Structural, magnetic and magnetocaloric properties of Heusler alloys Ni50Mn38Sb12 with boron addition

We report on the structural, magnetic and magnetocaloric properties of the Ni₅₀Mn₃₈Sb₁₂B_x alloys in term of boron addition with x = 1, 3 and 5. We have found that both the paramagnetic-ferromagnetic austenitic transition (T_C) and the ferromagnetic-antiferromagnetic martensitic transition (T_M) are sensitively influenced by the boron addition: T_C tends to increase, while T_M decreases with increasing boron concentration. Temperature dependent X-ray diffraction in the range of 200-500 K clearly shows an evolution of the structural transformation from orthorhombic to cubic structure phase transition on heating for the x = 1 and 3 samples. Strikingly, the addition of boron atoms into the lattice favours the ferromagnetic ordering relatively to the antiferromagnetic arrangement below T_M. This consequently affects on the magneto-structural transition as well as on the size of magnetocaloric effect.     

Pulsed laser ablated off-stoichiometric thin films of the Heusler alloy Co2TiSn on Si (100) substrate

We report the growth of thin films of ferromagnetic Heusler alloy Co₂TiSn on Si (100) substrate using KrF excimer pulsed laser ablation. Films of thicknesses ranging from 8 to 220 nm were deposited on Si (100) substrate heated up to 200 ± 10 °C, with an aim to study the structural, morphological and magnetic properties. The grown films are off-stoichiometric, polycrystalline, having single-phase with high degree of (220) texturing. Angle dependent fluorescence measurements suggest no segregation of alloying elements as a function of depth. X-ray reflectivity measurements indicate that all the films are having low density layer at the top as well as at the film-substrate interface. Magneto optical Kerr effect measurements at room temperature reveal clear hysteresis loops suggesting ferromagnetic behavior of the films. Thermal annealing at temperature ≥ 220 °C suggest transformation of Co₂TiSn phase into cobalt silicide phase, which confirms the necessity of low substrate temperature (< 220 °C) to produce such single-phase Co₂TiSn films.     

条纹图形Heusler合金CoFeMnSi薄膜的制备及其磁特性研究

以CoFeMnSi作为研究对象,对其进行图形化设计,以研究图形化CoFeMnSi薄膜的磁学特性。利用感光溶胶-凝胶法和激光干涉法制得条纹图形ZrO2薄膜,之后利用磁控溅射法在其表面溅射沉积CoFeMnSi,以达到制得图形化CoFeMnSi磁性薄膜的目的,并对其表面形貌和磁学特性进行了表征。采用金相显微镜分析验证CoFeMnSi薄膜继承了ZrO2的条纹图形结构,条纹图形周期约为2μm;面内磁性测量显示薄膜398 kA/m磁场下的磁化强度与外磁场和条纹夹角θ呈180°周期性变化关系,且磁化强度介于390~440 kA/m;利用CoFeMnSi平膜面内磁化强度及面外磁化强度与外加磁场方向的变化关系,解释了条形薄膜磁化强度的θ角度依赖关系;采用磁力显微镜观察到磁畴结构形态为蜂窝状,磁畴尺寸大约1~2μm。     

Effect of substituting sp-element on half metallic ferromagnetism in NiCrSi Heusler alloy

The investigation of the electronic and magnetic properties of semi Heusler alloy, NiCrSi and alloys obtained by substituting Si with Ge and P (i.e. NiCrGe and NiCrP) has been presented. The present work aims to search the new candidates showing half metallic (HM) ferromagnetism using all-electron full potential linearized augmented plane wave (FPLAPW) method within generalized gradient approximation (GGA) formalism for exchange and correlation (XC) effects. We predict that these alloys are true half metallic ferromagnets (HMFs), showing an indirect band gap in minority density of states (DOS). This gap is largest in case of NiCrSi (0.91 eV). The substitution of sp-element in NiCrSi results in the minority band gap reduction. The main contribution in total magnetic moment comes from Cr-3d states. Moreover, it has an integer value in present alloys as expected in accordance with the Slater-Pauling (SP) rule.     

Microstructure and thermoelectric properties of Heusler Fe2VAl thin-films

Heusler-type Fe₂VAl thin-films were prepared on zirconia substrate by radio-frequency magnetron sputtering. The effect of deposition temperature on crystal structure and electrical properties was investigated. The degree of Heusler-type ordering in crystal structure was enhanced by the increase in deposition temperature. The enhanced Heusler-type ordering contributed to the formation of a steep density of state derived from pseudogap opening near Fermi level, resulting in large Seebeck coefficient. Since the film consisted of submicrometer-size grains, the electrical conductivity and the thermal conductivity of the film were both reduced by the grain boundary effect. The estimated thermoelectric power factor was 1.0 mW/mK² at 350 K.     

