[Show abstract][Hide abstract] ABSTRACT: The solid solution strengthening of a-Ti was investigated in respect of dislocation nucleation and dissociation in all four active glide modes. A series of TiþX alloys (X ¼ Al, Sn, V, Zr and O) was selected to analyze the impact of solute valence structure (Al, Sn e p type elements, V, Zr e d type elements) and lattice site (interstitial O) on the mechanisms responsible for variation of mechanical properties. The computational procedure relied on the generalized stacking fault energy (GSFE) concept combined with the nudged elastic band method that enables full atomic relaxation and determination of the true, minimum energy GSFE path. Additionally, various concentrations of solutes and their distance to the glide plane were considered as well. Our study revealed a strong, nonlinear influence of X position on GSFE and migration of O atoms during the crystal slip. These new phenomena allowed one to determine three solution strengthening mechanisms: (I) hindrance of prismatic dislocation emission and reconfiguration of 1/3 <1120> screw dislocation cores (p type solutes), (II) hindrance of prismatic dislocation emission (V) and SFE reduction in other modes (both d type solutes) and (III) suppression of dislocation nucleation in all modes caused by O. We found that the stacking faults formed by the single partial dislocations have a thickness of few atomic layers and exhibit a highly non-uniform structure. Their ability to accommodate the lattice deformation introduced by solute elements greatly affects the stacking fault energies of the a-Ti alloys.
[Show abstract][Hide abstract] ABSTRACT: The use of the pulse plasma sintering technique for CoSb3 thermoelectric material consolidation is reported in this work. The influence of sintering temperature on the microstructure and material properties such as the See-beck coefficient, electrical resistivity, and thermal conductivity has been investigated. It is shown that, for samples fabricated at 923 K and 973 K, there were no significant differences in the average grain size or final phase composition. In both cases, a fine-grained polycrystalline structure of the compacts with density nearly equal to the theoretical value was achieved. Both samples were composed almost uniquely of CoSb3 phase. The measured thermoelectric parameters such as the Seebeck coefficient, electrical, and thermal conductivity showed similar dependence on temperature. For both samples, the Seebeck coefficient was negative at room temperature and showed a transition from n-to p-type conduction over the temperature range of 400 K to 460 K. The measured minimum thermal conductivity values, 4 W m-1 K-1 to 5 W m-1 K-1 at 723 K, are typical for undoped bulk CoSb3. A maximum ZT value of 0.08 at 623 K was obtained for the sample consolidated at 923 K for 5 min. The results of this work are very promising from the point of view of use of pulse plasma sintering as an alternative method for fabrication of a broad range of thermoelectric materials in the future.
Journal of Electronic Materials 09/2015; DOI:10.1007/s11664-015-4037-5 · 1.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Abstract Laser engineered net shaping (LENS) was used to produce thin-walled samples of the high-entropy alloy AlCoCrFeNi from a prealloyed powder. To determine the effect of the cooling rate during solidification on the microstructure of the alloy, different laser scanning rates were used. A microstructural study of the surfaces of the sample walls was performed using X-ray diffraction analysis and optical and scanning/transmission electron microscopy. The crystal structure of the alloy was determined to be a body-centred cubic (bcc)-derivative B2-ordered type. The microstructure of the alloy produced by LENS was dendritic. Further, it was found that with an increase in the laser scanning rate from 2.5 to 40 mm s-1, the average grain size decreased from 108.3 ± 32.4 μm to 30.6 ± 9.2 μm. The maximum cooling rate achieved during the laser cladding of the alloy was 44 × 103 K s-1. The electron microscopy study of the alloy showed the presence of precipitates. The morphology of the disordered bcc (Fe, Cr)-rich precipitates in the ordered B2 (Al, Ni)-rich matrix changed in the dendritic and interdendritic regions from fine and spherical (with a diameter of less 100 nm) to spinodal (with the thickness being less than 100 nm). The LENS- produced AlCoCrFeNi alloy exhibited an average microhardness of approximately 543 HV0.5; this was approximately 13% higher than the hardness in the as-cast state and can be attributed to the grain refinemet in the LENS- produced alloy. Moreover, it was found that increasing the cooling rate during laser cladding increasess the microhardness of the alloy.
