Synthesis and characterization of mechanically alloyed and shock-consolidated nanocrystalline NiAl intermetallic

The synthesis, microstructural characterization and microhardness of nanocrystalline B2-phase NiAl intermetallic are discussed in this paper. Nanophase NiAl powders were prepared by mechanical alloying of elemental Ni and Al powders under an argon atmosphere for different times (0–48 h). The alloyed nanocrystalline powders were then consolidated by shock compaction at a peak pressure of 4–6 GPa, to 83% dense compacts. Characterization by transmission electron microscopy (TEM) revealed that the microstructure of the shock-consolidated sample was retained at the nanoscale. The average crystallite size measurements revealed that mechanically alloyed NiAl grain size decreased from 48±27 to 9±3 nm with increasing mechanical alloying time from 8 to 48 h. The long-range-order parameters of powders mechanically alloyed for different times were determined, and were observed to vary between 0.82 for 5 h and 0.63 for 48 h of milling time. Following shock compaction, the long-range-order parameter was determined to be 0.76, 0.69 and 0.66, respectively, for the 16, 24 and 48 h alloyed specimens. Both the mechanically alloyed nanocrystalline NiAl powder and the shock-consolidated bulk specimen showed evidence of grain boundary dislocations, subgrains, and distorted regions. A large number of grain boundaries and defects were observed via high resolution TEM (HRTEM). Shear bands were also observed in the mechanically alloyed NiAl intermetallic powders and in the shock-consolidated compacts. Microhardness measurements of shock-consolidated material showed increasing microhardness with increasing crystallite size refinement, following Hall–Petch behavior.     

PAS consolidating behavior of mechanically alloyed nanocrystalline NiAl intermetallic compound

In this study, nanocrystalline NiAl intermetallic compound was obtained by mechanical alloying and PAS (plasma activated sintering method). Nanocrystalline NiAl powder was fabricated after 30 hr of milling with 2 wt.% stearic acid added as a PCA (process control agent) to the Ni-50at%Al composition. The grain size of the nanocrystalline NiAl powder was about 10 nm. Nanocrystalline NiAl powder was consolidated at 1000°C, 1100°C, 1200°C and 1300°C for 2 min with 30 MPa compressive force. The surface morphology of the NiAl consolidated at 1300°C was very regular and dense, above 96% of theoretical density (5.9 g/cm³). Al₄C₃ was observed in the NiAl consolidated at 1300°C by TEM analysis. It is thought that the carbons came from the stearic acid during the MA process and the graphite mold during the PAS process. The grain size of the NiAl consolidated at 1300°C did not increase but the grain shape became flat due to compressive force.     

Fe-29Ni-17Co合金的热变形行为

在实验温度为900-1200℃,应变率为0.001-1 s-1的条件下,对Fe-29Ni-17Co合金进行热压缩试验。热压缩过程中的动态再结晶导致材料的流变软化。用双曲正弦方程分析材料的流变行为,并确定了相应的材料常数A, n 和α,其中得到的表观活化能为423 kJ/mol。材料的应力峰值与稳态应力对Zener-Hollomon参数显示出简单的指数依赖关系。用Avrami方程分析了动态再结晶动力学,得到相应的指数为2.7,高于文献报道的2,这与连续动态再结晶机制有关。采用Cingara方程搭建峰值流变曲线,得到应变指数c约为0.85,比有关报道的不锈钢的c值0.2高,这点更加强了关于进行Fe-29Ni-17Co合金动态回复或连续动态再结晶研究的想法。     

Nanocrystalline FeAl matrix composites reinforced with TiC obtained by hot-pressing consolidation of mechanically alloyed powders

Bulk FeAl matrix nanocomposites reinforced with 50%, 30% and 10% of TiC were produced by hot-pressing consolidation of mechanically alloyed nanocrystalline Fe(Al)–TiC composite powders containing different amounts of reinforcement. The powders were compacted at 1000 °C under a pressure of 7.7 GPa. Structural investigations of the consolidated materials revealed that transformation of the Fe(Al) solid solution into FeAl intermetallic had occurred during sintering and that both the matrix and the reinforcement remained nanocrystalline, with the mean crystallite size of about 20 nm. We infer that applying of a high pressure hinders grain growth at elevated temperature. The microhardness of the produced materials is relatively high, ranging between 1363 and 1608 HV0.2, depending on the amount of TiC. The density of the obtained bulk materials is very close to 100% of the “ideal” value and the open porosity is zero. These results as well as structural investigations show that the quality of compaction with preserving of the nanometric grain size of the composite constituents is satisfactory.     

