Nanoindentation studies on amorphous, nanoquasicrystalline and nanocrystalline Zr8oPt2o and Zr75Pd25 alloys

Nanoindentation studies on rapidly solidified Zr80Pt20 and Zr75Pd25 binary alloys with nanocrystalline, nanoquasicrystalline, and amorphous microstructures are reported. The results indicate that the hardness and elastic modulus are the highest for a mixture of amorphous and nanoquasicrystalline state among the various microstructures studied. Nanoquasicrystalline phase has high hardness and elastic modulus in comparison to amorphous and nanocrystalline phases. The hardness to modulus ratio is close to 0.1 in both the alloys, irrespective of the phase/phase mixture studied indicating that the bonding in these alloys is of covalent nature. In Zr80Pt20, all the phases/phase mixtures have higher hardness and modulus in comparison to similar microstructures in Zr75Pd25 due to higher bond energies caused by more negative heat of mixing in the former case.     

铝基非晶合金的研究与发展

A1基非晶及纳米复合A1非晶合金,作为新兴的工程材料,已经引起人们广泛的关注,近年来发展十分迅速。简述了A1基非晶合金的形成机制、晶化动力学及热稳定性等方面的研究概况及进展,并就其性能与传统A1合金进行了对比。     

铝基非晶材料研究与再制造应用前景

发现于20世纪60年代的铝基非晶合金作为一种低密度材料拥有着较高的比强度,而且与传统晶态材料相比,呈现出长程无序、短程有序的原子排列特点,其内部不存在晶界、位错等较易引发失效的缺陷结构,表现出高硬度和优异的防腐、耐磨等性能,受到了国内外众多学者的广泛关注.起初,这类材料由于受到制备工艺的限制,表现为非晶与纳米晶共存的结构.随着科技发展,科学家们开发了一系列具有完全非晶结构的铝基合金体系.这些材料在具有较高的机械强度的同时能够表现出良好的韧性,使人们对其非晶形成能力、制备方法及应用推广等方面产生了较大的兴趣.对铝基非晶合金非晶形成能力的研究,学者们通常基于块体非晶合金非晶形成能力的经验判据,以及其他一些新提出的判定方法,如蒸发焓、费米层电子态密度、原子扩散以及析出相熔点等.但是,由于铝基非晶合金过冷液相区间较窄以及Al元素化学活性较强,因此铝基非晶合金的非晶形成能力普遍较弱.虽然人们在元素种类及含量变化对铝基非晶合金非晶形成能力的影响等方面做了大量的研究工作,但是目前仍未形成具有普适性的或更加精确的铝基非晶形成能力判定方法,未来仍需借助高性能材料模拟计算和机器学习等技术来进行完善.铝基非晶合金非晶形成能力较弱,以及其对外界条件的影响较为敏感,导致其在制备过程中易发生晶化,从而使获得的材料尺寸维度普遍较低.目前,铝基非晶合金的常见制备方法可按照其形态(粉状、块体、涂层等)来进行划分.粉状铝基非晶合金的制备方法主要为气雾化法和机械合金化法;块体铝基非晶合金的制备方法主要为直接凝固法和粉末冶金法;涂层类铝基非晶合金的制备方法主要包括激光熔覆、爆炸喷涂、冷喷涂、超音速火焰喷涂和电弧喷涂.相对而言,铝基非晶涂层制备技术不会受到工件尺寸的限制,工艺简单、操作方便,且适合于户外大面积施工,在表面防护与再制造工程领域更具应用潜力.尤其是在大型舰船、飞机、海洋设施等高附加值零件的再制造领域里,铝基非晶涂层制备技术的大规模推广应用将会带来巨大的经济效益.本文介绍了铝基非晶合金的发展过程、非晶形成能力、制备方法等内容,总结了铝基非晶涂层在再制造领域的应用前景,并展望了铝基非晶合金的未来研究方向.     

