1,354,103 research outputs found
Fabrication, mechanical properties and in vitro degradation behavior of newly developed ZnAg alloys for degradable implant applications
No full textZn and Zn-based alloys have been recognized as highly promising biodegradable materials for orthopedic implants and cardiovascular stents, due to their proved biocompatibility and, more importantly, lower corrosion rates compared to Mg alloys. However, pure Zn has poor mechanical properties. In this study, Ag is used as a promising alloying element to improve the mechanical properties of the Zn matrix as well as its biocompatibility and antibacterial properties. Accordingly, we design three ZnAg alloys with Ag content ranging from 2.5 to 7.0wt% and investigate the influence of the Ag content on mechanical and corrosion behavior of the alloys. The alloys are developed by casting process and homogenized at 410°C for 6h and 12h, followed by hot extrusion at 250°C with extrusion ratio of 14:1. Degradation behavior is assessed by electrochemical and static immersion tests in Hank's modified solution. Microstructural analysis reveals that hot extrusion significantly reduces the grain size of the alloys. Zn-7.0%Ag alloy shows a reasonably equiaxed and considerably refined microstructure with mean grain size of 1.5μm. Tensile tests at room temperature suggest that increasing the Ag content steadily enhances the tensile strength, while it does not affect the tensile ductility significantly. Zn-7.0%Ag shows high yield strength and ultimate tensile strength of 236MPa and 287MPa, respectively, which is due to the grain refinement and high volume fraction of fine AgZn3 particles precipitating along the grain boundaries during the extrusion process. Among all these alloys, Zn-7.0%Ag displayed superplasticity over a wide range of strain rates (from 5×10(-4)s(-1) to 1.0×10(-2)s(-1)) providing the possibility of exploiting forming processes at rapid rates and/or even at lower temperatures. In addition, extruded alloys exhibit slightly faster degradation rate than pure Zn. X-ray diffraction results show the presence of ZnO and Zn(OH)2 on the degraded surfaces. Moreover, scanning electron microscopy imaging reveals that micro-galvanic corrosion is more pronounced on the alloys with higher Ag content due to the higher volume fraction of AgZn3 particles. [Abstract copyright: Copyright © 2017 Elsevier B.V. All rights reserved.
Microstructural, texture, plastic anisotropy and superplasticity development of ZK60 alloy during equal channel angular extrusion processing
No full textIn this study, equal channel angular pressing (ECAP) was exploited to refine the grain size of a ZK60 magnesium alloy in multi-processing steps, namely at temperatures of 250 ̊C, 200 ̊C and 150 ̊C, producing an ultrafine-grained (UFG) structure. The microstructural development and texture evolution during ECAP were systemically investigated by electron backscattered diffraction (EBSD) analysis. The microstructure of the ECAP processed alloy was remarkably refined to an average grain size of 600 nm. During ECAP process the original fiber texture of the as-extruded alloy was gradually weakened and eventually replaced by a stronger texture component coinciding with ECAP shear plane. The ECAP processed material showed a proper balance of tensile as well as compression strength and tensile ductility at room temperature. Yield strength of 273 and 253 MPa in tension and compression, respectively, ultimate tensile strength of 298 MPa and fracture elongation of about 30% were obtained in the UFG alloy. A transition from ductile–brittle to ductile fracture consisting of very fine and equiaxed dimples was also found in the ECAP processed material. Compared to the as-received alloy, a combination of grain refinement and texture development in the UFG alloy gave rise to a notable reduction in mechanical asymmetric behavior at room temperature. The superplastic behavior of the as-extruded and ECAP processed alloy was also investigated at 200 ̊C with strain rate of 1.0×10-3 s-1.
The concurrent effect of grain boundary sliding and favorable basal texture in the UFG alloy led to an achievement of elongation value of about 300% while, under similar testing conditions, the elongation of about 140% was obtained in the as-extruded alloy
Development and characterization of a novel high entropy alloy strengthened through concurrent spinodal decomposition and precipitation
No full textStrengthening mechanisms, which are commonly exploited by conventional alloys, can be effectively incorporated in high entropy alloys (HEAs) to improve their mechanical behaviour. In this light, compositional modification of equiatomic HEAs can be pursued in order to obtain specific microstructural features. Herein, a face centred cubic CoCuFeMnNi alloy was modified by the addition of a proper amount of Ti; a dedicated thermal treatment allowed to concurrently activate two different phase formation mechanisms, i.e. precipitation and spinodal decomposition. This resulted in a nanostructured microstructure, characterized by the presence of periodic modulations of Cu content and by nanosized coherent L12 Ni3Ti precipitates. Such microstructure resulted in a more than 100 % increase of yield strength after ageing treatment and allowed to retain a satisfactory ductility. Advanced microstructural characterization, coupled with the application of semi-empirical models allowed to understand the role of each microstructural feature in determining the alloy's mechanical strength
Development and characterization of a novel high entropy alloy strengthened through concurrent spinodal decomposition and precipitation
No full textStrengthening mechanisms, which are commonly exploited by conventional alloys, can be effectively incorporated in high entropy alloys (HEAs) to improve their mechanical behaviour. In this light, compositional modification of equiatomic HEAs can be pursued in order to obtain specific microstructural features. Herein, a face centred cubic CoCuFeMnNi alloy was modified by the addition of a proper amount of Ti; a dedicated thermal treatment allowed to concurrently activate two different phase formation mechanisms, i.e. precipitation and spinodal decomposition. This resulted in a nanostructured microstructure, characterized by the presence of periodic modulations of Cu content and by nanosized coherent L12 Ni3Ti precipitates. Such microstructure resulted in a more than 100 % increase of yield strength after ageing treatment and allowed to retain a satisfactory ductility. Advanced microstructural characterization, coupled with the application of semi-empirical models allowed to understand the role of each microstructural feature in determining the alloy’s mechanical strength
