Combustion synthesis immiscible nanostructured Fe–Cu alloy

A bulk immiscible nanostructured Fe₆₀Cu₄₀ (ratio of atom) alloy is fabricated by a combustion synthesis combining rapid solidification technique that is convenient, inexpensive and capable of being scaled up for tailoring the bulk nanostructured materials. The Fe₆₀Cu₄₀ alloy is composed of dendrites (Fe(Cu) solid solution) with size of a few micron and matrix (Cu(Fe) solid solution) with size of about 30 nm. Owing to large superheat and rapid solidification process, there is no large-scale macroscopical separation in the Fe₆₀Cu₄₀ alloy.     

The melting behaviour of extruded Mg–8%Al–2%Zn alloy

Microscopical techniques were used to provide the microstructural details and identify mechanisms governing phase and morphological transformations during the heating of Mg–8%Al–2%Zn pellets in solid and semisolid states. It was found that an as-extruded matrix of equiaxed α-Mg grains with a twinning substructure, was thermally unstable and experienced complete recrystallization after reheating to 200 °C. The precipitates of the Mg₁₇Al₁₂ phase and augmented concentrations of alloying elements within migrating grain boundaries and triple junctions played a key role in transformation of the equiaxed grains into thixotropic structures during partial melting. A direct link exists between sizes of equiaxed grains in the solid state and unmelted particles in the semisolid slurry. Although the morphology of primary solid particles did not change during the melting progress, the rate of particles’ coarsening at high temperatures and their internal microstructure in subsequently solidified alloy were influenced by the solid–liquid ratio. The importance of these findings for semi-solid injection molding practice is emphasized.     

Abnormal grain growth in Al–3.5Cu

Significant abnormal grain growth has been observed in an Al–3.5 wt.% Cu alloy at temperatures where the volume fraction of small CuAl₂ particles was less than about 0.01. The initial fine-grained material had a weak crystallographic texture and there was no indication that any special boundaries were involved in the abnormal growth. Island grains isolated within the abnormal grains also showed no indication of special orientation relationships with their surrounding grains. Measurements indicated that the island grains initially had a size advantage over other matrix grains. The fraction of pinning phase was much lower at abnormal grain boundaries than at boundaries in the fine-grained matrix into which they were growing. A variety of simulations were made, including attempts to model that difference in pinning phase distribution, but none of these were successful in predicting abnormal grain growth.     

冷喷涂SiC/Al纳米复合涂层的组织结构与性能

为制备基体相晶粒细小、增强相均匀分布的SiC/Al纳米复合涂层,以Al、SiC为原料,采用高能球磨法获得SiC颗粒弥散分布的纳米晶Al基复合材料粉末,利用冷喷涂技术低温成型制备了SiC/Al纳米复合涂层,分析了SiC含量对复合涂层相结构、晶粒尺寸、微观结构、硬度及磨损性能的影响规律。结果表明：冷喷涂可实现球磨纳米晶复合粉末结构的原位移植,所制备SiC/Al纳米复合涂层组织致密,微米及亚微米级SiC弥散分布在纳米晶Al（约80 nm）基体之上;SiC颗粒对Al基体有明显强化作用,冷喷涂SiC/Al纳米复合涂层的硬度随SiC体积分数的增加而显著增加,50% SiC/Al纳米复合涂层的硬度高达515 HV_0.3,约为Al块材的13倍;冷喷涂SiC/Al纳米复合涂层的耐磨损性能随着SiC含量增加而显著提高,涂层磨损失效机制为磨粒对基体的切削犁沟变形。     

