Hydrogen solubility and diffusivity in amorphous La14Ni86 films

Hydrogen solubility and diffusivity in Pd-coated amorphous La₁₄Ni₈₆ films have been studied by thermal desorption spectroscopy (TDS). The TDS spectra were analyzed according to the statistical Harris and thermodynamic Kirchheim models. For H-loading pressures up to 100 mbar, hydrogen essentially occupies La₂Ni₂ and LaNi₃ tetrahedral sites at room temperature. Only hydrogen in LaNi₃ sites can be thermally desorbed before crystallization of the films takes place. Progressive filling of LaNi₃ sites with increasing hydrogen pressure occurs in agreement with the thermodynamic model. The density of LaNi₃ sites is well characterized by a Gaussian distribution with a mean energy of −0.44 eV. As regards diffusivity, hydrogen atoms jump from mean energy LaNi₃ sites with an activation enthalpy of 0.47±0.03 eV. Nonetheless, discrepancies in the width of the site energy distribution were found when it was independently evaluated from solubility and diffusivity analyses.     

Ni radiotracer diffusion in B2 ordered NiFeAl alloys

Ni bulk diffusion was measured in the B2 ordered NiFeAl alloys with a constant Al content of approximately 50 at.%. The ⁶³Ni radioisotope in combination with the radiotracer serial sectioning technique was applied at higher temperatures and the secondary ion mass spectrometry (⁶⁴Ni isotope) was used in a low temperature range. Arrhenius-type temperature dependencies were established for all studied compositions in the temperature interval from 1050 to 1500 K. As the Fe content x in the Ni_50−xFe_xAl₅₀ ternary alloys increases, the Ni diffusivity generally increases along the quasi-binary section between stoichiometric NiAl and FeAl. The activation enthalpy Q of Ni diffusion changes strongly non-monotonically in the ternary alloys between the binary end-members NiAl and FeAl revealing a pronounced minimum at the Ni₄₀Fe₁₀Al₅₀ composition and a conspicuous maximum around the Ni₂₅Fe₂₅Al₅₀ composition.     

B对无序态和有序态Ni_3Fe合金环境氢脆的影响

为了阐明金属间化合物中H_2所致脆性的机理及B在抑制这类氢脆的作用,研究了B对无序态和有序态Ni_3Fe合金氢脆的影响及其预渗氧后断口沿晶深度的影响.结果表明:B能同时降低H在无序态和有序态Ni_3Fe合金中的表观扩散系数,而且不管是否添加B,H在无序态Ni_3Fe合金中的表观扩散系数始终大于H在有序态Ni_3Fe合金中的表观扩散系数.因此,不能简单地从B影响Ni_3Fe合金中H扩散来解释B能明显抑制有序态Ni_3Fe合金在H_2中的环境氢脆.     

Fe10Ni90和Fe90Ni10合金结晶过程微观结构的模拟

以101 K/s、1O10 K/s两种冷速对富Ni的Fe10Ni90合金和富Fe的Fe90Ni10合金进行快速凝固,并采用原子平均势能、双体分布函数、配位数、3D可视化、最大标准团簇等方法研究了快速凝固过程中两种合金的微观结构随温度快速降低的演变.结果表明:在1010 K/s冷速下,两种合金的结晶温度一致,但Fe10N...     

H2-induced environmental embrittlement in ordered and disordered Ni3Fe: An electronic structure approach

The different behaviors in H₂-induced environmental embrittlement in ordered and disordered Ni₃Fe are associated with differences in their electronic structures. The experimental study on electronic structures of ordered and disordered Ni₃Fe has been carried out by electron energy-loss spectroscopy (EELS). The onset energy of Ni L_2,3 edges from ordered phase is 0.3 eV lower than that from disordered phase, while the 3d occupancy of Ni atoms in ordered phase is 0.07 electrons/atom less than that in disordered phase. Severe H₂-induced environmental embrittlement in ordered phase is attributed to rather negative dissociative adsorption energy of hydrogen at surfaces, which arises from upward shifting of the valence band center of Ni.     

Magnetic-induced tricritical point in alloys and the low-temperature Fe−Ni and Fe−Ni−Cr phase diagrams

Phase equilibria at temperatures above and below the tricritical point of binary alloys are discussed in terms of thermodynamic principles. The existence of a second spinodal curve for the disordered phase within the magnetic-induced miscibility gap is rationalized. The relative interplay of the chemical and magnetic contributions to the stabilities of various types of phase equilibria in binary alloys is also discussed. The effect of magnetic contribution to the Fe?Ni and Fe-Ni-Cr phase stabilities at low temperatures is reviewed. The calculated phase diagrams of the Fe?Ni and Fe-Ni-Cr systems considering the magnetic and nonmagnetic terms are compared with experimental results. These results were obtained either from meteorites or from synthesized alloys subjected to irradiation. The meteorites, which have cooled slowly in space, may have achieved metastable/stable equilibrium conditions. The many defects introduced by irradiation for the synthesized alloys may have helped to achieve metastable/stable equilibrium conditions.     

