Effects of Minor Element Additions to the Nanocrystalline FeAl Intermetallic Alloy Obtained by Mechanical Alloying

Chemical, microstructural, and mechanical properties of nanocrystalline FeAl intermetallic alloys with Li, Ce, and Ni additions have been assessed. Mechanical alloying and sintering procedures were used to produce and consolidate the alloys. The sintering procedure was based on room temperature uniaxial pressing followed by annealing in air of the pressed specimens. The mechanically alloyed powders have a microstructure consisting of micrometer-size particles that contain FeAl intermetallic nanocrystals. The three minor additional elements form a solid solution with the B2 intermetallic structure of the FeAl alloy. Densification greater than 90% has been obtained. The hardness values are higher than those obtained from specimens produced with conventional casting procedures. High resolution transmission electron microscopy images showed clusters of less than 5 nm with well-defined structure corresponding to Fe₃Al.     

机械合金化-热压烧结制备金属间化合物Fe_3Si

采用机械合金化(MA)和真空热压烧结(HP)法制备金属间化合物Fe3Si。X射线衍射(XRD)、扫描电镜(SEM)、差热分析(DTA)和振动样品磁强计(VSM)分别用于分析化合物的物相、显微形貌、致密度和磁学性质。研究表明球磨55h可达到完全合金化,Si溶入Fe中形成饱和固溶体α-Fe(Si),晶粒尺寸约7～8nm。热压烧结后,α-Fe(Si)固溶体发生有序转变生成Fe3Si。磁性能测量表明:样品的矫顽力随烧结温度的升高而减小;随烧结时间的延长而减小;饱和磁化强度随烧结时间的延长而增大。     

X-ray Diffraction Investigation of the Formation of Nanostructured Metastable Phases During Short-Duration Mechanical Alloying of Cu-Al Powder Mixtures

Mechanical alloying is a powder processing technique used to process materials farther from equilibrium state. This technique is mainly used to process difficult to alloy materials in which the solid solubility is limited, and to process materials where non-equilibrium phases cannot be produced at room temperature through conventional processing techniques. In the present work, mechanical alloying/milling of selected compositions in the Al-Cu binary alloy system was carried out at a ball-to-powder weight ratio (BPR) of 2 : 1, to investigate alloying and subsequent heat treatment on microstructural changes as a result of short milling times. Copper-aluminum powder mixtures containing 5, 20, and 40 wt% Al (11, 37, and 61 at% Al, respectively) were subjected to mechanical alloying, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC), after mechanical alloying and subsequent heat treatment. Nanometer-sized grains were observed in the as-milled crystalline powders in all compositions. Crystallite sizes were calculated using the Scherrer formula and found to be in the order of 10-20 nm after 360 minutes of milling time for all compositions. The XRD data show considerable solid solubility extension in these powders, and formation of intermetallic phases due to mechanical alloying and subsequent annealing. These changes are discussed in the context of the Al-Cu phase diagram.     

Nanoscale Grain Growth Behaviour of CoAl Intermetallic Synthesized by Mechanical Alloying

Grain growth behaviour of the nanocrystalline CoAl intermetallic compound synthesized by mechanical alloying has been studied by isothermal annealing at different temperatures and durations. X-ray diffraction method was employed to investigate structural evolutions during mechanical alloying and annealing processes. The disordered CoAl phase with the grain size of about 6 nm was formed via a gradual reaction during mechanical alloying. The results of isothermal annealing showed that the grain growth behaviour can be explained by the parabolic grain growth law. The grains were at nanometric scale after isothermal annealing up to 0·7 T _m. The grain growth exponent remained constant above 873 K indicating that grain growth mechanism does not change at high temperatures. The calculated activation energy indicated that the grain growth mechanism in the disordered CoAl phase at high temperatures was diffusing Co and Al atoms in two separate sublattices. Furthermore, an equation has been suggested to describe the grain growth kinetics of nanocrystalline CoAl under isothermal annealing at temperatures above 873 K (T/T _m ≥ 0·5).     

Recent Developments in Nanostructured Materials

Nanostructured materials with extremely fine grain sizes, typically less than 100 nm, have been shown to exhibit extremely high strength and hardness, increased diffusivity, useful sintering characteristics, and other unusual properties. Significant developments have taken place in recent years in trying to achieve high strength and ductility at the same time. New and potential applications are also being sought after. Several commercial ventures have also started (estimated to be around 1000 in different parts of the world). This overview will present the recent developments in our understanding of the mechanical behavior and molecular dynamics simulation studies, highlight the present unresolved issues, and discuss some of the promising applications for these novel and exciting materials.     

