Effects of La content on the glass transition and crystallization process of Al–Ni–La amorphous alloys

Effects of La content on the glass transition and crystallization process of Al_94−xNi₆La_x (x = 3–9) amorphous alloys were investigated by X-ray diffraction and differential scanning calorimeter. The results show that the thermal stability increases with increasing the La content. The crystallization changes from a two-stage process without glass transition at x = 3–6 to a three-stage one with obvious glass transition at x = 7–9. The first crystallization process results in precipitation of single fcc-Al at x = 3–5, fcc-Al plus metastable phase(s) at x = 6 and 7, and single metastable phase at x = 8 and 9. The first crystallization process at x = 4 and 5 is the growth of quenched-in nuclei, whereas that at x = 6, 7 and 9 is the diffusion-controlled growth with a decreasing, constant and increasing nucleation rate, respectively. The activation energy for the first crystallization process is larger in the eutectic reaction than that in the primary reaction, and is the highest when the number of the products is the most.     

Local structure variations in Al89La6Ni5 metallic glass

Reduced density functions (RDFs) from electron diffraction measurement of an Al₈₉La₆Ni₅ metallic glass were obtained and analysed using Reverse Monte Carlo (RMC) refinement accompanied by density functional theory calculations. Three different RDFs were observed from regions in close proximity (within microns) of each other, with the first/second peaks respectively located at 2.76/3.32, 2.90/3.46 and 2.96/3.55 Å (all ±0.02 Å). The first type of RDF was found to be the more prevalent. X-ray dispersive spectroscopy and electron energy loss spectroscopy showed no measurable composition variations on this scale. According to the RMC refinements, the variations in the peaks are due to slight lengthening in the mean Al–Al and larger lengthening of the Al–La first-neighbour contacts, while the mean Al–Ni contact shortens. These local structure variations in a nominally homogeneous Al₈₉La₆Ni₅ metallic glass are likely to be important to the understanding of metallic glass properties.     

Al88Ni6La6非晶及其晶化薄带的腐蚀行为研究

采用单辊甩带法制备出完全非晶态的Al88Ni6La6薄带，并用X射线衍射（XRD）仪和示差扫描量热计（Dsc）对该非晶薄带的非晶特性及其晶化过程进行了研究。该非晶薄带的晶化过程分为两步，其晶化温度约为239℃和332℃。在0．01mol／L NaCl溶液中对完全非晶态的A188Ni6La6试样及其部分晶化和完全晶化试样的电化学腐蚀行为进行了研究。结果表明，部分晶化的试样表现出最佳抗电化学腐蚀性能，而经过完全晶化的试样，其抗腐蚀性能明显下降。     

Analyzing the Effects of Milling and Sintering Parameters on Crystalline Phase Evolution and Mechanical Properties of Al86Ni8Y6 and Al86Ni6Y4.5Co2La1.5 Amorphous Ribbons

In the present study, Al₈₆Ni₈Y₆ and Al₈₆Ni₆Y_4.5Co₂La_1.5 bulk amorphous nanocomposites were synthesized by spark plasma sintering of milled melt spun ribbon particles. The as-cast ribbons were of near amorphous nature with minute amount of FCC Al embedded in the amorphous matrix. Milling of the ribbons resulted in partial devitrification due to mechanical crystallization. The milled ribbon particles were sintered in the temperature and pressure range of 300-500 °C and 500-700 MPa, respectively. It was observed that nominal amount of amorphous phase was retained at 500 °C and 500 MPa. With increase in sintering pressure and decrease in sintering temperature, the amount of crystalline phase evolution decreased, and maximum amount of amorphous phase was retained at 300 °C and 700 MPa. The microstructure consisting of amorphous phase embedded with hard intermetallic phases led to increase in the nanohardness of Al₈₆Ni₈Y₆ and Al₈₆Ni₆Y_4.5Co₂La_1.5 as-cast ribbons from 3.26 ± 0.59 GPa and 3.81 ± 0.58 GPa to 6.06 ± 0.70 GPa and 6.14 ± 0.82 GPa, respectively, for the corresponding consolidated amorphous nanocomposite. Microhardness of the three and five component system bulk samples was 4.19 ± 0.13 GPa and 3.6 ± 0.13 GPa, respectively.     

