Studies of grain boundary regions in deformed polycrystalline aluminum using spherical nanoindentation

Abstract

IntroductionMesoscale interfaces in materials, including grain boundaries, play a significant role in determining the mechanical properties of materials. The Hall-Petch effect [1,2], discovered over sixty years ago, provides a robust but empirical relationship between the macroscopic yield strength and average grain size in a material. However, there is still very little understanding about the precise role of grain boundaries during plastic deformation and there is no consensus on the correct physics-based model for predicting this effect [3,4]. Moreover, the Hall-Petch effect, captures only the homogenized effect of grain boundaries on the macroscale mechanical properties of metallic materials. In other words, this simple model does not pay attention to the difference in the behavior of the different type of boundaries. Various criteria have been put forth to describe the behavior of grain boundaries based on grain boundary character [5-7], interfacial structure (CSL boundaries) [7-9], grain boundary energy (special boundaries) [10,11], and slip transfer rules [12][13][14]. However, these dependencies are not yet established quantitatively. An accurate understanding of the role of grain boundaries is critical for the development of robust physics-based crystal plasticity models [15-18] to accurately predict material behavior during and following plastic deformation of polycrystalline materials.Lack of rigorous protocols for reliably capturing the mechanical response at the submicron length scales of interest constitutes the foremost technical gap in efforts aimed at advancing the fundamental understanding of the role of grain boundaries during plastic deformation. Approaches such as in-situ testing of micro-pillars [19][20][21][22] have an inherent drawback in that these methods require sophisticated equipment and are effort intensive, and therefore cannot be used to test a large number of interfaces in a cost and time efficient manner.Alternatively, instrumented indentation testing (both nano and micro) [23][24][25][26], when combined with improved data analyses protocols provides a high throughput approach suitable for addressing this challenge. The use of nanoindentation testing for characterizing grain boundary regions has been reviewed in Ref. [27].

Mechanical Characterization of Mesoscale Interfaces Using Indentation Techniques

“…This fraction can then be interpreted as the fractional increase in the local slip resistance at the indentation site in the deformed sample. 75,76 The viability of these new protocols was recently demonstrated by Vachhani et al 22 on a selected set of grain boundaries in a coarse-grained Al sample. Figure 4 summarizes the results of this study.…”

“…Studies of grain boundary regions in plane strain compression deformed polycrystalline Al samples using spherical nanoindentation stress-strain protocols combined with electron backscattered diffraction. 22 The deformed coarse-grained sample is shown with the specific grain boundary regions studied. For each grain boundary studied, the local percentage increase in critical resolved shear stress is measured as a function of the distance from the grain boundary (on both sides).…”

“…22,24 More importantly, in these studies, the objective of the spherical indentation stress-strain protocols is to lightly probe the local mechanical response in the grain boundary regions by comparing their local indentation stress-strain responses in the undeformed and deformed samples, where the deformation is imposed following standard practices in bulk forming of metals (e.g., simple compression, channel-die compression). By carefully comparing the changes in the grain boundary regions between the undeformed and the deformed conditions, it is hoped to reconstruct the roles of the different grain boundaries in the macroscopically imposed bulk plastic deformation on the sample.…”

“…19,20 However, the most commonly used formulations of the Hall-Petch effect treat all grain boundaries as the same, i.e., they do not discriminate between the different types of grain boundaries in modeling their role in spite of the large body of evidence. [21][22][23][24] In polymer composites, significant effort has been focused on improving wettability and chemical bonding at or near the interface region. [25][26][27][28][29][30][31][32] Generally referred as the interphase, this region exhibits a transition between the properties of the matrix and the reinforcement phases.…”

Gradientes de orientación en cercanías del borde de grano estudiados por EBSD

“…KAMAYA et al [24,25] establecieron un parámetro similar al KAM para la determinación de desorientación local, y observaron localización de desorientación en zonas cercanas a los bordes de grano en acero inoxidable tipo 316 deformado en tracción hasta 30%. Recientemente, VACHHANI et al [26] estudiaron aluminio policristalino y reportaron variaciones en el endurecimiento en una zona de 10 μm desde los bordes de grano, relacionadas con la acumulación de dislocaciones.…”