Enhanced cryogenic and ambient temperature mechanical properties of CoCuFeMnNi high entropy alloy through controlled heat treatment
No full textDedicated thermal treatments can improve the mechanical behaviour of high entropy alloys (HEAs) by producing nanostructured microstructures with improved characteristics. Herein, the inherent metastability of an equiatomic CoCuFeMnNi alloy was exploited to induce the formation of secondary phases upon ageing treatment. Advanced characterization techniques, namely high resolution synchrotron X-ray diffraction and aberration corrected scanning transmission electron microscopy, allowed to describe the decomposition of the supersaturated solid solution. Nanometric rounded Cu-rich clusters in the solution treated alloy and coherent, regularly oriented Cu-rich discs in the peak-aged condition were possibly produced by spinodal decomposition. An almost 100% enhancement of mechanical strength was obtained thanks to the modulation of composition. Moreover, mechanical behaviour at cryogenic temperature was improved by ageing, both in terms of strength and ductility. Plastic deformation took place by dislocation slip, regardless of the testing temperature.(c) 2022 Elsevier B.V. All rights reserved
Enhanced cryogenic and ambient temperature mechanical properties of CoCuFeMnNi high entropy alloy through controlled heat treatment
No full textDedicated thermal treatments can improve the mechanical behaviour of high entropy alloys (HEAs) by producing nanostructured microstructures with improved characteristics. Herein, the inherent metastability of an equiatomic CoCuFeMnNi alloy was exploited to induce the formation of secondary phases upon ageing treatment. Advanced characterization techniques, namely high resolution synchrotron X-ray diffraction and aberration corrected scanning transmission electron microscopy, allowed to describe the decomposition of the supersaturated solid solution. Nanometric rounded Cu-rich clusters in the solution treated alloy and coherent, regularly oriented Cu-rich discs in the peak-aged condition were possibly produced by spinodal decomposition. An almost 100% enhancement of mechanical strength was obtained thanks to the modulation of composition. Moreover, mechanical behaviour at cryogenic temperature was improved by ageing, both in terms of strength and ductility. Plastic deformation took place by dislocation slip, regardless of the testing temperature.(c) 2022 Elsevier B.V. All rights reserved
Novel Zn-based alloys for biodegradable stent applications: Design,development and in vitro degradation
No full textThe search for a degradable metal simultaneously showing mechanical properties equal or higher to that of stainless steel and uniform degradation is still an open challenge. Several magnesium-based alloys have been studied, but their degradation rate has proved to be too fast and rarely homogeneous. Fe-based alloys show appropriate mechanical properties but very low degradation rate. In the present work, four novel Zn–Mg and two Zn–Al binary alloys were investigated as potential biodegradable materials for stent applications. The alloys were developed by casting process and homogenized at 350 °C for 48 h followed by hot extrusion at 250 °C. Tube extrusion was performed at 300 °C to produce tubes with outer/inner diameter of 4/1.5 mm as precursors for biodegradable stents. Corrosion tests were performed using Hanks׳ modified solution. Extruded alloys exhibited slightly superior corrosion resistance and slower degradation rate than those of their cast counterparts, but all had corrosion rates roughly half that of a standard purity Mg control. Hot extrusion of Zn–Mg alloys shifted the corrosion regime from localized pitting to more uniform erosion, mainly due to the refinement of second phase particles. Zn–0.5Mg is the most promising material for stent applications with a good combination of strength, ductility, strain hardening exponent and an appropriate rate of loss of mechanical integrity during degradation. An EBSD analysis in the vicinity of the laser cut Zn–0.5Mg tube found no grain coarsening or texture modification confirming that, after laser cutting, the grain size and texture orientation of the final stent remains unchanged. This work shows the potential for Zn alloys to be considered for stent applications
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Microstructure, mechanical behavior and low temperature superplasticity of ECAP processed ZM21 Mg alloy
No full textIn this study, ultra-fine grained ZM21 Mg alloy was obtained through two-stage equal channel angular pressing process (ECAP) at temperatures of 200 and 150 degrees C. For each stage four passes were used. Plastic behavior, mechanical asymmetry and low temperature superplasticity of ultra-fine grained ZM21 alloy were investigated as a function of processing condition with particular attention to microstructural and texture evolution. Microstructural observations showed that after the first stage of ECAP an equiaxed ultra-fine grain (UFG) structure with average size of 700 nm was obtained. Additional stage did not cause any further grain refinement. However, Electron Backscattered Diffraction analysis showed that the original extrusion fiber texture evolved into a new one featuring a favorable alignment of the basal planes along ECAP shear planes. Such a preferential alignment provided a considerably higher Schmid factor value of 0.32, resulting in a remarkable loss in tensile yield stress, from 212 to 110 MPa and an improvement of the tensile fracture elongation, from 24% to 40%. Tensile and compression tests at room temperature revealed that yielding asymmetry could be alleviated by either weakening of basal plane fiber texture or by grain refinement. Tensile tests at 150 degrees C showed that texture supplies a significant contribution to plastic flow and elongation, making dislocation slip the dominant mechanism for deformation, while grain boundary sliding was not actively operated at this temperature. However, at 200 degrees C the effect of texture on fracture elongation of UFG alloys was subtle and the impact of grain size became more important. Hence, UFG samples exhibited maximum elongation values exceeding 370% at a strain rate of 5.0 x 10(-4) s(-1), confirming that the flow stress has notable texture dependence, while superplastic ductility was strongly influenced by grain size, being detectable only in UFG samples
- …
