Papers by Keyword: Nanocrystallisation

Abstract: Thermomechanical treatment was applied to a binary NiTi alloy in order to improve its functional properties by forming nanocrystalline structure of the alloy. The alloy deformation was obtained by cold rolling combined with transverse movement of the rolls. This technique allowed us to obtain high strain (c ≈ 6) for the relatively large specimens. Subsequently, the samples were annealed in the temperature range 300 -500oC in order to form a nano-, submicro –and/or microcrystalline structure. The evolution of the structure and associated changes of the transformation sequences and functional properties were studied with the use of TEM, X-ray phase analysis, DSC and bend and free recovery ASTM tests. A mixed amorphous/crystalline structure was obtained after severe deformation, the martensitic transformation was completely suppressed in the sample. Annealing at lower temperatures caused formation of nanocrystalline structure that grew to the microcrystalline and finally well-defined polygonized structure in annealed at 500oC specimens.

Abstract: The effect of severe plastic deformation in Bridgman’s chamber on magnetic properties of the amorphous alloys of metal-metalloid type Ni44Fe29Co15Si2B10, Fe50Ni33B17, Fe70Cr15B15 obtained by the melt quenching was studied. The substantial alteration of saturation magnetization depending on the number of ferromagnetic and antiferromagnetic components in the alloy was revealed. It was supposed that the internal separation into nanoscaled areas enriched and depleted by ferromagnetic components took place in the amorphous matrix under the action of severe plastic deformation.

Abstract: Precompacts out of immiscible systems CuCr (75/25 wt%) and WCu (80/20 wt%), respectively, were made by pressing mixed powders and sintering. By applying different strains and hydrostatic pressures of HPT at room temperature, disc-shaped samples with a diameter of 8 mm were produced. They were investigated by Light Microscopy, Scanning-Electron Microscopy using Back-Scattered Electrons, and X-ray Line Profile Analysis. In addition, Vickers microhardness data were collected. Both systems showed the highest microhardness at a shear strain of about γ = 170. The density (for the case of Cu25Cr) of the consolidated material could be increased to the theoretical value. Microhardness and grain sizes were studied individually for each of the phases, too.

Abstract: A nanostructured surface layer was induced on zircaloy-4 by the method of surface mechanical attrition treatment (SMAT). X-ray diffraction and microhardness tester were applied to identify the average grain size and hardness of specimen processed for different duration, transmission electron microscopy and high-resolution transmission electron microscopy were adopted to observe the microstructure of specimen. The results showed that the surface hardness enhanced gradually and then stabilized with the processing duration increasing, while the average grain size declined gradually, to the minimum 20 nm at 15 min, then increased. The formation of nanocrystalline was due to the mechanical twins and dislocation slip.

Abstract: In order to enhance the fatigue limit of titanium (TC4), the combined shot peening were employed. Firstly, nano-crystallization in surface layer of TC4 were carried out by high-energy shot peening, and then the surface repair shot peening with small balls were introduced for repairing the surface damage by high-energy shot peening, and reducing the surface roughness, thus to enhance the quality of nanocrystalline surface layer of TC4. The fatigue tests were performed using the specimens treated by the combined shot peening. In addition, the influence factors of fatigue limit, the behavior of fatigue crack initiation and propagation etc. are discussed. The results showed that 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 compound shot peening process is increased by 34% (165MPa). The main reason of fatigue improvement is considered to be that the combined shot peening process may effectively repair the surface damage, and also there is the certain relationship between the fatigue limit improvement and the surface roughness reducing.

Abstract: Surface grain refinement is a relatively new process aimed to enhance mechanical material properties. In this study Al7075-T6 bars have been shot peened with parameters much different from those of conventional shot peening. Microstructure gradient has been observed by means of transmission electron microscopy (TEM), X-ray diffraction (XRD) and nano indentation tester. A fine grained layer was found on top surface of the specimens. Measurements indicate of notable improvements in cases of hardness and elastic modulus in comparison with untreated material. XRD results also show significant depth affected both in terms of residual stress and FWHM. The results imply that using optimized shot peening parameters, it would be possible to create a fine grained layer on surface of component and consequently increase fatigue life and any other property affected by grain size.

Abstract: Combined amorphous and nanocrystalline films attract an attention due to possibility to improve toughness, wear resistance and other properties which are important in nanomaterials and processes in energy systems. One of the known film fabrication methods is the magnetron sputtering deposition in the presence of an additional external magnetic field with inductions of up to 0.3 T as it was demonstrated by an example of the B4C-TiB2 system. The induction effects on the amorphous films hardness, growth rate and nanocrystallization process were investigated and discussed. The deposition under an additional external magnetic field with induction more than 0.2 T leads to the of nc-inclusions formation as was determined by X-ray diffraction, microdiffraction and high resolution transmission electron microscopy methods. The target and film compositions were determined by energy disperse spectroscopy and Rutherford backscattering spectroscopy. Compared to the target composition, some enrichment by nonmetallic elements (carbon and boron) was fixed in films.

Abstract: The enhancement of the fatigue strength is significant for the engineering applications of commercial pure titanium and its alloys. The paper investigated improvement of fatigue strength for commercial pure titanium by combined high-energy shot peening. Firstly, nano-crystallization in surface layer of pure titanium was carried out by high-energy shot peening, and then the shot peening with small diameter shots was introduced to degrade the surface roughness, enhancing the quality of the nano-grained surface. The fatigue limit of pure titanium by high-energy shot peening turns out to be increased by 34%, and the fatigue limit of pure titanium by compound high-energy shot peening turns out to be increased by 52.3%, according to the results. Effective factors such as surface states etc. to fatigue life, the fatigue crack initiation and propagation behaviors were also discussed in this paper.

Abstract: Effect of copper addition in a Metallic glass 2714A on the nanocrystallization characteristics have been examined in this study. Amorphous ribbon of the alloy composition Co64.5 Fe3.5 Si16.5 B13.5 Ni1Cu1 were prepared by melt spinning technique. Nanocrystallization kinetics was studied using differential scanning calorimeter technique. The kinetic parameters such as activation energy and Avrami exponent were determined using two different non-isothermal analysis methods. The kinetic behavior of individual crystallization event has been rationalized on the basis of these results. The role of addition of copper on the crystallization behavior has been understood by comparing with Metallic glass 2714A. The isothermally annealed nanocrystallized microstructures were characterized by X-ray diffraction.

Abstract: Electropulse current effect during cold rolling on deformability, nanostructure formation and mechanical behavior in coarse-grained (CG) and ultrafine-grained (UFG) TiNi alloys enriched by nickel is investigated. The UFG sample subjected to cold rolling with current has a fracture strain (е = 1.91) which is higher than that without a current (е = 0.59). As a result of cold rolling with a current and a subsequent annealing at 400-450 °C, nanostructure is formed in both alloys, which leads to a significant enhancement of yield and ultimate stresses. It has been shown that the efficiency of electroplastic rolling depends on the purity of the alloys.