The effect of heat treatment on the structure and magnetic properties of mechanically alloyed Fe-45%Ni nanostructured powders

Magnetic iron-nickel alloys generally called permalloys are of great interest due to their magnetic properties. Fe-45%Ni alloy is one of the major iron-nickel compositions, well-known for high flux density, low coercivity and their responsiveness to the magnetic annealing. In this study, nanocrystalline Fe-45%Ni alloy powders were prepared by mechanical alloying process using a planetary high-energy ball mill under an argon atmosphere. The synthesized powders were heat treated at different temperatures using a vacuum furnace. The structural properties of the as-milled and the post-heat treated powders were studied by means of X-ray diffraction (XRD) technique and transmission electron microscopy (TEM). The magnetic measurements on the powders were carried out using a vibrating sample magnetometer (VSM). The results showed that the lattice strain decreases and the crystallite size increases with annealing temperature. It was also found that the variation of coercivity is dominated by the removal of residual stress at low annealing temperatures, whereas the value of coercivity depends on the crystallite size at higher annealing temperatures.     

Oxidation behavior of Mn and Mo alloyed Fe-16Ni-(5-8)Cr-3.2Si-1.0Al

Oxidation tests were conducted on a master alloy, Fe-16Ni-(5–8)Cr-3Si-lA1, to which (0–4) wt/o pct Mn and/or Mo were added. Tests were conducted at temperatures ranging from 1073–1273 K for times up to 1000 hr. Additions of Mn resulted in formation of a dual oxide structure and decreased oxidation protection. Addition of Mo significantly improved oxidation protection by formation of an intermetallic Fe(Mo)Si precipitate that eventually formed a protective SiO₂ oxide sublayer. Increasing the Cr concentration in an alloy containing both Mn and Mo resulted in a slight increase in weight gain. To first order, the apparent oxidation activation energy for all the alloys was nearly constant, 121 kJ/mol, suggesting that the same mechanism controlled the oxidation for all compositions. The oxidation protection was related to the alloy components and concentration.     

热压致密化块体纳米晶Ag50Ni50合金的显微组织

采用机械合金化,随后在620℃,58MPa下热压制备了致密的块体亚稳态纳米晶Ag50Ni50合金,用X射线衍射仪、扫描电镜和透射电镜研究了其组织结构.晶格常数的精确测定结果表明,球磨200h后Ag在Ni中固溶度为4.96%±0.21%(摩尔分数),Ni在Ag中固溶度为0.84%±0.30%(摩尔分数),热压以后仍具有过饱和固溶度,Ag在Ni中为0.45%±0.11%(摩尔分数),在相近温度下稳定化处理后进一步脱溶.600～700℃时Ag在Ni中固溶度为(0.21～0.24)%±0.11%(摩尔分数),Ni在Ag中固溶度小于0.1%(摩尔分数).机械合金化粉末的晶粒度约为6nm,热压后长大至40～60nm,退火后又长大至100～110nm.探讨了晶粒度与内应力同时作用下X射线衍射峰的展宽问题和纳米晶结构对快速致密化的作用.     

Effect of oxide species and thermomechanical treatments on the strength properties of mechanically alloyed Fe-17%Cr ferritic ODS materials

The effects of several oxide species, such as Y₂0₃, Zr0₂ and MgO, and the thermomechanical treatment (TMT) after the mechanical alloying (MA) process on the strength properties of Fe-17%Cr ferritic ODS (oxide dispersion strengthening) MA materials were investigated. Y20, showed the most uniform dispersion of the finest particles among me above oxides, but the microstructural evolution during the TMT had a larger effect on the strengthening of the alloys than the fine and uniform dispersion of the Y₂O₃ particles had.     

