17 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
Origin of improved tunability and loss in N2 annealed barium strontium titanate films
No full textBarium strontium titanate (BSTO) thin films were deposited on Pt(111) by high throughput evaporative physical vapor deposition and then annealed at 650 °C for 30 min under N2 atmosphere. Using advanced transmission electron microscopy, energy-dispersive x-ray spectroscopy and electron energy-loss spectroscopy, we directly show that not only does N substitute for O in the BSTO lattice but that it also compensates for Ti3+ ions, suppressing conductivity, thereby reducing dielectric loss and enhancing dielectric tunability. However, this effect is negated near the film edge where we speculate that exposed Pt acts as a reservoir of adsorbed/absorbed O and alters the local N2 concentration during annealing
Synthesis, mechanical properties and corrosion behavior of powder metallurgy processed Fe/Mg2Si composites for biodegradable implant applications
No full textRecently, Fe and Fe-based alloys have shown their potential as degradable materials for biomedical applications. Nevertheless, the slow corrosion rate limits their performance in certain situations. The shift to iron matrix composites represents a possible approach, not only to improve the mechanical properties, but also to accelerate and tune the corrosion rate in a physiological environment. In this work, Fe-based composites reinforced by Mg2Si particles were proposed. The initial powders were prepared by different combinations of mixing and milling processes, and finally consolidated by hot rolling. The influence of the microstructure on mechanical properties and corrosion behavior of Fe/Mg2Si was investigated. Scanning electron microscopy and X-ray diffraction were used for the assessment of the composite structure. Tensile and hardness tests were performed to characterize the mechanical properties. Potentiodynamic and static corrosion tests were carried out to investigate the corrosion behavior in a pseudo-physiological environment. Samples with smaller Mg2Si particles showed a more homogenous distribution of the reinforcement. Yield and ultimate tensile strength increased when compared to those of pure Fe (from 400 MPa and 416 MPa to 523 MPa and 630 MPa, respectively). Electrochemical measurements and immersion tests indicated that the addition of Mg2Si could increase the corrosion rate of Fe even twice (from 0.14 to 0.28 mm·yearâ 1). It was found that the preparation method of the initial composite powders played a major role in the corrosion process as well as in the corrosion mechanism of the final composite
Effects of superplastic forming on modification of surface properties of Ti alloys for biomedical applications
No full textIn the present work, both the surface chemical contamination and the mechanical alteration of Titanium Grade 5 and Grade 23 plates subjected to superplastic forming for the manufacturing of highly customized biomedical prostheses have been investigated. As case study, a cranial implant was considered. Free Inflation tests were conducted in order to determine the material behavior to be implemented in the numerical model used for simulating the implant manufacturing by superplastic forming. Glow Discharge Optical Emission Spectrometry analyses, nano-indentation tests and metallographic analyses allowed to relate the mechanical alteration to the Oxygen enrichment due to the environmental exposition during processing. Finally, real implants were produced and Cytotoxicity tests were conducted on the most oxidized part in order to determine the effect of the oxygen enrichment on the cells viability
Oxidation behaviour of titanium alloys for superplastic forming applications
No full textLAUREA MAGISTRALELe leghe di Titanio (Ti), offrono una combinazione unica di proprietà fisico-meccaniche oltre ad un’eccellente resistenza alla corrosione. Tali caratteristiche le rendono appetibili per una gran quantità di applicazioni critiche o gravose come quella aerospaziale. L’estensione dell’utilizzo delle leghe di Titanio ad altri settori (auto motive, industria chimica, settore energetico, marina, settore biomedicale, sport e architettura) comporta un continuo incentivo a perfezionare lo studio della metallurgia del Titanio, a potenziare i processi produttivi, a migliorare la qualità dei componenti; tutti i sopracitati obiettivi hanno come minimo comun denominatore il contenimento dei costi. La realizzazione di componenti ingegneristici (anche di forma complessa) richiede, per queste leghe, vaste lavorazioni meccaniche, implicando anche articolate sequenze di lavorazione per l’ottenimento della microstruttura desiderata. Ogni progresso o innovazione nella progettazione e nell’ottimizzazione dei processi deformativi del Titanio implicherà un notevole risparmio economico e andrà ad aumentare la vita utile dei componenti. La chiave di questo processo di innovazione è nel comprendere la risposta intrinseca del materiale alle condizioni di esercizio imposte, oltre a sviluppare un dettagliato processo di analisi. Tali scopi sono inoltre perseguibili integrando le informazioni sulla microstruttura del materiale con la simulazione dei processi produttivi. Come comunemente noto, l’evoluzione della microstruttura del Titanio durante le lavorazioni a caldo è molto sensibile ai parametri di processo come temperatura, velocità di deformazione, deformazione e “storia” del materiale (a partire dalle iniziali condizioni microstrutturali). Le leghe di Titanio, a causa della complessità micro strutturale, sono in generale molto difficili da lavorare rispetto agli acciai; di conseguenza è necessario un controllo costante dei parametri di processo per evitare il possibile insorgere di difetti di produzione.
