Influence of an accumulative roll bonding (ARB) process on the properties of AA5083 Al-Mg alloy sheets

In this study, fully annealed AA5083 type alloy sheets with 1 mm in thickness were processed by accumulative roll bonding (ARB) at room temperature, up to 6 ARB cycles. It was found that microstructure was refined and mechanical properties were significantly improved with ARB processing. The maximum achieved values of hardness and tensile strength were two and three times greater than that of the initial material, respectively. This was attributed to the intensive strain hardening and grain size refinement which occurred during ARB deformation. However, the uniform elongation values were decreased by increasing the number of ARB cycles, and early fracture was registered. SEM fractography of fractured surfaces after tensile tests revealed a typical ductile fracture of ARB processed specimens, which was changed with ARB deformation. It was observed that ductile area on the fractured surfaces and the amount of necking, which occured before fracture, were decreased with increasing the number of ARB cycles.  http://dx.doi.org/10.5937/metmateng1404285

Synthesis and characterization of Co2+ doped TiO2 nanoparticles

Scrolled titania nanotubes were synthesized by a hydrothermal method using commercial TiO2 powder in proton-deficient aqueous systems. In presence of CoCl2 extended hydrothermal treatment of TiO2 nanotubes lead to formation Co2+ doped TiO2 nanoparticles. TEM and HRTEM measurement were used for determining the shape, dimension and structure of doped nanoaprticles. XRD confirmed anatase crystalline structure od Co2+ doped TiO2. The presence of Co2+ ions did not induce distortions of the overall anatase crystal structure of TiO2 matrix. The concentration of Co2+ ions was 1.5% compared with the amount of Ti4+ ions.Physical chemistry 2008 : 9th international conference on fundamental and applied aspects of physical chemistry; Belgrade (Serbia); 24-28 September 200

Prepoznavanje faza koje sadrže Fe u mikrostrukturi livene legure AA6026 i njihova evolucija tokom postupka homogenizacije

The Fe-bearing intermetallic phases present in the as-cast AA6026 alloy and their evolution during homogenization treatments at 480-550°C were investigated using optical microscopy, SEM, and TEM techniques in combination with EDS analysis. In addition to the α-Al(Fe,Mn)Si phase with dendritic morphology, two types of plate-like Fe-bearing microconstituents were revealed in the microstructure of the as-cast alloy. The EDS microanalysis and electron diffraction showed that one set of platelets represented thin sections of α-Al(Fe,Mn)Si microconstituent. The other set of plate-like microconstituents was identified as a tetragonal, silicon-rich δ-Al4(Fe,Mn)Si2 phase. The formation of the δ-Al4(Fe,Mn)Si2 phase was attributed to the chemical composition of the alloy. During homogenization, the metastable δ-Al4(Fe,Mn)Si2 transformed into the α-Al(Fe,Mn)Si phase and fragmented. The dendritic α-Al(Fe,Mn)Si microconstituents underwent fragmentation. However, while the α-Al(Fe,Mn)Si microconstituents preserved a b.c.c. crystal lattice throughout the process, the product of the transformation of the δ-Al4(Fe,Mn)Si2 phase exhibited primitive cubic lattice.Ispitivanje prisustva faza koje sadrže Fe u mikrostrukturi livene legure AA6026, kao i njihova evolucija tokom postupaka homogenizacije na temperaturama od 480-550 °C, sprovedena su korišćenjem optičke mikroskopije, skenirajuće elektronske mikroskopije (SEM) i transmisione elektronske mikroskopije (TEM) u kombinaciji sa analizom EDS-a. Osim faze α-Al(Fe,Mn)Si sa dendritskom morfologijom, identifikovane su dve vrste pločastih mikrokonstituenata koji sadrže Fe u mikrostrukturi livene legure. EDS mikroanaliza i elektronska difrakcija pokazali su da jedan skup ploča predstavlja samo tanke sekcije mikrokonstituente α-Al(Fe,Mn)Si. Drugi skup pločastih mikrokonstituenata identifikovan je kao četvrtasta, silicijumom bogata faza δ-Al4 (Fe,Mn)Si2 . Formiranje faze δ-Al4 (Fe,Mn)Si2 povezano je sa hemijskim sastavom legure. Tokom homogenizacije, metastabilna faza δ-Al4 (Fe,Mn)Si2 transformisala se u fazu α-Al(Fe,Mn)Si i fragmentirala. Dendritski mikrokonstituenti α-Al(Fe,Mn)Si takođe su fragmentirali. Međutim, dok su mikrokonstituenti α-Al(Fe,Mn)Si sačuvali kubnu kristalnu rešetku tipa b.c.c. tokom procesa, proizvod transformacije faze δ-Al4 (Fe,Mn)Si2 pokazao je prostu kubnu rešetku