Martensitic transformation,crystal structure and shape memory effect in Ni55−xMn25Ga20Cox alloys

ABSTRACT

The effect of cobalt addition instead of nickel on the crystal structure, martensitic transformation behaviour and shape memory effect were investigated. Within the analysed range of chemical composition, a single non-modulated martensite was detected at ambient temperature. Cobalt addition modified the lattice parameters and, thus, affected the tetragonality of the martensite unit cell. The hysteresis of martensitic transformation was differently affected by the type of heat treatment applied. For furnace cooled samples, the hysteresis decreased from 50°C to 30°C; in the case of water quenched samples, the hysteresis sharply increased up to 60°C. The shape memory effect, measured as the recoverable strain upon annealing after compression tests, reached a fully recoverable deformation at 10?at.-% of cobalt.

This paper is part of a Thematic Issue on The Crystallographic Aspects of Metallic Alloys.     

Ultrafast heating effect on transient magnetic properties of L10-FePt thin films with perpendicular anisotropy

Laser-induced ultrafast magnetization dynamics and transient coercivity behavior in perpendicular magnetized L1₀-FePt films are investigated using the time-resolved polar magneto-optical Kerr technique. The magnetization after photo-excitation shows a dramatic reduction on the picosecond time scale. In contrast, the coercivity shows a weak decrease, accompanied by a skewed Kerr loop shape for low and intermediate fluences. The results can be interpreted by the laser-induced non-uniform demagnetization due to the weakened coupling between FePt grains of different size and/or components. The remaining coercivity vanishes when the film is fully demagnetized at higher fluence. We claim that the remaining coercivity can be manipulated by employing appropriate laser energy and film thickness, which may be helpful for application in heat-assisted magnetic recording.     

Electronic band structure calculations on thin films of the L21 full Heusler alloys X2YSi (X, Y = Mn, Fe, and Co): Toward spintronic materials

To design half-metallic materials in thin film form for spintronic devices, the electronic structures of full Heusler alloys (Mn₂FeSi, Fe₂MnSi, Fe₂FeSi, Fe₂CoSi, and Co₂FeSi) with an L2₁ structure have been investigated using density functional theory calculations with Gaussian-type functions in a periodic boundary condition. Considering the metal composition, layer thickness, and orbital symmetries, a 5-layered Co₂FeSi thin film, whose surface consists of a Si layer, was found to have stable half-metallic nature with a band gap of ca. 0.6 eV in the minority spin state. Using the group theory, the difference between electronic structures in bulk and thin film conditions was discussed.     

The effect of microstructure on the mechanical properties of two-phase titanium alloys

This article presents the results of investigations of microstructure and mechanical properties of two-phase +β titanium alloys with different volume fraction of the β-phase. Microstructure of the specimens was examined using an optical microscope. Fracture surfaces were observed by SEM technique. The influence of the microstructure and phase composition on the mechanical properties of the alloys was studied. Static tensile tests, hardness tests and fatigue investigations were performed. It was noticed that the volume fraction and chemical composition of the β-phase has a significant effect on mechanical properties and cracking process during fatigue.     

Effect of Ga irradiation on the magneto-optic spectra of Pt/Co/Pt sandwiches

Interfacial changes in rf sputtered Pt/Co(2.6 nm)/Pt sandwiches grown onto sapphire (Al₂O₃) substrates induced by irradiation of 30 keV Ga⁺ ions at low dose (10¹⁴ ions/cm²) have been investigated by magneto-optic polar Kerr rotation (PKR) spectroscopy between 1 and 5 eV. The irradiation resulted in an increase of PKR over the whole spectral range. The measured PKR spectra were compared with those computed from the transfer matrix formalism using known polar Kerr rotation and ellipticity spectra for Co and five Co_xPt_1 − x alloys. The comparison between measured and computed PKR spectra provided an in-depth profile of Co and Pt ion distributions across the sandwich and confirmed that irradiation favors alloying in the vicinity of the two interfaces. These results are in a good agreement with the profile evaluated independently by TRIDYN simulations. Our results evidence an asymmetry in the irradiation effect due to an excess of Pt-Co alloying at the upper interface. Moreover, the observation of a negative PKR peak around 3.2 eV states definitively the presence of a chemically ordered Co_0.75Pt_0.25 alloy phase inside the irradiated film structure.     