Journal of Alloys and Compounds 07/2015; 648:751-758. DOI:10.1016/j.jallcom.2015.05.144 · 3.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The paper presents the results of Molecular Dynamics (MD) studies of the thermal properties of Cu and Ag composites with single- (SLG) and multi-layered (MLG) graphene. We show that the thermal boundary conductance (TBC) of the metal-graphene interface drops significantly for the systems containing more than one layer of graphene. It is also concluded that the TBC for a single graphene layer is significantly higher for silver than for copper. For both systems, however, we found that the interface is a barrier for heat transfer with the thermal conductance being at least two orders of magnitude lower than for metal. Moreover, we found that the TBC decreases with an increase in the number of graphene layers. The interfacial effect becomes negligible for a thickness bigger than two graphene layers. Above this thickness the thermal conductivity of the region of multilayered graphene is not influenced by the interface and becomes similar to that of graphite. The results are compared with available experimental data and discussed in terms of the rules for designing composites of a high thermal conductivity.
[Show abstract][Hide abstract] ABSTRACT: The structure and corrosion resistance of Grade 2 titanium subjected to the hydroextrusion processes were examined. The microstructure was characterized using optical microscopy and transmission electron microscopy. The corrosion resistance was determined using the impedance and potentiodynamic
methods, in 0.1 M H2SO4 solutions and an acidified 0.1 M NaCl solution with a pH of 4.2, at ambient temperature. Nanohardness tests were performed under a load of 100 mN. It has been demonstrated that the hydroextrusion method makes it possible to obtain relatively homogeneous
nanocrystalline titanium Grade 2 with an increased hardness, the elastic modulus almost unchanged with respect to that of the initial structure and a lower corrosion resistance.
Journal of Nanoscience and Nanotechnology 07/2015; 15(7):4992-4998. DOI:10.1166/jnn.2015.10027 · 1.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The interactions of prime interstitial and alloying elements in hexagonal Ti were investigated using a density functional theory calculations. The binding energies of oxygen with all substitution elements whose solubility limit in α-Ti is greater than 3 at% were calculated. The investigations performed reveal no attraction between Zn, Zr, Ag and O, and strong O–Sc and O–Sn binding. It was found that the O–X clustering mechanism is based on a direct and long-range O–X interaction, both controlled by valence structure and electronegativity of substitational elements. The single crystal and isotropic elastic constants together with Pugh's plasticity criterion were calculated for Ti with multiple point defects to evaluate their impact on mechanical properties. The results obtained reveal that a low concentration of O improves ductility in Ti + Sc solid solutions and increases the brittleness of Ti + Sn alloys. The diverse effect on ductility is due to different chemical bond types in the vicinity of O. The results show that the interstitial-substitational elements clustering effect may be used to optimize mechanical properties of α-Ti alloys.
[Show abstract][Hide abstract] ABSTRACT: Corrosion behavior of a metallic biomaterial is an important characteristic because the biocompatibility of a biomedical grade metallic implant is primarily related to corrosion behavior. The aim of this research was to study the effect of Equal Channel Angular Pressing (ECAP) process on corrosion behavior of the AISI 316L type austenitic stainless steel. ECAP was conducted on an AISI 316L stainless steel up to eight passes. Scanning transmission electron microscopy (STEM) technique was utilized to study the microstructure of the as-received material and the samples subjected to ECAP. Electrochemical corrosion polarization and electrochemical impedance spectroscopy tests were performed in Ringer solution in order to determine and compare the corrosion behavior of initial coarse-grained and ECAP-ed specimens as an indication of biocompatibility. The results showed that an ultrafine-grained/ nanocrystalline 316L stainless steel with a mean grain size of about 78 nm was obtained after performing the eight passes of ECAP. The corrosion resistance of 316L stainless steel was improved considerably by increasing of the number of ECAP passes. After performing the eight passes of ECAP process, the corrosion rate of 316L stainless steel measured to be 0.42 μA.cm2 which is significantly lower than that of initial coarse-grained material (3.12 μA.cm2).