Dislocation structures in Zr3Al-based alloys

Transmission electron microscopy has been used to investigate dislocation structures in deformed binary and ternary Zr₃Al-based alloys. In the binary alloy deformed at temperatures between 293 and 673 K the dislocations in the Zr₃Al phase consisted of a/31 1 2-type partial dislocations bounding superlattice intrinsic stacking fault on {1 1 1} planes. The {111} a/31 1 ¯2 stacking fault energy was approximately 2mJ m^–2 at 673 K. In binary specimens deformed between 873 and 1073 K cube slip predominated. Dislocations consisted mainly of a/2 1 1 0 pairs separated by antiphase boundary. For this temperature range the {1 0 0} a/201 1 antiphase boundary energy was between 30 and 45 mJ m^–2. Alloying with niobium or titanium was found to increase the {111} a/31 1¯2 stacking fault energy and thus increase the propensity for antiphase boundary-type dissociation.     

Low-cycle fatigue in Ni3Al-based alloys

The low-cycle fatigue properties of hot-extruded powders of a Ni₃Al-based alloy, IC 218, with nominal composition Ni-16.5Al-8.0Cr-0.4Zr-0.1B (at %) have been evaluated at room temperature. Tests were conducted under total strain conditions in a laboratory air environment. Results indicate that the low-cycle fatigue performance of the PM processed IC 218 nickel aluminide is superior to other structural alloys especially at higher strain amplitudes. These results are explained in terms of the high ductility of the fine-grain material and good crack growth propagation resistance in these alloys. Stress response curves for annealed IC 218 alloys indicate considerable cyclic hardening followed by cyclic softening. The onset of cyclic softening is found to occur at a constant cumulative plastic strain. The critical cumulative plastic strain criteria are verified for step-loaded IC 218 nickel aluminide coupons.     

Crystallization of Al-based Amorphous Alloys in Good Conductivity Solution

The corrosion-induced crystallization of Al94 exNixGd6（x=6 and 10, in at.%） metallic glasses as well as phase separation, oxidation and cracking in good conductivity solution has been investigated by various techniques.The transmission electronic microscopy（TEM） result reveals that crystalline intermetallics and oxides present on the electrochemically thinned hole edge, and the phase separation occurs in the matrix of the as-spun ribbons with the circumferential speed Rcof 29.3 m/s. In addition, the bending and cracking of the samples occur after corrosion. The influence of Ni content on the phase separation, bending and cracking can be explained by the fact that the percolation of the backbone clusters in the amorphous alloy melts and glasses is enhanced by increasing the composition of Ni.     

Grain-growth in nanocrystalline zirconium-based alloys

Crystallization and subsequent grain growth in nanocrystalline Fe₃₃Zr₆₇ and (Fe, Co)₃₃Zr₆₇ alloys were studied by TEM, differential scanning calorimetry and X-ray diffraction. The grain-growth data for both alloys obtained over a wide temperature range (about 200 °C) were fitted to different kinetic equations (with different grain-growth exponent, n). The model with n=3 (Equations 4 and 5) was found to predict in the best way the isothermal experimental data. This result gives strong evidence that crystallization (in our case by a polymorphic reaction) is indeed observed of a glass into a nanocrystalline material prior to the coarsening, rather than grain growth in an extremely fine-grained material which was never glassy at all. The activation energies for grain-growth, — 260±25 kJ mol^–1, were found to be practically the same for both systems. Additional information about the crystal growth kinetics of the nanocrystals in the amorphous matrix was obtained for (Fe, Co)₃₃Zr₆₇ glass.     

Amorphous and nanocrystalline silicon growth on carbon nanotube substrates

In this study, smooth and conformal hydrogenated silicon thin films are examined and analyzed on various multi-walled carbon nanotube (MWCNT) substrates. The films are deposited using radio-frequency plasma-enhanced chemical vapor deposition with He dilution and parameters that are heavily in the γ regime. It is proposed that high-energy plasmas with limited penetration depth can induce crystallization to occur on MWCNT substrates of varying active surface areas. The samples presented exhibit properties that are promising for energy applications, including photovoltaics and lithium-ion batteries and have been studied using scanning electron microscopy, Raman spectroscopy, X-ray diffraction, UV-Vis spectrophotometry, four-point probe measurements, and Fourier transform infrared spectroscopy.     

