Microstructure,tensile properties and fracture behaviour of aluminium alloy 7150

A study has been made to understand the microstructure, tensile properties and fracture characteristics of aluminium alloy 7150. Detailed optical and transmission electron microscopical observations were used to analyse the intrinsic microstructural features of the alloy in the T77 condition. The alloy was deformed to failure over a range of strain rates in environments of 3.5% sodium chloride solution and laboratory air. The environment was found to have little influence on strength of the alloy. The strength only marginally increased with an increase in strain rate. However, for all strain rates, the ductility of the alloy degraded in the aggressive environment. The ratio of strain to failure in sodium chloride solution to that in laboratory air indicates that the alloy is only mildly susceptible to stress corrosion cracking. The fracture behaviour was different in the two environments. However, in a given environment the fracture behaviour was essentially the same. In the aggressive environment fracture was predominantly intergranular while fracture revealed a ductile transgranular failure in laboratory air. An attempt is made to discuss the kinetics of the fracture process in terms of competing mechanistic effects involving intrinsic microstructural features, matrix deformation characteristics, environment and strain rate.     

Synthesis,Photophysical Properties and Incorporation of a Highly Emissive and Environment‐Sensitive Uridine Analogue Based on the Lucifer Chromophore

The majority of fluorescent nucleoside analogues used in nucleic acid studies have excitation maxima in the UV region and show very low fluorescence within oligonucleotides (ONs); hence, they cannot be utilised with certain fluorescence methods and for cell‐based analysis. Here, we describe the synthesis, photophysical properties and incorporation of a highly emissive and environment‐sensitive uridine analogue, derived by attaching a Lucifer chromophore (1,8‐naphthalimide core) at the 5‐position of uracil. The emissive nucleoside displays excitation and emission maxima in the visible region and exhibits high quantum yield. Importantly, when incorporated into ON duplexes it retains appreciable fluorescence efficiency and is sensitive to the neighbouring base environment. Notably, the nucleoside signals the presence of purine repeats in ON duplexes with an enhancement in fluorescence intensity, a property rarely displayed by other nucleoside analogues.     

The presence and consequences of precipitatefree zones in an aluminium-copper-lithium alloy

The addition of lithium to aluminium alloys has the potential for providing a class of high-strength alloys with exceptional properties suitable for aerospace applications. Potential candidates are precipitation hardenable and belong to the Al-Li-Cu family. The intrinsic microstructural features have a pronounced influence on the mechanical response of these alloys. In this work, the mechanisms responsible for the formation of precipitate-free zones along grain boundaries in precipitation-strengthened lithium-containing aluminium alloys were examined. The influence of grain morphology and the nature and type of precipitate coverage at the grain boundary in controlling the formation of these zones was analysed. The presence and influence of these zones along the grain boundaries on mechanical properties was studied for an Al-4.5Cu-1.21 Li alloy. It was found that while strength is comparable with existing high-strength alloys, the ductility decreases due to the presence of precipitate-free zones. The degradation in ductility is attributed to the particular mode of plastic deformation of this alloy, and to the restriction of plastic deformation in narrow planar zones along the grain boundaries. Fracture occurs when a critical local strain is reached in these zones. The overall consequences of precipitate-free zones along grain boundaries on mechanical properties are discussed in the light of competing effects involving the nature of matrix-strengthening precipitates, grain-boundary particles and deformation characteristics. Previous address: Materials Modification Inc., Falls Church, Virginia 22044, USA Previous address: Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.