Resumo em Inglês:

A numerical simulation method was used to analyze the solidification process of Zr-based bulk metallic glass (BMG) during the horizontal continuous casting (HCC) process. The large-scale general-purpose finite element analysis software ANSYS was adopted to develop a temperature field model. The variation of temperature field in the specific area at different time was studied. Its accuracy was verified by the experimental results of Zr48Cu36Ag8Al8 bulk metallic glass continuous casting solidification process. The effects of casting parameters include intermittent casting procedure (drawing and stopping), casting speed, pouring temperature and cooling rate. The results show that the optimum casting speed range is 1-2 mm/s, pouring temperature is 1223 K and cooling rate of 10 mm rod casting is 5 L/min respectively. A fully Zr-based bulk metallic glass whose diameter is 10 mm and length has no limitation has been successfully fabricated.

Resumo em Inglês:

In this paper we investigated the microstructure and phase transition of atomized Fe-(1.3, 7.9, 11.7 wt.%) Mn alloy powders. The results show that the main phases of Fe-1.3Mn, Fe-7.9Mn and Fe-11.7Mn powders are ferrite, α’-martensite+austenite, α’-martensite+ε-martensite+austenite, respectively. The δ-ferrite in the Fe-1.3Mn powder is the high-temperature δ-ferrite directly formed from liquid, companying by a small number of nanometer sized austenite particles precipitated from the ferrite matrix. In the Fe-11.7Mn powder, the γ-austenite, ε-martensite and α’-martensite are found in the same region and have the K-S orientation relationship, suggesting phase transitions of γ-austenite → ε-martensite → α’-martensite and γ → α’-martensite.

Resumo em Inglês:

On the basis of compression tests, uni-axial compression deformation and fracture behaviors of NiTiZrAlCuSi bulk metallic glass samples with different dimensions have been studied by numerical simulation approaches. Finite element models for compression simulation have been developed using ABAQUS software. Shear damage models were employed to reproduce the compression deformation and fracture process of the Ni-based amorphous alloys. Mises stress distribution and equivalent plastic strain obtained numerically during compression were analyzed. The simulation results were consistent with those of the compression experiment tests. The smaller of the samples exhibited larger plasticity and higher fracture strength when compared with the bigger one.

Resumo em Inglês:

Direct laser fabrication (DLF) is one of the advanced near-net shape processing technologies. The variations of laser parameters can change the energy distribution within a laser scan, and thus affect the cooling rate, and resulting microstructure of deposited material. In this study, the effect of the laser parameters on the microstructure of additive laser fabricated Ti6Al4V alloy was investigated. It was found that huge columnar grains formed during laser fabrication of Ti6Al4V alloy for a wide range of processing conditions, and the laser power, scanning speed and duration time were had a significant effect on the refinement of the prior grains. Gradient morphologies were obtained from a complex thermal history.

Resumo em Inglês:

We here report on a comparative study of the microstructure and magnetic properties of as-cast and SPCJA-ed Co-based wires for potential sensor applications. Experimental results indicate that both as-cast and SPCJA-ed wires exhibit typical amorphous feature. There also exists the enhanced local ordering degree of atomic arrangement and obvious transformation (maximum magnetic permeability and minimum saturation magnetization) of magnetic properties after SPCJA treatment, resulting from variation of magnetic moment exchange coupling and magnetic anisotropy by coactions of high-density pulsed magnetic field energy and thermal activation energy. Moreover, SPCJA treatment can drastically improve the GMI property of melt-extracted wires. GMI ratio [ΔZ/Z0]max of 166.07% and field response sensitivity ξmax of 413.76%/Oe by more than 2.25 times and 1.73 times of as-cast wire at 5MHz. Therefore, the SPCJA-ed wire exhibits the improved GMI and magnetic properties at 5MHz~10MHz in contrast to AC annealed wire, which is more suitable for GMI sensor applications working at low-applied-frequency and relatively low-working-magnetic field.

