoidable problem. Moreover, the low ductility of the AMCs limits the welding process window. As the hard materials such as Ferro-Titanit alloy, cermet, and WC/Co were applied to produce the welding tools, the wearing of the tools was significantly reduced and the sound joints could be achieved at high welding speed for the AMCs with low reinforcement volume fraction. In this article, current state of understanding and development of welding tool wearing and FSW parameters of AMCs are viewed. Furthermore, the factors affecting the microstructure and mechanical properties of the joints are evaluated in detail.
39 citations
TL;DR: In this paper, a carbon nanotube-reinforced 2009Al (CNT/2009Al) composites with aligned CNTs were fabricated by friction stir processing (FSP) and FSP-rolling, respectively.
Abstract: Carbon nanotube-reinforced 2009Al (CNT/2009Al) composites with randomly oriented CNTs and aligned CNTs were fabricated by friction stir processing (FSP) and FSP-rolling, respectively. The CNT/2009Al composites with aligned CNTs showed much better tensile properties at room temperature and elevated temperature compared with those with the randomly oriented CNTs, which is mainly attributed to larger equivalent aspect ratio of the CNTs and avoidance of preferential fracture problems. However, much finer grain size was not beneficial to obtaining high strength above 473 K. The aligned CNTs resulted in tensile anisotropy, with the best tensile properties being achieved along the direction of CNT aligning. As the off-axis angle increased, the tensile properties were reduced due to the weakening of the load transfer ability. Furthermore, aligned CNTs resulted in much lower coefficient of thermal expansion compared with randomly oriented CNTs.
37 citations
TL;DR: In this article, composites with silicon carbide particle (SiCp) as reinforcement and AZ91 magnesium alloy as matrix have been synthesized using liquid metal stir-casting technique with optimized processing conditions.
Abstract: In the present investigation, composites with silicon carbide particle (SiCp) as reinforcement and AZ91 magnesium alloy as matrix have been synthesized using liquid metal stir-casting technique with optimized processing conditions. The composites with good particle distribution in the matrix, and better grain refinement and good interfacial bonding between the matrix and reinforcement have been obtained. The effect of SiCp content on the physical, mechanical, and tribological properties of Mg-based metal matrix composite (MMC) is studied with respect to particle distribution, grain refinement, and particle/matrix interfacial reactions. The electrical conductivity, coefficient of thermal expansion, micro- as well as macro-hardness, tensile and compressive properties, and the fracture behavior of the composites along with dry sliding wear of the composites have been evaluated and compared with the base alloy.
TL;DR: The AISI316L stainless steel composites reinforced with 2, 4, 6, and 8 vol% titanium diboride (TiB2) particles were sintered by the high pressure-high temperature method.
Abstract: The AISI316L stainless steel composites reinforced with 2, 4, 6, and 8 vol% titanium diboride (TiB2) particles were sintered by the high pressure-high temperature method. Ball-on-disk method was carried out to study wear behavior of the composites. Tests were carried out at room temperature. The TiB2 particles improved the hardness and tribological properties of the composites. The friction coefficient of the composites decreased with the increasing content of TiB2. The reduction of the wear rate with the increasing of the content of TiB2 particles in the steel matrix was also observed. It is demonstrated that the friction coefficient of composites with the same content of TiB2 particles depend on the sintering conditions.
TL;DR: In this paper, the effect of laser power, defocusing distance, and welding speed on the welded joint appearance was examined, and the microstructure and mechanical properties on the typical butt joints were investigated.
Abstract: A 4 kW fiber laser was chosen to weld the new hot-rolled nano-scale precipitation-strengthened steel with a thickness of 4.5 mm. The effect of laser power, defocusing distance, and welding speed on the welded joint appearance was examined, and the microstructure and mechanical properties on the typical butt joints were investigated. Results showed that increasing laser welding power may cause faster downward flow of molten metal to produce greater root humping. With the welding speed increasing, the average welding seam (WS) width decreased, and the average WS and heat-affected zone (HAZ) hardness increased. The microstructures of WS, fusion line, and coarse grain heat-affected zone were lath martensite, but the growth direction of the original austenite grain boundaries was significantly different. The microstructures of fine grain heat-affected zone were ferrite and martensite, and the microstructure of mixed grain heat-affected zone contained ferrite, massive M/A island, and a small amount of martensite. The micro-hardness values of WS, HAZ, and base metal (BM) were 358, 302, and 265 HV, respectively. The butt joint fracture at the BM far from the WS and the welded joint tensile strength are observed to follow proportional relationship with hardness.