Ion channeling study of epitaxy of iron based Heusler alloy films on Ge(111)

We have investigated perfection of atomic rows on iron-based Heusler alloy films on Ge(111) planes by using ion channeling technique in order to find the dominant factors for the perfection. Fe₃Si/Ge(111) and Fe₂CoSi/Ge(111) have a high quality of atomic rows at the heterointerface like that of perfect crystals. Fe_3−xMn_xSi/Ge(111) (x = 0.84, 0.72 and 0.36) interfaces have imperfection of atomic rows which may be controlled by both the lattice mismatch with the Ge substrate and the Mn-Si pairs due to the site disorder in the film with the Mn content x > 0.75. Analysis of axial channeling parameters employed in this study is very useful for quantitative evaluation of perfection of atomic rows at the heterointerface.     

On enhancing the mechanical properties of aluminum P/M alloys

Sintered aluminum alloys are an attractive material for the automobile industry, both because of the low specific gravity and high strength-to-weight ratio of aluminum itself, and the fabrication advantages associated with a powder metallurgy process. However, properties such as impact, stiffness, corrosion and wear resistance are often poor, thereby restricting the widespread use of these materials. Recent work by the authors has shown that hardness, wear resistance and tensile properties of a P/M Al–Cu–Mg ternary master alloy can be improved using a novel diffusion/supersolidus liquid phase sintering process. Improvements were due to in-situ microalloying during sintering, in particular, the influence of Ag and Sn. To complement this work, the present investigation addresses the response of a commercial alloy, AA2014, to the microalloying process. Results show that sintered densities for the commercial alloy were relatively unaffected by the presence of either Ag or Sn, and were superior to the ternary master alloy. Hardness and tensile properties were also improved relative to those obtained for the ternary, and were comparable to wrought 2014. Examination of final microstructure of Ag modified AA2014 using TEM showed the presence of Ω as the principal precipitate, but only after extended sintering times. This particular precipitate is believed to contribute to enhanced hardness. The apparent absence of Ω for short sintering times was due to the presence of silicon in the commercial product. However, the corrosion behavior of the P/M AA2014 was superior to the wrought product and thus the process is presented as a potential P/M alternative to using ingot metallurgy techniques for microalloying.     

Influence of alloying and secondary annealing on anneal hardening effect at sintered copper alloys

This paper reports results of investigation carried out on sintered copper alloys (Cu, 8 at%; Zn, Ni, Al and Cu-Au with 4 at%Au). The alloys were subjected to cold rolling (30, 50 and 70%) and annealed isochronally up to recrystallization temperature. Changes in hardness and electrical conductivity were followed in order to investigate the anneal hardening effect. This effect was observed after secondary annealing also. Au and Al have been found to be more effective in inducing anneal hardening effect.     

First-principles study: Effect of relaxation on the geometrical structure of La0.5Co4Sb12

In this study, the internal parameters of La-filled skutterudite La_0.5Co₄Sb₁₂ were relaxed following the damped Newton dynamic schemes. The relaxation results show that the distances and the bond lengths of atoms around the filling atom slightly change. The NN (nearest neighbor) Sb atom shows outward relaxation. Contrarily, the NNN (next nearest neighbor) Co atom and the TNN (third nearest neighbor) Sb atom show inward relaxations. The relaxation of internal parameters resulting from La filling has significant effects on the bonding charge density resulting in the change of charge distribution and bond length as well as bond angle. Based on the internal parameters after relaxation, the electronic structures of La_0.5Co₄Sb₁₂ were calculated by utilizing the FLAPW (Full Potential Linear Augmented Plane Wave) based on the DFT (Density Functional Theory) of the first-principles in detail. As a comparison, the band structure of CoSb₃ was calculated. The effects of La filling on the electronic structure and electrical conduction of La_0.5Co₄Sb₁₂ were discussed.     

First-principles study on the mechanical,thermal properties and hydrogen behavior of ternary V-Ni-M alloys

Studying hydrogen behavior in alloys and the mechanical properties of alloys are essential to various practical uses,such as separation membranes,as well as hydrogen embrittlement protection.In order to further develop the non-Pd-alloy membranes used in hydrogen separation,the mechanical,thermal properties of V14NiM(M= Al,Fe,Si,Ti,Zn)and hydrogen solubility and diffusion behaviors of V-based ternary alloys were studied by first principles calculation.The results indicated that the hydrogen solution energies of V-Ni-M are greater than pure vanadium.And the mono-vacancy in pure vanadium can capture 6 H atoms while the V-Ni-M alloys can only capture 5 H atoms.Therefore,the V-Ni-M alloys exhibit lower solubility of hydrogen and higher brittleness resistance to embrittlement compared with pure vanadium.And the diffusion coefficients of V-Ni-M alloys are smaller than that of pure vanadium thanks to smaller hydrogen solubility.The hydrogen solubility and hydrogen permeability can maintain relatively balanced.The study of mechanical properties suggests that the V-Ni-Ti has the best resistance to deformation and pure vanadium has the best ductility.Moreover,V-Ni-Si alloy has the smallest thermal expansion coefficient in the temperature range of 473-723 K,which is the temperature of hydrogen separation,indicating that V-Ni-Si is the best for hydrogen separation according to thermal properties.     