Resumo em Inglês:

Microstructures of CuCr25 and CuCr50 alloys treated by high current pulsed electron beam (HCPEB) were investigated in this work. The microstructure and solidification behavior of the Cr-rich phases were characterized by scanning electron microscopy (SEM). Results show that a remelting layer of 3~5 μm is formed on the surface of Cu-Cr alloys. The microstructure of the remelting layer reveals that both the fine dispersion of Cr-rich spheroids and the craters appear after HCPEB treatment. This means that metastable liquid phase separation occurs during rapid solidification under HCPEB treatment. In addition, the appearance of relatively large craters in the subsurface of Cr-rich particles with the distance about 5-10 μm provides direct evidences supporting results reported by other researchers in terms of numerical simulation temperature field of HCPEB treatments.

Resumo em Inglês:

Fe-based magnetic metallic amorphous and nanocomposites have excellent soft magnetic properties including greater magnetization and magnetic permeability compared with crystalline alloys, especially at high operation frequency and temperature. The high magnetic field (HMF) up to 12T is introduced to the heat annealing of Fe71(Nb0.8Zr0.2)6B23 bulk metallic glass (BMG). The 12T HMF annealing shows the effect of improve the thermal stability of amorphous state during annealing in the BMG's supercooled liquid region as compared with the annealing without magnetic field. The HMF annealing inhibits the brittleness of BMG during the structural relaxation, due to the increment of the activation energy under the HMF. The HMF annealing also results in squared hysteresis loops after the structural relaxation and lower coercivity.

Resumo em Inglês:

Effects of forming pores in Zr70Cu30 metallic glass on the deformation behaviour is investigated through molecular dynamics simulations. The formation of pores leads to only a small reduction in strength, but dramatically enhanced plasticity in compression. The large plasticity of glass is attributed to the large effective free space induced by forming pores. It can also promote formation of crystalline phases in the amorphous matrix during deformation. Simulation reproduces the images of the evolution of pores in the metallic glass. The simulation results are in good agreement with experimental results.

Resumo em Inglês:

The Gd55Al20Co25 amorphous wires exhibit a relatively strong magnetocaloric effect (MCE). These melt-extracted amorphous wires display second-order magnetic transition (SOMT) and the value of maximal magnetic entropy (–ΔSm) for the melt-extracted wires is calculated to be ~9.7 J·kg–1·K–1 around the Curie point (TC) of ~110 K with applied field of 5 T. Moreover, the melt-extracted amorphous wires show a high refrigerating efficiency with a relatively large cooling power (RCP, ~804 J·kg–1) and refrigeration capacity (RC, ~580 J·kg–1) under an applied magnetic field of 5T due to the broad paramagnetic-ferromagnetic (PM-FM) region associated with amorphous alloys. These favorable properties make melt-extracted Gd-based amorphous wires to be the potential refrigerant for magnetic refrigeration (MR) of liquid oxygen.

Resumo em Inglês:

The present investigation aims at understanding the effect of size on melting behaviour of free standing alloy nanoparticles. Alloy nanoparticles of Bi1-xSbx (x=0.12, 0.25, 0.50 and 0.75) have been synthesized by solvothermal route using sodium borohydride (NaBH4) as a reducing agent. The samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The melting behaviour of alloys was determined by differential scanning calorimetry (DSC). The formation of different solid solution phases of Bi-Sb alloy has been confirmed by detailed XRD studies. Uniform cuboctahedral as well as spherical morphology of alloy nanoparticles has been observed in TEM with average particle size of 40±9 nm in Bi0.75Sb0.25 specimen. DSC studies indicate the depression in the melting temperature of the nanoparticles as compared to the bulk alloy of same composition.

Resumo em Inglês:

Ni48Mn26.4Ga19.7Fe5.9 microwires with grain size of 1-3 micron were successfully fabricated by melt-extraction. The superelastic effects in the microwires under various temperatures and loads were systematically demonstrated. The as-extracted microwires displayed partial superelasticity when attended at relatively high temperature. The critical stress for stress-induced martensite formation increases linearly with temperature and follows the Clausius-Clapeyron relationship. The temperature dependence of the as-extracted polycrystalline Ni48Mn26.4Ga19.7Fe5.9 microwires is 16.4 MPa/K, which is higher compared with Ni-Mn-Ga single crystals. In addition, the as-extracted microwires display excellent shape memory behavior with the recovery strain and recovery ratio of 1.26% and 86%, respectively, when the total strain reaches 1.47% at 310 K.