Influence of metastable Al9Ni2 phase on the sequence of phase transformations initiated by heating of Al/Ni multilayer foils produced by EBPVD method

Al/Ni multilayer foils (MF) undergo a cascade of phase transformations at heating, initiated by diffusion interaction of Al and Ni layers. It is found that phase transformations sequence at initial stages depends on the method of producing MF: at sputtering or ion-beam deposition of elements, metastable Al₉Ni₂ phase forms at phase transformations initial stages, and in the case of MF produced by electron-beam physical vapour deposition (EBPVD) method or cold rolling of laminates, this is Al₃Ni phase. Such a difference in phase transformations sequence is associated with the influence of the method of MF production on the possibility of intermixed zone (IZ) formation on layer interfaces. In the study it was suggested that such anomalously high diffusion mobility of atoms can be achieved in the presence of excess vacancies in MF structure. With this purpose, MF structure was produced by high-rate (up to 30 nm/s) layer-by-layer deposition of elements by EBPVD method. Phase transformations and MF were studied by the method of X-ray diffraction (XRD) and differential-thermal analysis (DTA). It is shown that irrespective of MF composition and modulation period, at their heating phase transformations start with formation of metastable Al₉Ni₂ phase. At further MF heating, a stable Al₃Ni phase forms alongside Al₉Ni₂ phase. Later on, Al₉Ni₂ and Al₃Ni phases turn into stable intermetallics characteristic of MF chemical composition.     

Al/Ni formation reactions: characterization of the metastable Al9Ni2 phase and analysis of its formation

The metastable phase Al₉Ni₂ was investigated in order to characterize the thermodynamics and kinetics of its formation. Here, Al₉Ni₂ was observed as the first phase to form in a series of sputter-deposited Al/Ni multilayer foils, but it did not form in foils with a very small bilayer period (12.5 nm). In these foils, the stable phase Al₃Ni was the first phase observed. Using differential scanning calorimetry, the heat of formation and the Gibbs free energy for the formation of Al₉Ni₂ were both calculated to be −28 kJ/mole·atom. The average activation energy for Al₉Ni₂ was determined using a Kissinger analysis and was found to vary with bilayer thickness, with an average value of 1.58 ± 0.06 eV. In light of these experimental results, a simple nucleation model based on thermodynamics and diffusional intermixing is proposed to explain why Al₉Ni₂ forms before Al₃Ni in most cases, except in foils with very small bilayers where it is absent.     

Effect of ball milling on structure and thermal stability of Al84Gd6Ni7Co3 glassy powders

The influence of ball milling on microstructure and thermal stability of the gas-atomized Al₈₄Gd₆Ni₇Co₃ glassy powder has been investigated as a function of the milling time. The results show that the traces of crystalline phases present in the as-atomized powder decrease gradually with increasing the milling time. The thermal stability of the fcc-Al primary phase increases while the thermal stability of the intermetallic phases decreases with increasing milling. Moreover, significant improvement in hardness occurs after milling, which is attributed to the amorphization of the residual crystalline phases present in the as-atomized powder. These results demonstrate that milling is an effective way for amorphizing the residual crystalline present in the amorphous matrix and to control the thermal stability of the material.     

The effect of Cr2O3 addition on crystallization and properties of La2O3-containing diopside glass-ceramics

We report on the effect of La₂O₃ and Cr₂O₃ on the structural, thermal and crystallization behavior of diopside-based glasses and on the processing, microstructure and properties of the sintered glass-ceramics (GCs). The structural features and thermal behavior of the glasses were investigated by Fourier transform infrared spectroscopy, density measurements, dilatometry and differential thermal analysis. The sintering, crystallization, microstructure and properties of the GCs were investigated under different heat treatment conditions (800–1000 °C; 1–300 h). The good matching of thermal expansion coefficients and the strong, but not reactive, adhesion to yttria-stabilized zirconia and Crofer 22 APU alloy, in conjunction with a low level of electrical conductivity, indicates that the investigated GCs are suitable candidates for further experimentation as solid oxide fuel cell sealants.     

Stability of Al-rich glasses in the Al–La–Ni system

The stability of Al-rich glasses in the Al–La–Ni ternary system has been measured. Glasses with critical thicknesses ranging from 270 to 780 μm have been achieved. An in-situ two-phase amorphous region containing as much as ∼10 vol.% of nanocrystals was also observed with critical thicknesses ranging from 420 to 950 μm. Measurements of T_g, T_x and T_ℓ, as well as other empirical measures of thermal stability based on these quantities, confirm the unusual thermal stability of these glasses. These results suggest that bulk Al-based glasses with a maximum critical thickness exceeding 1 mm may be possible in alloys based on this system.     

