Effect of Ga addition on the microstructure and magnetic properties of hydrogenation–disproportionation–desorption–recombination processed Nd–Fe–B powder

Microstructural evolution of hydrogenation–disproportionation–desorption–recombination processed Nd_12.5Fe_72.8Co_8.0B_6.5Ga_0.2 powder has been investigated in relation to coercivity development during the desorption recombination process. Coercivity increases when residual NdH₂ is completely dehydrogenated and the decomposed Nd is segregated at the grain boundaries of the Nd₂Fe₁₄B phase. Three-dimensional atom probe (3DAP) analysis indicates the segregation of Ga at the grain boundary. The reason for the enhancement of the coercivity by the trace addition Ga is discussed based on the 3DAP results.     

Variation of structure and mechanical properties of Ti–6Al–4V with isothermal hydrogenation treatment

Grain refinement of Ti–6Al–4V (Ti64) was achieved by means of nucleation and growth of hydride phases associated with repeated isothermal hydrogenation (RIH). With a hydrogen loading of 0.7 H/M at 600 °C, a refined nanostructure (～50 nm) in the α matrix resulted mainly from the formation of platelet β_H and δ by the RIH treatment. The first cycle of RIH significantly increased the hardness and yield compressive stress, but further RIH cycling became less effective. For RIH with a hydrogen loading of 0.5 H/M at 750 °C, the refined structure was eliminated, and the compressive yield stress and microhardness of Ti64 were lowered.     

Evolution of the fine structure of Pd–5.3 at % Ιn–0.5 at % Ru solid solution after hydrogenation

Fine-structure parameters, such as the effective coherent domain size D_hkl and microstrain ε_hkl, of Ρd–5.3 at % Ιn–0.5 at % Ru alloy foil, which was subjected to electrolytic hydrogenation and subsequent prolonged (for 55000 h) relaxation, have been determined using X-ray diffraction data. The hydrogenation and subsequent relaxation of the alloy have been found to result in the refinement of coherent domains CD(hkl) and increase in the microstrains ε_hkl.     

Phase components of the sintered Mg–x wt% LaNi5 (x=20–50) composites and their hydrogenation properties

Mg–x wt% LaNi₅ (x=20–50) composite materials were prepared by sintering powder mixtures of magnesium and LaNi₅ at 973 K for 1 h. The microstructures were studied using optical microscopy, X-ray diffraction and scanning electron microscopy. After sintering, the samples with x=20 and 30 consisted of Mg, Mg₂Ni and LaMg₁₂. As x increased to 40, the La₂Mg₁₇ phase started to form. With further increase in x to 50, Mg and LaMg₁₂ vanished, and Mg₂Ni and La₂Mg₁₇ were the major phases. The phase abundance as a function of composition was determined by Rietveld analysis. It is also found that LaMg₁₂ is not a well-defined stoichiometric compound but exists within a composition range. Compared with pure magnesium, all samples were more easily activated under hydrogen pressure of 5 MPa at 623 K. From the viewpoint of pressure–composition isotherms (P–C isotherms) of hydrogenation, the sintered Mg–x wt% LaNi₅ (x=20–50) composites work as a mixture of pure magnesium and Mg₂Ni.     

Phase relations and hydrogenation behavior of （La1-xMgx）3Al alloys

The crystal structures and hydrogenation behavior of （La1-xMgx）3Al （x = 0.1, 0.2, 0.3, and 0.4） alloys were investigated. It was found that the alloys with x = 0.1 and 0.2 consist of La（Mg,Al）, La, and a novel phase. The novel phase was determined as La2Al. It is shown that the amount of La2Al decreases as the Mg content increases. When x increases to 0.3, only La（Mg,Al） and a small amount of La2Al exist. When x is 0.4, La2Al phase disappears and the alloy contains both La（Mg,Al） and La（Al,Mg）2 Laves phase. The （Lao.9MgoA）3Al and （La0.TMg0.3）3Al alloys can be decomposed into LaH3, MgH2, and La2Al5 by the absorption of hydrogen at 473 K.     

Density functional theory study on hydrogenation mechanism in catalyst-activated Mg（0001） surface

A small amount of Fe3O4 catalyst is known to substantially improve the adsorption and desorption thermodynamics and kinetics of Mg-based materials. Using density functional theory in combination with nudged elastic band method, the dissociative chemisorptions of hydrogen on both pure and Fe-doped Mg(0001) surfaces were studied. The adsorption energy calculations show that a weakly physisorbed state above pure and Fe-doped Mg surface atoms can serve as a precursor state to dissociative chemisorption. Then, the dissociation pathway of H2 and the relative barrier were investigated. The calculated dissociation barrier (1.08 eV) of hydrogen molecule on a pure Mg(0001) surface is in good agreement with comparable experimental and theoretical studies. For the Fe-doped Mg(0001) surface, the activated barrier decreases to 0.101 eV due to the strong interaction between the s orbital of H and the d orbital of Fe.     