Resumo em Inglês:

Gd60Al20Co20 amorphous wires with smooth surfaces and circular cross-sections were fabricated by a melt-extraction technique. The mechanical properties of the extracted microwires were evaluated by tensile tests and their fracture reliability was estimated by using Lognormal and two- or three- parameter Weibull analysis, respectively. The microwires exhibit a tensile fracture strength ranging from ~788 to ~1196 MPa, with a mean value of 1008 MPa and a standard variance of 121 MPa. Lognormal method of statistical analysis presents that the average stress of microwires is ~1012 MPa. Weibull statistical analysis indicates that the two-parameter tensile Weibull modulus is 8.5 and the three-parameter Weibull modulus is 5.3 with a threshold value ~365 MPa for as-extracted amorphous microwires. Our results show that the extracted Gd-based wires possess excellent tensile properties and high fracture reliability, with a high potential for applications in and magnetic refrigeration.

Resumo em Inglês:

Giant magneto-impedance (GMI) effect and domain transformation for melt-extracted Co68.15Fe4.35Si12.25B13.25Nb1Cu1 amorphous wires have been induced by a cryogenic Joule annealing (CJA) treatment with different DC current amplitude (0~350 mA) for 240s. Experimental results indicate that the maximum GMI ratio ([ΔΖ∕Ζ0]max) achieves to 188.1% with exciting field increasing to 1.8 Oe monotonically for 300 mA annealing treated wires, which can attribute to the surface complex domain structure change formed by CJA treatment. The liquid nitrogen can protect the wire from crystallization when applied large DC currents. Moreover, the CJA treatment can improve the response sensitivities effectively. These remarkable characteristics make the melt-extracted microwires by CJA tailoring as the promising candidate material for small-sized magnetic sensors.

Resumo em Inglês:

Molecular-dynamic simulations have been used to study the structure evolution in iron melts rapidly cooled under different pressures. An extreme cooling rate (4×1012K/s) was adopted in the cooling process. The simulation results show that at the ambient pressure, martensitic transformation happened. However, at a pressure of 1.4GPa, the system passes from bcc structure to a less closed packed structure which is composed of both full icosahedra clusters and bcc structure. With the increase of pressures, an amorphous state was observed, and a compacted local structure with more defected icosahedra is obtained. This work contributes to a better understanding about the dynamics of phase transitions in iron under high pressure, especially during the extremely fast cooling process.

Resumo em Inglês:

Crystallization kinetics and nanocrystalline induced brittleness in an in-situ dendrites reinforced Ti44Zr20Nb12Cu5Be19 bulk metallic glass (BMG) composites were investigated. The activated energy of the present Ti-based metallic glass matrix is obtained to be about 201 KJ/mol. As the annealing temperature rises, the annihilation of free volume is believed to cause the increase of hardness and the decrease of plasticity. Brittle fracture occurs after the precipitation of nanocrystalline, which can be ascribed to the formation of the microcracks in the matrix during deformation.

Resumo em Inglês:

The distribution model of temperature for the billets was established based on the porosity formation mechanism. By calculating the numerical relationship between the temperature of the deposition layer and the formation ability of the porosity, the distribution of porosity for spray formed 7075 aluminum billets was predicted and the influence of superheat on the formation probability of porosity was analyzed. The results show that a denser billet can be obtained with the average temperature of 813 K of the droplets and a liquid fraction of 50%. The porosity would increase whether the temperature further increased or decreased. Finally, the 7075 aluminum alloy was prepared by spray deposition technique with the optimum process parameters. It can be found that the distribution and types of the porosity were different at different locations. These results are in good agreement with the porosity distribution law.

Resumo em Inglês:

Bulk nanocrystalline Fe-Ni alloys with Ni content of 40-55 at.%, grain size of 12-15 nm, and hardness of 3.9-4.6 were prepared by an electrodeposition. The decrease in the hardness values as grain size decreased was discussed in terms of grain refinement effect, solid solution strengthening, and grain boundary relaxation strengthening. It was found that the change of Ni content could not significant solid solution strengthening, and there were no difference in the state of grain boundary for each sample. Micro X-ray diffraction analysis on the sample after tensile tests showed that the (200) texture was developed but full width at half maximum was not changed. This indicated the potency that the grain boundary activity would be induced by a tensile loading. The softening behavior of electrodeposited Fe–Ni alloys would be related to the transition of the dominant deformation mechanisms.