[Show abstract][Hide abstract] ABSTRACT: Biodegradable polymeric fibers with nano and submicron diameters, produced by electrospinning can be used as scaffolds in tissue engineering. However is necessary to characterize their mechanical properties especially at the nanoscale. The PeakForce Quantitative NanoMechanics (PF-QNM) is recently developed AFM mode, which allows to probe mechanical properties of the material, such as: reduced Youngs modulus, deformation, adhesion, and dissipation, simultaneously with topographical imaging. In this paper we are presenting results of PF-QNM characterization of two kinds of electrospun fibers: PCL and PCL/HAp. The average calculated from DMT theory Young modulus was 3 ± 1 MPa for PCL mesh and 17 ± 3 MPa for PCL+HAp mesh. 1. Introduction Biodegradable polymeric fibers, produced by electrospinning method, with nano and submicron diameters can be used as scaffolds in tissue engineering. It is possible due to their interfibrous pore size, high surface area to volume ratio, immunogenicity, biodegradability and structural similarity to the extracellular matrix (ECM) 1). In physiological conditions fibers are subjected to stresses and strains from the surrounding biological environment. Such stresses can cause permanent deformation or even failure to scaffold structure. Therefore, there is a growing need to characterize their mechanical properties, especially at the nanoscale. Atomic force microscopy (AFM) has emerged as a powerful tool in the imaging of cells and biomaterials and probing selected mechanical properties under physiological conditions 2,3,4). The PeakForce Quantitative NanoMechanics (PF-QNM) is recently developed AFM mode, which allows to measure mechanical properties of the material, such as: reduced Youngs modulus, deformation, adhesion, and dissipation, simultaneously with topographical imaging 5) .
[Show abstract][Hide abstract] ABSTRACT: The Portevin–Le Chatelier effect manifests itself as an unstable plastic flow which occurs during tensile tests of some dilute materials in a certain range of temperatures and strain rates. This phenomenon is also exceptionally intense in nickel based superalloys used in the aerospace industry. The aim of this research was to investigate the Portevin–Le Chatelier effect in Inconel 718 solution strengthened superalloy. The tested material was subjected to tensile tests carried out within the temperature range 250–600 °C with three different strain rates 2×10−3 s−1, 10−2 s−1 and 5×10−2 s−1. The strain curves were analyzed in terms of intensity and statistical behavior. A quantitative method to describe the phenomenon was used in the study. The results indicate the presence of clear trends with temperature and strain rates. Additional optical observations were carried out to assess the changes of the microstructure.
Materials Science and Engineering A 12/2014; 619:158–164. DOI:10.1016/j.msea.2014.09.075 · 2.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study concerns imaging of the structure of materials using AFM tapping (TM) and phase imaging (PI) mode, using probes modified with Focused Ion Beam (FIB). Three kinds of modifications were applied–thinning of the cantilever, sharpening of the tip and combination of these two modifications. Probes shaped in that way were used for AFM investigations with Bruker AFM Nanoscope 8. As a testing material, titanium roughness standard supplied by Bruker was used. The results show that performed modifications influence the oscillation of the probes. In particular thinning of the cantilever enables one to acquire higher self-resonant frequencies, which can be advantageous for improving the quality of imaging in PI mode. It was found that sharpening the tip improves imaging resolution in tapping mode, which is consistent with existing knowledge, but lowered the quality of high frequency topography images. In this paper the Finite Element Method (FEM) was used to explain the results obtained experimentally.
[Show abstract][Hide abstract] ABSTRACT: The melting of aluminum thin film was studied by a molecular dynamics (MD) simulation technique. The effect of the grain size and type of confinement was investigated for aluminum film with a constant thickness of 4 nm. The results show that coherent intercrystalline interface suppress the transition of solid aluminum into liquid, while free-surface gives melting point depression. The mechanism of melting of polycrystalline aluminum thin film was investigated. It was found that melting starts at grain boundaries and propagates to grain interiors. The melting point was calculated from the Lindemann index criterion, taking into account only atoms near to grain boundaries. This made it possible to extend melting point calculations to bigger grains, which require a long time (in the MD scale) to be fully molten. The results show that 4 nm thick film of aluminum melts at a temperature lower than the melting point of bulk aluminum (933 K) only when the grain size is reduced to 6 nm. (C) 2014 AIP Publishing LLC.
[Show abstract][Hide abstract] ABSTRACT: The aim of this research was to investigate the influence of a geometrical notch on the Portevin–Le Chatelier effect. This phenomenon manifests itself as the serrated stress–strain curve obtained for certain materials when they undergo plastic deformation during tensile tests. The Portevin–Le Chatelier effect can often be observed in aluminum alloys especially those with an addition of magnesium. The material examined in the present experiments was a model Al–3Mg alloy. Samples were prepared with notches cut in the specimens with different depths and shapes. The results clearly indicate that the notch has a significant influence on the Portevin–Le Chatelier effect. With increasing notch depth, the stress amplitude of the serrations increases together with their frequency and the course of the serrations changes.