Grain size-dependent Mg/Si ratio effect on the microstructure and mechanical/electrical properties of Al-Mg-Si-Sc alloys

Al-Mg-Si-Sc alloys with different Mg/Si ratio (<1.73 in wt.% vs>1.73 in wt.%) and different grain size (coarse grains vs ultrafine grains) were prepared, which allowed to investigate the grain size-dependent Mg/Si ratio effect on the microstructural evolution and concomitantly on the hardness and electrical conductivity when subjected to aging at 200 °C. In the coarse-grained Al-Mg-Sc-Sc alloys, the β″ precipitation within the grain interior and also the precipitation hardening were highly dependent on the Mg/Si ratio, while the electrical conductivity was slightly affected by the Mg/Si ratio. A promoted β″ precipitation was found in the case of Si excess (Mg/Si ratio <1.73), much greater than in the case of Mg excess (Mg/Si ratio>1.73). While in the ultrafine-grained Al-Mg-Si-Sc alloys, the electrical conductivity rather than the hardness was more sensitive to the Mg/Si ratio. The alloy with Si excess displayed electrical conductivity much higher than its counterpart with Mg excess. This is rationalized by the grain boundary precipitation promoted by Si, which reduced the solute atoms and precipitates within the grain interior. Age softening was found in the ultrafine-grained alloy with Si excess, but the ultrafine-grained alloy with Mg excess held the hardness almost unchanged during the aging. The hardness-conductivity correlation is comprehensively discussed by considering the coupling effect of Mg/Si ratio and grain size. A strategy to simultaneously increase the hardness/strength and electrical conductivity is proposed for the Al-Mg-Si-Sc alloys, based on present understanding of the predominant factors on strengthening and conductivity, respectively.     

时效工艺对Cu-Te-Cr合金性能的影响

研究了在不同时效温度下,时效时间对Cu-Te-Cr合金力学性能和电学性能的影响.利用扫描电镜和能谱仪分析了析出相的形貌、组成及其分布.研究表明,随着时效时间的延长,合金硬度和导电率都先快速上升,然后缓慢下降,出现一个类似峰值点.且温度越高,达到峰值所需的时间越短;随着Te含量的增加,合金硬度减小,而导电率则相对提高.这是由于随时效时间的延长和时效温度的提高,合金发生晶粒长大和第二相析出、从而提高合金硬度和导电率;随时效的进一步进行,部分第二相发生重溶,晶粒进一步长大,合金硬度和导电率缓慢下降.     

钛合金精密铸造陶瓷型壳性能的研究

获得高质量精密铸造钛合金铸件的关键是制备出高质量的陶瓷型壳 .本文对Y/Y、ZY/ZY和TJ/ZC等三种面层材料的性能进行了研究 ,主要包括TG/DTA测试、型壳强度测试以及热膨胀性能测试 .实验表明 ,TJ/ZC材料是钛合金精密铸造理想的面层材料 .     

Cr/Mo比对FeCoMoCrZr非晶合金晶化过程及磁性能的影响

采用单辊急冷法制备了一系列不同Cr/Mo比的FeCoMoCrZr非晶薄带,并对该系非晶合金进行等温热处理。用XRD和VSM研究Cr/Mo比的变化对(Fe0.58Co0.42)73Mo17-xCrxZr10系非晶合金晶化过程和磁性能的影响。结果表明,x在9~17之间变化时,所制备的合金薄带为非晶结构;(Fe0.58Co0.42)73Mo5Cr12Zr10非晶合金的晶化过程为:Am→α-Fe(Co)+CrFe4+Fe23Zr6+Cr2Mo,(Fe0.58Co0.42)73Cr17Zr10非晶合金的晶化过程为:Am→α-Fe(Co)+Am′→α-Fe(Co)+CrFe4+Fe3Ni2+未知相;Cr/Mo比例的增加降低了合金的热稳定性,促进了退火后α-Fe(Co)相的析出。两种合金的饱和磁化强度Ms随退火温度的变化趋势相同但幅度不同,在低于晶化峰值温度Tp退火,(Fe0.58Co0.42)73Mo5Cr12Zr10合金的Ms随退火温度的升高缓慢上升;而(Fe0.58Co0.42)73Cr17Zr10合金的Ms随退火温度的升高快速大幅上升。