Amorphization of the intermetallic compounds Co2Zr and Fe2Zr under mechanical grinding

The crystalline intermetallic compounds Co₂Zr and Fe₂Zr were produced in the stoichiometric composition and milled in a planetary ball mill for different milling periods. The samples were investigated in respect to the question if a crystal-to-glass transition occurs due to the milling process. Three different experimental methods were used for this study: X-ray diffraction, Mößbauer spectroscopy, and measurements of the specific heat capacityc _p . The intermetallic compound Fe₂Zr is very suitable for this study since it is ferromagnetic at room temperature. Thus it shows characteristic features in the Mößbauer spectrum and in the measurement of the specific heat capacityc _p . The investigation shows that the intermetallic compounds Co₂Zr and Fe₂Zr undergo a crystal-to-glass transition under mechanical grinding but the X-ray diffraction patterns show that the transformation is not complete. Even after long milling periods always an amount of a crystalline phase is present in the milled samples. In comparison the mechanically ground samples show the same properties as mechanically alloyed powder mixtures of the two elements of the same chemical composition. A probable explanation for the development of an amorphous phase by mechanical grinding of the crystalline compounds Co₂Zr and Fe₂Zr is the accumulation of internal strain in the crystalline grains. Another possible explanation, the addition of iron impurities to the crystalline compounds due to the wear debris of the milling equipment, seems to be improbable since the intermetallic phases Co₂Zr and Fe₂Zr show extended existance ranges in the equilibrium phase diagrams and hence are stable in respect to a variation in the composition.     

Comparison of the mechanical grinding of the crystalline compounds CoAl and CoZr

We report on a detailed comparison of the behavior of the CsCl-type compounds CoAl and CoZr under mechanical grinding in a shaker mill. The compound CoZr undergoes a crystal-to-glass transition whereas the compound CoAl develops into a nanocrystalline state. The samples are investigated by means of X-ray diffraction. For CoAl, a nearly complete disordering takes place. For CoZr, a crystal-to-glass transition occurs without detectable decrease of the long range order as long as Bragg-peaks are observed. Therefore the defects created in the compound CoZr must be very high-energetic in contrast to the defects in the compound CoAl. A simple model is given that can explain the observed crystal-to-glass transition for CoZr via a grain boundary mechanism.     

Photoacoustic study of nanocrystalline silicon produced by mechanical grinding

Mechanical grinding (MG) was used to produce nanocrystalline silicon and its thermal and transport properties were investigated by photoacoustic absorption spectroscopy (PAS). The experimental results suggest that in as-milled nanocrystalline silicon for 10 h the heat transfer through the crystalline and interfacial components is similar, and after annealed at 470 °C the heat transfer is controlled by crystalline component.     

Magnetocaloric properties of amorphous GdNiAl obtained by mechanical grinding

An amorphous GdNiAl sample was prepared by mechanical grinding performed on a crystallised intermetallic. The treatment changes greatly the ferromagnetic behaviour of this compound; its Curie temperature decreases from 57 K (unmilled sample) to 29 K (milled sample). Specific heat and magnetisation measurements reveal that amorphous GdNiAl exhibits an interesting magnetocaloric effect; for an applied magnetic field of 5 T a change of 8.9 J/kgK is observed at 36 K for the isothermal magnetic entropy. PACS 75.30.Sg; 65.40.+g; 75.50.Kj     

Amorphization of solids by fast neutrons

The diffraction patterns of amorphous solids prepared in the traditional way and by fast neutron bombardment are systematized. It is found that the diffraction patterns of these two classes of materials are different. In the case of a single crystal of titanium nickelide we have demonstrated here for the first time that these radiation-modified solids belong to the class of amorphous materials of distortion type. Fiz. Tverd. Tela (St. Petersburg) 40, 1584–1588 (September 1998)     

铟钇金属间化合物力学性能的第一性原理计算

本文基于密度泛函理论的平面波超软赝势方法,对铟钇(In-Y)金属间化合物的力学结构稳定性、弹性性质和热力学性能进行了研究.通过结构优化得到了In_3Y、InY、InY_2三种金属间化合物的晶格常数,发现与实验值比较吻合.弹性常数的计算结果表明In-Y金属间化合物的结构是稳定的,由弹性常数推算出In_3Y、InY、InY_2三种合金的体积模量、杨氏模量、剪切模量、泊松比和各向异性等力学性质,发现InY_2合金的体积模量、杨氏模量、剪切模量要比其它两种的值大,其抗形变能力更强.本文还预测了In-Y金属间化合物的热力学性质,如德拜温度、热导率,通过第一性原理计算得到的In-Y金属间化合物的力学和热力学性质为In-Y合金材料的实际应用和材料设计提供了参考.     