Spherical assemblies of titania nanotubes generated through aerosol processing

We report on the possibility to build hierarchically organized three-dimensional (3D) titania spherical particles having high surface-to-volume-ratio, by aerosol processing of nanotubular building blocks. Morphology and crystal structure of titania based spherical assemblies, obtained in the temperature range from 150 to 650°C, were characterized by means of scanning and transmission electron microscopy, x-ray powder diffraction and Raman spectroscopy. Initial shape of 1D building units, nanotubes, was well preserved in the spherical assemblies processed at 150 and 450°C. Processing at 650°C resulted in a collapse of the nanotubular building blocks and formation of the assemblies of irregularly shaped TiO2 nanoparticles. Structural analysis revealed several phase transitions in titania spherical assemblies in course with the temperature increase indicating possibility of in-situ phase composition adjustment during aerosol processing. © 2015 Elsevier Ltd and Techna Group S.r.l

Room temperature ferromagnetism in Cu2+ doped TiO2 nanocrystals

Hydrothermal synthesis of Cu2+ doped TiO2 nanocrystals, using dispersion of titania nanotubes in the presence of Cu2+ ions as precursor, which showed room temperature ferromagnetism, is reported. Morphology of Cu2+ doped TiO2 nanocrystals was characterized by transmission electron microscopy (TEM). An average size of polygonal nanocrystals mainly of square/rectangular shape was around 13 nm. The x-ray powder diffraction (XRPD) analysis of resultant powder confirmed the anatase crystal phase of Cu2+ doped TiO2 nanocrystals. Room temperature ferromagnetic ordering with saturation magnetic moment of the order of 2 x 10-2 μB per Cu atom was observed

Атомистички и кристалографски феномени при контракцији нанозрна

Међуповршине, односно границе зрна, имају кључну улогу у дефинисању јединствених својстава металних материјала са нанокристалном као и микроструктуром ултрафиних зрна. Услед ефекта мале величине, атомска структура и покретљивост граница зрна одређују стабилност нанокристалне микроструктуре. У овом прилогу је дат приказ феномена који се односе на механизам и кинетику миграције граница зрна у танким филмовима злата под утицајем капиларних сила. Контракција циркуларних нанозрна у танким филмовима тзв. мазед бикристалне морфологије – поликристалне структуре у којој су присутне само две оријентације зрна – на повишеним температурама карактеризована је in situ електронском микроскопијом, конвенционалном и високе резолуције. Карактеризација је указала да је висок степен анизотропије својствен нанозрнима, односно изражена склоност ка образовању пљосни (фацета) на одређеним типовима кристалографских равни са ниским Милеровим индексима. Утврђено је да механизам миграције пљосни укључује колективно кретање атома, односно, регруписање атома са померајима мањим од параметра кристалне решетке, клизање и миграцију степеница. In situ микроскопија високе резолуције је показала да је миграција контролисана нуклеацијом и пропагацијом степеница и дисконекција, тј. дефеката који поред компоненте степенице имају и дислокациону компоненту, како латерално тако и у правцу перпендикуларном на површину филма. Непостојна кинетика контакције је у сагласности са механизмом миграције контролисаним нуклеацијом степеница. Детаљна анализа дефеката на међуповршинама показала је присуство степеница различитих висина и дислокационог садржаја. Висина степенице/дисконекције ограничава њену мобилност у {110} филмовима, док такав утицај није запажен у {111} филмовима. У току контракције није примећена ротација зрна.Nanocrystalline and ultrafine-grained metallic materials are considered to be interface-controlled as key factors in determining their unique properties are associated with grain boundaries. Due to the small size effect, atomic structure and mobility of grain boundaries have an important influence on the stability of a nanocrystalline structure. In this contribution, the phenomena related to the mechanism and kinetics of capillary driven grain boundary migration in thin Au films are reviewed. The shrinking of island grains in {110} and {111} mazed bicrystal thin films at elevated temperatures was observed by conventional and high-resolution in situ electron microscopy. Measurements of grain boundary anisotropy showed the preference for low index facets. Observed migration mechanisms of the facets involve collective motion of the atoms such as regrouping by atomic shuffles, glide and step migration. In situ HREM revealed that the facet migration is controlled by the nucleation and propagation of the steps/disconnections - by lateral motion as well as the propagation of buried steps perpendicular to the film surface. The erratic kinetic of grain shrinkage is consistent with migration mechanisms controlled by the step nucleation. Detailed analysis of the interfacial defects showed a range of step heights and dislocation contents. The step height limits the mobility of steps/disconnections in {110} films, while no such influence was observed in {111} films. There was no measurable grain rotation during the grain shrinkage.Lecture series / Serbian Academy of Sciences and Arts ; book