Iron grain boundary diffusion in pure and in Cr,Fe and Zr-doped Ni3Al alloys

The grain-boundary diffusion (GBD) of ⁵⁹Fe in pure and in Cr-, Fe- and Zr-doped Ni₃Al alloys was studied at temperatures between 873 and 1273 K using the serial sectioning method. The grain-boundary diffusivities P (P is a product of grain-boundary width δ, and the diffusion coefficient in the grain-boundary D_b) were obtained from the penetration profiles using both the Le Claire’s analysis and direct fit of Suzuoka’s solution. The activation enthalpies of ⁵⁹Fe GBD were somewhat lower in comparison with analogous values for ⁵¹Cr GBD, reported in our previous paper.     

Sm12.7Fe86.3Nb1合金的氢处理

Sm12.7Fe86.3Nb1合金在不同温度下氢爆(HD)及氢化-歧化-解吸-再复合(HDDR)处理时,100℃时可氢化形成Sm2(Fe,Nb)17Hy,随着温度升高氢化速度加快,到400℃时单胞体积最大膨胀了3.38%.超过500℃时Sm2(Fe,Nb)17Hy H2→SmHy α-Fe(Nb)发生歧化,直到900℃仍旧存在,故氢爆温度应低于500℃.解吸与再复合过程在超过700℃时可能以SmHy α-Fe(Nb)→Sm2(Fe,Nb)17 H2方式进行.在连续的HDDR处理过程中,吸氢-歧化在升温(400℃/h)的过程中即已完成,而解吸-再复合在保温时与歧化阶段达到平衡,即SmHy α-Fe(Nb)←→Sm2(Fe,Nb)17 H2,抽真空是使该反应向右进行的主要驱动力.在HDDR过程中破坏试样的原颗粒尺寸会残留较多的软磁α-Fe相而恶化氮化后的磁性能,HDDR后残留的α-Fe相含量均高于退火态的残留量,2次循环后磁粉的矫顽力较高.HDDR使粉末颗粒表面产生裂纹,再复合后的Sm2(Fe,Nb)17颗粒细小均匀,尺寸分布在几十纳米到300 nm之间.     

Hydrogen diffusion in Mg–H and Mg–Ni–H alloys

Coefficients of hydrogen diffusion in Mg₂NiH₄ (D_I), MgH₂ (D_M), and in (Mg + Mg₂Ni)−H eutectic (D_E) were measured in the temperature interval 449–723 K. Experimental material was prepared by two techniques: by melting and casting and by ball-milling and compacting into pellets. Hydrogen charging was carried out from the hydrogen gas phase at high temperature and pressure. The Mg₂NiH₄ pellets were prepared in different regimes resulting either in a structure with a high fraction or with a low fraction of twinned low-temperature phase LT2. It was found that the LT2 slows down the hydrogen desorption rate considerably – values of D_I in low-temperature untwinned phase LT1 are by a multiplication factor of about 20 higher than those in the twinned phase LT2. Obtained values of D_M are significantly lower than the literature values reported for the pure Mg. Hydrogen diffusion coefficients in interphase boundary were estimated from D_E.     

B掺杂p型SiGe合金的制备与热电性能表征

以一定化学计量比均匀混合的Si、Ge、B混合粉末为原材料,使用放电等离子烧结(SPS)一步法合金化制备了p型Si₈₀Ge₂₀B_x(x=0.5,1.0,2.0)合金热电材料,并对样品的组成、微观形貌、热电性能进行了表征与分析。结果表明,放电等离子烧结过程实现原位合金化并烧结为块体材料。随着B掺杂量的增加,电导率明显提升,热导率显著下降,当温度为950K时,热导率为1.79W/(m·K)。在1050K时,ZT值达到了0.899。球磨和掺杂的协同作用使得SiGe合金基体内产生不同类型的缺陷特征而散射不同波长的声子,导致硅锗合金热导率的降低。     

Anodic behavior of Fe40Ni40B20 and Fe39Ni39B10Si12 amorphous alloys

Amorphous alloys Fe₄₀Ni₄₀B₂₀ and Fe₃₉Ni₃₉B₁₀Si₁₂ exhibit a typical active/passive transition when anodically polarized in borate and/or phthalate buffer solutions, at pH = 4.0; 5.3;6.3 and 8.4. There is no large effect of Si on the anodic behaviour of the alloys in these media. Both alloys undergo a typical pitting corrosion in chloride containing solutions, but, the Si bearing alloy is more resistant to the breakdown of passivity. AES analysis revealed a large enrichment in Si within the passivating film formed on the Si bearing alloy, which most probably accounts for its enhanced stability.     