Enrichment of Ni-Mo-V via pyrometallurgical reduction from spent hydrogenation catalysts and the multi-reaction mechanism

Spent hydrogenation catalysts are important secondary resources due to richness in the valuable metals of Ni,Mo and V.Recovery of valuable metals from spent catalysts has high economic value and environmental benefits since they are hazardous wastes as well.Traditional recycling processes including hydrometallurgical leaching and soda roasting-leaching have disadvantages such as generating large amounts of wastewater,long process,and low recovery efficiency of valuable metals.Thus,this paper pro...     

Microstructural design of hardfacing Ni–Cr–B–Si–C alloys

This work reports the procedure for selection of alloying elements to refine the microstructure of hardfacing Ni–Cr–B–Si–C alloys by providing in situ formed nucleation agents. It is concluded that the refining element should be able to spontaneously produce precipitates at high temperatures with little solubility in their Cr-rich counterparts. After exploring the theoretical backgrounds on how to select the refining element, Nb and Zr were selected and the phase formation reactions of Zr- or Nb-modified Ni–Cr–B–Si–C alloys were calculated using Thermo-Calc® simulations. Detailed microstructural analyses of the rapidly solidified samples deposited from the modified alloys showed that addition of Nb in specific quantities induces a significant microstructural refinement in the original Ni–Cr–B–Si–C alloy without deteriorating its high hardness. The Nb-modified alloy could be used to further investigate the viability of microstructural refinement as an effective toughening mechanism for Ni–Cr–B–Si–C and similar alloy systems.     

Machining of a Zr–Ti–Al–Cu–Ni metallic glass

Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ metallic glass machining chips were characterized using SEM, X-ray diffraction and nano-indentation. Above a threshold cutting speed, oxidation of the Zr produces high flash temperatures and causes crystallization. The chip morphology was unique and showed the presence of shear bands, void formation and viscous flow.     

High-Temperature Structural Stabilities of Ni-Based SingleCrystal Superalloys Ni–Co–Cr–Mo–W–Al–Ti–Ta with Varying Co Contents

It has been recently pointed out that the compositions of industrial alloys are originated from cluster-plus-glueatom structure units in solid solutions. Specifically for Ni-based superalloys, after properly grouping the alloying elements into Al, Ni-like(■), r-forming Cr-like(■) and c-forming Cr-like(■), the optimal formula for single-crystal superalloys is established [Al–Ni_(12)](Al_1■~_(0:5) ■_(1:5)). The Co substitutions for Ni at the shell sites are conducted on the basis of the first-generation single-crystal superalloy AM3, formulated as [Al–■_(12)Co_x](Al_1Ti_(0.25)Ta_(0.25)Cr_1W_(0.25)Mo_(0.25)), with x = 1.5, 1.75, 2 and 2.5(the corresponding weight percents of Co are 9.43, 11.0, 12.57 and 15.71, respectively). The900 ℃ long-term aging follows the Lifshitz–Slyozov–Wagner theory(LSW theory), and the Co content does not have noticeable influence on the coarsening rate of c0. The microstructure and creep behavior of the four(001) single-crystal alloys are investigated. The creep rupture lifetime is reduced as Co increases. The alloy with the lowest Co(9.43 Co) shows the longest lifetime of about 350 h at 1050 ℃/120 MPa, and all the samples show N-type rafting after creep tests.     

Sluggish diffusion in Co–Cr–Fe–Mn–Ni high-entropy alloys

Sluggish diffusion kinetics is an important contributor to the outstanding properties of high-entropy alloys. However, the diffusion kinetics in high-entropy alloys has never been probed directly. Here, the diffusion couple method was used to measure the diffusion parameters of Co, Cr, Fe, Mn and Ni in ideal-solution-like Co–Cr–Fe–Mn–Ni alloys. These parameters were compared with those in various conventional face-centered cubic metals. The results show that the diffusion coefficients in the Co–Cr–Fe–Mn–Ni alloys are indeed lower than those in the reference metals. Correspondingly, the activation energies in the high-entropy alloys are higher than those in the reference metals. Moreover, the trend of the normalized activation energy is positively related to the number of composing elements in the matrix. A quasi-chemical model is proposed to analyze the fluctuation of lattice potential energy in different matrices and to explain the observed trend in activation energies. Greater fluctuation of lattice potential energy produces more significant atomic traps and blocks, leading to higher activation energies, and thus accounts for the sluggish diffusion in high-entropy alloys.     