快速凝固AZ91D镁合金的相结构及位错

采用TEM和XRD分析技术研究了快速凝固AZ91D镁合金的相结构及位错。实验发现，在急冷快速凝固条件下，AZ91D镁合金惯常发生的L→α-Mg Mg17Al12共晶反应在很大程度上被抑制，形成了以过饱和的α-Mg为主相的快速凝固组织。凝固组织由尺寸为0．8gm～6gm的α-Mg晶粒、离散分布于晶内和晶界上的尺度在5nm-70nm之间的少量的椭球形Mg17Al12和极少量的多角形Al8Mn5颗粒、以及弥散分布于晶内的非常细小的不规则形态β-AlMg相组成。β-AlMg相为从α-Mg晶内过饱和析出的、逐步向Mg17Al12相过渡的亚稳定组态。在a-Mg晶粒内存在着大量的位错缠结和位错胞，位错胞尺寸在0．2μm～0．74μm之间；高密度位错主要存在于在α-Mg的(002)密排晶面上，且呈现出明显的方向性。     

FeNiCrCoAl3颗粒增强2024铝合基复合材料的显微组织和力学性能

将不同比例的2024商业铝粉和由气雾化法制得的FeNiCrCoAl,高熵合金粉球磨不同时间,然后,将混合粉末通过热挤出方法成型。通过XRD、SEM和TEM方法研究球磨粉和烧结后合金的显微组织,并通过应力测试机测试挤出样品的力学性能。结果表明：球磨后,粉末的晶粒尺寸减小,显微组织发生变化。混合粉末经过48h球磨后,颗粒平均直径约为30nm;粉末在热挤出后,晶粒尺寸约为70nm。在适当条件下,热挤出合金的压缩强度达到710MPa。通过对样品组织和性能关系的分析发现：强度的增加主要归因于纳米a（A1）和FeNiCrCoAl,颗粒以及析出的超细二次相Al6Fe相和富Fe相。     

搅拌摩擦加工原位反应制备Al_3Ti-Al表面复合层

通过在铝合金1100-H14表面加工矩形凹槽并添加微米级钛粉再进行搅拌摩擦加工(friction stir processing,FSP)的方法,在铝合金表面获得Al3Ti-Al复合层.采用扫描电镜(SEM),能谱分析(EDS)以及X射线衍射(XRD)对表面复合层微观结构及相组成进行了分析,并对复合层的显微硬度进行了检测.结果表明,在FSP强烈的热、力耦合作用下,钛粉产生了碎化,破碎后的钛颗粒与铝产生快速原位反应,生成微米和亚微米级Al3Ti颗粒,残留的钛颗粒和细小的Al3Ti颗粒一同均匀地分布于铝合金基体中,从而使得铝合金表面的硬度得到提高,其平均值达到了71.39HV,为基体硬度的2.1倍.     

Microstructure evolution and thermal stability of nanocrystalline Cu-Nb alloys during heat treatment

The microstructure evolution and high thermal stability of the mechanically-alloyed supersaturated nanocrystalline Cu-10%Nb alloy during subsequent heat treatment were investigated by X-ray diffractometry and transmission electron microscopy (TEM). The results show that no significant change of the microstructure of the solid solution can be detected after annealing at 300-400 ℃. The pronounced phase separation can be detected at 700 ℃. After annealing for 30 min at 900 ℃, almost all the Nb atoms precipitate from the solid solution, and the average Cu grain size is about 37 nm. As the solute atoms hinder the migration of fcc phase, at Cu grain boundaries, no significant grain growth occurs before large amount of Nb atoms precipitates from Cu matrix, and the decrease of internal strain and density of dislocation is small. Furthermore, the nanosized Nb precipitates can also help to reduce the Cu grains growth through precipitating pinning effect. Therefore, the mechanically-alloyed nanocrystalline Cu-Nb alloys have a high thermal stability. And the contaminations brought into the Cu matrix by milling can influence the phase formation and the thermal stability of Cu-Nb alloys during heat treatment.     

