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Aluminium foils (99.99% purity) and single crystals (99.999% purity) were charged with hydrogen using a gas plasma method and electrochemical methods, resulting in the introduction of a large amount of hydrogen. X‐ray diffraction measurements indicated that within experimental error there was a zero change in lattice parameter after plasma charging. This result is contradictory to almost all other face‐centred cubic (f.c.c.) materials, which exhibit a lattice expansion when the hydrogen enters the lattice interstitially. It is hypothesized that the hydrogen does not enter the lattice as an interstitial solute, but instead forms an H–vacancy complex at the surface that diffuses into the volume and then clusters to form H2 bubbles. Small‐ and ultra‐small‐angle neutron scattering (SANS, USANS) and small‐angle X‐ray scattering (SAXS) were primarily employed to study the nature and agglomeration of the H–vacancy complexes in the Al–H system. The SAXS results were ambiguous owing to double Bragg scattering, but the SANS and USANS investigation, coupled with results from inelastic neutron scattering, and transmission and scanning electron microscopy, revealed the existence of a large size distribution of hydrogen bubbles on the surface and in the bulk of the Al–H system. The relative change in lattice parameter is calculated from the pressure in a bubble of average volume and is compared with the experimentally determined value.

This paper investigates the potential of mechanical tensioning (MT) to reduce the magnitude of residual stresses in welds and to eliminate buckling distortion. Both friction stir (FSW) and arc welds have been produced from the aluminium alloy AA2024, with different levels of tensile stress applied along the weld line either during or after welding. The resulting welds have been characterised in terms of out of plane distortion, residual stresses and microstructure. Buckling distortion was eliminated by stretching plates to between 35 and 70% of the yield stress of the material during welding. For each set of welding parameters investigated, an optimum tensioning stress has been identified, which eliminates the tensile residual stress peak across the weld zone, along with distortion. This optimum tensioning stress increases in line with the heat input of the welding process. When MT stresses are increased beyond this optimum value, then distortion arises once more and a band of compressive stress is formed across the weld zone.

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During the last 20 years, research and development efforts have been undertaken to develop g-TiAl based alloys as a replacement for Ni-based superalloys for high-temperature applications in turbine blades of advanced aero engines and turbo-chargers of automotive combustion engines. [1] Intermetallic g-TiAl based alloys are intended for use in the temperature range from 600 to 900 8C and there is growing demand for alloys which can be deformed easily by cost-effective forming operations. One possible approach is to design alloys with microstructures in which homogenously distributed b and g-phase are the main constituents. [2] Therefore, these alloys are named b/g-alloys.[3] Since Mo is a strong b stabilizer the ternary system Ti-Al-Mo is well suited for studying this type of alloys. [4] Despite the importance of Mo as an alloying element, the knowledge of its effect on phase diagram, transformation temperatures, and order/ disorder transition temperatures is limited. In many advanced multi-phase TiAl alloys three intermetallic phases, g, a 2 , and b o , are the dominating microstructural constituents, all of which are ordered at room temperature. At elevated temperatures the ordered hexagonal a 2 -phase (D0 19 ) disorders to a (A3) and the ordered cubic b o -phase (B2) disorders to the body-centered cubic b-phase (A2) while the g-phase (L1 0 ) remains ordered up to its dissolution temperature (T g ). The phase fractions present have a strong influence on the mechanical properties of the material and on the processing characteristics at hot-working temperatures. High b/b o -phase contents, for instance, improve the deformation behavior during hot-working but simultaneously decrease the creep resistance if prevailing at service temperature. [5] Additionally, the room temperature ductility is negatively affected by high b o -phase fractions. [6] In the present work, sections of the ternary phase diagram Ti-Al-Mo, obtained by thermodynamic calculations and COMMUNICATION Being a strong b stabilizer, Mo has gained importance as an alloying element for so-called b/g-TiAl alloys. Intermetallic TiAl-based alloys which contain a significant volume fraction of the body-centered cubic b-phase at elevated temperatures have proven to exhibit good processing characteristics during hot-working. Unfortunately, the effect of Mo on the appearing phases and their temperature dependence is not well known. In this work, sections of the Ti-Al-Mo ternary phase diagram derived from thermodynamic calculations as well as experimental data are presented. The phase transition temperatures stated in these phase diagrams are compared with the results of high-temperature diffraction studies using high-energy synchrotron radiation. Additionally, the disordering temperature of the b o -phase is determined. 306 wileyonlinelibrary.com ß

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Intermetallic γ‐TiAl based alloys are a novel class of lightweight structural materials that exhibit excellent high‐temperature strength while having low density. These properties make them ideal candidates for replacing dense Ni base alloys currently used in the temperature range from 550 to 750 °C. Therefore, extensive research activities were conducted during the last 20 years to make this innovative class of materials fit for service. In this task, diffraction methods have been an important tool for promoting the development of TiAl alloys. The ability to perform experiments in situ and to determine phase fractions even in cases where two phases are present in ultrafine lamellar structures are only two examples for applications in which diffraction methods are indispensable. In this work, a review is given concerning the use of diffraction methods in the development of TiAl alloys. Different methods are introduced and highlighted by examples. This review lists the advantages of diffraction experiments and critically discusses the limits of the individual methods.