Processing,characterization, room temperature mechanical properties and fracture behavior of hot extruded multi-scale B4C reinforced 5083 aluminum alloy based composites

Microstructural characteristics and mechanical behavior of hot extruded Al5083/B₄C nanocomposites were studied. Al5083 and Al5083/B₄C powders were milled for 50 h under argon atmosphere in attrition mill with rotational speed of 400 r/min. For increasing the elongation, milled powders were mixed with 30% and 50% unmilled aluminum powder (mass fraction) with mean particle size of >100 μm and <100 μm and then consolidated by hot pressing and hot extrusion with 9:1 extrusion ratio. Hot extruded samples were studied by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), tensile and hardness tests. The results showed that mechanical milling process and presence of B₄C particles increase the yield strength of Al5083 alloy from 130 to 566 MPa but strongly decrease elongation (from 11.3% to 0.49%). Adding <100 μm unmilled particles enhanced the ductility and reduced tensile strength and hardness, but using the >100 μm unmilled particles reduced the tensile strength and ductility at the same time. By increasing the content of unmilled particles failure mechanism changed from brittle to ductile.     

Corrosion behavior of ultra-fine grained industrial pure Al fabricated by ECAP

Corrosion behavior of ultra-fine grained(UFG) industrial Al fabricated by equal channel angular pressing(ECAP) for 16 pass times was investigated by potentiodynamic polarization test, potentiostatic polarization test, electrochemical impedance spectroscopy(EIS) measurement, immersion test and surface analyses (OM and SEM). The microstructures including grain size, grain boundaries and dislocations were also observed by TEM. The results show that the UFG industrial pure Al has more positive pitting potential, less corrosion current density and five times larger passive film resistance compared with the coarse grained(CG) one. It was found that the increased pitting resistance is profited from the more stable passive film kept in the Cl⁻ aggressive solution due to more grain boundaries, larger fraction of non-equilibrium grain boundaries and residual stress of the UFG industrial pure Al.     

Precipitation phenomena in an ultrafine-grained Al alloy

To provide insight into the influence of length scales on precipitation phenomena, a bulk Al 7075 alloy with an ultrafine grain (UFG) structure was fabricated via cryomilling, hot isostatic pressing and extrusion, followed by solution treatment and artificial aging. The precipitation behavior in the UFG material was studied by transmission electron microscopy (TEM) and compared to its coarse grain (CG) counterpart. Results indicate that there are important differences in the size, chemistry and spatial distribution of precipitates between these two bulk materials. Differences in grain size were noted to influence the dislocation substructure, which in turn governed precipitation kinetics. Specifically, a high volume of grain boundaries (GBs) leads to a low concentration of vacancies in the UFG material, which hinders homogeneous nucleation of precipitates during aging. The nucleation of Guinier–Preston (GP) zones in UFG after extrusion is influenced by the presence of dislocations, which provide heterogeneous nucleation sites. The density of precipitates in the UFG was comparatively lower than that in the CG counterpart, because of the lower dislocation density in the grain interior. Solution treatment promoted recovery of dislocations in the UFG material. Then GP zones and nanoscale platelet η′ phase precipitates nucleated homogeneously during artificial aging. In comparison, the network of dislocations that remained in the CG material after solution treatment facilitated the formation of plate-like η′ phase precipitates around dislocations during artificial aging. Moreover, copious GP zones were observed in the CG interior after artificial aging with a comparatively higher density than those in the UFG.     

A novel ultrafine-grained Ni3Al with increased cyclic oxidation resistance

An ultrafine-grained (UFG) Ni₃Al was fabricated by annealing an electrodeposited Ni–Al composite in vacuum at 600 °C for 2 h. The UFG Ni₃Al, compared to a compositional-similar but coarse-grained (CG) alloy prepared by arc-melting, exhibited a greatly increased cyclic oxidation resistance at 900 °C. Microstructural investigation showed that the CG alloy grew a scale with a high susceptibility to buckling and cracking because of the formation of large voids at the scale/metal interface, but that the UFG alloy grew an adherent scale, because its typical structure prevented the formation of the interface void during oxidation.     