The formation, structure and properties of nanocrystalline Ni-Mo-B alloys

The structure, structure evolution and microhardness of nanocrystalline Ni-Mo-B alloys were studied by X-ray diffraction, differential scanning calorimetry, transmission and high resolution electron microscopy and microhardness measurements. The nanocrystalline structure was produced by controlled crystallization of amorphous alloys. The annealed samples consist of the FCC nanocrystals with the amorphous regions between them. The grain size of the nanocrystals is about 20 nm and depends on the chemical composition of the alloy. The chemical composition of the amorphous phase between the nanocrystals changes at the annealing. A slight grain growth was observed when the annealing time increases. The diffusion of Mo and B from FCC to the amorphous phase occurs at the annealing. It results in the lattice parameter change. The microhardness of the alloys increases during the annealing. The microhardness values are the same in all alloys before the nanocrystalline structure decomposition. The microhardness is inconsistent with the Petch-Hall equation. The microhardness of the alloys is determined by the microhardness of the amorphous phase bands located between the nanocrystalline grains.     

热喷涂工艺制备铝基非晶态合金材料研究进展

传统的铝基非晶态合金的制备方法有急冷法和机械合金化法.急冷法使用方便,冷却速率快,可获得非晶,可实现连续生产;机械合金化法具有设备简单,操作方便,易工业化,合金成分范围相对较宽等优点.但是这两种方法只限于制备非晶条带、薄片或粉末,而且生产制备周期较长,生产效率较低.热喷涂技术作为一种新兴的制备工艺,可获得非晶涂层,实现材料制备与成形一体化,明显缩减生产周期,已成为可以工业化应用的重要制备技术.     

Precipitation phase transformation in nanocrystalline Fe-Mo alloys

Precipitation phase transformation was studied in nanocrystalline Fe-rich Fe-Mo alloys with the use of X-ray diffraction and M?ssbauer spectroscopy. Alloys up to 5 at% Mo in Fe were synthesized by mechanical alloying and formed in alpha phase bcc solid solutions with average grain sizes in the range of 10-13 nm. The precipitation transformation (alpha-->alpha + lambda) was found to proceed via a Mo clustering that was correlated with the size of the nanograins. This was understood in terms of the Gibbs Thomson effect with a concept of negative surface energy contribution to the Gibbs free energy of mixing in a nanocrystalline alloy with positive internal energy of mixing. This contribution increased the stability of the solid solution for nanosized grains, and the Mo precipitation started once the grains grew beyond a critical size. We argue that the Mo precipitation takes place in the grain boundary regions, and the Mo-rich lambda phase also precipitates directly in the grain boundary regions, in contrast to the microcrystalline alloys, where the Mo clusters formed within the grains and were first dissolved in the Fe matrix before the lambda phase was formed.     

Structure and coercivity of nanocrystalline Fe-Si-B-Nb-Cu alloys

Crystallization behaviour and magnetic properties of melt-spun Fe-Si-B-Nb-Cu alloys have been investigated. It is found that the primary phase changes from α-Fe(Si) to Fe₃Si (DO₃) on increasing the Si content The coercivity of the alloys containing the Fe₃Si phase is significantly lower as compared to the alloy containing α-Fe(Si) phase. A heat treatment temperature-time-coercivity map has been obtained for optimization of the coercivity.     

Mechanical and fracture behavior of powder metalurgy processed Ti3Al-based alloys

This work considers structural and compression mechanical properties of three Ti₃Al-based alloys processed by powder metallurgy. Mechanically alloyed powders were compacted by hot-pressing to non-porous homogenous compacts. Prior to compression tests, all compacts were homogenized by a solution treatment at 1050°C (α + β region) for 1h, followed by water quenching. The compression tests were performed from room temperature to 500°C in vacuum at a strain rate of 2.4 × 10⁻³ s⁻¹. Detailed microstructural characterization has been evaluated by scanning electron microscopy (SEM), followed by electron dispersive spectroscopy (EDS) and X-ray diffraction analysis. Fracture topography was examined by SEM. The Ti₃Al-Nb alloy exhibits the highest ductility in the whole temperature range, whereas addition of Mo to Ti₃Al-Nb alloy yields the highest ultimate compression strength. A correlation between ductility and the fracture mode exists for all materials.     