Homogenization of an al-mg alloy and alligatoring failure: alloy ductility and fracture

High-strength Al-Mg alloys have a propensity toward hot fracture and failure by alligatoring during hot rolling. In this study the effect of homogenization conditions on the susceptibility of an Al-5.1Mg-0.7Mn alloy toward alligatoring was investigated. It was found that the plates homogenized in a temperature range of 460-520 degrees C were prone toward alligatoring, but once homogenized at 550 degrees C they were not prone toward it any longer. The characterization of the fracture showed a predominance of the intergranular ductile fracture, but the type of the constitutive particles filling the voids changed with the homogenization regime. Grain decohesion and grain-boundary embrittlement show that certain thermal treatments resulted in a microstructure that promoted slip localization

Homogenization of an al-mg alloy and alligatoring failure: influence of the microstructure

In this study the influence of the microstructure of Al-5.1Mg-0.7Mn alloy on the propensity towards an alligatoring failure was evaluated. The morphology of the constituent particles appears to be the key factor, with dissolution and fragmentation of fibrous Mg-Si-Sr and transformation of the Al-6(Fe,Mn) into more compact shapes taking place at homogenization temperatures above 520 degrees C, contributing to improved ductility and diminished propensity towards alligatoring. Homogenization at temperatures below 500 degrees C gave rise to a non-uniform precipitation of the dispersoids. Such a microstructure favored localized slip that induced stress concentration at the grain boundaries and triggered grain boundary embrittlement

Electron microscopy observations of twin-twin intersections in a particle hardened copper-titanium alloy

The twin-twin intersections in the precipitation hardened two-phase copper-titanium alloys were studied using conventional and high-resolution electron microscopy. The incident twin propagation through an obstacle twin was accommodated by both slip and twinning. The ledge associated with the twin-twin intersections was associated with complex sessile configurations. The existence of an accommodation mechanism without shear transfer on conventional {111} planes in the obstacle twin led to the formation of strong rotation fields and activation of secondary twinning in the obstacle twin

Influence of cold-rolling deformation and annealing temperature on the grain growth of al-4%mg-mn alloy

The propensity of an alloy toward an abnormal grain growth (AGG) severely limits the annealing temperature range, capability for hot forming, and deteriorates mechanical properties. In this work, the results of a study of the effect of thermo-mechanical processing (TMP) of Al-Mg alloy AA5182 on the AGG are reported. TMP included cold rolling with a reduction range of 40-85 % followed by isochronal annealing (1 h) in a temperature range of 350-520 degrees C or isothermal treatment at 480 degrees C for different periods of time. The microstructure was characterized using optical microscopy in polarized light and an FEG SEM. The results showed that an increase in the degree of reduction lowers the annealing temperature for the onset of the AGG, while an increase in the annealing temperature increases the extent of the AGG for a given cold reduction. The abnormal grains tended to form in the region close to the surface of the plates, and with extended annealing bands. develope parallel to the surfaces in the rolling direction. However, no appreciable grain growth occurred in the plate center. The AGG and grain-boundary mobility showed a strong anisotropy with a much faster lateral growth than the motion of the growth front normal to the plate surface. Such anisotropy was attributed to the rod-like shape and alignment of Al6Mn dispersoids through Zener pinning