Il Titanio e le sue leghe, sono comunemente utilizzati in svariate applicazioni biomedicali. Si tratta infatti di un elemento biocompatibile, con ottime proprietà di “osseo-integrazione” e di durata all’interno dell’organismo. Inoltre, non essendo il Titanio ferromagnetico, rende possibile lo svolgimento di analisi mediante risonanze magnetiche. La lega Ti-6Al-4V è da tempo una delle più utilizzate in campo biomedicale.
Il presente lavoro di tesi, contribuisce collateralmente ad un progetto nazionale che coinvolge diverse università (Politecnico di Milano, Politecnico di Bari, Università di Cosenza). Il progetto è finalizzato alla progettazione e alla produzione di protesi craniche in Ti-6Al-4V ELI (Grado 23), una variante del Ti-6Al-4V con una minor percentuale di ossigeno iniziale. Le protesi, la cui forma è stata riprodotta dalla scansione di un modello di cranio presente nei laboratori di ingegneria biomedica del Politecnico di Milano saranno realizzate mediante formatura superplastica. Tale processo consiste nel deformare una lamina di materiale portata a temperatura superplastica (nel nostro caso 850°C) mediante la pressione di un gas; aderendo ad uno stampo precedentemente progettato, la lamina assume la forma desiderata per il componente. Tale processo consente dunque di ottenere in modo agevole componenti di forma complessa. Prima di procedere alla realizzazione delle protesi è stato però necessario svolgere un’analisi preliminare sulle leghe a disposizione (Ti-6Al-4V e Ti-6Al-4V ELI). Con la supervisione del Professor Vedani (relatore del presente lavoro), sono state svolte diverse analisi sui materiali. La prima analisi condotta è stata la GDOES (“Glow discharge emission spectroscopy”). Tale tecnologia consente di osservare la composizione chimica degli elementi costituenti la lega, in termini di percentuale in peso e atomica lungo lo spessore dei campioni. In particolare, sono state analizzate le curve dell’Ossigeno presente nel materiale. Dal momento che l’analisi GDOES impone l’utilizzo di campioni piatti, non è stato possibile utilizzare campioni deformati superplasticamente (a causa della curvatura delle superfici). Tuttavia sono state parzialmente riprodotte le condizioni di formatura superplastica sui campioni, ponendo gli stessi in un forno ed esponendoli a temperatura superplastica per diversi intervalli temporali. In tal modo, non è stato considerato nell’analisi il contributo de formativo del gas. L’obiettivo di questa prima indagine è stato quello di ricercare una correlazione tra il tempo di permanenza in forno a temperatura superplastica e lo spessore dello strato ossidato dei campioni. E’ stata eseguita un’analisi di regressione per trovare il modello che meglio riproducesse il fenomeno ossidativo. Il miglior modello trovato è un modello del secondo ordine senza termine costante. Dai dati ricavati dall’analisi GDOES, è emerso un comportamento analogo delle due leghe. In particolare, le curve dell’ossigeno in funzione dello spessore dei campioni hanno evidenziato un trend analogo: i campioni esposti a temperatura superplastica per intervalli di tempo minore (18-36 minuti), mostrano un picco iniziale del quantitativo di Ossigeno seguito da una rapida decrescita del quantitativo del medesimo; per tali campioni è stato dunque rinvenuto uno strato ossidato molto piccolo. I campioni esposti a temperatura superplastica per intervalli di tempo maggiori (fino a 180 minuti), hanno evidenziato invece un andamento asintotico tendente valori molto piccoli di ossigeno ma rinvenuti a profondità molto maggiori. I dati rinvenuti dall’analisi GDOES sono stati utilizzati per generare altri grafici attraverso i quali è stato possibile studiare la variazione della percentuale in peso e atomica di ossigeno al variare dello spessore dei provini. L’analisi successiva è stata finalizzata a comprendere e giustificare i dati ricavati dall’analisi iniziale. Nello specifico, è stata svolta un’analisi micro strutturale al SEM, sui provini soggetti alle condizioni “limite” (intervalli di tempo minimo e massimo di permanenza in forno). In tal modo è stato possibile osservare come il tempo di esposizione a temperatura superplastica potesse influenzare la microstruttura dei provini. E’ stata dunque osservata l’evoluzione micro strutturale dei provini a partire dal metallo base. L’analisi al microscopio elettronico ha evidenziato una microstruttura a grana allungata e di dimensioni maggiori per i provini rimasti in forno per il tempo minore (18 minuti) ed una microstruttura globulare per provini rimasti in forno per il tempo maggiore (180 minuti). Per completare l’analisi, i provini (piegati a 90 ° prima di essere inglobati e lucidati per lo studio al SEM), sono stati sottoposti ad un’indagine visiva delle cricche superficiali. E’ emerso che i provini di entrambe le leghe esposti a temperatura superplastica per il tempo minimo hanno evidenziato la formazione di cricche molto piccole e poco profonde. I provini sottoposti a temperatura superplastica per il tempo massimo hanno invece evidenziato la formazione di cricche più ampie e profonde. Lo strato ossidato è risultato dunque essere più fragile per tale condizione. Tali analisi, saranno di supporto per ottimizzare il processo ed ottenere migliori risultati in termini di qualità del prodotto finale, nel rispetto dei vincoli e delle normative in termini di citotossicità.Titanium alloys