ANALISI DEL COMPORTAMENTO PLASTICO DI UN ACCIAIO AUSTENITICO DEFORMATO IN TORSIONE AD ALTA TEMPERATURA

Il comportamento meccanico dell’acciaio austenitico AISI 316L deformato in torsione nell’intervallo di temperature 850-1100°C con velocità di deformazione equivalente 0,0001-0,006 s-1 è stato analizzato ed un modello che lega il comportamento meccanico con l’evoluzione microstrutturale è proposto. Il materiale a temperature superiori a 900°C ha presentato un comportamento meccanico anomalo, mostrando una significativa riduzione dello sforzo di flusso plastico fino a circa 40 % in seguito alla riduzione della dimensione media del grano da 100 a 30 ?m. Congiuntamente all’anomalo comportamento meccanico, il materiale ha esibito un’evoluzione della struttura del grano inusuale durante la deformazione: la forma dei grani è evoluta da quella equiassica precedente alla deformazione a quella romboidale (con bordi di grano allineati con le direzioni degli sforzi principali di taglio in torsione), formando bordi di grano con punti tripli aventi angoli di 90° e 180°, e bordi di grano con gomiti di 90°, configurazioni che sono in disaccordo con la minimizzazione dell’energia superficiale dei bordi di grano. Le evidenze sperimentali dell’evoluzione microstrutturale, razionalizzata in [1], hanno portato a concludere che la formazione della struttura romboidale è avvenuta in seguito alla migrazione dei bordi di grano indotta dalla deformazione in accordo con la configurazione degli sforzi principali per la torsione. Secondo il modello qualitativo proposto nella presente memoria, il flusso plastico è stato affetto dalla migrazione dei bordi di grano, la quale, avvenendo, ha causato la riduzione della densità dislocativa. Tale riduzione ha determinato per la lega con dimensione media del grano di 30 ?m una diminuzione significativa dello sforzo di flusso plastico rispetto alla lega con grano dalla dimensione media di 100 ?m, dove la diminuzione non si è manifestata a causa dell’inferiore valore del rapporto fra la superficie dei bordi di grano ed il volume del materiale

Microstructure modelling of hot deformation of Al–1%Mg alloy

This study presents the application of the finite elementmethod and intelligent systems techniques to the prediction of microstructural mapping for aluminium alloys. Here, the material within each finite element is defined using a hybrid model. The hybrid model is based on neuro-fuzzy and physically based components and it has been combined with the finite element technique. The model simulates the evolution of the internal state variables (i.e. dislocation density, subgrain size and subgrain boundary misorientation) and their effect on the recrystallisation behaviour of the stock. This paper presents the theory behind the model development, the integration between the numerical techniques, and the application of the technique to a hot rolling operation using aluminium, 1 wt% magnesium alloy. Furthermore, experimental data from plane strain compression (PSC) tests and rolling are used to validate the modelling outcome. The results show that the recrystallisation kinetics agree well with the experimental results for different annealing times. This hybrid approach has proved to be more accurate than conventional methods using empirical equations

Strength enhancement of aluminium honeycombs caused by entrapped air underdynamic out-of-plane compression

The out-of-plane crushing behaviour of aluminium hexagonal honeycombs containing different percentages of holes (i.e., the fraction of penetrated cells to the total) was extensively investigated over a wide range of strain rates where each test was conducted at constant compression velocity. Strength enhancement due to the increase of the strain rate and the entrapped air was studied. It is found that the strain hardening of honeycomb structures during the dynamic crush is mostly attributed to the pressure change caused by the entrapped air. The leaking rate, δ̇, was then studied and found to be dependent on the strain and strain rate, and independent of the wall thickness to edge length ratio, t/l. An empirical constitutive relation describing the plastic collapse stress in relation to the t/l ratio, the strain and strain rate is proposed, which agrees well with the experimental results. © 2012 Elsevier Ltd. All rights reserved.S. Xu, J.H. Beynon, D. Ruan, T.X. Y

Non-dissociative single-electron capture by CO2+ from rare gases

The state-selective non-dissociative single-electron capture reactions of 6 keV CO2+ ions from rare gas (He, Ne, Ar, Kr, Xe) targets are investigated by translational energy spectroscopy. Capture from CO2+ (3Π) into doublet states of CO+ dominates. However, there are firm indications for the involvement of electronically excited CO2+ (1.7 eV above CO2+ (3Π) in the projectile beam, and also for the formation of quartet states of CO+. Landau-Zener reaction windows, derived specifically for atomic ion-atom systems, are applied to this diatomic dication-atom series, giving excellent agreement with experiment

Non-dissociative single electron capture by NO2+ from noble gases

The state-selective non-dissociative single-electron capture reactions of 6 keV NO2+ ions from noble gas (He, Ne, Ar, Kr and Xe) targets are investigated by translational energy spectroscopy. Capture occurs predominantly from ground state NO2+ (X 2Σ+). For the NO+ (X 1Σ+) formation a shift of the peak in the translational energy distribution was found (1.3 to 1.7 eV) which suggests higher vibrational excitation of the molecular product than expected from the mutual position of potential energy curves of the NO system. A simple model assuming a dynamical perturbation of the NO+ curve in the presence of He+ is suggested, which accounts for the shift

Translational energy loss of H+ fragments from capture-dissociation of H+2 in collision with rare gas atoms

Translational energy spectra have been obtained for capture-dissociation of 6 keV H+2 in collision with inert gas targets. Energy loss measurements provide further evidence in support of a double-collision mechanism for the production of H+ and H− fragments via an intermediate H∗2 molecule

Characterisation of cluster ions formed from inert gas atoms plus small molecules and radicals

The formation of ions incorporating inert gas atoms has been studied under conditions similar to those used in the classic work of Field and Franklin and their collaborators. Characterisation of these ions by techniques of accurate mass measurement, deuterium substitution, and mass-analysed ion kinetic energy spectroscopy has been achieved. For the first time, ions formed by electron ionization of mixtures of ammonia plus inert gas are reported

Interference peaks in translational energy loss spectra. An example in the spectrum of NO2+·

Translation energy loss (TEL) spectra for NO2+· in collision with helium were obtained using the methods of O'Keefe et al. [2]. The results are in generally satisfactory agreement with the previous work, and are compared with other information on states of NO2+·, particularly data from double-charge-transfer spectroscopy which are in serious disagreement with the present and previous TEL results. In the course of the present work and in other translational energy studies in this laboratory, a problem, inherent in TEL spectroscopy conducted on a double-focusing mass spectrometer, that can lead to spurious peaks, became apparent and is discussed