TL;DR: In this paper, the fabrication method, microstructure, interface reaction, modeling, and physical and mechanical properties of the composites are addressed, with particular emphasis on fabrication and micro-structure of aluminum matrix composites reinforced with shape memory alloys.
Abstract: Metal-matrix composites reinforced with shape memory alloys (SMA, including long fiber, short fiber, and particle) are “intelligent materials” with many special physical and mechanical properties, such as high damping property, high tensile strength, and fatigue resistance. In this review article, the fabrication method, microstructure, interface reaction, modeling, and physical and mechanical properties of the composites are addressed. Particular emphasis has been given to (a) fabrication and microstructure of aluminum matrix composites reinforced with SMAs, and (b) shape memory effect on the physical and mechanical properties of the composites. While the bulk of the information is related to aluminum matrix composites, important results are now available for other metal-matrix composites.
TL;DR: In this paper, an Ag/ZnO nanocomposite has been synthesized and characterized for investigating its photocatalytic activity, and the average particle size was found to be around 20-30nm.
Abstract: A Ag/ZnO nanocomposite has been synthesized and characterized for investigating its photocatalytic activity. The morphology and particle size of the Ag/ZnO was studied by scanning electron microscope (SEM) and the microstructure of the as-synthesized nanocomposite was confirmed by X-ray diffraction (XRD) analysis. The elemental composition of the metal oxide was determined by energy dispersion spectrometry (EDS). Diffuse reflectance spectra (DRS), Photoluminescence (PL) spectra, and Fourier transform infrared spectroscopy (FTIR) were also studied for characterizing the nanocomposite. The average particle size was found to be around 20–30 nm. Photocatalytic activity of Ag/ZnO has been investigated over methyl violet (6B) dye under UV and visible light irradiation. The degradation of methyl violet (6B) dye using ZnO and Ag/ZnO was compared and found that Ag/ZnO composite is more efficient than ZnO. The rate of disappearance of dye was monitored spectrophotometrically in the maximum visible absorption wavelength and the extent of degradation was discussed in terms of Langmuir–Hinshelwood model. The Ag/ZnO composite was found capable of degrading the industrial dye effluent. Effect of H2O2 addition on dye degradation by the Ag/ZnO was investigated and Ag/ZnO was found to be an effective antimicrobial agent. Reusability of Ag/ZnO catalyst was also tested.
TL;DR: In this paper, the effects of the rotating speed and the time of ball milling were analyzed based on the microstructure evolution and properties of the few-layer graphene (FLG) and copper powders to produce homogeneous composite powders.
Abstract: Graphene-reinforced copper composites recently have attracted more attention, since they exhibited excellent mechanical properties and could be used widely in many fields. Few-layer graphene (FLG) and copper powder were mixed by ball milling to produce homogeneous composite powders. Then, FLG-reinforced copper composites (FLG/Cu) were fabricated by spark plasma sintering (SPS) using the composite powders with a FLG volume fraction of 2.4 vol%. The effects of the rotating speed and the time of ball milling were analyzed based on the microstructure evolution and properties of the FLG/Cu composites. Obvious strengthening effect of FLG was found for the composites, and the conductance of the composite reaches 70.4% of IACS. The yield strength of the composite produced by ball milling at a speed of 100 r/min for 4 h is 376 MPa, which is 2.5 times higher than that of copper and higher than that of copper composite enhanced by 5 vol% CNTs (360 MPa). The defects produced in FLG with the increase of rotating speed and time could reduce the mechanical and conductive properties of the composites.