Characterization of stacking faults on basal planes in intermetallic compounds La5Ni19 and La2Ni7

Stacking faults on basal planes as well as intergrowth structures in La₅Ni₁₉ and La₂Ni₇ have been characterized by high-resolution transmission electron microscopy (HREM) and electron diffraction. Most stacking faults observed in La₅Ni₁₉ are of the inter-block-layer type in which the faulting in the stacking sequence of block layers occurs without changing the number of unit layers in each block layer. In contrast, most stacking faults observed in La₂Ni₇ are of the intra-block-layer type in which the faulting occurs in the number of unit layers in block layers, although some stacking faults of the inter-block-layer type are also observed. Intergrowth structures are commonly observed in as-cast alloys with compositions corresponding to La₅Ni₁₉ and La₂Ni₇. The implication of these results for increasing the hydrogen absorption rate of LaNi₅-based alloys without significantly affecting hydrogen absorption/desorption properties described in terms of pressure-composition isotherms are discussed.     

熔体结构变化对Al85Ni5Y8Co2非晶合金晶化行为的影响

采用四电极电阻法研究Al85Ni5Y8Co2合金熔体电阻率,并结合X射线衍射仪(XRD)和差示扫描量热仪(DSC)探索熔体结构变化对Al85Ni5Y8Co2非晶晶化行为的影响。结果表明:Al85Ni5Y8Co2合金在1135~1357 K温度区间内发生熔体结构变化,而熔体结构变化导致Al85Ni5Y8Co2非晶晶化行为存在差异;在熔体温度为1673 K下制备的非晶薄带第一晶化较1323 K和1523 K的明显提前,对激活能的计算证实其初晶化更容易析出FCC-Al,且更难析出Al2Y3、Al13Co4、Al9Co2和AlNi等脆性金属间化合物;通过XRD的测试发现高的熔体温度可能会导致熔体中原子团簇的破坏,从而引起非晶晶化行为的改变。     

Performances of micro-textured WC-10Ni3Al cemented carbides cutting tool in turning of Ti6Al4V

Cutting performances of micro-textured WC-10Ni₃Al cutting tools compared with micro-textured WC-8Co cutting tools in turning of Ti6Al4V was investigated in this study. Cutting forces, cutting temperature, and tool life based on the criterion of a 300 μm flank wear were measured. The wear tracks of the rake face and flank face for micro-textured WC-10Ni₃Al cutting tools were analyzed. It is found that WC-10Ni₃Al cutting tools had smaller heat damages during LST compared with WC-8Co cutting tools, which was benefit for avoiding premature tool failure during Ti6Al4V machining process. Micro-textures on the rake face could effectively reduce cutting forces, cutting temperature, adhesion on the rake face, and hence increase tool life, especially at higher cutting speed.     

Gd3Al2-xSix系合金磁热效应的研究

采用真空高频磁悬浮炉制备出一系列Gd3Al2-xSix (x=0,0.1,0.2,0.5,0.8)合金.通过Si对Al的部分替代,研究了Si元素微量替代对Gd3Al2合金磁热效应的影响.结果表明,Gd3Al2-xSix系合金的结构与Gd3Al2相同;随着Si含量的增加,Gd3Al2-xSix系合金的居里温度和磁热效应发生了变化,这说明Si对Al的部分替代改变了GdAl2合金的磁热效应.     

Gd3Al2系合金磁热效应的比较

用真空高频磁悬浮炉制备了Gd3＋xAl2-x（x=0,0.1,0.2,0.3）合金、Gd3-XZnxAl2（x=0、0.1、0.3、0.6）合金和Gd3-xMnxAl2（x=0、0.1、0.2、0.3、0.6）合金。用直接测量法测量了在1.5 T磁场下这3组合金的磁热效应。结果表明,在1.5 T磁场下,Gd3＋xAl2-x合金中X=0.1时,其最大绝热温度变化ΔTad最高,为2.1 K;Gd3-xMnxAl2合金中Gd2.7Mn0.3Al2的居里温度Tc最高,达到290 K。     

Al、Ti掺杂对Mg2Ni合金相结构稳定性的影响及其微观机理

采用第一性原理赝势平面波方法,研究元素Al和Ti掺杂对Mg2Ni储氢合金相结构稳定性的影响及其微观机理.结果显示:在掺杂浓度x＝0～0.5范围内,所形成的Mg2Ni型Mg2-xMxNi(M＝Al,Ti)固溶体合金的相结构稳定性随Al掺杂浓度的增大而增强,随Ti掺杂浓度的增大而减弱,且Mg2-xMxNi(M＝Al,Ti)固溶体合金相对于立方结构的Mg3MNi2(M＝Al,Ti)化合物呈现热力学不稳定性,极易分解成由立方结构Mg3MNi2(M＝Al,Ti)和六方结构Mg2Ni组成的复合相,计算结果与实验结果吻合.电子结构分析表明,Al、Ti掺杂Mg2Ni储氢合金的相结构稳定性与体系在低能级区的成键电子数密切相关.     

