Papers by Keyword: Nanocrystallisation

Abstract: This work studies the production of melt spun Fe₇₈Si₉B₁₃ ribbons with amorphous or nanocrystalline structure. The main objective is the preservation of the amorphous structure after obtaining powders by mechanical milling of the ribbons, as well as the study of the influence of the milling conditions on the size distribution and structure of the obtained powders. In order to obtain high quality amorphous ribbons, the wheel rotation speed, crucible-wheel distance, melt homogenization time, ejection pressure and the ejection temperature were optimized in the melt spinning process. Different mills were used for powder production, studying the size distribution, efficiency, and preservation of the amorphous character as a function of the milling time. Ribbons and powders were characterized by X-ray diffraction (XRD) and electron microscopy (SEM and TEM); laser diffraction was used for powder granulometry.

Abstract: Ultrasonic Shot Peening (USP) - a novel route of surface modification was employed on biomedical grade Nickel free high nitrogen stainless steel (18Cr-21Mn-0.65N-balance Fe) to obtain a surface nanostructure without changing its chemical composition and microstructural phase transformation. Hardened steel shots of diameter 2 mm and 3mm were repeatedly impacted on the specimen surface at a constant frequency of 20KHz for 2 and 8 minutes duration. Coarse surface grains of size 36±6µm transformed into nanocrystalline grains of size 13-18 nm. The deformed layer resulted by USP treatment increased with increase in shot diameter and duration of USP. The microstructure was characterized by using optical microscope, SEM, XRD and TEM technique. The hardness and roughness of the treated surface was also found to be strongly dependent on the USP process parameters.

Abstract: In this paper, the effects of Surface Mechanical Attrition Treatment on the high-temperature oxidation of AISI 316L austenitic stainless steel are investigated. Samples treated with different conditions were oxidized at 650°C in order to study the effect of this type of nanocrystallisation on the oxidation resistance of the alloy concerned. X-ray diffraction and in-situ Raman spectroscopy were used to identify the oxides formed at the surface. The results indicate the presence of hematite and chromium oxides. Experimental results obtained by Raman spectroscopy were also used to study the stress evolution in Cr₂O₃ films during isothermal conditions.

Abstract: The results presented here concern two NiTi alloys (near-equiatomic NiTi and Ni-rich alloy) subjected to plastic deformation by compression combined with reversion oscillating torsion. The maximal strain obtained was ε_c = 6.20. Finally the alloys were annealed at the temperature range 300 – 500°C for 1 hour. The structure of the as-prepared alloys was studied with the use of temperature X-ray diffraction and TEM observations. Also the DSC and bend and free recovery ASTM tests were carried out. It was found that the structure consists of a mixture of highly deformed B2 parent phase and B19’ martensite. The TEM studies revealed some amorphous areas in the most strained region of the samples. Annealing at lower temperatures caused formation of nanocrystalline structure that grew to the microcrystalline and finally well-defined polygonized structure in annealed at 500°C specimens. Multi-stage transformation was observed in the annealed at lower temperatures samples.

Abstract: The ultrasonic impact treatment on U70 raill steel was carried out under different technology by using HJ-II ultrasonic impact machine. The electric current was 1.5 A, the amplitude was 24 µm and the impact frequency was 10000 HZ during the treatment, the peening duration was 60 min. The influence of impact on microstructure was observed with the scanning electron microscope of 6360LA type and high resolution transmission electron microscope of JEM-2100 type, and the hardness of impacted surface was also researched before and after impact treatment. The wear resistance was tested by using M-2000 type wearing machine. The experimental results indicate that the surface strength of rail steel can be improved evidently by using ultrasonic impact treatment. The surface microstructure of the rail steel could be refined to nanoscale and the hardness and wear resistance of impact layer were significantly improved through the ultrasonic impact technology. Compared to the specimen without treatment, its surface hardness was increased 26.4%, and the wear resistance was increased 40.63%. The depth of plastic deformation zones of the specimen with 60 min treatment is nearly 100µm. In the plastic deformation zone, ferrite is refined to nanoscale grains so that the matrix is strengthened.

