Determination of lattice orientation in aluminium alloy grains by low energy gallium ion-channelling
Introduction
The metal matrix composite (MMC), AMC225xe is manufactured by a powder metallurgy route to achieve, through solid state processing, a homogeneous dispersion of the SiC reinforcement phase in an aluminium alloy (AA2124) metal matrix. The gas atomized aluminium alloy powder is blended with the ultrafine SiC reinforcement in a proprietary high energy mixing process known as mechanical alloying (MA) which imparts a significant amount of cold work to the material through cold welding and cold fracturing processes. The resultant powder is degassed and then consolidated by hot isostatic pressing (HIP) into a 100% theoretical density billet in which the average aluminium grain size is (0.92 ± 0.01) μm and a SiC particulate size of (3 ± 2) μm although almost 6% of the particulates have sizes less than one micron due to the MA process.
Powder metallurgy MMC parts are fabricated by forging, rolling or extrusion processes into complex shapes at temperatures between 300 °C and 600 °C. In the literature it has been established that F.C.C. materials demonstrate a fibre texture during the extrusion process that combines the 〈1 1 1〉 and 〈1 0 0〉 textures parallel to the fibre (extrusion) axis [1]. Processed parts are used in performance-driven applications of industries including aerospace, automotive, electronics, high speed machinery and sports equipment to provide enhanced stiffness and fatigue performance over conventional materials. In such applications ductility and fracture toughness are also key requirements and justify the expense over cast MMCs. For example, the fatigue performance is comparable to that of a conventional titanium alloy with a 35% density saving. The aerospace industry requires both high performance and reliability so the technical understanding of these alloys, their behavior and performance after secondary processing is limited by the application of traditional theories to the increased complexity of the microstructure of these MMCs.
Imaging the microstructural evolution in the MMC due to secondary processing such as extrusion using conventional techniques is difficult due to artifacts arising from traditional metallographic sample preparation techniques [2]. Additionally, the refined nature of the grain structure of a particulate reinforced MMC means that high resolutions are necessary in order to observe even basic metallographic features such as grain/sub-grain structures. Optical microscopy permits the observation of the reinforcement phase but does not allow for grain structure analysis. Scanning electron microscopy (SEM) reveals details such as the presence of the intermetallic particles but similarly cannot provide grain structure information, even through electron backscattered detection (EBSD) techniques due to resolution limitations. TEM does provide the opportunity for detailed microstructural characterization but with the usual limitations of specimen preparation and minimal area selection.
Traditionally FIB microscopy has been used in the semiconductor industry for circuit modification and defect analysis where the standard procedures for site-specific TEM specimen preparation [3], [4], [5] have also been applied to metallic systems for grain size and aspect ratio determination [6], [7]. The FIB technique for sample preparation and imaging has been developed to study the fine microstructural detail of the MMC that cannot be observed by optical microscopy or in a conventional SEM. The FIB microscope is used with low incident ion currents so that very little material is sputtered and imaging is achieved by collecting secondary electrons or secondary ions to a resolution of 14 nm. As the tilt angle of the grain with respect to the FIB beam is changed so the secondary electron intensity varies according to the aluminium lattice orientation of the grain. The channelling contrast can be rationalised by reference to the model of ion-channelling of low energy ions [8], [9].