Reinforcement of Al–Fe–Ni alloys with the in situ formation of composite materials

One of the most effective methods for the improvement of the mechanical properties of metals is their reinforcement with non-metallic materials. In the present work powder of K₂TiF₆ and KBF₄ was added in an Al–Fe–Ni alloy while the alloy was in liquid form at 1060 °C with a 5 wt.% mixture of powders and with simultaneous stirring for 30 min. The liquid was squeeze-casted at 150 bar. The as-cast specimens were examined with electron microscopy and X-ray diffraction. SEM analysis revealed that the as-formed material is composed by needle-like crystallites along with a dentritic form and an interdendritic phase. The composition of the needle-like crystallites may presumably be expressed by the formula (Fe-Ni)Al₃. The rest of the matrix consists of almost pure Al grown dentritically, while the interdendritic phase contains Fe and Ni dissolved in Al. EDS analysis also proved the existence of spots with high Ti concentration, which probably refer to the Ti–B compounds. Finally TEM verified the presence of nanocrystals in the matrix.     

A new consideration on reinforcement distribution in the different planes of nanostructured metal matrix composite sheets prepared by accumulative roll bonding (ARB)

Recently, a number of novel methods based on accumulative roll bonding (ARB) have been introduced to produce particulate metal matrix composites. Nonetheless, the microstructure uniformity from the reinforcement distribution viewpoint in the different planes of ARBed composite sheets has not been focused up to date. This paper aims to compare the evolution of the B₄C particles distribution in nanostructured Al-10 vol.% B₄C composites prepared by ARB in the rolling direction-normal direction (RD-ND) and transverse direction-normal direction (TD-ND) planes. From optical microscopic evaluations quantified by the radial distribution function analysis, it is realized that the homogeneity in the RD-ND planes is in excess of the TD-ND planes. In addition, transmission electron microscopy reveals the development of nanostructures in the Al matrix after seven ARB passes.     

Phase identification of Al–B4C ceramic composites synthesized by reaction hot-press sintering

A series of boron carbide (B₄C) matrix composites with different contents of Al, were synthesized by reaction hot-press sintering with milled B₄C and pure metallic Al powder at 1600 °C for 1 h. X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to identify the phase constituent of the milled powders and the composites. The results have shown that parts of B₄C and Al particles were oxidized to boron oxide (B₂O₃) and alumina (Al₂O₃) during the milling. Thermit reaction occurred and B₂O₃ was reverted during hot-press sintering. A ternary phase of Al boron carbide (Al₈B₄C₇) was found in the composites, and the B₄C transformed to a rich boron phase (B_6.5C) because of the superfluous boron in the system.     

Corrosion of ultra-fine grained copper fabricated by equal-channel angular pressing

Corrosion of ultra-fine grain (UFG) copper fabricated by equal-channel angular pressing (ECAP) has been investigated in comparison with that in recrystallized coarse grain (CG) copper. Corrosion current was estimated by a Tafel extrapolation method to examine the kinetics of corrosion in a modified Livingstone etchant, which is sensitive to dislocations and grain boundaries. UFG copper exhibited a lower corrosion current in comparison with that in its recrystallized coarse grain (CG) counterpart despite the fact that the dislocation density and total fraction of grain boundaries are much greater in UFG copper than in CG copper. Corrosion damage on the surface of UFG copper is macroscopically rather uniform whereas obvious attack at grain boundaries and selective corrosion of some grain interiors were observed in CG copper.     