Vacancies effect on the mechanical properties in B2 FeAl intermetallic by the first-principles study

ABSTRACT

The geometric structures, electronic and mechanical properties of the high vacancy concentration intermetallic FeAl (experimental value: 3.3 at.% at 1451?K) were investigated by first-principles calculations based on density functional theory. The FeAl structures of different vacancy concentration with minimum energy were addressed, which shows that vacancies of iron (V_Fe) are more favourable and tend to gather together. For mechanical properties, both Young's modulus and elastic constants show an overall downward trend as vacancy concentration increases, but increase abnormally with the vacancy concentration ranging from 3.7 at.% to 5.6 at.%. All can be explained by the strength of Al–Fe bond, in other words, the Al–Fe interaction. Interestingly enough, intermetallic FeAl shows a transfer from the brittle manner to ductile manner, which also behaves as an important feature of FeAl in experiments. All the mechanical properties agree well with experimental data, indicating the reasonable vacancy model of FeAl intermetallic.     

Magnetic phase transitions in layered intermetallic compounds

Magnetic, magnetoelastic, and magnetotransport properties have been studied for the RMn₂Si₂ and RMn₆Sn₆ (R is a rare earth metal) intermetallic compounds with natural layered structure. The compounds exhibit wide variety of magnetic structures and magnetic phase transitions. Substitution of different R atoms allows us to modify the interatomic distances and interlayer exchange interactions thus providing the transition from antiferromagnetic to ferromagnetic state. Near the boundary of this transition the magnetic structures are very sensitive to the external field, temperature and pressure. The field-induced transitions are accompanied by considerable change in the sample size and resistivity. It has been shown that various magnetic structures and magnetic phase transitions observed in the layered compounds arise as a result of competition of the Mn–Mn and Mn–R exchange interactions.     

Transformation of graphite structure under mechanical grinding

Using x-ray diffractometry and computer simulation, the process of graphite structure transformation under mechanical grinding is investigated. It is established that all of main structural parameters vary over grinding time and are interrelated. The major parameter, whose variation affects the changes of the other, is the average size of the regions of coherent scattering along the crystallographic axis a, L_a. Reduction in the grain size is the main change under mechanical grinding, the change in the lattice parameters being its consequence. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 29–34, August, 2006.     

Persistent local chemical bonds in intermetallic phase formation

We found a direct evidence for the existence of the local chemical Bi–In bonds in the BiIn₂ melt. These bonds are strong and prevail, dominating the structure evolution of the intermetallic clusters. From the local structure of the melt-quenched BiIn₂ ribbon, the chemical Bi–In bonds strengthen compared with those in the equilibrium solidified alloy. The chemical bonds in BiIn₂ melt retain to solid during a rapid quenching process. The results suggest that the intermetallic clusters in the melt evolve into the as-quenched intermetallic phase, and the intermetallic phase originates from the chemical bonds between unlike atoms in the melt. The chemical bonds preserve the chemical ordered clusters and dominate the clusters evolution.     

Thermodynamics and point defects in the B2 intermetallic phase PdIn

The X-ray and bulk densities of PdIn alloys have been determined at ambient temperature on samples annealed at 1273 and 1373 K and quenched in water-ice mixtures. From these measurements the vacancy concentrations in this intermetallic phase have been obtained as a function of In concentration at these two temperatures. In addition, a generalized thermodynamic model is presented which considers the existence of antisite and vacancy defects on both sublattices without any dilute solution approximations. This model uses three energy parameters, whose values have been obtained for PdIn using the measured vacancy concentrations and the chemical potentials of In. This model is able to describe all the available vacancy data and is suitable for calculating these values at compositions and temperatures when such data are not available. In addition, the model permits the calculation of the concentrations of the minority point defects. It is shown that dilute-solution approximations, which have invariably been used in this kind of modelling, are unsatisfactory. The present results and their analysis confirm that PdIn is a near-triple-defect type of intermetallic and not a near-Schottky type, as has been claimed recently.     

Structure and mechanical properties of intermetallic magnetron-sputtered coating based on the Ni-Al system

The intermetallic coating phase composition, fine structure, and surface morphology have been investigated by electron microscopy and X-ray diffraction. It is shown that Ni₃Al is the primary phase in the intermetallic coating for all samples studied. The NiAl and AlFe₃C phases are present in small amounts. It is established that ion beam treatment of the coating changes its structural characteristics and radically transforms the character of coating wear.     