Strengthening mechanisms of an Al-Mg-Sc-Zr alloy

As a step toward developing an Al-Mg-Sc-Zr alloy for use up to 200 °C, the mechanisms responsible for alloy strengthening were identified for Al-6Mg-2Sc-1Zr (wt%) (Al-6.7Mg-1.2Sc-0.3Zr (at%)). The current work quantifies the active strengthening mechanisms at room temperature and explicitly considers solid solution strengthening, grain boundary strengthening, and Al₃(Sc,Zr) precipitate strengthening. Existing strengthening models, together with data from microstructural characterization were used to determine the magnitude of individual contributions. Strengthening due to the sub-micron grain size was the largest contribution to alloy strength, followed in decreasing order by precipitate strengthening and solid solution strengthening. Tensile yield strengths, 540–640 MPa (78–93 ksi), measured at room temperature agree well with predicted values. Model predictions showed that increasing the precipitate size from 7.5 nm to 20–25 nm and increasing the volume fraction of these particles from 0.015–0.025 up to 0.035 could produce a material with a yield strength of 865 MPa (125 ksi).     

Mechanical alloying of lanthana-bearing nanostructured ferritic steels

A novel nanostructured ferritic steel powder with the nominal composition Fe–14Cr–1Ti–0.3Mo–0.5La₂O₃ (wt.%) was developed via high energy ball milling. La₂O₃ was added to this alloy instead of the traditionally used Y₂O₃. The effects of varying the ball milling parameters, such as milling time, steel ball size and ball to powder ratio, on the mechanical properties and microstructural characteristics of the as-milled powder were investigated. Nanocrystallites of a body-centered cubic ferritic solid solution matrix with a mean size of approximately 20 nm were observed by transmission electron microscopy. Nanoscale characterization of the as-milled powder by local electrode atom probe tomography revealed the formation of Cr–Ti–La–O-enriched nanoclusters during mechanical alloying. The Cr:Ti:La:O ratio is considered “non-stoichiometric”. The average size (radius) of the nanoclusters was about 1 nm, with number density of 3.7 × 10²⁴ m^?3. The mechanism for formation of nanoclusters in the as-milled powder is discussed. La₂O₃ appears to be a promising alternative rare earth oxide for future nanostructured ferritic steels.     

Influence of zirconium content on microstructure and creep properties of Mo–9Si–8B alloys

Mo–Si–B alloys with a molybdenum solid solution accompanied by two intermetallic phases and Mo₅SiB₂ are a prominent example for a potential new high temperature structural material. In this study the influence of 1, 2 and 4 at.% zirconium on microstructure and creep properties of Mo–9Si–8B (at.%) alloys produced by spark plasma sintering is investigated. Creep experiments have been carried out at temperatures of 1100 °C up to 1250 °C in vacuum. The samples exhibit sub-micron grain sizes as small as 450 nm due to the chosen production route. With addition of 1 at.% zirconium, formation of SiO₂ on the grain boundaries can be prevented, thereby enhancing grain boundary strength and creep properties significantly. Moreover ZrO₂ particles also enhance creep resistance of the molybdenum solid solution. Creep deformation is a combination of dislocation creep in the grains including dislocation-particle interaction and grain boundary sliding leading to intergranular fracture surfaces. It is promising to use grain size adjustments in order to balance the creep and oxidation resistance of the investigated material.     

Characteristics of nano particles and their effect on the formation of nanostructures in air plasma spraying WC-17Co coating

WC-17Co nanostructured coating was prepared by means of air plasma spraying technology. Microstructures and compositions of the nano WC-Co powder and coating were analyzed using SEM (Scanning Electron Microscopy), and XRD (X-ray Diffraction), etc. The average grain size of the coating was measured using XRD. The mechanism of nanostructure formation and the properties of the nanostructured coating were investigated. The results show that the size of original particles is about 50-500 nm. Finer sub-particles of 2-5 nm are found to exist in the original particles. These sub-particles can act as crystallization nuclei and make the grains much finer during the plasma spraying process, which is beneficial to the formation of nanostructure in the coating. Both amorphous and nanostructured phases can be identified in the coating. The nanostructured coating is mainly composed of WC, W₂C and some amorphous phases. The nanostructured WC-Co coating has a good mechanical property combination. Nanostructured coating possesses good combination properties of micro-hardness, fracture toughness and bonding strength.     