机械合金化制备金属铌的难熔化合物

金属难熔化合物材料因其具有良好的高温热稳定性能和力学性能，而呈现出广阔的应用前景。本文借助于机械合金化技术成功地制备出铌的难熔化合物ＮｂＣ、ＮｂＢ，并对其合成过程进行了分析。     

Ni—Ti—Cu粉末的机械合金化

用ＴＥＭ研究了Ｎｉ４０Ｔｉ５０Ｃｕ１０粉末在球磨过程中，颗粒尺寸及微观结构的变化。球磨６０小时后，得到非晶相粉末。用ＸＲＤ研究了球磨工艺条件对衍射峰相对强度和晶格常数的影响，结果表明，提高球磨转速和球料比，加快颗粒细化和合金化过程，合金化过程中，Ｃｕ和Ｎｉ原子向Ｔｉ中扩散速率较快，这与Ｔｉ的原子半径较大有关。ＤＳＣ测定了晶化转变开始温度为５１１℃。     

Mechanical alloying and microstructure of a Nb–20% V–15% Al alloy

A niobium-based alloy with 20% V–15% Al (at.%) was synthesized by mechanical alloying of elemental powders. During milling, the splitting of Nb X-ray peaks into two components was observed. Each component was found to correspond to a niobium solid solution (Nb_I and Nb_II) with a different lattice parameter. The intensities of Nb_I peaks on X-ray diffraction patterns decreased with the milling time and disappeared completely after 180 h of milling while the intensities of Nb_II peaks gradually increased. The powders were hot pressed and microstructural and phase analyses of the consolidated material were carried out. The microstructure consisted of Nb solid solution, Nb₃Al-base intermetallic with the A15 crystal structure and dispersoid Al₂O₃. Also an unexpected, detrimental, Nb₂Al-base σ phase was found. The volume fraction of the σ phase depended on the temperature of consolidation.     

Alloying behaviour in nanocrystalline materials during mechanical alloying

The alloying behaviour in a number of systems such as Cu-Ni, Cu-Zn, Cu-Al, Ni-Al, Nb-Al has been studied to understand the mechanism as well as the kinetics of alloying during mechanical alloying (MA). The results show that nanocrystallization is a prerequisite for alloying in all the systems during MA. The mechanism of alloying appears to be a strong function of the enthalpy of formation of the phase and the energy of ordering in case of intermetallic compounds. Solid solutions (Cu-Ni), intermetallic compounds with low ordering energies (such as Ni₃Al which forms in a disordered state during MA) and compounds with low enthalpy of formation (Cu-Zn, Al₃Nb) form by continuous diffusive mixing. Compounds with high enthalpy of formation and high ordering energies form by a new mechanism christened as discontinuous additive mixing. When the intermetallic gets disordered, its formation mechanism changes from discontinuous additive mixing to continuous diffusive one. A rigorous mathematical model, based on iso-concentration contour migration method, has been developed to predict the kinetics of diffusive intermixing in binary systems during MA. Based on the results of Cu-Ni, Cu-Zn and Cu-Al systems, an effective temperature (T _eff) has been proposed that can simulate the observed alloying kinetics. TheT _eff for the systems studied is found to lie between 0·42–0·52T ₁.     

Phase Development and Crystallization Kinetics of NiTi Prepared by Mechanical Alloying

NiTi alloy is produced by mechanical alloying(MA). It becomes amorphous after milling for enough time, such as 100 h in this paper. DSC measurement shows that the crystallization temperature is 676 K for the amorphous powder. Activation energy of crystallization is 199.98kJ/mol for MA powder, which is lower than that of amorphous prepared by magnetron sputtering.Avrami parameter of crystallization is 1.07.     

机械合金化法制备W—Cu合金的研究

采用高能球磨的机械合金化法制备W—Cu合金,对其在实验过程中的工艺条件进行了研究。结果表明：随着球磨时间的延长,粉末的粒度逐渐细化,比表面积逐渐增大。球磨过程分为三个阶段。最佳工艺条件是：当球料比为5：1,W：Cu为8：2,球磨时间达到30h时,铜粉和钨粉基本形成固溶体。球磨过程加入少量的无水乙醇,防止在球磨过程中粉末被氧化。     

Cover Picture: Recent Developments in Nanostructured Materials (Adv. Eng. Mater. 11/2005)

The cover shows transmission electron micrographs of typical microstructures in as‐deposited copper samples. More about recent developments in nanostructured materials research can be found in the review paper by C. Suryanarayana on page 983. Nanostructured materials with extremely fine grain sizes, typically less than 100 nm, have been shown to exhibit extremely high strength and hardness, increased diffusivity, useful sintering characteristics, and other unusual properties. Significant developments have taken place in recent years in trying to achieve high strength and ductility at the same time. New and potential applications are also being sought after. Several commercial ventures have also started (estimated to be around 1000 in different parts of the world). This overview will present the recent developments in our understanding of the mechanical behavior and molecular dynamics simulation studies, highlight the present unresolved issues, and discuss some of the promising applications for these novel and exciting materials.     