The degradation of corrosion resistance for Al 5083 alloy after thermal and superplastic forming processes

Corrosion behavior, particularly the intergranular corrosion susceptibility of a superplastic Al 5083 alloy (denoted as Al 5083S) and a non-superplastic Al 5083 alloy (denoted as Al 5083N) with various thermal processes and a superplastic forming process, has been systematically evaluated. The nitric acid mass loss test (NAMLT) according to ASTM G 67 indicated that the weight loss of Al 5083S was larger than that of Al 5083N, which was due to the finer grain size in the former alloy. It also showed that superplastically formed specimens of Al 5083S and the specimens of Al 5083S and Al 5083N treated with the same thermal process as the superplastically formed specimens suffered from severe intergranular corrosion. The serious intergranular corrosion of these specimens was attributed to the formation of continuous β (Mg₂Al₃) precipitates at grain boundaries, i.e., the sensitization effect. Such a detrimental effect can be eliminated by a postforming annealing treatment at 345 °C for 1 h. Furthermore, electrochemical measurements in a 3.5 wt.%NaCl solution also revealed that the sensitized specimens possessed more active corrosion potential (E_corr), breakdown potential (E_b), and protection potential (E_pp), as well as higher corrosion current density (i_corr) and passive current density (i_p), than those of the as-received specimens. Experimental results also showed that the corrosion resistance of the superplastically formed specimen was the worst among all specimens, which was attributed to the formation of cavities during the superplastic forming in addition to the sensitization effect caused by the thermal processing. The influences of both detrimental effects on the corrosion resistance of the Al 5083 specimens were also discussed.     

In situ thermomechanical processing to avoid grain boundary precipitation and strength-ductility loss of age hardening alloys

To avoid grain boundary (GB) precipitation during aging, a new strategy of in situ thermomechanical processing for age hardening alloys was proposed. Specifically, high-density nanoscale precipitates were introduced into ultrafine grain (UFG) interiors of 7075Al alloy by equal-channel-angular (ECAP) processing at 250 °C for 8 passes, thus avoiding GB precipitation. Tensile test results indicated that the UFG 7075Al alloy exhibits superior mechanical properties (yield strength of 350 MPa, ultimate tensile strength of 500 MPa, uniform elongation of 18% and tensile ductility of 19%) compared with the UFG 1050Al counterpart (yield strength of 170 MPa, ultimate tensile strength of 180 MPa, uniform elongation of 2.5% and tensile ductility of 7%). Fracture surface morphology studies revealed numerous homogeneous micro shear bands in necking shrinkage areas of both UFG 7075Al and 1050Al alloys, which are controlled by cooperative GB sliding. Moreover, the introduction of nanoscale precipitates in UFG 7075Al matrix weakened the tendency of shear fracture, resulting in a higher tensile ductility and more homogeneous deformation. Different from the GB precipitation during postmortem aging, in situ thermomechanical treatment dynamically formed GBs after precipitation, thus avoiding precipitation on GBs.     

Effects of solute Mg on grain boundary and dislocation dynamics during nanoindentation of Al–Mg thin films

Using in situ nanoindentation in a transmission electron microscope (TEM) the indentation-induced plasticity in ultrafine-grained Al and Al–Mg thin films has been studied, together with conventional quantitative ex situ nanoindentations. Extensive grain boundary motion has been observed in pure Al, whereas Mg solutes effectively pin high-angle grain boundaries in the Al–Mg alloy films. The proposed mechanism for this pinning is a change in the atomic structure of the boundaries, possibly aided by solute drag on extrinsic grain boundary dislocations. The mobility of low-angle boundaries is not affected by the presence of Mg. Based on the direct observations of incipient plasticity in Al and Al–Mg, it was concluded that solute drag accounts for the absence of discrete strain bursts in indentation of Al–Mg.     

Structure and phases in nickel-base self-fluxing alloy coating containing high chromium and boron

The structure of a nickel-base, self-fluxing alloy coating, containing chromium and boron thermal sprayed and fused, was investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). A lumpy M₆C carbide, a rodlike M₃B₂ boride of tetragonal structure, a rodlike M₇C₃ carbide of hexagonal structure, and a Ni-Ni₃B eutectic phase formed in the coating after fusing. Metals of M₆C, M₃B₂, and M₇C₃ phases are composed of chromium, molybdenum, and nickel; chromium and molybdenum; and mainly chromium, respectively. The nickel phase in the coating has the L1₂ type superlattice structure.     

