This chapter reviews the precipitation and precipitate phases that occur during heat treatments in multi-component Al-Li based alloys. It describes aspects related to nucleation, growth, morphology and orientation relationships of the strengthening precipitates δ’ and T1, the toughening precipitate S’ and the recrystallisation-inhibiting precipitate β’. Equilibrium precipitate phases such as T2, which are deleterious to the mechanical and corrosion properties of the alloys, are also described. It is shown that careful alloy chemistry control, two-step homogenization and controlled stretching prior to ageing can be employed to improve the volume fraction and distribution of the precipitate phases. All these processing aspects are necessary to achieve optimum combinations of properties for the alloys.
The formation of metastable and equilibrium phases in binary Al–Li, ternary Al–Li–Mg and Al–Li–Cu, and quaternary Al–Cu–Li–Mg alloys has been studied by using a variety of experimental techniques including differential scanning calorimetry, electrical resistivity, X-ray diffraction, conventional and high-resolution electron microscopy and 3D atom probe measurements. Al3Li (δ′) is the strengthening phase in binary Al–Li and ternary Al–Li–Mg alloys. Mg reduces the solubility of Li in Al and also substitutes for Li in δ′. The characteristics of θ′ (and θ) and T1 phases in Al–Li–Cu alloys and the composition limits where these phases are formed are well understood. For low Li contents (1.4–1.5%). Formation of T1 is promoted by small additions of Ag and Mg and by cold work prior to artificial aging. Zr forms the metastable β′ (Al3Zr) phase, which has an appreciable effect on retarding recrystallization besides providing nucleation sites for composite δ′ particles. Sc and Yb additions behave in a similar way; the added advantage is improved creep strength. The available information from phase equilibria studies of Al–Li–Cu–Mg alloys is somewhat limited, but sufficient to give an indication of the desirable solution treatment and aging temperatures and the phases formed at these temperatures. 3D atom probe studies suggest the involvement of Mg atoms in the formation of clusters which lead to the formation of the T1 phase, during artificial ageing of aging of quenched Al–Cu–Mg–Ag alloys. All these aspects are covered in detail, with specific reference to different commercial and semi-commercial Al–Li alloys, wherever possible.
This Occasional Paper provides an overview of the main challenges facing Hong Kong SAR as it continues to become more closely integrated with the mainland of China. Section I provides an overview of recent macroeconomic developments and the main policy issues in Hong Kong SAR. Section II examines various aspects of the ongoing integration with the mainland, and the associated implications for the structure of the economy, and for macroeconomic and structural policies. Section III examines the medium-term fiscal outlook under different policy scenarios and discusses alternative policy options to restore fiscal balance. Section IV reviews recent developments in the real estate sector and their macroeconomic impacts. Section V presents an econome tric analysis of deflation and its determinants. Section VI examines the factors behind, and the implications of, rising wage inequality in Hong Kong SAR. Section VII presents an overview of recent developments in the financial sector and provides an assessment of Hong Kong SAR’s prospects as an international financial center.
The emergence of Al–Li alloys as potential light metal, for safe use in a spectrum of aircraft structures and related aerospace applications has in recent years engendered an unprecedented widespread interest aimed at studying, understanding and improving their mechanical properties. In this chapter, we present and discuss some of the key aspects relevant to aluminum-lithium alloys, spanning the specific domain of precipitation kinetics as influenced by composition and heat treatment, intrinsic microstructural features and their effects, the fundamental mechanisms contributing to strength, ductility, fracture toughness, and overall anisotropy in mechanical properties of these alloys. The tensile behavior of representative first generation, second generation and third generation aluminum-lithium alloys is also presented and briefly discussed. Microstructural influences on mechanical properties are examined with specific reference to matrix microstructural features, dislocation-microstructural feature interaction, and matrix slip characteristics.
Airworthiness regulatory bodies are authorised and responsible for verifying and ensuring the safety and reliability of aircraft. There are many civil and military aviation organisations and regulatory bodies. The functions and responsibilities of several of these organizations are summarised in this chapter. Owing to the importance of aircraft structural fatigue, a survey of fatigue design philosophies is also given. This is followed by (i) a discussion of the airworthiness certification methodology for materials and structures, starting with the initial mill products and proceeding via incremental levels to the finished aircraft; and (ii) an example of material certification for an aluminium–lithium (Al-Li) alloy that is a candidate for use in the airframes of light combat aircraft (LCA).
Introduction: the BRICS as a club -- Global power shift: the BRICS, building capabilities for influence -- BRICS collective financial statecraft: four cases -- Motives for BRICS collaboration: views from the five capitals -- Conclusion: whither the BRICS?
The Civil War in the West, 1863, by Andrew N. Morris, is the latest addition to the Center of Military History's U.S. Army Campaigns of the Civil War series. In 1863, Union and Confederate forces fought for control of Chattanooga, a key rail center. The Confederates were victorious at nearby Chickamauga in September. However, renewed fighting in Chattanooga that November provided Union troops a victory, control of the city, and drove the Confederates south into Georgia. The Union success left its armies poised to invade the Deep South the following year.
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