Evidence for the formation mechanism of nanoscale microstructures in cryomilled Zn powder

Nanocrystalline Zn powder has been synthesized by a cryomilling method. The average grain size decreased exponentially with the cryomilling time and reached a minimum average grain size of around 17 nm. Large numbers of small grains (2∼6 nm) have been found in the very early stages of cryomilling. Dynamic recrystallization was used to explain the observed phenomena. The exothermic peaks revealed in the differential scanning calorimetry (DSC) results were correlated with the release of microstrain as confirmed by the x-ray diffraction measurements.     

Phase precipitation behavior and tensile properties of as-cast Ni-based superalloy during heat treatment

The phase precipitation behavior and tensile properties of an as-cast Ni-based alloy, IN617B alloy, after solution heat treatment and long-term aging treatment were investigated. Ti(C,N), M₆C and M₂₃C₆ are the primary precipitates in as-cast microstructure. After solution heat treatment, most of carbides dissolve into the matrix except a few fine Ti(C,N) within grains. During long-term aging at 700 °C, the phase precipitation behaviors of the alloy are characterized as follows: (1) M₂₃C₆ carbides at grain boundaries (GBs) transform from film-like shape to cellular shape and gradually coarsen due to the decrease of the surface energy and element aggregation to GBs; (2) M₂₃C₆ carbides within grains have a bar-like morphology with a preferential growth direction [110] and have a cube-on-cube coherent orientation relationship with the matrix γ; (3) γ′ particles inhibit the coarsening of M₂₃C₆ within grains by constraining the diffusion of formation elements. Furthermore, the tensile strength of the alloy obviously increases, but the ductility significantly decreases after the aging for 5000 h. The alloy has a relatively stable microstructure which guarantees the excellent tensile properties during long-term aging.     

Nanostructured yttria stabilized zirconia coatings deposited by air plasma spraying

Nanostructured yttria partially stabilized zirconia coatings were deposited by air plasma spraying with reconstituted nanosized powder. The microstructures and phase compositions of the powder and the as-sprayed nanostructured coatings were characterized by transmission electron microscopy（TEM）, scanning electron microscopy（SEM） and X-ray diffxaction（XRD）. The results demonstrate that the microstructure of as-sprayed nanostructured zirconia coating exhibits a unique tri-modal distribution including the initial nanostructure of the powder, equiaxed grains and columnar grains. Air plasma sprayed nanostructured zirconia coatings consist of only the nontransformable tetragonal phase, though the reconstituted nanostructured powder shows the presence of the monoclinic, the tetragonal and the cubic phases. The mean grain size of the coating is about 42 nm.     

Tungsten–vanadium–yttria alloys for fusion power reactors (I): Microstructural characterization

This study and a second part dedicated to the mechanical characterization provide a better knowledge of tungsten (W)–vanadium (V) alloys reinforced with yttrium oxide (Y₂O₃) particles, which have been scarcely investigated. Two W alloys (W-2 or 4 wt.% V-0.5 wt.% Y₂O₃) and a pure W material processed by powder metallurgy and consolidated by hot isostatic pressing were analysed. Along this part, the microstructure of the materials at room temperature is mainly analysed with a field emission scanning electron microscope.The densification in the compacts shows an increase with the V and Y₂O₃ additions. Porosity is reduced because of the formation of a W–V solid solution and V pools that fill the pores between the grains, although such effect is mainly observed in the W2V0.5Y alloy. The microstructure of pure W is composed of coarse polyhedral grains whereas a few coarse W grains, V pools and a nanostructured area, composed of fine W grains with dispersed Y, form the alloys. In contrast to previously studied W-4 wt.% V alloys, the V pools exhibit a reduction in the oxygen content, which prevents the formation of acicular oxide structures. Finally, the refinement of the microstructure induced by the addition of V and Y₂O₃ was analysed by electron backscattered diffraction measurements. Pure W presents high amount of grains over 1 μm (around 60% of the volume fraction) and only 2% below 100 nm. In the new alloys meanwhile, the population of micron size grains is highly reduced to less than 10% and grains smaller than 100 nm represent the 20%.     