5083铝合金慢应变速率拉伸下的应力腐蚀行为

采用慢应变速率拉伸(SSRT)测试法及扫描电镜(SEM)与透射电镜(TEM)分析手段,研究了预变形量及应变速率对5083铝合金在空气和3.5%NaCl溶液中应力腐蚀开裂(SCC)行为的影响。结果表明:在空气中,5083铝合金几乎不发生SCC现象,其断口均呈现韧性断裂形貌;在3.5%NaCl溶液中,5083铝合金具有SCC敏感性,其断口呈现局部沿晶界或相界断裂形貌,随着应变速率的减小,应力腐蚀敏感指数(ISSRT)增大,当应变速率减至1×10-6s-1时,其应力腐蚀断口呈现典型的解理脆断特征;(Mg5Al8)等第二相析出及预变形后位错增多是导致5083铝合金SCC敏感性增大的主要原因。     

Microstructure and corrosion performance of a cold sprayed aluminium coating on AZ91D magnesium alloy

A pure Al coating was deposited on AZ91D magnesium alloy through cold spray (CS) technique. The microstructure of the coating was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the grain interfaces and subgrains formed close to the particle/particle boundaries. Electrochemical tests revealed that the cold sprayed pure Al coating had better pitting corrosion resistance than bulk pure Al with similar purity in neutral 3.5 wt.% NaCl solution. In addition, a mass-transfer step was found to be involved in the corrosion during 10 days immersion.     

Understanding the effect of nitrogen in austenitic stainless steel on the intergranular stress corrosion crack growth rate in high temperature pure water

Understanding the effect of nitrogen content on the crack growth rate (CGR) due to intergranular stress corrosion cracking (IGSCC) in high temperature (288 °C) pure water, in non-sensitised and strain-hardened stainless steel (SS) type 304 LN was the focus of this study. Non-sensitised SS containing two different levels of nitrogen (0.08 and 0.16 wt.%) in the solution annealed condition was strain-hardened by cross-rolling at 200 °C (warm rolling). It has earlier been reported that SS with a higher nitrogen level in the warm rolled condition has a higher CGR in high temperature pure water. Tensile testing was carried out using both the SS in the warm rolled as well as in the solution annealed condition at 288 °C. Samples were prepared for transmission electron microscopy (TEM) from the warm rolled SS and from the tensile tested (at 288 °C) specimens. TEM studies indicated that twinning and shear band formation were the major modes of deformation due to rolling at 200 °C and these feature were observed to terminate at grain boundaries, leading to regions of higher strain and stresses at grain boundaries. Higher nitrogen SS has higher grain boundary strain and stresses making the grain boundary regions more susceptible to IGSCC, resulting in higher CGR values. At 288 °C dislocation entanglement and cross-slip were the predominant modes of deformation.     

Interfacial reaction process of the hot-pressed WC/2024Al composite

12 vol%WCp/2024Al composite was fabricated from mixed powders by hot-pressing at various tempera-tures. Investigation of the interfacial reaction between the WC phase and the Al alloy matrix was performed by X-ray diffraction （XRD）, transmission electron microscope （TEM） and energy dispersive spectroscopy （EDS）. A multiple layer interface structure, which is composed of Al/ WAl12/AlnC3/WC, is found to form by the interfacial reaction during hot-pressing. Further study shows that the AlaC3 layer forms along with a given crystal orientation of WC phase and might retard the interfacial reaction process.     