TL;DR: In this paper, the theoretical research and application of variable gauge rolling (VGR) are reviewed and two basic equations, VGRf and VGR-s, were deduced.
Abstract: Variable gauge rolling (VGR) is a new technology for producing the materials which have the advantage of lightweight due to optimized thickness according to load distribution. The new progresses in the theoretical research and application of VGR are reviewed in this paper. Two basic equations, VGR-f and VGR-s, were deduced. The former is a new differential equation of force equilibrium, and the latter is a new form of formula for the law of mass conservation. Both of them provide a new base for the development of VGR analysis. As the examples of VGR’s application, tailor rolled blank (TRB) and longitudinal profile (LP) plate are introduced. Now TRBs are only produced in Germany and China, and have been used in the automotive manufacturing to play an important role in lightweight design. LP plates have been used in shipbuilding and bridge construction, and promised a bright prospect in reducing construction weight. In addition, new technologies and applications of VGR emerge constantly. Tailor welded strips and tailor rolled strips with variable thickness across the width can be used for progressive die and roll forming. The 3D profiled blank can be obtained by two-step rolling process. Tailor tubes with the variable wall thickness are an efficient way to reduce the weight. The blank with tailored thickness and mechanical property is also under development. Above products based on the tailored ideas provide a new materials-warehouse for the designers to select so as to meet the needs of weight reducing and material saving.
TL;DR: In this paper, the thermal resistance of low-melting-temperature alloy (LMTA) thermal interface materials (TIMs) was measured by laser flash method before and after different stages of heating.
Abstract: Thermal resistance of low-melting-temperature alloy (LMTA) thermal interface materials (TIMs) was measured by laser flash method before and after different stages of heating. The results showed that the thermal performance of the LMTA TIMs was degraded during the heating process. It is suggested that the degradation may mainly be attributed to the interfacial reaction between the Cu and the molten LMTAs. Due to the fast growth rate of intermetallic compound (IMC) at the solid–liquid interface, a thick brittle IMC is layer formed at the interface, which makes cracks easy to initiate and expand. Otherwise, the losses of indium and tin contents in the LMTA during the interfacial reaction will make the melting point of the TIM layer increase, and so, the TIM layer will not melt at the operating temperature.
TL;DR: In this article, the fracture morphologies of several advanced high-strength steels (DP590, DP780, DP980, M1180, and M1300) formed in uniaxial tension and piercing were observed by scanning electron microscope, and then quantitatively analyzed by image processing technique.
Abstract: The fracture morphologies of several advanced high-strength steels (DP590, DP780, DP980, M1180, and M1300) formed in uniaxial tension and piercing were observed by scanning electron microscope, and then quantitatively analyzed by image processing technique. The tension-induced fractographs are dominated by obvious uniform or bimodal size dimples, while shearing-induced fractographs have smooth surfaces and few dimples. The fracture zone of higher grade DP steels is smoother. As for M1180 and M1300, the fracture zones consist of very small dimples and smooth brittle surfaces. The dimple size of M1300(~1.2 μm) is smaller than that of M1180(~1.6 μm). Moreover, in the tensile fracture, the quantitative correlation between average dimple diameter (d) and tensile strength (σ) can be represented by d = 10,502.32σ
−1.21. However, the relation between dimple density and tensile strength is not monotonic due to the appearance of bimodal size dimples with increase of tensile strength. For shearing-induced fracture during piercing, the fitted empirical model between the percentage of burnish zone (f) and tensile strength can be described as f = 239.9σ
−0.36.
TL;DR: In this article, the microstructure and electrochemical behaviors of the CoCrCuFeNiNb high-entropy alloys coatings were investigated in detail, and the experimental results indicated that the coating consists of a simple fcc solid solution phase and an order (CoCr)Nb-type Laves phase.