Abstract: The amorphous Fe₇₈Si9B13 alloy was treated by low frequency pulse magnetic field (LFPMF). The microstructure was observed by Mössbauer spectroscopy and Transmission Electron Microscope. The activation energy of nanocrystallization of amorphous alloy Fe78Si9B13 treated by low frequency pulse magnetic field was determined by differential scanning calorimetry (DSC). The results indicated that the activation energy was decreased from 433.6kJ.mol-1 to less than 200kJ.mol-1 after LFPMF. The nucleating rate of α-Fe(Si) was increased and there was only single phase α-Fe(Si) crystalline formed.

Abstract: Continuous CO₂ laser irradiation is exploited to realize nanocrystallization of Fe based amorphous alloys Fe_73.5Cu₁Nb₃Si_13.5B₉ by overlapping of the heated areas, and to get homogeneous ultrafine grains of about 10nm with a bcc α-Fe(Si) structure, which is the foundation of the excellent soft magnetic property. X-ray diffraction and transmission electron microscopy(TEM) are used for microstructure analysis and observation, and the results show that irradiation power of 71w~99w gives the heat shock to the transformation from the metastable amorphous state to crystallized state, and proper time of about 20~30 seconds is necessary for optimum microstructure and soft magnetic properties.

Abstract: We prepared glasses in 30BaO-(15+x)TiO₂-(55–x)GeO₂ composition (x=0, 10, 20, 35) and examined their crystallization behavior. Relation of crystallizing-phase to optical transparency in the resulting crystallized sample was also considered. Although the glass samples with x=0-20, which precipitate fresnoite-type Ba₂TiGe₂O₈ as main phase, lost transparency after crystallization, the glass with x=30, which precipitates BaTi₄O₉ phase, kept good transparency. Dependence of transparency on the x is also discussed.

Abstract: We present the magnetic characterization of nanocrystalline amorphous ribbons with nominal composition Fe_73.5B₉Si_13.5Mo₃Cu₁, obtained by melt spinning. An annealing treatment was made on the alloys during 10, 20, 40, 50, 60 and 70 minutes at 400°C, in order to induce a nanocrystalline phase. Measurements of magnetization were made to evaluate the coercive field (H_C) as a function of annealing time. Results of vibrating sample magnetometry (VSM) show two important changes: a decrease in H_C value as annealing time increases, until reaching a minimum value of 31 Oe, corresponding to the amorphous ribbon with annealing time of 20 minutes. This behavior indicates a soft magnetic state suggesting a structural relaxation. At this time, it is observed the best magnetic properties. In the second change, H_C increases until reaching a maximum value of 34 Oe, corresponding to 60 minutes. This increase indicates the onset of nanocrystallization state for the FeMo phase. In order to evaluate the magnetic anisotropy field (H_K), measurements of low field absorption (LFA) were carried up by means of an electronic paramagnetic resonance (EPR) spectrometer modified. The peak-to-peak linewidth (ΔH_PP-LFA) of LFA signal is associated with H_K, therefore H_K can be evaluated during the different annealing times.

Abstract: The rotary bending fatigue tests were to investigate the effect of the combined shot peening for surface nanocrystallization (CSPN) on the fatigue limit of titanium (TC4). CSPN formed nano-crystallization in surface layer of TC4, and may reduce and reducing the surface damage of TC4 by high energy shot peening. In the case of equiaxed microstructure, the fatigue limit of TC4 specimen treated by high-energy shot peening is increased by 13%, while the fatigue limit of TC4 specimen treated by the CSPN is increased by 34%. But in the case of lamellar microstructure, the fatigue limits of the specimens by either the surface treatment process both of the high energy shot peening and CSPN. The main reason of fatigue improvement change is due to that the effect of surface nanocrystallization and surface damage repairing by CSPN is changed between the different microstructure of TC4.