Dynamic Dislocation-Defect Analysis and SAXS Study of Nanovoid Formation in Aluminum Alloys

Abstract

Crystalline defects other than the essential dislocations are produced by dislocation intersections resulting in debris, which can transform into loops, point defects, and∕or nanovoids. The stress concentrations ahead of slip clusters promote void formation leading to incipient cracks. To evaluate the progression of these processes during deformation, dynamic dislocation-defect analysis was applied to nominally pure aluminum, Al–Mg, and Al–Cu alloys. In the case of nanovoid formation, small angle X-ray scattering (SAXS) was used to quantitatively assess if the void size and its volume fraction can be determined to directly correlate with the measured thermodynamic response values. The SAXS signal from the nanovoids in nominally pure aluminum is distinctly measurable. On the other hand, thermomechanical processing of even nominally pure aluminum results in the formation of nanoprecipitates, which requires future calibration.

Nanovoid characterization of nominally pure aluminium using synchrotron small angle X-ray Scattering (SAXS) methods

“…This assumption is confirmed by previously obtained TEM images [29] of quench voids in Al and approximate slopes of "À4" consistently obtained from the ln[I(q)] vs. lnq plot (Figure 2(b)), as predicted by Porod's law [17,18]. It should also be acknowledged that in addition to the presence of 3-D voids, dislocation loops will also be present in the quenched Al samples.…”

“…Following the original work of Kiritani [4] as adapted by Westfall et al [29], a thermal processing method was used to produce a stable population of nanovoids in two nominally pure Al samples: 99.988 and 99.995 at.%, to be designated as 3 and 4 N, respectively. The samples were received in the form of cold rolled 120μm thick foils from Toyo Aluminum.…”

“…In addition, it has long been known that careful sample preparation, coupled with thorough scrutiny of the scattering data to identify multiple scattering artefacts [21], is essential. Past applications of the SAXS measurement tool to the study of nanostructures in metals (Antion et al [22]; Deschamps et al [23][24][25]; Dumont et al [26,27]; Werenskiold et al [28]; Westfall et al [29]) have used Guinier and Porod analysis methods in combination with detailed modelling of the heterogeneous particle microstructure to interpret the measured SAXS data in terms of particle size, shape, interface structure and relative volume fraction. Most of this work has focused on the characterization of particle features in precipitation-strengthened aluminium alloy systems.…”

“…The data from different SDD (low-q/high-q), in absolute intensity units, can then be directly combined. The resulting combined q range is approximately three times that obtained previously by Westfall et al [29]. A He ion counter located upstream from the final set of shaping slits is used to obtain a pre-sample measure of the incident beam.…”

“…The quench and bath temperatures were chosen to optimize the thermally induced vacancy concentration; faster quench rates to lower bath temperatures have been shown to maximize the number of thermal vacancies while reducing vacancy agglomeration [31]. Consequently, the level of void scattering is expected to increase with increasing quench temperature and quench rate [29]. Sample naming follows the convention [Al][quench temperature], where [Al] is either 3 or 4 N. The quench rates obtained ranged from 10 3 to 10 4°C /s ( Table 1).…”