Characterization of nanocrystalline Al-based alloy produced by mechanical milling followed by cold-pressing consolidation

In the present study high energy mechanical milling followed by high-pressing consolidation has been used to obtain bulk nanocrystalline aluminum based alloy. Fully dense disks with homogenous microstructure were obtained. X-ray analysis indicates that high strain imposed by mechanical milling and severe plastic deformation (SPD) processing causes microstructure showing a mixture of a significant lower value of the crystallite size (around 60 nm) and the creation of a large number of linear defects which induces microstrains (around 0.7%). In addition, the elevated value of hardness which was obtained for the consolidated disks and the favorable comparison with the value measured for the same as-received alloy confirm the potential of the SPD technique when it is collected with high energy mechanical milling for producing higher performance materials. On the basis of calorimetric measurements, as a function of consolidation stresses, we can say that the thermal behaviour was characterized by a significant heat release and grain growth during the heating cycle. Moreover, the noticed broad endothermic peak can be attributed to some precipitants containing magnesium.     

Hard magnetic nanocrystalline alloys of Fe-O system

The structure and magnetic properties of nanocrystalline Fe-O alloys produced by high-energy ball milling and subsequent low-temperature annealing were investigated. The Fe₂O₃, FeO and Fe powders as well as their mixtures were used as starting materials. The structure was studied by X-ray diffraction analysis, Mössbauer spectroscopy and scanning electron microscopy. The magnetic properties were measured in vibrating sample magnetometers at room temperature and 4.2 K. The nanocrystalline composite alloys obtained as a result of the milling contained FeO and α-Fe with an average crystallite size of 15-20 nm as well as an amorphous phase, which was identified as a solid solution of oxygen in iron. However, alloys subjected to subsequent annealing contained only α-Fe and Fe₃O₄ with an average crystallite size of about 20 nm. Unlike the starting materials the produced powders had properties which are characteristic of hard magnetic materials. For example, the powder produced by the milling of Fe₂O₃+50% α-Fe mixture followed by annealing had the following properties at 300 K: intrinsic coercive force μ₀ H _c = 0.067 T, remanence B _r = 0.48 T, energy product (BH)_max = 9 kJ/m³.     

Modelling thermodynamics of nanocrystalline binary interstitial alloys

Grain boundary (GB) segregation in nanocrystalline alloys can cause reduction of GB energy, which leads to thermodynamic stabilization of nanostructures. This effect has been modelled intensively. However, the previous modelling works were limited to substitutional alloy systems. In this work, thermodynamics of nanocrystalline binary interstitial alloy systems was modelled based on a two-sublattice model proposed by Hillert [M. Hillert, et al. Acta Chem. Scand., 24 (1970) 3618] and an atomic configuration for nanocrystalline systems proposed by Trelewicz and Schuh [J.R. Trelewicz, et al. Physical Review B, 79 (2009) 094112]. The modelling calculations agree with the reported experimental data, indicating that the current thermodynamic model is capable of accounting for the alloying effect in the nanocrystalline binary interstitial alloys.     

Vacuum arc characteristics on nanocrystalline CuCr alloys

 Vacuum arcs generated on nanocrystalline CuCr₅ alloys were observed by a digital high-speed video camera. Experimental results show that nanosized Cr particles have strong influence on arc characteristics. The arcs are much more stable on nanocrystalline CuCr₅ alloys and the chopping current is about 0.8 A, which is much lower than that on coarse crystalline CuCr₅ alloys. Spots can directionally move a long distance on nanocrystalline CuCr alloy whereas the spot motion is totally random walk on Cr particles for coarse crystalline CuCr alloy.