For gallium ion energies of less than 100 keV, the nuclear stopping is larger than electronic stopping by an order of magnitude and the predominant effect is sputtering. Modelling the channelling of low energy gallium ions (30 keV) in the F.C.C. crystal lattice of aluminium to determine the channelling contrast is based on the Lindhard–Onderlinden approach describing the directional variations in sputtering for single crystal targets. Whenever an incident beam is aligned with a direction [uvw] along a row or string of atoms at intervals tuvw in the aluminium lattice and within a critical angle fψc (1 < f < 2), the model [9] proposes its division into two beams: one aligned beam of fraction (1 − χuvw) and one random or non-aligned beam of fraction χuvw. Lindhard’s treatment [8] is based on a Thomas-Fermi screened Coulomb potential between the interacting ion and target atoms of atomic numbers Z1 and Z2. The upper ion energy limit E′ for Lindhard’s low ion energy approximation is given by E′ = 2Z1Z2e2tuvw/4πε0a2 which for 30 keV Ga+ ions into aluminium is calculated to be ∼2.3 MeV where the Bohr screening parameter, a = a0 0.8853 (Z12/3 + Z22/3)−1/2 and a0 is the Bohr radius (0.529177 Å). Table 1 lists the calculated critical angles, ψc for selected lattice directions [uvw] in single crystal aluminium illustrated in the schematic, Fig. 1 where ψc = (3a2Z1Z2e2/4πε0E tuvw3)1/4 and E is the ion energy (30 keV) of the incident gallium ions. As the atom spacing, tuvw increases so the critical angle ψc decreases.
Onderlinden [9] suggested from the comparison of measured and calculated ψc for 2–20 keV Ar+ on Cu, Au, Pb and Al that the sputter yield in channelling directions γuvw is caused by ions that have collided in the first few topmost layers forming the random beam component of the incident beam, χuvw. The channelled fraction (1 − χuvw) does not contribute to sputtering or secondary electron emission. Formally γuvw ∝ χuvwγamorphous where the amorphous sputtering yield, γamorphous is dependent on the angle of incidence and the energy of the incident ion E. The fraction of the incident beam that is non-channelled and becomes the random beam for a direction [uvw], is given by the fraction of collision cross-sections, π(ψctuvw)2/(Ntuvw)−1 = πNψc2tuvw3 where N is the number density for aluminium atoms in the lattice. For the selected lattice directions [uvw], this value is given in Table 1 and it is particularly small for the strongly channelled [1 1 0] direction. If the polar angle from the normal along a channelling direction [uvw] is ψ, then the non-channelling fraction is given by . This function is plotted in Fig. 3. for the 〈1 0 0〉 tilt directions and shows the strongly dependent variations that are the cause of severe surface roughening of aluminium during sputtering for depth profiling during Auger or SIMS analysis [10].
The variation of the channelling contrast recorded by imaging the induced secondary electron yield has the same dispersion χuvw as the variation for sputtering with lattice orientation and is used in this study to determine the lattice orientation of aluminium grains in the MMC after secondary processing by extrusion.
Section snippets
Materials and methods
A metal matrix composite comprising of an aluminium alloy (AA2124, Cu ∼4.0 wt%, Mg ∼1.4 wt%, Mn ∼0.6 wt%) reinforced with 3 μm ceramic particulate SiC at 25 vol% was secondary processed by extrusion at elevated temperatures (350, 450 and 550 °C) and three deformation ratios (5:1, 10:1 and 20:1) and then heat treated. Samples for micro-textural analysis were examined in a low energy scanning ion microscope (FEI FIB200) equipped with a liquid metal gallium ion source.
Polished sections of the alloy were
Microstructural texture of MMC
A typical secondary electron (SE) image of a polished MMC section is shown in the image in Fig. 2(a) where the darkest contrast features are the SiC ceramic reinforcement particles in the aluminium alloy matrix whose grains are visible as the mid-to-light contrast. The corresponding grain boundary map from the ImageJ analysis is shown in the line drawing in Fig. 2(b). The intense white spots seen mainly either at the grain boundaries or in isolated clumps within the Aluminium grains are
Conclusions
Low energy gallium ion beam analysis has successfully been used to determine the micro-texture of grain orientation in this fine-grained composite using the SiC particulate as an internal reference and the Lindhard–Onderlinden model for directional variation in sputtering for single crystal targets. The results are in broad agreement with the macro-texture data obtained through XRD analysis. The main limitation of the method is the deterioration of the surface during analysis due to sputtering.
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