Materials Science Forum Vols. 519-521

Abstract: Crack turning or delamination behavior of AA 2050-T87 and AA 7050-T7451 ESE(T) and hourglass coupons under cyclic fatigue conditions is presented. Fatigue crack growth rate curves, fracture surface examinations, and the preferred manner of crack growth for each alloy are discussed in an effort to better understand fatigue crack growth behavior of aluminum-lithium alloys in structural components under service conditions.

Abstract: Discs of Al85Ce8Ni5Co2 amorphous alloy were severely deformed by high pressure torsion. Severe plastic deformation exceeding equivalent strain of 8.2 induces the formation of nanocrystalline fcc-Al in a more stable residual amorphous matrix. Calorimetric and X-ray diffraction measurements revealed that the deformed outer part of the disc crystallizes into a mixture of equilibrium phases during the first thermal event. However, in the amorphous ribbon the same crystalline mixture develops only after the second stage.

Abstract: Aluminum foam is a class of porous materials; in which closed pores are produced by a gas generation in liquid (or semi-liquid) aluminum. Aluminum foams are, generally, fabricated by heating a foamable precursor (a powder compact consisting of aluminum and TiH2 powders). Decomposition of TiH2, which is followed by a hydrogen gas release, produces bubbles in molten aluminum. In this research, aluminum foam was fabricated with the help of a chemical exothermic reaction. Titanium and boron carbide (B4C) powders were blended in the Al-TiH2 precursor as reactive powder elements. When one end of the precursor was heated, a strong exothermic reaction between titanium and B4C took place (3Ti + B4C 􀃆 2TiB2 +TiC + 761KJ), and the neighboring part of the precursor was heated by the heat of reaction. Hence, once the reaction happens at the end of the precursor, it propagates spontaneously throughout the precursor. The blowing process takes place at the same time as the reaction because aluminum melts and TiH2 decomposes by the heat of reaction. The advantage of this process is that the energy to make aluminum foam is not necessarily supplied form the external source, but generated form inside of the precursor. Therefore the blowing process is self sustainable (Self-Blowing Process). In this work, the effect of processing parameters on the Self-Blowing Process was observed. The processing parameters we focused on were blending ratio of the starting powders (aluminum, TiH2, titanium, B4C) and heating methods.

Abstract: To obtain further progress and a more detailed understanding of the mechanisms involved in recrystallisation, new and more accurate techniques such as in-situ observations are necessary. This innovative method has been used to monitor the recrystallisation process in a FEGSEM equipped with hot stage. Observations are done in backscatter mode with particular attention to orientation contrast. EBSD maps of the observed areas can be acquired before and after recrystallisation. Details of the movement of the interfaces between the recrystallised region and the parent structure are recorded and analysed. The results show that the grain boundaries observed do not move smoothly but with a jerky motion. The recrystallising front sweeps through small areas, corresponding to single sub-grains or small groups of them, very rapidly and then stops at other sub-grain boundaries for varying time before progressing to the following area.

Abstract: This paper presents two studies illustrating the possibilities of Small-Angle X-ray Scattering for characterising quantitatively the state of precipitation in aluminium alloys. In the first example, maps are presented, of precipitate size and volume fraction in the cross-section of friction stir welds of AA7449 alloy. It is shown that the influence of welding speed on the distribution of mechanical properties can be understood using this microstructural data. In the second example, the precipitation kinetics in an Al-Zr-Sc alloy is evaluated by in-situ small angle X-ray scattering. Evidence is given for the heterogeneous chemical structure of the Al3(Zr,Sc) precipitates, consisting of a Zr-rich shell surrounding a Sc-rich core. It is shown that this particular distribution results in a very good resistance to coarsening of the precipitate microstructure.

Abstract: Nuclear magnetic resonance (NMR) is shown to be a sensitive metallurgical characterization technique for metastable phase development during early stage aging of the high purity experimental alloy Al(1.05Cu 1.7Mg) (at.%). It is shown, using 63Cu NMR and positron annihilation lifetime spectroscopy (PALS), how room temperature structural evolution proceeds in Al(1.05Cu 1.7Mg) prepared in the form of supersaturated solid solution. The combination of NMR and PALS allows identification of solute aggregate chemistry and defect kinetics during natural aging. Guinier-Preston-Bagaryatsky (GPB) zone formation is detected by NMR within 0.5 h of quench, and the percentage of Cu atoms in GPB zones increases to ~80% within 50 h at room temperature, with the residual 20% of Cu atoms remaining in solid solution. The formation of GPB zones corresponds with the Vickers hardness, in fact the hardness is shown to directly depend on the amount of total Cu partitioned to GPB zones. The vacancy kinetics, as measured by PALS, show an exponential decay in time following the quench with the majority of decay complete within 50 h supporting the notion that early stage hardening by GPB zone formation is controlled by defect concentration and availability.

Abstract: An X-ray microtomography combined with hard X-ray imaging microscopy, that potentially has a spatial resolution of the order of 10 to 100 nm, has been applied to the three-dimensional observation of internal microstructural features in overaged Al-Ag alloys. A Fresnel zone plate is used as an objective with a magnification of 49.3 times. Imaging of resolution test patterns has indicated spatial resolutions of around 180 and 200 nm in the vertical and horizontal directions, respectively. This paper reports the first impression of the microstructural imaging by means of such a high-resolution imaging microtomography. Precipitate microstructures are readily observed and quantified in terms of volume fraction and orientation. Conventional microtomography with a simple projection geometry is also applied for comparison purpose at the highest resolution level currently available at a third generation synchrotron facility. It would appear that the present technique provides a unique potential to observe the 3-D geometry and spatial distribution of nanoscopic features inside samples that are several orders of magnitude thicker than thin-foil specimens for TEM observation.

Abstract: This paper generally presents different techniques available to image the microstructure of materials in three dimensions (3D) at different scales. It then focuses on the use of the more versatile of these techniques for aluminum alloys : X-ray tomography. The paper describes the recent improvements (spatial and the temporal resolution, grain imaging). Electron tomography is also presented as a promising technique to improve the spatial resolution.

Abstract: Ultrasonic velocity and attenuation measurements are powerful tools to infer much information about the microstructure and properties of aluminum and its alloys. Laser-ultrasonics is a technology that enables doing these measurements remotely, in-situ or inline and in a fraction of a second. Therefore, it is possible to characterize the thermomechanical processing of aluminum alloys with unprecedented time resolution. This paper reviews the physical principles that allow relating velocity and attenuation measurements to various materials properties and microstructural features such as elastic moduli, crystallographic distribution orientation (texture), residual stresses, recrystallization and dislocations. In-situ (in laboratory furnaces) and in-line measurement examples from the Industrial Materials Institute research group are reviewed and presented.