The influences of multiscale-sized second-phase particles on fracture behaviour of overaged 7000 alloys

To identify the most important parameters of multiscale microstructural features influencing the fracture modes and resistance to damage, detailed microstructural and fractographic analysis of overaged 7000 alloy plates are performed using the broken plane-strain fracture toughness, K-Ic, test specimens. The geometric characteristics of differently sized second-phase particles are changed by the compositional variations. It was found that the fracture process involves three main micromechanisms. The dominant fracture mode changes with alloy purity, leading to fracture toughness degradation. Quantitative description of fractures by profilometry confirmed that crack initiation and propagation is fostered by the coarse Fe- and Si-rich particles. (C) 2009 Elsevier B. V. All rights reservedMesomechanics 2009 Conference, Jun 24-26, 2009, Oxford, Englan

Micromechanical modelling of fracture toughness in overaged 7000 alloy forgings

In this article, a multiple micromechanisms-based model that quantitatively relates the plane-strain fracture toughness, KIC, of overaged 7000 alloy forgings to their microstructural attributes, fracture surface morphology and basic tensile properties is developed. To verify the proposed model, extensive microstructural and fractographic analyses along with mechanical tests are carried out using three industrially produced alloys with different contents of Fe and Si impurities. The fracture mechanisms are identified and individual contributions to the overall fracture are quantitatively assessed. The fracture toughness is then calculated using the experimentally obtained average values of relevant mechanical properties, area fractions of main fracture modes and microstructural parameters such as volume fraction of coarse intermetallic (IM) particles, their size and spacing, density of intragranular precipitates, number and width of the precipitate-free zones (PFZ). The proposed model correctly predicts the effect of individual microstructural parameters on the overall fracture behaviour

Fractographic analysis of fatigue damage in 7000 aluminium alloys

In this paper, an attempt is made to correlate the fatigue damage in 7000 aluminium alloys with different impurity contents to the microstructural features and to explain their interdependence through fractographic observations. The Paris constants of these alloys in the form of hot-forged plates subjected to the overaged T73 temper are evaluated and differences in the fatigue crack growth rate described by striation spacing measurements. Scanning electron microscopy analysis of fatigue fracture surfaces revealed that the type and morphological parameters of coarse intermetallic particles play a critical role in fatigue crack growth behaviour. The elemental distribution determined by means of energy-dispersive spectroscopy analysis showed that the fractured particles accelerating the crack advances are larger particles of Fe-rich phases. The fatigue crack growth rate increases considerably with increasing amounts of these particles. The smaller η, S and Mg2Si particles contribute beneficially to fatigue life

Effect of Alloy Purity on Fracture Behavior of Overaged 7000 Alloy Plates

The article deals with the relations between microstructural parameters, microscopic fracture mechanisms, and resultant plane-strain fracture toughness, K(IC), of hot-forged 7000 alloy plates in overaged condition. Three industrially produced alloys with the (Zn + Mg + Cu) content of similar to 11.5 mass% and different contents of (Fe + Si) impurities (from 0.23 to 0.37 mass%) were used. The variation in alloy purity generated differences in the geometric characteristics of coarse intermetallic phase particles, exerting significant effect on fracture behavior of these alloys. To evaluate the fracture mechanism, characterization of the broken K(IC) test specimen surfaces was performed using scanning electron microscopy. Three micromechanisms governing fracture process were identified. The extent of primary voiding at coarse intermetallic phase particles increases systematically with the increase of (Fe + Si) level, whereas the extent of intergranular/transgranular fracture decreases. This observation was confirmed by a quantitative description of fractures by means of a profilometric method. It was found that morphological parameters of the fracture profiles clearly differentiate between different fracture modes and offer conclusive proof that coarse voiding at fractured intermetallic phase particles dominates fracture at low alloy purity. A higher fraction of coarse voiding is shown to result in an overall reduction of fracture toughness

The effect of coarse intermetallic particles on the fracture process in forged 7000 alloys

The effect of coarse intermetallic particles on the fracture process in 7000 alloy forgings was investigated using three alloys with different (Fe + Si) impurity levels. The intermetallic particles were identified by selective etching and energy dispersive spectroscopy analysis conducted on a scanning electron microscope. Their geometrical parameters were estimated by image analysis and then correlated with area fractions of different fracture modes on the broken fracture toughness test specimens. It was found that the dominant fracture mode varies with the (Fe + Si) content. The coarse voiding at large intermetallic particles increases systematically with an increase of the impurity level, which in turn increases the amount and size of particles containing Fe and Si while decreasing their spacing. That the crack nucleation and propagation are accelerated by these particles was revealed by in situ scanning electron microscopy observation of the fracture process

