Influence of low-temperature annealing on magnetic properties of (Nd0.625Ni0.375)85Al15 metallic glass

After a review of the selection process of (Nd_0.625Ni_0.375)₈₅Al₁₅ as a metallic glass with a relatively high glass-forming ability, we investigate the influences of its phase transitions by duplicating the heating process of the isochronal thermal analysis with low-temperature annealings. The structure, thermal stability and magnetic properties are characterized. And the influences on magnetic properties are particularly discussed with emphasis. Both the annealing processes, to the glass-transition temperature and to the onset temperature of crystallization, bring about a higher coercivity of the sample and a higher freezing temperature of the spin-glass-state. For the sample annealed to the onset temperature of crystallization, the influence is quite obvious and is ascribed to the formation of ferrimagnetic Nd₇Ni₃ phase, as detected by XRD. For the sample annealed to the glass-transition temperature, the indistinct influence is further identified with the analysis of the frequency dependence of the spin-glass-state, and it is mainly attributed to the change of the short-range order in the amorphous matrix.     

Structural and magnetic nanoclusters in Cu50Zr50−xGdx (x = 5 at.%) metallic glasses

It is shown that phase-separated metallic glasses on the nanoscale can be prepared by rapid quenching of Cu₅₀Zr_50−xGd_x melts with a low concentration of gadolinium (x = 5 at.%). Gd-enriched clusters of 2 nm size are formed as early stages of decomposition in the deeply undercooled melt. The key physical parameter to obtaining such a nanoclustered microstructure upon quenching is the critical temperature of liquid-liquid phase separation which has to be close to the glass transition temperature. Thus, the thermodynamic properties of the liquid phase even in the metastable deeply undercooled melt essentially determine the structure formation. Analysis of the spatial atomic arrangement by atom probe tomography after annealing in the supercooled liquid state provides direct evidence of the spinodal character of the decomposition by uphill diffusion. The Gd-enriched nanoclusters exhibit ferromagnetic ordering below 50 K and the cluster size regime derived from magnetization measurements is in good agreement with that obtained from atom probe tomography investigations. The first stage of crystallization of Cu₅₀Zr₄₅Gd₅ glass is observed to be Ostwald-type ripening on a nanoscale. The phase-separated glass acts as a precursor for the formation of a metastable nanocrystalline structure.     

Microstructural evolution of a Ni-based superalloy (617B) at 700 °C studied by electron microscopy and atom probe tomography

We report on the microstructural evolution of a polycrystalline Ni-based superalloy (Alloy 617B) for power plant applications at a service temperature of 700 °C. The formation of secondary M₂₃C₆-carbides close to grain boundaries (GBs) and around primary Ti(C,N) particles is observed upon annealing at 700 °C, where γ′ is found to nucleate heterogeneously at M₂₃C₆ carbides. Using atom probe tomography, elemental partitioning to the phases and composition profiles across phase and grain boundaries are determined. Enrichments of B at γ/M₂₃C₆ and γ′/M₂₃C₆ interfaces as well as at grain boundaries are detected, while no B enrichment is found at γ/γ′ interfaces. It is suggested that segregation of B in conjunction with γ′ formation stabilizes a network of secondary M₂₃C₆ precipitates near GBs and thus increases the creep rupture life of Alloy 617B. Calculations of the equilibrium phase compositions by Thermo-Calc confirm the chemical compositions measured by atom probe tomography.     

Investigation of the thermal and magnetic properties of Fe61Co10Zr2 Fe61Co10Zr2.5Hf2.5Me2W2B20 (Me = Y, Nb, W, Ti, Mo, Ni) bulk amorphous alloys obtained by an induction suction method

The microstructure, magnetic properties and thermal stability of Fe₆₁Co₁₀Zr_2.5Hf_2.5Me₂W₂B₂₀ (Me = Y, Nb, W, Ti, Mo, Ni) alloys were investigated. The samples were obtained by an induction suction method as 0.5 mm thickness plates. The microstructure was examined using X-ray diffraction and Mössbauer spectroscopy. It was shown that the investigated samples have amorphous structure throughout the volumes of the samples. The magnetic properties were measured using a Vibrating Sample Magnetometer. The investigated alloys are soft magnetic materials with low coercivity field (from 5.8 A/m to 54 A/m) and high saturation of the magnetization (from 0.87 T to 1.26 T). The studies of thermal stability were performed using a differential scanning calorimeter. It was shown that the addition of respective atoms led to changes of Curie temperature in the range from 497 to 587 K, depending on the composition of the alloys.