(ZrB_2+Al_2O_3+Al_3Zr)_p/A356复合材料的原位直接反应法制备及微观组织(英文)

采用A356-(K2ZrF6+KBF4+Na2B4O7)作为熔体直接反应体系制备(ZrB2+Al2O3+Al3Zr)/A356复合材料。利用XRD、SEM和TEM等测试技术研究复合材料的相组成和微观组织。结果表明,复合材料增强相由ZrB2和Al2O3陶瓷相颗粒和Al3Zr金属间化合物相颗粒组成。ZrB2颗粒易团聚形成颗粒团簇并沿α(Al)合金晶界分布;ZrB2颗粒的微观形貌为六边形,尺寸在50nm左右。TEM研究发现,Al3Zr颗粒以小面形式生长,其长径比约为20;Al2O3颗粒形貌为长方体状和椭圆状,尺寸约为0.1μm。此外,基体与颗粒的相界面干净,无界面反应物生成。     

Nucleation and growth of the γ′(AlAg2) precipitate in Al–Ag(–Cu) alloys

Precipitation of the γ′(AlAg₂) phase was investigated in Al–Ag(–Cu) alloys using high-resolution transmission electron microscopy and scanning transmission electron microscopy. Precipitation commenced with segregation of Ag to stacking faults, followed by thickening in steps corresponding to single unit cell height ledges. In conjunction with gradual segregation of Ag and Al into ordered layers, this yielded γ′ phase platelets with a thickness of either 2 or 3 × the AlAg₂c-lattice parameter. Plates with a thickness of 2c(AlAg₂) could not achieve self-accommodation of the shape strain for transformation. Further thickening of the precipitates was slow, despite considerable Ag segregation around the precipitates. Growth by the addition of single unit cell height ledges is expected to lead to an additional shear strain energy barrier to ledge nucleation and this may contribute to a process of nucleation-limited growth.     

Microstructural and mechanical behavior of blended powder semisolid formed Al7075/B4C composites under different experimental conditions

This work aimed to fabricate B₄C reinforced aluminum matrix composites via blended powder semisolid forming that is an implementation of the benefits of semisolid forming to the powder metallurgy. Al7075 elements were incrementally added to ethanol solution under mechanical mixing. Al7075 constituents and B₄C particles were blended in a high energy ball mill. Cold compacted Al7075/B₄C blends were pressed at semisolid state. The effects of the size of the matrix (20, 45 and 63 μm), reinforcing volume fraction (5%, 10% and 20%) and semisolid compaction pressure (50 and 100 MPa) on the morphology, microstructure, density, hardness, compression and bending strength were thoroughly analyzed. Experimental results revealed that the highest microstructural uniformity was achieved when large B₄C particles (45 μm) were distributed within the small particles (20 μm) of the matrix phase. Composites with matrix particles larger than reinforcing phase indicated agglomerations in loadings more than 10% (volume fraction). Agglomerated regions resisted against penetration of the liquid phase to the pores and lowered the density and strength of these composites. Composites with 20 μm Al7075 and 20% (volume fraction) 45 μm B₄C powder pressed under 100 MPa exhibited the highest values of hardness (HV 190) and compressive strength (336 MPa).     

Microstructure and wear performance of Al5083/CeO2/SiC mono and hybrid surface composites fabricated by friction stir processing

Friction stir processing (FSP) was utilized to produce surface composites by incorporating nano-sized cerium oxide (CeO₂) and silicon carbide (SiC) particles individually and in combined form into the Al5083 alloy matrix. The study signified the role of these reinforcements on microstructure and wear behavior of the resultant surface composite layers. The wear characteristics of the resultant mono and hybrid surface composite layers were investigated using a pin-on-disc wear tester at room temperature. The microstructural observations of FSPed regions and the worn out surfaces were performed by optical and scanning electron microscopy. Considerable grain refinement and uniform distribution of reinforcement particles were achieved inside the nugget zone. All the composite samples showed higher hardness and wear resistance compared to the base metal. Among the composite samples, the hybrid composite (Al5083/CeO₂/SiC) revealed the highest wear resistance and the lowest friction coefficient, whereas the Al5083/SiC composite exhibited the highest hardness, i.e., 1.5 times as hard as that of the Al5083 base metal. The enhancement in wear behavior of the hybrid composites was attributed to the solid lubrication effect provided by CeO₂ particles. The predominant wear mechanism was identified as severe adhesive in non-composite samples, which changed to abrasive wear and delamination in the presence of reinforcing particles.