Fe–Cr–Ti–Nb–V–C系堆焊层的组织及耐磨性研究

设计了Fe-Cr-Ti-Nb-V-C系堆焊层金属，并与Fe-Cr-W-Mo-C系进行对比，利用扫描电镜（SEM）、透射电镜（TEM）分析了堆焊层的组织，利用MM200型磨损试验机进行耐磨性对比，通过磨损面形貌观察，探讨了磨损机理。结果表明，由于形成弥散的MC型碳化物第二相及强韧的基体，Fe-Cr-Ti-Nb-V-C系堆焊层金属具有良好的耐磨性。     

Fatigue behavior of Zr–Ti–Ni–Cu–Be bulk-metallic glasses

The high-cycle fatigue (HCF) behavior of the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 (in at.%) bulk-metallic glass (BMG) was studied. Two batches of samples that are from different lots (Batches 59 and 94) are employed in present experiments. The HCF experiments were conducted, using an electrohydraulic machine at a frequency of 10 Hz with a R ratio of 0.1 in air at room temperature and under tension-tension loading, where R=σ_min./σ_max.. (σ_min. and σ_max. are the applied minimum and maximum stresses, respectively). A high-speed and high-sensitivity thermographic-infrared (IR) imaging system was employed for the nondestructive evaluation of temperature evolutions during fatigue testing. No distinct sparking phenomenon was observed at the final fracture moment for this alloy. The fatigue lifetime of Batch 59 is longer than that of Batch 94 at high stress levels (maximum stresses >864 MPa). Moreover, the fatigue-endurance limit of Batch 59 (703 MPa) is somewhat greater than that of Batch 94 (615 MPa). The vein pattern and liquid droplets were observed in the apparent-melting region along the edge of the fractured surfaces. The fracture morphology suggests that fatigue cracks initiated from casting defects, such as porosities and inclusions, which have an important effect on the fatigue behavior of BMGs.     

Double twinning in Ni–Mn–Ga–Co

Magnetic shape-memory alloys tend to deform via magnetic-field-induced and stress-induced twin-boundary motion. The rather low martensite transformation temperature of ternary Ni–Mn–Ga limits the operating temperature for potential applications. By alloying 5 at.% cobalt, the martensite transformation temperature and the Curie temperature was increased from 70 and 110 °C respectively up to 160 °C. In the single crystalline samples two non-modulated structures with tetragonal and orthorhombic lattices were found. The non-modulated orthorhombic structure has similar lattice parameters to the pseudo-orthorhombic 14M Ni–Mn–Ga phase. The single crystal specimen with the non-modulated orthorhombic structure exhibited a cyclic permutation of all three crystallographic axes in response to uniaxial loading. The parallelepiped-shaped sample was compressed repeatedly in all three directions. While maximizing work done by the load during deformation required three different martensite variants to result from deformation in three different directions, only two different martensite variants were found. The analysis of the sample shape revealed two variants mutually related through cyclic permutation of the lattice parameters, which cannot result from a single twinning event. The cyclic permutation is discussed in the light of Crocker’s double twinning mechanism.     

Peculiarities of ball-milling induced crystalline–amorphous transformation in Cu–Zr–Al–Ni–Ti alloys

An amorphization process in (Cu₄₉Zr_45−xAl_6+x)_100−y−zNi_yTi_z (x = 1, y, z = 0; 5; 10) induced by ball-milling is reported in the present work. The aim was investigation of the effect of Ni and Ti addition to Cu₄₉Zr₄₅Al₆ and Cu₄₉Zr₄₄Al₇ based alloys as well as type of initial phases on the amorphization processes. Also the milling time sufficient for obtaining fully amorphous state was determined. The entire milling process lasted 25 h. Drastic structural changes were observed in each alloy after first 5 h of milling. In most cases, after 15 h of milling the powders had fully amorphous structure according to XRD except for those ones, where TEM revealed a few nanosized crystalline particles in the amorphous matrix. In (Cu₄₉Zr₄₅Al₆)₈₀Ni₁₀Ti₁₀ alloy the amorphization process took place after 12 h of milling and the amorphous state was stable up to 25 h of milling. In the case of (Cu₄₉Zr₄₄Al₇)₈₀Ni₁₀Ti₁₀ alloy the powders have fully amorphous structure between 12 h and 15 h of milling.     