Resumo em Inglês:

The present work describes the synthesis and characterization of Ni45Fe5Mn40Sn10 Heusler alloys. The constituent pure metals were melted in radio frequency induction furnace to form the alloy and then cooled it quickly. The as-cast alloy was annealed at 850 °C for 24 h in vacuum and cooled in two different conditions i.e. self-cooling at room temperature in air and sudden cooling at liquid nitrogen temperature (quenched). The X-ray diffraction, scanning and transmission electron microscopic techniques have been used for structural/microstructural characterization as well as chemical analysis of the material. The effect of annealing and subsequent cooling in two different conditions has been described and discussed in the context of evolution of B2 and orthorhombic martensite with lattice parameter a= 0.65 nm, b=0.59 nm and c=0.56 nm.

Resumo em Inglês:

Systematic investigation was made on the magnetic property of a series of Nd2Fe14B/α-Fe nanocomposites prepared by crystallization of amorphous Nd8Fe84Ti2B6 ribbon at annealing temperatures in the range of 800-860 °C. Both remanence and energy product increase with increasing annealing temperature, reaching the maximum values at 850 °C. Coercivity remains around 5.5 kOe for the annealing temperatures above 800 °C. Although the smooth demagnetization curve indicates effective exchange coupling between the Nd2Fe14B/α-Fe dual phases, FORC diagram reveals the existence of α-Fe for the Nd8Fe84Ti2B6 ribbon annealed at 850 °C. In addition, the variation of microcoercivity at different locations indicates a distributed exchange interaction, which can be caused by the nonuniform microstructure. The magnetic property is also affected by the demagnetization effect caused by the particle shape, which is evidenced by the negative region in the first-order reversal curve (FORC) diagram.

Resumo em Inglês:

This work reports the effects of metallic glass precursors on the catalytic performance of nanoporous metals. Pd-based multicomponent nanoporous metals with similar nanoporous structure were successfully fabricated by electrochemically dealloying the Pd20Ni60P17B3 and Pd20Ni20Cu40P17B3 metallic glass precursors at the critical dealloying potentials. It was found that the glassy precursors with different chemical compositions result in different doping elements in the as-obtained nanoporous metals and thus lead to different catalytic activities.

Resumo em Inglês:

Iron nitride films were deposited by radio frequency (RF) reactive magnetron sputtering at different bias voltages. It was found that the composition, structure and magnetic properties of the iron nitride films were strongly dependent on the depositing bias voltage. With a bias voltage of –50 V, the iron nitride film is fully dense and has a homogenous microstructure consisting of α-Fe and Fe16N2, leading to a unique combination of high Ms of 1645 emu/cc, Hc of 5 Oe and μi of 1573.

Resumo em Inglês:

The microstructure and phase formation of rapid solidified Ag-rich Ag-Cu-Zr alloys were investigated. Two types of structure; interconnected- and droplet-type structures, were obtained due to phase separation mechanisms. The former was spinodal decomposition and the later was nucleation and growth mechanism. Depending on the alloy compositions, three crystalline phases; FCC-Ag, AgZr and Cu10Zr7 phases were observed along with an in-situ nanocrystalline/amorphous composite. Vickers hardness testing indicated a significant increase of hardness in the nanocrystalline/amorphous-composite alloy.

Resumo em Inglês:

A melt-spun Fe90Si5B5 alloy ribbon consists of bcc-Fe(Si) + Fe3B + amorphous phase and exhibits good bending ductility, high tensile fracture strength above 1000 MPa, high corrosion resistance and unique magnetic properties as exemplified for high saturation magnetization exceeding 1.9 T, moderately high initial permeability of about 150 and low coercivity of 745 A/m which are attractive for magnetic sensors utilizing a nearly constant high permeability with applied field up to coercivity. Besides, the tensile fracture strength and elongation increase significantly to 1286 MPa and 0.62%, respectively, after annealing for 900 s at 823 K. The Fe-Si-B alloy ribbons are attractive as a new type of sensor material with features of high bending ductility, high tensile strength and elongation, relatively good corrosion resistanceand unique soft magnetic properties with very high saturation magnetization.