Abstract: The CoCrCuFeNiNb high-entropy alloys coatings were prepared by using plasma-transferred arc cladding process. The microstructure and electrochemical behaviors of the coating were investigated in detail. The experimental results indicated that the coating consists of a simple fcc solid solution phase and an order (CoCr)Nb-type Laves phase. The polarization curves, obtained in 1 and 6 mol/L hydrochloric acid solutions, clearly indicated that the general corrosion resistance of the coating at ambient temperature was better than that of 304 stainless steel. The coating displayed a lower corrosion current and lower corrosion rate. Electrochemical impedance spectroscopy demonstrated that the impedance of the coating was significantly higher than that of the 304 stainless steel.
TL;DR: In this paper, the effect of deep cryogenic treatment (DCT) on the microstructures and mechanical behavior of ultrafine-grained WC-12Co cemented carbide was investigated by using XRD, SEM, and DSC.
Abstract: Effect of deep cryogenic treatment (DCT) on the microstructures and mechanical behavior of ultrafine-grained WC-12Co cemented carbide was investigated by using XRD, SEM, and DSC. The phase transformations of pure Co and binder phase Co in cemented carbide were analyzed in detail to correlate the strengthening mechanism with its α → e phase transition. The results show that DCT resulted in a slight increase in hardness and bending strength of ultrafine-grained WC-12Co cemented carbide. For the ultrafine-grained cemented carbide after DCT, there is no significant change in the microstructure and the elemental distribution of the cemented carbides, but the fractured morphology shows a feature of plastic deformation. In the cases of pure Co and the binder phase Co in WC-12Co cemented carbide, they exhibit different features of phase transformation. The improvement of mechanical property of cemented carbide can be attributed to the increased amount of e-Co in WC-12Co composites after DCT.
TL;DR: In this article, a new technique to produce ultra-fine grained tubular specimen has been proposed, and the experiments have been performed using equal channel angular pressing (ECAP) with an angle of 90° between two intersecting channels and also the use of rubber pad as a mandrel during process.
Abstract: A new technique to produce ultra-fine grained tubular specimen has been proposed, and the experiments have been performed using equal channel angular pressing (ECAP) with an angle of 90° between two intersecting channels and also the use of rubber pad as a mandrel during process. Commercial purity copper tubes have been pressed up to three passes through four different fundamental routes (A, BA, BC, and C) directions of which are identified in the text below. The influence of each route on the value, distribution, and homogeneity of hardness has been investigated by applying Vickers micro-hardness measurements at various locations of the tube’s transverse planes. Significant enhancement of the hardness is observed after the first pass ECAP. Also, routes C and BC show, respectively, better average hardness magnitude and hardness distribution uniformity. In addition, the results indicate that there is about 50% and 62% reduction of the grain size, compared to the annealed condition, following ECAP process of the copper tube sample after the first and the third pass via route BC.
TL;DR: In this article, discontinuous pitch-based carbon fiber reinforced aluminum matrix (Al-CF) composites with aluminum-silicon alloy (Al−Si) were fabricated through hot pressing.
Abstract: The constant increase in power and heat flux densities encountered in electronic devices fuels a rising demand for lightweight heat sink materials with suitable thermal properties. In this study, discontinuous pitch-based carbon fiber reinforced aluminum matrix (Al-CF) composites with aluminum–silicon alloy (Al–Si) were fabricated through hot pressing. The small amount of Al–Si contributed to enhance the sintering process in order to achieve fully dense Al–CF composites. A thermal conductivity and CTE of 258 W/(m K) and 7.0 × 10−6/K in the in-plane direction of the carbon fibers were obtained for a (Al95 vol% + Al–Si5 vol%)-CF50 vol% composite. Carbon fiber provides the reducing of CTE while the conservation of thermal conductivity and weight of Al. The achieved CTEs satisfy the standard requirements for a heat sink material, which furthermore possess a specific thermal conductivity of 109 W cm3/(m K g). This simple process allows the low-cost fabrication of Al–CF composite, which is applicable for a lightweight heat sink material.
TL;DR: The effects of shot peening and plasma electrolytic oxidation (PEO) on the intergranular corrosion behavior of AA 7A85-T7452 were investigated by electrochemical polarization and electrochemical impedance tests as mentioned in this paper.
