Ruolo dell’instabilità microstrutturale nel creep: casi di studio

Il creep dei materiali metallici viene molto spesso studiato analizzando in maniera accurata i risultati delle prove meccaniche di laboratorio, senza però tener conto che in maniera molto qualitativa di quanto avviene durante l’esposizione ad alta temperatura. I risultati delle prove meccaniche, per esempio sotto forma di tempo a rottura in funzione della tensione applicata, sono spesso descritti attraverso relazioni del tutto empiriche, tipicamente equazioni polinomiali, che per la loro stessa natura presentano molti problemi in fase di estrapolazione. Un approccio più sistematico consiglierebbe invece un’approfondita analisi delle correlazioni fra evoluzione microstrutturale e risposta meccanica, con lo scopo di arrivare ad equazioni costitutive basate sulla fisica dei fenomeni. Nonostante tale approccio possa apparire complesso e dispendioso, e per tale motivo esso venga quasi costantemente accantonato, si può dimostrare come una sua applicazione garantirebbe sostanziali vantaggi rispetto ai modelli convenzionali più o meno empirici. A questo scopo il presente lavoro presenta due casi di studio apparentemente molto diversi, cioè il creep delle leghe di magnesio e quello degli acciai 9Cr. Si dimostrerà come, pur tenendo conto delle ovvie differenze, la stessa metodologia possa essere utilizzata ottenendo una qualità della descrizione dei risultati sperimentali che, persino per alcuni utilizzatori, potrebbe risultare addirittura sorprendente

Deformazione plastica a caldo e mappe di processo

L’introduzione di materiali ad elevate prestazioni, sia ferrosi che non ferrosi, pone spesso problemirilevanti quando si tratti di identificare le condizioni ottimali di lavorazione a caldo. Prove su impiantosono ovviamente molto complesse e costose, e vengono sempre più spesso sostituite da prove simulativein laboratorio. I dati ottenuti da queste prove (usualmente di compressione o torsione) vengono utilizzatiper ricavare le equazioni costitutive, che a loro volta permettono di stimare i carichi di lavorazione o dieffettuare simulazioni FEM. A fianco di questa che è la procedura classica per determinare la rispostaalla lavorazione a caldo di un materiale, recentemente è stato messo a punto un metodo complementareche si basa sull’ottenimento delle mappe di efficienza e di stabilità. Su tali mappe vengono riportatii valori di un indice di efficienza che rappresenta l’abilità del materiale nell’assorbire energia,convertendola in fenomeni microstrutturali. A queste mappe ne vengono sovrapposte altre in cui,attraverso opportuni parametri di stabilità, si possono identificare quelle zone in cui l’assorbimentodi energia avviene per fenomeni “positivi” (che innalzano quindi la lavorabilità,come la ricristallizzazione), da quelle in cui l’energia viene assorbita per il danneggiamento.In questo modo risulta possibile, almeno in teoria, identificare la finestra di lavorabilità ottimale.Questo lavoro ha lo scopo di illustrare sinteticamente questa procedura,descrivendone sia i vantaggi che i potenziali svantaggi

Draft Seagrass Bibliography Data Base

The following report describes in detail the elements of a literature search performed for the Southwest Florida Water Management District (SWFWMD), Department of Surface Water Improvement and Management (SWIM). The work was performed in accordance with the agreement between the District and Mote Marine Laboratory (MML), and comprises Task 3.0 (Literature Search) of the project entitled, Tampa Bay Water Quality Assessment- Determination of Environmental Requirements of Selected Populations

Oxidation behaviour of nanostructured Ti-B-N based coatings

Nanostructured Ti-B-N based coatings are widely employed in many applications for the excellent propertiessuch as high hardness, low friction, good resistance to wear and corrosion. In addition, if some alloyingelements such Al and Si are incorporated an oxidation improved resistance is achieved. In the present work,Ti-B-N, Ti-Si-B-N and Ti-Al-Si-B-N coatings deposited by Ion Implantation Assisted Magnetron Sputtering(IIAMS) have been investigated. All the coatings were deposited on single-crystal Si-[100]. To evaluate theoxidation behavior, the coatings were annealed in air at 700 and 900°C for 4 hours. Ti-B-N coatings oxidizescompletely after annealing at 700°C for 4 hours, while a layered structure with well-definite interface isproduced on Ti-Si-B-N and Ti-Al-Si-B-N based coatings. Upon annealing at 900°C, the Si-doped film showedsignificantly better oxidation resistance compared to that of Ti-B-N and Ti-Al-Si-B-N coatings

Investigation of the temperature-relatedwear performance of hard nanostructured coatings deposited on a s600 high speed steel

Thin hard coatings are widely known as key elements in many industrial fields, from equipment for metal machining to dental implants and orthopedic prosthesis. When it comes to machining and cutting tools, thin hard coatings are crucial for decreasing the coefficient of friction (COF) and for protecting tools against oxidation. The aim of this work was to evaluate the tribological performance of two commercially available thin hard coatings deposited by physical vapor deposition (PVD) on a high speed tool steel (S600) under extreme working conditions. For this purpose, pin-on-disc wear tests were carried out either at room temperature (293 K) or at high temperature (873 K) against alumina (Al2O3) balls. Two thin hard nitrogen-rich coatings were considered: a multilayer AlTiCrN and a superlattice (nanolayered) CrN/NbN. The surface and microstructure characterization were performed by optical profilometry, field-emission gun scanning electron microscopy (FEGSEM), and energy dispersive spectroscopy (EDS).Funding: This research was made possible by an NPRP award NPRP 5-423-2-167 from the Qatar National Research Fund (a member of The Qatar Foundation)

Plant biodiversity of mountain grasslands as influenced by dairy farm management in the Eastern Alps

It has been widely demonstrated that farm management affects the plant species composition of grassland. The present study aimed to investigate the effect of farm management on plant species richness and composition in forty-nine small-scale farms breeding dairy cattle, located in the Eastern Italian Alps at two levels of precision: plot and farm levels. Data on housing system, quality scheme, farm productivity, income from milk yield and livestock density were collected through interviews with farmers. In each farm, botanical surveys were carried out in different plots representing the botanical composition of the farmland vegetation. Elevation, slope, type of use, number of hay cuts and type of fertilisation were also recorded. The botanical surveys of the plots on each farm were analysed to describe plant composition at the plot level, then merged to describe plant composition at the farm level. At both levels, grassland botanical composition was found to be affected by farm management. At the plot level, meadows cut 2 and 3 times per year did not exhibit any differences in plant richness, but they differed in plant species, botanical family and phytosociological class composition, with a general simplification of botanical composition. We found fewer phytosociological classes but not fewer plant species or botanical families in plots fertilised with slurry than in plots fertilised with manure or not fertilised, and a change in the botanical composition due to changes in the relative abundance of plant species. At the farm level, we observed a decrease in the number of plant species and phytosociological classes, and changes in plant composition, with increasing milk yield and livestock density. Changes in botanical composition were less evident at the farm level than at the plot level. However, protecting farms and their economic viability is a means of maintaining biodiversity at the plot level