B. Noble

The elastic modulus of aluminium-lithium alloys

Youngs modulus measurements have been made on Al-Li alloys containing up to 32 at % lithium, in an attempt to determine the cause of the high modulus that characterizes this potentially important alloy system. In alloys of commercial interest (7–11 at %, 2–3 wt % lithium) the modulus is in the range 79 to 83 GPa, the actual value depending on heat-treatment conditions. The major contribution to this increased modulus arises from lithium in solid solution. The Youngs moduli of the Al3 Li and AlLi intermetallic phases are estimated to be 96 GPa and 105 GPa respectively. Additions of magnesium to the Al-Li system produce a small decrease of the modulus, e.g. 4.5 at % (4 wt %) magnesium reduces the modulus by approximately 2 GPa.

Stress corrosion cracking of GRP pultruded rods in acid environments

Stress corrosion cracking of GRP pultruded rods has been investigated in 0.0001 to 5.0 N hydrochloric acid environments under bending and tensile loading modes. Crack initiation takes place at exposed glass fibres in the surface of the rod, and crack propagation is planar and at right angles to the rod axis. Leaching of calcium and aluminium from the fibres takes place during the cracking process, and time-to-failure is dependent on the acid concentration, the stress level and the ease of access of the acid to the glass fibre surface. Possible mechanisms of crack propagation through the glass fibres and resin are discussed.

On the α(Al)/δ′(Al3Li) metastable solvus in aluminium–lithium alloys

Data for the {alpha}/{delta}{prime}(Al{sub 3}Li) solvus reported in the literature shows an extremely wide scatter. An attempt has therefore been made to redefine this solvus boundary using electrical resistivity measurements for low lithium concentrations and differential scanning calorimetry for high lithium concentrations. The concentration, C{sub e}, at the {alpha}/{delta}{prime} boundary is closely defined over the range 2--13 at.% Li by the equation 1nC{sub e}(at.%) = 4.176 {minus} RT/9180 and found to be in good agreement with values calculated from thermodynamic data. Zirconium additions of up to 0.05 at.% do not affect the position of the {alpha}/{delta}{prime} boundary. A by-product of the investigation was the resistivity of the {delta}{prime}(Al{sub 3}Li) phase, which was calculated to be 41 n{Omega}m.

Failure of high voltage electrical insulators with pultruded glass fibre-reinforced plastic cores

Abstract A group of ten 275 kV insulators exhibiting various states of electrical and mechanical damage were withdrawn from service, and subjected to comprehensive optical and scanning electron microscopy to determine the failure sequence. The insulators were of two types, suspension and tension. Both types suffered progressive electrical damage starting at the end fitting and progressing along the interface between the core and the shedding, eventually producing electrical punctures between sheds. Tension insulators, which carried higher mechanical loads, also suffered cracking between sheds which appears to admit moisture. The combined effect of electrical activity and moisture appears to be similar to acid stress corrosion in producing brittle fracture of the pultruded rod.

Low-temperature interface reaction in aluminium-silicon carbide particulate composites produced by mechanical alloying

An experimental investigation has been carried out on the reaction that takes place between 3 and 20 μm SiC particles and the aluminium alloy 1050 matrix of composite materials prepared by a mechanical alloying process. The work is different from that undertaken by other researchers in that the SiC-Al interface reaction has been studied in the temperature range 853–933 K, i.e., with the matrix initially in the solid state. Differential thermal analysis, X-ray diffraction and scanning electron microscopy all show that the SiC-Al reaction initiates in the solid state at temperatures as low as 883 K. The reaction produces Al4C3 and Si, the latter entering into solid solution in the aluminium matrix. At temperatures exceeding 903 K, the reaction produces a liquid phase and at this stage the speed of the interface reaction increases significantly. The results are discussed in terms of Al-Si-C metastable equilibrium and the kinetics of the reaction.

Use of the Gibbs–Thompson relation to obtain the interfacial energy of δ′ precipitates in Al–Li alloys

Abstract The δ′ phase in Al–Li alloys containing 6.4–7.6 at.% Li has been allowed to coarsen for several hundred hours at temperatures of 130–150°C. The size of the δ′ precipitates has been measured by transmission electron microscopy (TEM). The concentration of lithium in solid solution in equilibrium with δ′ particles of various sizes has been measured by an electrical resistivity technique. The resulting TEM and resistivity data are used to solve the Gibbs–Thompson equation to give a value for the α/δ′ interfacial free energy. A value of 0.041±0.01 J m −2 has been obtained for this interfacial energy and this has been compared with the range of values reported in the literature.

Microstructural stability of binary AlLi alloys at low temperatures

Abstract Binary AlLi alloys containing 6.5, 9.5 and 11.5 at.%Li have been aged for 24 h at 423 K and then subjected to long term exposure at 343, 373 and 403 K. Proof stress and fracture energy have been measured as a function of exposure time, showing that little or no embrittlement occurs at 343 K, but the alloys undergo increasing degrees of embrittlement as the exposure temperature is raised to 403 K. Transmission electron microscopy and electrical resistivity measurements show that an exposure temperature of 343 K results in very slow precipitation of fine δ′, but at high exposure temperatures the dominant process is growth of pre-existing δ′ particles that have been produced by the 423 K pre-age. These results are discussed in terms of the kinetics of precipitation and the importance of quenched-in vacancies.

Influence of zinc to magnesium ratio and total solute content on the strength and toughness of 7xxx series alloys

A systematic study of the precipitation processes in high strength Al-Zn-Mg-Cu alloys has been conducted. A series of experimental alloys was prepared with varying Zn:Mg ratio, but equivalent total solute content, to be representative of those used in applications which demand a combination of strength, fracture toughness and resistance to environmentally sensitive cracking mechanisms. Artificial ageing curves were constructed for each alloy, based upon 7xxx series duplex treatments widely used in industry. Ageing kinetics were investigated beyond peak strength into the overaged condition, since this is known to promote the most favourable balance of properties. Differential scanning calorimetry (DSC) and transmission election microscopy (TEM) observations were made on the alloys throughout the ageing process to fully understand the precipitation events occuring. For a particular overageing treatment, higher Zn:Mg ratio alloys were consistently found to be at a more advanced stage of precipitation while higher strengths were retained at low Zn:Mg ratios. Grain boundary characteristics, such as particle size and precipitate free zone width, were also influenced by Zn:Mg ratio at a given strength level. This paper provides new understanding of precipitation kinetics and the control of important microstructural features which influence the balance of properties in 7xxx series aluminium alloys.

Fatigue-crack initiation in IMI 829 caused by high-temperature fretting

Fretting fatigue tests have been carried out on a recently developed near-α creep-resistant alloy, IMI 829. The fretting fatigue characteristics have been determined at temperatures of 400 and 600° C under conditions of high and low fatigue stresses. In each case fatigue-crack initiation has been examined and related to the fretting damage on the surface of the specimen, and the microstructure of the alloy.