Characteristics of mechanically alloyed Cu, C and Cr mixed powders

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机械合金化Mo-Cu粉末的烧结特性

采用高能球磨方法制备了超细Mo-Cu复合粉末,通过X射线衍射,金相显微镜,扫描电镜和透射电镜等方法研究分析了所制备Mo-Cu粉末的烧结致密化、显微组织及性能的变化,并与未球磨粉末的烧结试样进行了比较.结果表明:经机械合金化的Mo-Cu粉末处于非平衡储能状态,烧结活性较高,使致密化温度降低80～100℃;其成形压坯在1250℃下烧结1.5h后,性能较佳,相对密度达到97.9%,且烧结体Mo、Cu两相分布均匀,其硬度、电导率、热导率分别达到70.10HRB、23.17ms/m、179.33W/(m·K),与未球磨粉末烧结体性能相比,都有了显著提高;过高的烧结温度与过长烧结时间,会引起Mo晶粒的明显长大.     

溅射Ni-16Co-26Cr-10Al纳米晶涂层的熔盐热腐蚀性能

利用磁控溅射技术在铸造Ni-16Co-26Cr-10Al合金上溅射了相同成分的纳米晶涂层,并进行900℃热腐蚀实验,结果表明:溅射Ni-16Co-26Cr-10Al纳米晶涂层由于晶粒尺寸细小,促进了保护性氧化物形成元素Al沿原"短路扩散通道-晶界"向氧化前沿的扩散,从而促进了保护性Al2O3的快速形成,使得溅射Ni-16Co-26Cr-10Al纳米晶涂层表现出比M38合金更优异的抗热腐蚀性能。     

Microstructure development in mechanically alloyed yttria dispersed austenitic steels

Austenitic oxide dispersion strengthened (ODS) alloys containing 0.5 and 5 wt.% yttria were prepared from elemental powders (Fe–20% Ni–14% Cr–2.5% Mo–2.5% Al–2% Mn) by mechanical alloying. The powders were analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy and transmission electron microscopy (TEM), paying particular attention to the behavior of yttria. XRD and high-resolution TEM analyses show that yttria does not form a solid solution with austenite; unlike in ferritic ODS alloys, where it dissolves. Milling induces the formation of the thermodynamically favorable yttrium aluminum perovskite (YAP). Alumina from the aluminum powder in the starting blend, formed in the initial stages of milling using oxygen available from the other elemental powders, combines with yttria to form YAP. The yttria content does not affect alloy formation but reduces the crystallite size and strain significantly in the 5% yttria composition. TEM analysis of hot-pressed compacts reveals nanocrystalline particles of yttria, yttrium aluminum garnet and YAP.     

机械合金化Cu-Cr粉末的制备及性质研究

采用机械合金化法制备铜铬粉末,研究了不同球磨时间对粉末颗粒结构、晶粒粒度、显微硬度和表面形貌的影响,用XRD、SEM方法表征Cu-15wt%Cr粉末在不同球磨时间下形成固溶体的结构和表面形貌.结果表明:高能球磨形成了铬固溶于铜的过饱和固溶体,随球磨时间的延长,晶粒逐渐细化,点阵畸变愈来愈大,球磨60h后,粉末呈近球形,畸变率为0.271%,显微硬度为349.18HV.     

铁-镍双掺杂方钴矿的合成与热电性能

采用微波加热合成结合放电等离子体烧结制备了铁-镍双掺杂方钴矿Co_(3.8-x)Fe_xNi_(0.2)Sb_(12) (x=0.05, 0.10, 0.15, 0.20)块体材料,并对其物相组成、晶粒尺寸、元素分布、热电性能等进行了系统研究。X射线衍射分析表明,样品X射线衍射峰与单相CoSb_3相符;场发射扫描电镜分析表明,样品晶粒尺寸为1～3μm、平均尺寸为1～2μm,各元素均匀分布;电性能分析表明,Ni/Fe双掺杂对电输运性能有进一步改善,最高功率因子为2.667×10~3μW·(m·K~2)~(-1);热性能分析表明,Fe掺杂对晶格热导率影响较小,晶格热导率与晶粒尺寸有关,主要热输运机制为晶界散射,Co_(3.65)Fe_(0.15)Ni_(0.2)Sb_(12)的最小晶格热导率为2.8 W·(m·K)~(-1)。Co_(3.7)Fe_(0.1)Ni_(0.2)Sb_(12)在773 K获得最大热电优值0.50,显著高于传统方法制备的Ni/Fe单掺杂或者双掺杂样品。     