退火温度对Al、Cu、Ni诱导Si薄膜晶化进程的影响

基于金属诱导晶化方法,利用直流磁控溅射离子镀技术制备了Al-Si、Cu-Si和Ni-Si薄膜.采用真空退火炉和X射线衍射仪于不同温度下对样品进行了退火试验并分析了退火后薄膜物相结构的变化规律.结果表明,Al诱导SAi薄膜晶化的效果最好,Cu次之,Ni诱导Si薄膜晶化的效果较差;AI可在退火温度为400℃时诱导Si薄膜晶化,且随退火温度的升高Si的平均晶粒尺寸增大;Cu-Si薄膜的内应力较大和Ni-Si薄膜中Ni/Si界面处难以形成NiSi2是Cu、Ni诱导Si薄膜晶化效果较差的主要原因.     

稀土掺杂Gd2 O3对YSZ/（Ni，Al）热障涂层组织与性能的影响

目的提高YSZ/(Ni,Al)复合涂层与基体的结合强度和抗高温氧化性。方法采用电泳沉积的方法,在Inconel 600高温合金表面上沉积YSZ/(Ni,Al)复合涂层和掺杂稀土Gd2O3-YSZ/(Ni,Al)(简称G-YSZ/(Al,Ni))复合涂层,后进行真空烧结,然后对制备好的热障复合涂层进行划痕实验和等温循环氧化实验。通过对样品进行等温循环氧化实验,获取不同氧化时间段的复合涂层样品,并采用SEM和XRD对复合涂层组织和形貌进行分析。结果在1100℃等温氧化过程中,未掺杂稀土元素的氧化增重速率为0.0057 mg/mm2,而掺杂钆元素的氧化速率为0.0049 mg/mm2,氧化增重速率比未掺杂稀土YSZ/(Ni,Al)复合涂层的低。G-YSZ/(Ni,Al)热障复合涂层在等温氧化过程中颗粒长大较小、裂纹少、表面更加致密。真空烧结后的YSZ/(Al,Ni)复合涂层和G-YSZ/(Al,Ni)复合涂层与基体的结合强度约为4.0 N,氧化100 h后,掺杂稀土的G-YSZ/(Al,Ni)复合涂层结合强度为3.5 N,未掺杂稀土的YSZ/(Al,Ni)复合涂层与基体的结合强度为2.6 N。G-YSZ/(Ni,Al)热障复合涂层中存在Gd2Zr2O7相和稳定的Ni Al2O4相,Gd2Zr2O7相具有良好的稳定性以及耐高温氧化。结论掺杂稀土氧化钆的G-YSZ/(Al,Ni)涂层的抗高温氧化性能显著提高。在等温氧化过程中,掺杂稀土元素的G-YSZ/(Al,Ni)复合涂层,其颗粒趋向于均匀化,裂纹明显变少,使得涂层更加致密,表面更加平整。等温氧化100 h后,掺杂了稀土氧化钆的G-YSZ/(Al,Ni)复合涂层基体之间具有更好的结合力,抗剥落性和服役寿命较好。     

Effects of quenching conditions on crystallization behavior of amorphous Al90Ni5Ce5 alloys

X-ray diffraction and DSC were used to investigate the crystallization behavior of amorphous Al90Ni5Ce5 alloys at different quenching temperatures.All the amorphous Al90Ni5Ce5 alloys quenched at different temperatures crystallize by two stages.The first stage corresponds to FCC Al phase precipitating from the amorphous matrix.The crystallization onset temperature increases with increasing quenching temperature.The quenching temperature also influences the isothermal behaviors.At low quenching temperatures,the FCC Al precipitation is only through grain growth.At high quenching temperatures,the FCC Al precipitation is through growth of quenched-in Al unclei and nucleation and growth of new crystallites.The reason that the crystallization onset temperature varies with quenching temperature is likely as that the quenched-in Al nuclei decreases with increasing temperature.