Heat treatment induced intermetallic phase transition of arc-sprayed coating prepared by the wires combination of aluminum-cathode and steel-anode

A method to prepare intermetallic composite coatings employing the cost-efficient electric arc spraying twin wires assistant with suitable heat treatment was developed. In this study, a Fe-Al composite coating was produced by spraying twin wires, i.e. a carbon steel wire as the anode and an aluminum wire as the cathode. The inter-deposited Fe-Al coating was transformed in-situ to Fe-Al intermetallic composite coating after a post annealing treatment. The effect of annealing treatment conditions on phase composition, microstructure and mechanical properties of the coating was investigated by using XRD, SEM, EDS and OM as well as microhardness tester. The results show that the desirable intermetallic phases such as Fe₂Al₅, FeAl and Fe₃Al are obtained under the annealing condition. The main oxide in the coating is FeO which can partially transform to Fe₃O₄ up to the annealing condition.     

Amorphization of ice near the melting point

In addition to reflections of the hexagonal phase of ice I _h, the intense diffuse scattering of X-rays mainly due to the amorphization of ice is revealed on the X-ray diffraction patterns of water ice samples prepared at liquid nitrogen (studied by the authors earlier) and samples prepared at T = ?10°C (this work). The measurements are performed in the temperature range from ?25 to 0°C. The existence of reflections of the crystalline phase and intense diffuse scattering on the X-ray diffraction patterns makes it possible make a conclusion about the coexistence of crystalline and amorphous structures of ice. Splitting of the first maximum on the electron-density radial distribution function is detected on the basis of an X-ray diffraction pattern recorded at T = ?3°C. This splitting is explained by an increase in the interatomic distances between the nearest-neighbor atoms located at different levels. Similar splitting was also detected on a radial distribution function constructed using an X-ray diffraction pattern recorded at ?10°C.     

Amorphization of metals by ion implantation and ion mixing methods

In this paper, we present a review of the experimentally determined characteristics of solid-phase amorphization of metals and alloys under ion implantation and ion mixing conditions. For the first time we systematically consider the characteristic features and the thermodynamic, structural, and kinetic criteria for amorphization in different metallic systems, and we demonstrate the expediency of the following classification of alloys undergoing amorphization: metal-metalloid systems, intermetallics, heterophase alloys, alloys with high positive heat of mixing. We compare solid-phase amorphization under conditions of bombardment by beams of charged particles, thermal mixing of the alloy components, and mechanical alloying. We consider the phase and structural states preceding amorphization and the possibilities for predicting metallic systems which can undergo amorphization.V. D. Kuznets Siberian Physicotechnical Institute, Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 3–30, August, 1994.     

Structure and orientation of an intermetallic phase in a W-Ni-Co alloy

The aim of this investigation is crystal structure determination of an intermetallic phase formed in a W-Ni-Co alloy during a heat-treatment carried out at a temperature of 800°C. This intermetallic phase is expected to play a critical role on the final microstructure (fine tungsten particles in an FCC matrix that is present in between large tungsten grains) and thereby, on the properties of the alloy. 92W-5.3Ni-2.7Co alloy was prepared through powder metallurgy route (liquid phase sintering) followed by heat-treatment at 800°C for 5?h. The intermetallic phase formed at this temperature was characterised using transmission and scanning electron microscopes. The intermetallic phase was found to have orthorhombic crystal structure with Pnam (62) space group as determined using automated diffraction tomography along with precession electron diffraction. Chemical analysis in TEM suggested that the intermetallic phase is based on stoichiometry (Co,Ni)₂W. Orientation imaging of the phase was also carried out in TEM and EBSD to understand its evolution. Equiaxed or lath morphology of the intermetallic phase was found to depend on the crystallographic orientation relationship of the phase with the tungsten grains and the matrix phase.     

Amorphization in Fe-Cu system by tracing its microscopic features

Strong evidence that amorphization was achieved by ion-beam mixing in a Fe-Cu system that has a large positive heat of formation (+19kJ/mole) is reported. The composition of the films, Fe₇₀Cu₃₀, was chosen according to those of the localized amorphous phase determined by energy dispersive spectroscopy. The phase identification by transmission electron microscopy, as well as the results from Rutherford backscattering, indicated that the formation of an amorphous phase and the mixing efficiency by ion-beam mixing depended greatly on the alloy composition. Raman spectra were obtained from the metal-metal thin amorphous films in this study and the related results are discussed.Project supported in part by Science Fund of the Chinese Academy of Sciences