Microstructural, texture, plastic anisotropy and superplasticity development of ZK60 alloy during equal channel angular extrusion processing

In 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

On the preparation and characterization of thin NiTi shape memory alloy wires for MEMS

Shape memory alloy (SMA) wires are employed as actuators in small devices for consumer electronics, valves and automotive applications. Because of the continued miniaturization of all the industrial products, nowadays the tendency is to produce MEMS (micro electromechanical systems). Among the most promising functional MEMS materials, the thin SMA wires that are offering a rapid actuating response with high power/weigh ratio of the material, are attracting a world wide interest. This paper is aimed at showing the production process and the characterizations of thin NiTi shape memory wires. The activity was focused on drawing procedure and on functional and TEM characterizations of the final products. In particular, it was evaluated the performance of the SMA wires for actuators in terms of functional fatigue and thermo-mechanical properties by means of an experimental apparatus design ad hoc for these specific test

On the preparation and characterization of thin NiTi shape memory alloy wires for MEMS

Shape memory alloy (SMA) wires are employed as actuators in small devices for consumerelectronics, valves and automotive applications. Because of the continued miniaturization of all the industrialproducts, nowadays the tendency is to produce MEMS (micro electromechanical systems). Among the mostpromising functional MEMS materials, the thin SMA wires that are offering a rapid actuating response withhigh power/weigh ratio of the material, are attracting a world wide interest. This paper is aimed at showing theproduction process and the characterizations of thin NiTi shape memory wires. The activity was focused ondrawing procedure and on functional and TEM characterizations of the final products. In particular, it wasevaluated the performance of the SMA wires for actuators in terms of functional fatigue and thermo-mechanicalproperties by means of an experimental apparatus design ad hoc for these specific test

Design of Wear-Resistant Austenitic Steels for Selective Laser Melting

Type 316L stainless steel feedstock powder was modified by alloying with powders containing carbide/boride-forming elements to create improved wear-resistant austenitic alloys that can be readily processed by Selective Laser Melting. Fe-based alloys with high C, B, V, and Nb contents were thus produced, resulting in a microstructure that consisted of austenitic grains and a significant amount of hard carbides and borides. Heat treatments were performed to modify the carbide distribution and morphology. Optimal hard-phase spheroidization was achieved by annealing the proposed alloys at 1150 °C for 1 hour followed by water quenching. The total increase in hardness of samples containing 20 pct of C/B-rich alloy powder was of 82.7 pct while the wear resistance could be increased by a factor of 6

Ultrafine grained alloys produced by severe plastic deformation: issues onmicrostructural control and mechanical behaviour

A review is presented about recent results obtained on properties of ultrafine grained structural alloys and pure metals processed by severe plastic deformation. Carbon steels, aluminium alloys and pure nickel were investigated after processing by Equal Channel Angular Pressing. Data on microstructural evolution, aging kinetics, mechanical properties and recrystallization texture are discussed to draw a picture of advantages and drawbacks of such ultrafine materials.(*) This paper was presented at the "nd International Conference & Exhibition in Metallurgical Process Technology", Riva del Garda 19-21 Sett. 200

Metalli a struttura ultrafine ottenuti mediante deformazione plastica severa

Nella memoria viene presentata una breve rassegna sull’evoluzione microstrutturale, le proprietà e letecniche di produzione di metalli massivi a struttura ultrafine o nanostrutturati. Alcune caratteristichevengono esposte con il supporto di dati sperimentali ottenuti su leghe di alluminio e su acciai deformatimediante tecnica “Equal Channel Angular Pressing” o mediante laminazione a temperature pocosuperiori a quella ambiente. Si mette in evidenza come l’evoluzione della microstruttura con ladeformazione plastica severa a basse temperature segua dei principi comuni per i diversi metalli,schematizzabili secondo la teoria della ricristallizzazione continua. Esistono alcuni processi adottabili inlaboratorio ormai maturi per la sintesi di metalli ultrafini o nanostrutturati in forma massiva mentre sonoancora in fase di valutazione e sviluppo analoghe tecnologie che si possano inserire nelle linee diproduzione industriale dei metalli. Infine, i metalli ultrafini si caratterizzano per una struttura a granisubmicroscopici in uno stato molto lontano dall’equilibrio, specialmente per quanto concerne lacondizione dei bordi grano. La conseguente elevata reattività modifica profondamente le cinetiche didiffusione, di ingrossamento del grano e trasformazione di fase