[Show abstract][Hide abstract] ABSTRACT: Fabrication of scaffolds for tissue engineering (TE) applications becomes a very important research topic in present days. The aim of the study was to create and evaluate a hybrid polymeric 3D scaffold consisted of nano and microfibers, which could be used for bone tissue engineering. Hybrid structures were fabricated using rapid prototyping (RP) and electrospinning (ES) methods. Electrospun nanofibrous mats were incorporated between the microfibrous layers produced by RP technology. The nanofibers were made of poly(L-lactid) and polycaprolactone was used to fabricate microfibers. The micro- and nanostructures of the hybrid scaffolds were examined using scanning electron microscopy (SEM). X-ray microtomographical (μCT) analysis and the mechanical testing of the porous hybrid structures were performed using SkyScan 1172 machine, equipped with a material testing stage. The scanning electron microscopy and micro-tomography
analyses showed that obtained scaffolds are hybrid nanofibers/microfibers structures with high porosity and interconnected pores ranging from 10 to 500um. Although, connection between microfibrous layers and electrospun mats remained consistent under compression tests, addition of the nanofibrous mats affected the mechanical properties of the scaffold, particularly its elastic modulus. The results of the biocompatibility tests did not show any cytotoxic effects and no fibroblast after contact with the scaffold showed any damage of the cell body, the cells had proper morphologies and showed good proliferation. Summarizing, using RP technology and electrospinning method it is possible to fabricate biocompatible scaffolds with controllable geometrical parameters and good mechanical properties.
[Show abstract][Hide abstract] ABSTRACT: The aim of this work is the development of universal discrete models of composite materials, which can take into account their complex structure, and allow one to solve heat transfer problem as well as calculate effective thermal conductivity coefficient of those materials.
[Show abstract][Hide abstract] ABSTRACT: Precipitation strengthening of an ultrafine-grained Al-Mg-Si alloy has been studied using samples obtained by hydrostatic extrusion. It has been demonstrated that the microstructure after hydrostatic extrusion consists of two types of grains: (1) nano-sized free of dislocations and surrounded with high angle grain boundaries and (2) micron-sized with dislocation substructure. After ageing at 160 °C, small needle-like precipitates appear in grain interiors of both nano- and micron-sized grains, bringing about a significant strength improvement. However, the precipitates are smaller than those in their coarse grained counterparts. As a consequence, they constitute weaker barriers for dislocations and induce a lower strengthening effect. In addition, one may observe intensive precipitation at nano-grains boundaries, which further reduces the strengthening effect. It was also shown that peak ageing and overageing take place for much shorter time than in the case of coarse grained samples and are caused by the grain growth rather than a change in the precipitation state.
Materials Science and Engineering A 07/2014; 609:80–87. DOI:10.1016/j.msea.2014.04.092 · 2.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper the effect of production method and heat treatment of magnesium alloys to their corrosion resistance is presented. The study was carried out on the AZ91D alloy obtained by permanent mould casting, pressure die casting and pressure die casting with the subsequent heat treatment. Studies of the microstructure were carried out using a light microscope. The corrosion resistance was examined using the Potentiodynamic Test. The surface observations after corrosion tests were carried out using the Scanning Electron Microscope (SEM). It was stated that the best corrosion resistance is typical for AZ91D alloy produced by pressure die casting method with heat treatment. The alloy after permanent mould casting has got the worst corrosive properties.
[Show abstract][Hide abstract] ABSTRACT: The precipitation phenomena in 7475 nanograined aluminium alloy was analysed by means of microhardness measurements, small angle X-ray scattering and electron microscopy. The nanograined samples were obtained by the hydrostatic extrusion of solution annealed and water quenched samples. It has been established that low temperature ageing causes precipitation processes to occur. However, the precipitation phenomena in nanograined materials proceed differently to those in micrograined materials. Moreover, the particle strengthening is limited by enhanced grain boundary precipitation which does not contribute to an increase in strength (when dislocation slip is the dominant deformation mechanism) and by the smaller size of precipitates.