Freeform fabrication of Ti–Ni and Ti–Fe intermetallic alloys by 3D Micro Welding

A novel freeform fabrication method for refractory metals named 3D Micro Welding (3DMW) has been proposed, which is a rapid prototyping combined with TIG (Tungsten Inert Gas) welding. Formation of beads and small pin objects composed of the Ti–Ni and Ti–Fe intermetallic alloy phases was attempted. Morphological changes of their beads in diameter, height, and contact angle were investigated as a function of the pulsed-arc peak current. As the arc peak current increased from 5 A to 30 A, the height and the contact angle decreased, while the bead diameter increased in Ti–Ni and Ti–Fe systems. Ti, Ti₂Ni and TiNi phases were formed in the Ti–Ni system, while Ti, TiFe and TiFe₂ phases in the Ti–Fe system. Small pin objects of these two systems were made by stacking molten beads on a fixed z-axis and their microstructures were observed. There were micro-cracks in beads of both system, but no crack appeared in these small pin objects.     

Hydrogen storage properties of a Ni,Fe and Ti-added Mg-based alloy

Mg-5wt%Ni-2.5wt%Fe-2.5wt%Ti (referred to as Mg-5Ni-2.5Fe-2.5Ti) hydrogen storage material was prepared by reactive mechanical grinding, after which the hydrogen absorption and desorption kinetics were investigated using a Sievert-type volumetric apparatus. A nanocrystalline Mg-5Ni-2.5Fe-2.5Ti sample was prepared by reactive mechanical grinding and hydriding-dehydriding cycling. Analysis by the Williamson-Hall method from an XRD pattern of this sample after 10 hydriding-dehydriding cycles showed that the crystallite size of Mg was 37.0 nm and that its strain was 0.0407%. The activation of Mg-5Ni-2.5Fe-2.5Ti was completed after three hydriding-dehydriding cycles. The prepared Mg-5Ni-2.5Fe-2.5Ti sample had an effective hydrogen-storage capacity near 5 wt% H. The activated Mg-5Ni-2.5Fe-2.5Ti sample absorbed 4.37 and 4.90 wt% H for 5 and 60 min, respectively, at 593K under 12 bar H₂, and desorbed 1.69, 3.81, and 4.85 wt% H for 5, 10 and 60 min, respectively, at 593K under 1.0 bar H₂.     

Diffusion parameters of grain-growth inhibitors in WC based hardmetals with Co,Fe/Ni and Fe/Co/Ni binder alloys

The diffusion behaviour of the grain-growth inhibitors (GGI) Cr and V during early sintering stages from 950 to 1150 °C was investigated by means of diffusion couples of the type WC-GGI-binder/WC-binder. Besides Co, also alternative Fe/Ni and Fe/Co/Ni binder alloys were investigated. It was found that the diffusion in green bodies differs significantly from sintered hardmetals. Diffusivities in the binder phase were determined from diffusion couples prepared from model alloys and were found to be almost equal for Co and alternative binder alloys. The diffusion parameters determined from green bodies allowed to estimate the GGI distribution in a hardmetal during heat up. This was subsequently used to estimate an appropriate grain size of VC and Cr₃C₂ in hardmetals, which is required to ensure a sufficient GGI distribution during sintering before WC grain-growth initiates.     

Moisture-induced embrittlement mechanism for (Ni,Fe)Ti alloys

Recent experimental studies showed that the ductility of NiTi is not affected by moisture, while addition of iron beyond 9 a/o in NiTi leads to moisture-induced embrittlement. To explore the nature of this embrittlement, we studied the chemical interaction between water vapor and (Ni,Fe)Ti(110) surfaces with 5 a/o and 10 a/o Fe. Temperature-programmed desorption and X-ray photoelectron spectroscopy show that decomposition of water to produce atomic hydrogen occurs on both surfaces. Activation energy for surface diffusion was calculated by density functional theory, showing that addition of Fe decreases H surface diffusivity, in agreement with experiment. Together with the observation that addition of 9 a/o Fe increases the strength of NiTi, this indicates that moisture-induced embrittlement in higher strength NiTi alloys is not due to faster H surface diffusion, but lower critical hydrogen concentration required for embrittlement.