offer a unique combination of mechanical and physical properties and excellent corrosion resistance, which make them desirable for a variety of critical applications (e.g, automotive, chemical, energy, marine, biomedical, sports, and architecture). The current work, contributes collaterally to a national project that involves different universities, finalized to the design and production of cranial prostheses in Ti-6Al-4V ELI to be realized trough Superplastic Forming Process (SPF). Before proceeding with prostheses realization, a preliminary study on the material (Ti-6Al-4V and Ti-6Al-4V ELI), needed to be performed. Different analyses on samples exposed to superplastic temperature (850 °C), for different time intervals, have been executed to this purpose. The first analysis performed on titanium samples has been the Glow discharge optical emission spectrometry (GDOES) analysis. This technology has allowed to us to observe the chemical composition of the alloys elements, both in terms of atomic and weight percentage on surface and along the depth of the samples. The goal of the research was to found a correlation between the thickness of the oxide layer and the exposure time at superplastic temperature in the furnace. A regression analysis has been performed, in order to fit the best statistical model that could reproduce the oxidation behaviour of the alloys. Results have shown that a quadratic regression model (without constant term in the model) fits well the oxidation phenomenon. After this step, a microstructural analysis of the alloys has been executed, leading to observe how exposure time to SPT in the furnace, could affect the microstructure of the samples. This analysis could be useful to explain the oxidation behaviour of the samples too, giving some more information about the oxide layer formation and distribution along the thickness. Both alloys have shown the same behaviour, resulting in larger and oriented grains for shortest exposure times in the furnace and coarser grains for longest exposure times. The term of comparison has been the base material, whose microstructure has been formerly analyzed with SEM. Proceeding on this way, it has made it possible to get a complete overview on the phenomena occurring on the Titanium alloys. Moreover, to evaluate possible effects of the oxide layer on alloy ductility, a visual analysis of surface cracks has been performed on mechanically bent samples, with the purpose of finding a correlation between cracks dimensions and exposure time in the furnace. Moving from the shortest exposure time condition (18 minutes) towards the longest one (180 minutes),Ti-6Al-4V has shown an increase of cracks length (average value) of about one order of magnitude, while Ti-6Al-4V ELI has shown an increase of about 450 % of cracks length
Microstructure, texture evolution, mechanical properties and corrosion behavior of ECAP processed ZK60 magnesium alloy for biodegradable applications
No full textUltra-fine grained ZK60 Mg alloy was obtained by multi-pass equal-channel angular pressing at different temperatures of 250. °C, 200. °C and 150. °C. Microstructural observations showed a significant grain refinement after ECAP, leading to an equiaxed and ultrafine grain (UFG) structure with average size of 600. nm. The original extrusion fiber texture with planes oriented parallel to extrusion direction was gradually undermined during ECAP process and eventually it was substituted by a newly stronger texture component with considerably higher intensity, coinciding with ECAP shear plane. A combination of texture modification and grain refinement in UFG samples led to a marked reduction in mechanical asymmetric behavior compared to the as-received alloy, as well as adequate mechanical properties with about 100% improvement in elongation to failure while keeping relatively high tensile strength. Open circuit potential, potentiodynamic and weight loss measurements in a phosphate buffer solution electrolyte revealed an improved corrosion resistance of UFG alloy compared to the extruded one, stemming from a shift of corrosion regime from localized pitting in the as-received sample to a more uniform corrosion mode with reduced localized attack in ECAP processed alloy. Compression tests on immersed samples showed that the rate of loss of mechanical integrity in the UFG sample was lower than that in the as-received sample
Structural and mechanical characterization of Cr-Co alloy after oxygen plasma immersion ion implantion
No full textMicrostructural changes of ECAP-processed magnesium alloy AZ91 during cyclic loading at different stress-amplitude levels
No full textMicrostructural changes of magnesium alloy AZ91 after fatigue loading in the EX-ECAP state were evaluated using EBSD. It was found that both the number fraction of low-angle boundaries and parameter KAM decreased after the testing at a stress amplitude of 160 MPa but started to increase with the increasing stress amplitude. This behaviour can be explained with a mutual influence of dislocation accumulation (which is stronger with a higher stress amplitude) and dynamic softening (which is weaker with a decreasing number of cycles/cycles to failure). The average grain size remained almost unchanged except at a stress amplitude of 180 MPa, which could have been caused by certain conditions allowing an ideal development of both mentioned phenomena