Influence of notch radius and microstructure on the fracture behavior of Al-Zn-Mg-Cu alloys of different purity

The influence of notch radius on the fracture behavior of two high-strength Al-Zn-Mg-Cu alloys with different Fe content in the T73 condition was investigated. The fracture toughness tests were performed on non-fatigue-precracked notched bending specimens with different notch radii ranged from 0.15 mm to 1.0 mm. The obtained data were interpreted using the concept of Notch Fracture Mechanics combined with finite-element method (FEM) calculations. It was found that both alloys are very sensitive to the notch radius. The fracture toughness increases with increasing notch radius. For a given notch radii, the increase in fracture toughness is more significant for the more pure alloy. The fracture behavior of investigated alloys with respect to microstructural features and their relation with the fracture micromechanisms were analyzed

Phylogeographic Reconstruction of HIV Type 1B in Montenegro and the Balkan Region

Human immunodeficiency virus (HIV) is one of the most genetically variable human viruses as it is characterized by high rates of mutation, viral replication, and recombination. Phylodynamics is a powerful means of describing the behavior of an infection as a combination of evolutionary and ecological processes. Only a few studies of HIV-1 molecular epidemiology have so far been carried out in the Balkans. In this study, we used Bayesian methods to reconstruct the phylogeography and phylodynamics of HIV-1B in Montenegro and some other Balkan countries on the basis of pol gene sequences retrieved from a public database. The phylogenetic analysis showed that 43% of the isolates grouped in accordance with their geographic area, whereas the majority were interspersed in the tree, thus confirming the multiple introductions of HIV-1B in the Balkans. The Bayesian phylogeographic analysis suggested that HIV-1B entered the Balkans in the early 1970s probably through Greece and other Mediterranean tourist/travel destinations (such as Slovenia). Other Balkan countries, such as Bulgaria and Serbia, may have played an important role in spreading the infection to the entire Eastern Mediterranean area, and possibly to Northeast Europe. This suggests that the Balkans may have played a role as a "gateway" between Western and Eastern Europe

Hepatitis B virus genotype and subgenotype prevalence and distribution in Montenegro

The Mediterranean area and the Balkans in particular show the highest level of genetic heterogeneity of HBV in Europe. Data about the circulation of HBV genotypes in Montenegro are lacking. It was studied the prevalence and distribution of HBV genot/subgenotypes in a total of 150 HBV infected patients living in Montenegro. Phylogenetic analysis of 136 successfully amplified P sequences showed a high degree of genetic heterogeneity of HBV in Montenegro. Subgenotype D2 (36.8%) and D3 (32.3%) were the most prevalent, followed by genotype A (subgenotype A2 in all of the cases-19.8%). Eight patients were infected with recombinant strains. HBV-D1 which is the most spread HBV subgenotype in the south-eastern Mediterranean countries, seems to be relatively rare in Montenegro, suggesting a penetration of HBV more probably from North-East or West than from Eastern Mediterranean countries. The relatively different prevalence of D3 and A2 among subjects infected through sexual route, seems to confirm the association of these subgenotypes with different route of transmissions (mainly parenteral for D3 and mainly sexual for A2) even in Montenegro. The low prevalence of D2 among children and its absence in perinatal transmission, suggests that this subgenotype circulated prevalently in the past. If this is due to changes in the most prevalent way of transmission and in the recent different contacts of Montenegro with other European countries, it remains to be established by other larger studies

Genetic diversity of the haemagglutinin (HA) of human influenza a (H1N1) virus in montenegro : Focus on its origin and evolution.

In 2009 an influenza A epidemic caused by a swine origin H1N1strain, unusual in human hosts, has been described. The present research is aimed to perform the first phylogenetic investigation on the influenza virus A (H1N1) strains circulating in Montenegro, from December 1, 2009, when the first case of death due to H1N1 was confirmed, and the epidemic began causing a total of four fatalities. The phylogenetic analysis of the strains circulating showed the absence of a pure Montenegrin cluster, suggesting the occurrence of multiple re-introductions in that population from different areas till as far as the early 2010. The time to most recent common ancestor (TMRCA) for the complete dataset has been dated in early 2008, pre-dating the first Montenegrin identification of H1N1 infection. These data suggest that virus was spreading undetected, may be as a consequence of unidentified infections in returning travelers. Anyhow, the estimated TMRCA of Montenegrin strains is fully consistent to that found in different areas. Compatibly with the time coverage of the study period here analyzed, molecular dynamic of Montenegrin strains follows similar trend as in other countries