机械合金化法制备Ni-Nb纳米级非晶合金

本文采用机械合金化法（MA）,以元素粉末为原料,对成分为Ni60 Nb40 的混合粉末进行球磨,采取湿磨和逐步提高球磨速度的方法,制备纳米级非晶合金。利用X射线衍射技术对球磨产物进行物相分析。结果表明,采取湿磨和分步提高球磨速度手段会促进纳米级非晶态组织的形成。此非晶合金的形成是晶粒细化、球磨过程中粒子的表面能增大、缺陷和应力增强等多种作用的结果。     

Microstructural evolutions of nickel-aluminide nanocomposites during powder synthesis and thermal spray processes

The synthesis and microstructural evolutions of the NiAl-15 wt% (Al₂O₃–13% TiO₂) nanocomposite powders were studied. These nanocomposite powders are used as feedstock materials for thermal spray applications. These powders were prepared using high and low-energy mechanical milling of the Ni, Al powders and Al₂O₃–13% TiO₂ nanoparticle mixtures. High and low-energy ball-milled nanocomposite powders were also sprayed by means of high-velocity oxy fuel (HVOF) and air plasma spraying (APS) techniques respectively. The results showed that the formation of the NiAl intermetallic phase was noticed after 8 h of high-energy ball milling with nanometric grain sizes but in a low-energy ball mill, the powder particles contained only α-Ni solid solution with no trace of the intermetallic phase after 25 h of milling. The crystallite sizes in HVOF coating were in the nanometric range and the coating and feedstock powders showed the same phases. However, under the APS conditions, the coating was composed of the NiAl intermetallic phase in the α-Ni solid solution matrix. In both of the nanocomposite coatings, reinforcing nanoparticles (Al₂O₃–13% TiO₂) were located at the grain boundaries of the coatings and pinned the boundaries, therefore, the grain growth was prohibited during the thermal spraying processes.     

Microstructure and magnetic properties of nanostructured Fe–Co powders prepared by series of milling and annealing treatments

Fe_1−xCo_x(x = 0.1, 0.15, 0.2, 0.25, 0.3 and 0.5) powders were prepared by different milling-annealing treatments, and magnetic properties were investigated based on microstructure. Elevated heating times led to an increase in crystallite size, and decrease in lattice parameter. Up to 20 min annealing, series 3 powders showed a decrease in microstrain 2.5 times more than series 2. The coercivity (H_C) of 1-step milled and 60 min annealed Fe₅₀Co₅₀ alloys decreased rigorously from 60 Oe to 19 Oe due to strain relief (from 0.3% to 0.08%) and grain growth (from 30 nm to 40 nm). For series 2 alloys, the H_C (up to 60 min heating) increased from 72 Oe to 90 Oe, and decreased (up to 100 min heating) to 70 Oe. Compared to series 1, extra milling treatment of series 2 causes an increase in magnetization saturation (M_S) due to completion of alloying and grain refinement. Also, compared to series 2, extra annealing treatment for series 3 resulted in larger values of M_S caused by stain relief.     

Mechanical properties of nanostructured Mg–5 wt%Al–x wt%AlN composite synthesized from Mg chips

In the present investigation, Mg chips are recycled to produce nanostructured Mg–5wt%Al reinforced with 1, 2 and 5 wt% nanosized AlN particulates by mechanical milling (MM). Each batch of composite mixture was milled for different milling durations to produce different degrees of grain refinement. The mechanical properties such as tensile strength, ductility and hardness with respect to the effect of grain refinement, in other words, milling duration were studied. It was found that grain size played an important role in controlling ductility of the composites.     

高聚物的机械合金化

机构合金化是一种新型的固相加工法,它能用于制备许多加工困难的高性能聚合物,高分子合金和复合材料。综述了机械合金化高聚物的形貌,结构,性能等特点,阐述了高聚物机械合金化的机理。