HOT DUCTILITY OF MICROALLOYED STEELS

The loss in ductility experienced by microalloyed steels at temperatures generally ranging from 700 to 1100°C is a widely studied subject in steel research. The hot ductility behaviour of steels is usually measured through the reduction of area of the samples after hot tensile tests performed up to fracture. The so-called standard hot ductility curves are then obtained by testing the steel at different temperatures. With the aim of achieving more precise information about the hot deformability behaviour of microalloyed steels, interrupted hot-tensile tests followed by rapid quenching (i.e. to “freeze” the microstructure) were carried out. Several steels were characterized by this method, followed by metallurgical investigations of the strained samples in order to identify the damage mechanisms and the precipitates affecting hot ductility. The paper presents a summary of the results achieved about the effects of chemical composition on hot cracking, as affected by precipitation of secondary phases at austenite grain boundarie

Biodegradable magnesium coronary stents: Material, design and fabrication

Biodegradable cardiovascular stents in magnesium (Mg) alloys constitute a promising option for a less intrusive treatment, due to their high compatibility with the body tissue and intrinsic dissolution in body fluids. The design and fabrication aspects of this medical device require an integrated approach considering different aspects such as mechanical properties, corrosion behaviour and biocompatibility. This work gathers and summarises a multidisciplinary work carried out by three different research teams for the design and fabrication of Mg stents. In particular, the paper discusses the design of the novel stent mesh, the deformability study of the Mg alloys for tubular raw material and laser microcutting for the realisation of the stent mesh. Although, the results are not fully validated as the device has not been fully tested, they show the feasibility of the used approaches, as the first prototype stents in Mg alloy were produced successfully. © 2013 Copyright Taylor and Francis Group, LLC

High performance shape memory effect (hp-sme): un innovativo percorso termo-meccanico per lo sviluppo di attuatori sma ad elevate prestazioni

Recentemente è stato proposto un percorso termo-meccanico innovativo per l’utilizzo di leghe a memoria diforma basato sul ciclaggio termico della martensite indotta da sforzo. Questo fenomeno viene chiamato HighPerformance Shape Memory Effect (HP-SME). Questa soluzione consente di utilizzare elementi austenitici conun notevole incremento del carico necessario al funzionamento dell’attuatore a memoria di forma e di mantenereuna elevata capacità del materiale di recupero della deformazione. Attuatori basati su questo principio mostranoproprietà funzionali migliorate rispetto agli attuatori convenzionali basati sull’effetto a memoria di forma (Shapememory effect, SME). In questo lavoro vengono riportati i risultati dei test di recupero della deformazione a caricocostante di fili sottili austenitici e confrontati con quelli di fili martensitici dello stesso diametro

Compositi a matrice di alluminio rinforzati con nanoparticelle di allumina prodotti mediante un processo di metallurgia delle polveri

Sono stati prodotti fili di Al rinforzati con particelle nanometriche di Al2O3 mediante un processo di metallurgiadelle polveri basato sulla macinazione severa e sulla consolidazione di polveri tramite estrusione a caldo.Le billette nanocomposite sono state infine laminate a freddo. Questa procedura ha consentito di prepararenanocompositi caratterizzati da un rinforzo nanometrico omogeneamente disperso in una matrice metallica agrani ultrafini. La macinazione ha provocato la frammentazione dell’ossido superficiale che copriva le particelledi Al e la rottura dei cluster di nano-allumina aggiunti ex situ. I nanocompositi mostrano proprietà meccanichemigliorate in termini di durezza e resistenza a trazione. Essi esibiscono inoltre eccellenti capacità di damping adalta temperatura