Microstructure evolution and mechanical properties' improvement of NiAl–Cr(Mo)–Hf eutectic alloy during suction casting and subsequent HIP treatment

A NiAl–Cr(Mo)–Hf eutectic alloy was prepared by suction-casting technique and subsequently hot isostatic pressing treatment. Microstructure and mechanical tests were performed and the results revealed that the suction-cast alloy possessed fine NiAl/Cr(Mo) lamellar, large area fraction of eutectic cell and semi-continuously distributed Ni₂AlHf phase at the cell boundaries. After the HIP treatment, the Ni₂AlHf particles became fine and distributed evenly in the alloy. Moreover, some of the Ni₂AlHf particles along the eutectic cell boundaries were transformed into Hf solid solution phase. Compared with the conventionally cast alloy, the room-temperature compressive ductility and strength of the suction-cast alloy attained significant improvement. In addition, the room-temperature ductility and elevated temperature strength of suction-cast alloy were markedly enhanced by HIP treatment.     

Microstructure and properties of Cu−2Cr−1Nb alloy fabricated by spark plasma sintering

Cu?2Cr?1Nb alloy was fabricated by spark plasma sintering (SPS) using close coupled argon-atomized alloy powder as the raw material. The optimal SPS parameters obtained using the L₉(3⁴) orthogonal test were 950 °C, 50 MPa and 15 min, and the relative density of the as-sintered alloy was 99.8%. The rapid densification of SPS effectively inhibited the growth of the Cr₂Nb phase, and the atomized powder microstructure was maintained in the grains of the alloy matrix. Uniformly distributed multi-scale Cr₂Nb phases with grain sizes of 0.10?0.40 μm and 20?100 nm and fine grains of alloy matrix with an average size of 3.79 μm were obtained. After heat treatment at 500 °C for 2 h, the room temperature tensile strength, electrical conductivity, and thermal conductivity of the sintered Cu?2Cr?1Nb alloy were 332 MPa, 86.7% (IACS), and 323.1 W/(m·K), respectively, and the high temperature tensile strength (700 °C) was 76 MPa.     

Microstructural changes in cast martensitic steel after creep at 620°C

Microstructural changes in the cast steel GX12CrMoWVNbN10-1-1 (Fe–0.11 C–0.31 Si–0.89 Mn–9.57 Cr–0.66 Ni–1.01 Mo–1.00 W–0.21 V–0.06 Nb–0.05 Cu–0.05 N in wt %) have been investigated after tests for long-term strength at a temperature of 620°C in the range of stresses of 120–160 MPa. Upon short-term creep (up to 5000 h), the tempered troostite structure and distribution of particles of proeutectoid constituents change insignificantly, except for the precipitation of particles of the Laves phase ～100 nm in size along boundaries of laths, blocks, packets, and initial austenite grains. Upon long-term creep (to 10000 h), the tempered troostite partially transforms into the subgrain structure, which is accompanied by a decrease in the dislocation density from 6.4 × 10¹⁴ to 3.1 × 10¹³ m^–2 and connected with growth of sizes of M₂₃C₆ carbides of 105–150 nm and particles of the Laves phase to 380 nm, due to the dissolution of these particles located along path boundaries. Upon long-term creep, the average size of V(C,N) particles increases from 45 to 64 nm (while Nb(C,N) particles increase from 48 to 87 nm), and the Nb content in V-enriched carbonitrides and the V content in Nb-enriched M(C,N) particles substantially decrease. No formation of the Z phase has been revealed. The combination of M(C,N) nanoparticles with the presence of W in the solid solution has been found to be responsible for the enhanced high-temperature strength of the steel.