Abstract: The effects of shot peening (SP) and plasma electrolytic oxidation (PEO) on the intergranular corrosion behavior of the novel high strength aluminum alloy 7A85 (AA 7A85) were investigated by electrochemical polarization and electrochemical impedance tests. The intergranular corrosion mechanism of SP, PEO and PEO combined with sealing-treated AA 7A85 was studied by the metallographic analysis, residual stress testing, X-ray diffractometer analysis and scanning electron microscopy. The results show that AA 7A85-T7452 is very sensitive to intergranular corrosion. SP would significantly improve its intergranular corrosion resistance. This is attributed to the combination action of residual compressive stress and grain refinement. PEO would reduce the largest corrosion depth by 41.6%. Moreover, PEO without sealing did not eliminate the intergranular corrosion due to the existence of the micropores and microcracks in the oxide coating. However, PEO combined with the SiO2 sol–gel sealing treatment could effectively protect the AA 7A85-T7452 from intergranular corrosion because of the good corrosion resistance and barrier function of the sealed coating.
TL;DR: In this article, a novel method was developed to fabricate carbon nanotubes (CNTs)-reinforced Mg matrix composites, which consists of two steps: CNTs pre-dispersion by ball-milling and the ultrasonic melt processing.
Abstract: A novel method was developed to fabricate carbon nanotubes (CNTs)-reinforced Mg matrix composites. The method consists of two steps: CNTs pre-dispersion by ball-milling and the ultrasonic melt processing. Mechanical ball-milling effectively pre-dispersed CNTs on Zn flakes with suitable rotational speed and ball-milling time. Serious CNT entanglements were dispersed by the ball-milling. However, ball-milling for a long time at high speed would damage the morphology of CNTs. The ultrasonic overcame the poor wettability between Mg melt and CNTs and then dispersed pre-dispersed CNTs in the Mg melt. CNTs were distributed well in the composites and maintained integrated structure. CNTs significantly improved the mechanical properties of the matrix. The strengthening efficiency reached to 37.1, which proves the superiority of this novel method. Besides grain refinement, load transfer may make a great contribution to the improvement of the strength for the composites.
TL;DR: The microstructure, microsegregation, and mechanical properties of directional solidified Mg-3.0Nd-1.5Gd ternary alloys were experimentally studied as mentioned in this paper.
Abstract: The microstructure, microsegregation, and mechanical properties of directional solidified Mg–3.0Nd–1.5Gd ternary alloys were experimentally studied. Experimental results showed that the solidification microstructure was composed of dendrite primary α(Mg) phase and interdendritic α(Mg) + Mg12(Nd, Gd) eutectic and Mg5Gd phase. The primary dendrite arm spacing λ1 and secondary dendrite arm spacing λ2 were found to be depended on the cooling rate R in the form λ1 = 8.0415 × 10−6R−0.279 and λ2 = 6.8883 × 10−6R−0.205, respectively, under the constant temperature gradient of 40 K/mm and in the region of cooling rates from 0.4 to 4 K/s. The concentration profiles of Nd and Gd elements calculated by Scheil model were found to be deviated from the ones measured by EPMA to varying degrees, due to ignorance of the back diffusion of the solutes Nd and Gd within α(Mg) matrix. And microsegregation of Gd depended more on the growth rate, compared with Nd microsegregation. The directionally solidified experimental alloy exhibited higher strength than the non-directionally solidified alloy, and the tensile strength of the directionally solidified experimental alloy was improved, while the corresponding elongation decreased with the increase of growth rate.
TL;DR: In this article, the effect of austempering temperature, time, and route on bainitic lath width, volume fraction of retained Austenite, carbon concentration in retained austenite and nanohardness of retained Austinite was studied.