TEM microstructure of mechanically alloyed Ti-12Mg powders

The microstructures of mechanical alloyed（MA） Ti-12%Mg alloy powders were examined using a high resolution TEM （HRTEM）. The effect of MA atmospheres such as argon gas and liquid isopropyl alcohol on the resultant microstructure was investigated. Both the MA powders form a homogeneous Ti-Mg solid solution, but the oxidation behavior is distinguished. The phase change was studied as a function of milling conditions and annealing temperatures.     

High-velocity oxyfuel reactive spraying of mechanically alloyed Ni-Ti-C powders

Recently, there has been considerable interest in producing cermet coatings with nanoscale carbide grains in the size range 50 to 500 nm. In this article, the production of nanoscale TiC grains in a Ni-based alloy matrix by reactive high-velocity oxyfuel (HVOF) spraying of metastable Ni-Ti-C powder is reported. Mechanical alloying of a Ni(Cr) prealloyed powder and Ti and C elemental powders was performed in a planar-type ball mill, and materials were characterized in detail using x-ray diffraction (XRD) and scanning electron micros-copy (SEM). Phase changes were correlated with milling time and other processing conditions. Results show that, by the selection of appropriate conditions, a metastable Ni-Ti-C powder could be obtained with the nominal composition 50wt.%Ni-40wt.%Ti-10wt.%C. Following sieving and classification, powder was produced with a particle size range of −38 to 8 μm, which is suitable for HVOF spraying. Coatings, approximately 250 μm thick, were deposited by HVOF spraying onto mild steel substrates, and the microstructures formed were investigated. XRD showed that a self-propagating high-temperature synthesis (SHS) reaction had occurred in the powder particles during spraying and that the principal phases present in the coating were TiC and a Ni-rich solid solution; small quantities of NiTi, TiO₂, and NiTiO₃ were also present. SEM revealed that the coatings had a characteristic, splatlike morphology and that TiC formed as a nanoscale dispersion, with a size range of ∼50 to 200 nm, within solidified splats. The microstructures of these reactively sprayed Ni-TiC coatings are briefly compared with those observed in HVOF-sprayed coatings deposited using prereacted SHS powder. The original version of this paper was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

机械合金化法制备Cu-Nb合金过程中的形变孪生特性

采用机械合金化法制备纳米Cu-10%Nb合金,通过显微硬度测量以及高分辨透射电镜观察,对该合金粉末在室温球磨过程中的微观结构演变和形变孪生特性进行研究;利用局部应力集中模型分析形变孪晶的形核机制。结果表明:随着球磨时间的增加,该合金硬度(HV)不断升高,球磨120 h后可达4.8 GPa;该合金在球磨初期以位错胞结构为主;球磨50 h后,Cu平均晶粒尺寸减小至约50 nm,部分区域出现纳米形变孪晶;继续增加球磨时间,孪晶数量增加,孪晶界强化效果显著;由于孪生将促进纳米晶粒的进一步细化,球磨120 h后,纳米晶尺寸减小到20 nm以下。     

An ultrasonic study of relaxation processes in pure and mechanically alloyed tungsten

High-temperature internal friction in a pure tungsten single crystal, polycrystals with different grain sizes, and mechanically alloyed tungsten polycrystals was studied. Positron annihilation spectroscopy was used to prove that all studied materials contain a detectable amount of dislocations. Then, resonant ultrasound spectroscopy was applied to determine the internal friction evolution with temperature from room temperature up to 740 °C. For the pure tungsten samples, a sharp increase of internal friction was observed for temperatures above 470 °C (for the single crystal) or above 400 °C (for the polycrystals); the activation energy corresponding to this increase was the same as the activation energy for the ductile-to-brittle transition in tungsten reported in the literature. For the alloyed materials, direct observation of the onset of this relaxation mechanism was impossible due to additional effects resulting from the secondary phases in the material.