H13钢RE–N–C–S–V–Nb多元共渗成分优化

用RE-N-C-S-V-Nb盐浴多元共渗的方法对H13钢表面进行改性处理,形成了含有钒铌碳氮化物的高性能复合层。以多元共渗渗层厚度和显微硬度作为正交试验的参考指标,对多元共渗配方成分进行优化。根据正交试验中各因素水平对指标的影响结果得到最优的成分为:硼砂盐浴中,稀土质量分数5%,V2O5质量分数6%,Nb2O5质量分数6%;盐浴回火中,稀土质量分数1%,CNO-质量分数34%。利用X射线衍射仪(XRD)和扫描电子显微镜(SEM)对多元共渗层的组成和显微结构进行了分析和研究,结果表明:共渗后试样表面形成了NbN、VN、NbC、V8C7等高性能物相和组织,渗层厚度达100～120μm且组织致密并呈弥散性分布。与常规氮化处理的试样比较,高温氧化增重速率缓慢仅为氮化处理的1/4,微裂纹出现在热疲劳循环2 000次以上。     

Dynamic recrystallization behavior of Fe–20Cr–30Ni–0.6Nb–2Al–Mo alloy

Single-pass compression tests of an aluminaforming austenite(AFA) alloy(Fe–20Cr–30Ni–0.6Nb–2Al–Mo) were performed using a Gleeble-3500 thermal–mechanical simulator. By combining techniques of electron back-scattered diffraction(EBSD) and transmission electron microscopy(TEM), the dynamic recrystallization(DRX) behavior of the alloy at temperatures of 950–1100 ℃ and strain rates of 0.01–1.00 s~(-1) was investigated. The regression method was adopted to determine the thermal deformation activation energy and apparent stress index and to construct a thermal deformation constitutive model. Results reveal that the flow stress is strongly dependent on temperature and strain rate and it increases with temperature decreasing and strain rate increasing. The DRX phenomenon occurs more easily at comparably higher deformation temperatures and lower strain rates. Based on the method for solving the inflection point via cubic polynomial fitting of strain hardening rate(h) versus strain(e) curves, the ratio of critical strain(ec) to peak strain(ep) during DRX was precisely predicted. The nucleation mechanisms of DRX during thermal deformation mainly include the strain-induced grain boundary(GB)migration, grain fragmentation, and subgrain coalescence.     

Oxidation behavior of Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr hypereutectic alloys at 1250°C

Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere.Microstructural characteristics and oxidation resistance of the alloys at 1250 ℃ were investigated.The results show that,when the Cr content is 4 at%,the microstructures consist of(Nb,Ti)_(ss) and Nb_5Si_3;as Cr content increases to8 at%,C14 Laves phase Cr_2Nb is formed.The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features.The weight gains of the two alloys after oxidation at 1250℃ for 100 h are 235.61 and198.50 mg·cm~(-2),respectively.During oxidation,SiO_2,TiO_2,Nb_2O_5 and CrNbO_4 are formed at first.Then,Ti_2Nb_(10)O_(29) is formed after oxidation for 20 min and begins to change into TiNb_2O_7 as the oxidation proceeds.SiO_2 is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale.After oxidation for 100 h,oxidation products consist of SiO_2,TiNb_2O_7,Nb_2O_5 and CrNbO_4.     

Synthesis of Fe–Cr–Mo–C–B–P bulk metallic glasses with high corrosion resistance

Bulk metallic glasses with a maximum thickness (t_max) of 1.0–2.7 mm were synthesized in the Fe₄₃Cr₁₆Mo₁₆(C, B, P)₂₅ system over a wide composition range by copper mold casting. They exhibit a large supercooled liquid region (ΔT_x) of 40–90 K and a high reduced glass transition temperature (T_g/T_l) of 0.54–0.60, indicating high glass-forming ability (GFA) and high thermal stability of the supercooled liquid. The critical cooling rate for glass formation was evaluated to be of the order of 10² K s⁻¹. The bulk metallic glasses exhibited high corrosion resistance in aggressive HCl solutions. The alloying element P has a beneficial effect on corrosion resistance.     

Structure–property relationship of a spray formed Al–Y–Ni–Co alloy

The structure–property relationship of a spray formed Al–Y–Ni–Co alloy with two sets of processing conditions was investigated. Significant differences in tensile strength, yield strength, and high temperature ductility were observed with respect to the microstructural changes. Fracture toughness values were determined for both sets of specimens and found to be 7.5 and 5.8 MPa·m^1/2, respectively. Three intermetallic phases were observed in the matrix and constitute a volume fraction of approximately 75%. It is believed that the specimens failed during fracture toughness testing by the mechanism of cleavage, observed in the Al₃Y intermetallic particles.