Abstract: A low-temperature nanobainitic steel was obtained through one-step and two-step austempering. The effect of austempering temperature, time, and route on bainitic lath width, volume fraction of retained austenite, carbon concentration in retained austenite, and nanohardness of bainitic lath and retained austenite was studied. Results showed that the transformation kinetics was slowed down and the bainitic lath was refined as the austempering temperature decreased from 300 to 250 °C. Both coarser and finer bainitic laths were obtained with the two-step austempering, which was consistent with the lath size at 300 and 250 °C austempering, respectively. X-ray diffraction analysis showed that both volume fraction of retained austenite and its carbon concentration decreased with the decrease of austempering temperature for the one-step austempering, and especially the carbon concentration is obviously increased when the two-step austempering is adopted. The nanohardness of the bainitic lath in the sample after two-step austempering treated lies between that of the samples after 300 and 250 °C austempering treated. The product of tensile strength and total elongation of the two-step austempered sample is the highest, which increases monotonously with the product of retained austenite fraction and its carbon concentration. Higher strength–ductility balance may be resulted by the fine bainitic lath, high volume fraction, and high stability of retained austenite in the sample after two-step austempered.
TL;DR: In this article, the performance of friction stir welded (FSWed) high-strength 7075Al-T651 alloy was evaluated and the nugget zone was observed to consist of fine and equiaxed recrystallized grains with a low dislocation density and free of original precipitates, but containing uniformly distributed dispersoids.
Abstract: Microstructural evolution and strain hardening behavior of a friction stir welded (FSWed) high-strength 7075Al-T651 alloy were evaluated. The nugget zone was observed to consist of fine and equiaxed recrystallized grains with a low dislocation density and free of original precipitates, but containing uniformly distributed dispersoids. The strength, joint efficiency, and ductility of the FSWed joints increased with increasing welding speed. A joint efficiency of ~91% was achieved at a welding speed of 400 mm/min and rotational rate of 800 r/min, while the ductility remained basically the same as that of the base metal. There was no obvious strain rate sensitivity observed in both base metal and welded joints. While both the base metal and FSWed joints exhibited stage III and IV hardening characteristics, the hardening capacity, strain hardening exponent, and strain hardening rate all increased after friction stir welding.
TL;DR: In this paper, the corrosion process of new steels was studied using electrochemical impedance spectroscopy experiments during 72-h. The results indicated that the ferrite phase (both the proeectoid and eutectoid ferrite) dissolved preferentially.
Abstract: New steels with different carbon contents were self-developed by thermo-mechanical controlled processing. The effects of the carbon content and the microstructure on the corrosion properties of new steels were investigated by immersion test and SEM. The results indicated that the ferrite phase (both the proeutectoid and eutectoid ferrite) dissolved preferentially. Cementite reserved and accumulated on the surface. As carbon content increased, the content of ferrite decreased and cathode/anode area ratio increased. Therefore, the corrosion rate of new steels increased from 0.30 to 0.90 mm/years when the carbon content rose from 0.05 to 0.13 wt%. The corrosion process of new steels was studied using electrochemical impedance spectroscopy experiments during 72 h. It indicated that the impedance modulus |Z|0.01 Hz of the new steels reduces with the increase of the immersion time. While the corrosion process of the new steel with 0.11 wt% C developed faster than that with 0.07 wt% C, although their |Z|0.01 Hz was similar at the initial stage.
TL;DR: In this paper, the influence of cerium addition on the isothermal oxidation behavior of 00Cr17NbTi ferritic stainless steel was studied at temperature up to 1,000°C for 100h in air.
Abstract: The influence of cerium addition on the isothermal oxidation behavior of 00Cr17NbTi ferritic stainless steel was studied at temperature up to 1,000 °C for 100 h in air. The results show that cerium additions can reduce the grain size of this ferritic stainless steel, improve the diffusion of chromium and decrease the critical concentration of chromium to form protective Cr2O3 layer. With the increasing of cerium addition, the oxide particles become smaller and this can increase the rupture strength and spalling resistance of oxide layers. The transport mechanism through the oxide layer is varied from metal transport outward from steel to principally oxygen transport inward with the increase of cerium content, which leads to the lower oxidation rate and the better scale adherence of 00Cr17NbTi ferritic stainless steel.