Resumo em Inglês:

Glass formability and phase transformations in rapidly quenched ferromagnetic nano-structured (Fe85B15-xSix)98-yC2Cuy (where x=0; 5 and y=0; 1) systems were investigated. The consecutive crystallization stages of bcc-Fe and borides were determined by resistometry, differential scanning calorimetry and by thermogravimetry, where the values of important transformation parameters were estimated and mutually correlated with the results from the structure analysis by TEM and XRD. Morphology of the nano-sized Fe grains in amorphous matrix and their transformation to borides matrix was observed by TEM in dependence on the chemical composition and thermal treatment. The effects of systematic alloying on the transformation process and on the resulting structure have been correlated with selected magnetic properties of the samples after suitable annealing.

Resumo em Inglês:

Phase transformations and composition of Al-Fe-Si, Al-Co-Si and Al-Ni-Si based rapidly quenched ribbons have been investigated. Different Al-Si based alloys with varying Si content and additions of third metallic element, namely Al80Si20, Al60Si40, Al75Fe5Si20, Al70Fe10Si20, Al75Co5Si20, Al70Co10Si20, Al75Ni5Si20, Al70Ni10Si20 were analyzed. Variation of phase composition with elemental composition was observed. Evolution of phases was determined by resistometry, differential scanning calorimetry (DSC). In situ X-ray diffraction (XRD) and transmission electron microscope (TEM) records were observed during isothermal and isochronal annealing.

Resumo em Inglês:

Rapid solidification of the super high-temperature Al2O3/ZrO2 melt was studied by a novel method. The melt was prepared by explosion reaction using Al and Zr(NO3)4 as raw materials and then sprayed to Cu-plate. The heat transfer during the solidification process was analyzed by one-dimensional Finite Element Analysis. The cooling rate of the melt decreased with the increasing distance from the Cu-plate. According to the microstructure evolution, the coating can be divided into four areas: amorphous, nano-crystalline, cellular and dendrite crystal layer. The amorphous and nano-crystalline (50~100 nm) layers can be obtained at the cooling rate of about 1.68~7.76 × 105K/s and 0.48~1.68 × 105K/s, respectively, while the cooling rate of the cellular and dendrite crystal layers were about 0.26~0.48 × 105K/s and 0.14~0.26 × 105K/s, respectively. The microstructure of the hypoeutectic ceramics shows that the nano-crystalline, cellular and dendritic crystals of the Al2O3 phases were embedded in the Al2O3/ZrO2 eutectic matrix.

Resumo em Inglês:

The microstructural evolution of Cu-10at%C nanocomposite powder during high energy mechanical milling (HEMM) of Cu/graphite powder mixture was studied. It was found that the efficiency of milling in refining the microstructure of the copper matrix was very high with the presence of graphite in the Cu/graphite powder mixture. The average grain size of the Cu matrix reaches its minimum value of about 30 nm just after milling for 6 h, and the graphite particles were refined down to about 10~20 nm after milling for 24 h. It envisaged that the refining of graphite particles down to 10 nm would be accompanied by formation of nanograins of copper matrix, and it appears that high energy mechanical milling can greatly enhance the solubility of carbon in copper though it is very hard to determine the exact solubility.

Resumo em Inglês:

Two consolidated copper samples were prepared by consolidation of a gas atomized powder using two different routes of spark plasma sintering (SPS), respectively. The first route was direct SPS of copper powder at 800 °C for 10 minutes, and the second route was compacting copper powder by die-pressing to produce a powder compact followed by SPS of the powder compact at 800 °C for 10 minutes. The microstructures and tensile mechanical properties and the fracture surfaces of specimens have been studied. It was found that the level of consolidation of the sample made using Route 1 was higher than that of the sample made using Route 2, as reflected by the relatively higher strength and better ductility of the first sample than those of the second sample. The lower consolidation level of the second sample was depicted by the higher volume fraction of cavities in the fracture surfaces of specimens which were formed by separation of the powder particles at the weakly bonded interparticle boundaries. The grain sizes of the first sample were larger than those of the second sample. The reasons for the differences in the levels of consolidation and grain sizes associated with two SPS routes are discussed.