Papers by Author: M.G.D. Geers

Abstract: A multiscale constitutive framework for Ni-base superalloys has been developed, in which an efficient unit cell is adopted to describe the γ/γ’ microstructure morphology. The framework enables the prediction of the deformation and the creep and fatigue damage accumulation in CMSX-4 for a range of temperatures and stress levels. Moreover, the material microstructural degradation due to rafting and isotropic coarsening can be simulated, and the effects of this degradation on the alloy mechanical response can be quantified. The present paper focuses on the application of the model to real gas turbine components. A high pressure turbine blade finite element model is used to demonstrate the computational efficiency of the multiscale framework. Moreover, the location of critical regions and the life time are shown to differ from the results obtained from classical models that neglect the microstructure evolution.

Abstract: This work analyses those size effects that are encountered first upon downscaling, including grain boundary effects, free surface effects, grain statistics effects. The separate influence of first-order effects was carefully investigated from uniaxial tensile tests on high-purity aluminum specimens with a well-defined microstructure of through-thickness grains, whereby the total number of grains in the cross-section was reduced towards a single grain in a cross-section by, first, decreasing the film thickness and, second, for specimens with through-thickness grains decreasing the specimen width. In addition, 3D dislocation-field strain gradient plasticity simulations were employed to analyze the intrinsic size effects, using the grain size and texture as measured experimentally. The work shows that for miniaturized structures with a limited number of columnar grains a unique Hall-Petch relation does not exist, even though a grain boundary effect, i.e. a decrease in stress level (at a given strain) for decreasing grain boundary area per unit volume, is clearly present. When the microstructure is kept constant upon miniaturization, the free surface per unit area increases causing the stress level of the structure to decrease, the effect of which increases towards a single grain in the cross-section. In addition, the work shows that grain statistics effects also contribute to observed weakening, due to insufficient compensation of local (weaker) material properties by the surrounding material (i.e. grains). Finally, grain statistics also significantly increase the statistical variation in mechanical properties for small-sized structures, an effect that is especially important for the reliability of miniature components. The separate influence of these first-order effects as well as their interplay are explained in terms of the movement of the dislocations upon plastic flow.

Abstract: The introduction of advanced high strength steels, e.g., into the automotive industry initiated a huge interest in analyzing and understanding ductile fracture of sheet metals to greater details. This demands for the development of experimental methodologies that provide microvoid evolution parameters, which also serve as crucial input parameters for advanced forming simulation that can predict damage evolution. Therefore, this work scrutinizes the reliability and applicability of an increasingly popular damage characterization methodology, in which microindentation tests are carried out to measure hardness and elastic modulus degradation as a function of accumulated strain, relating this degradation to damage evolution. To accomplish this goal, this methodology is applied to several different sheet metals of different formability (an interstitial-free steel, a dual phase steel, an aluminum-magnesium-silicon alloy and a ferritic stainless steel). To analyze and verify the results of indentation based methodology, damage evolution in these metals is monitored also via different experimental techniques, i.e. scanning electron microscopy, micro-ct tomography and sensitive density measurement. Moreover, finite element simulations are carried out to understand the effect of void accumulation in the degradation of hardness and elastic modulus. In the case of using the hardness as a damage probe, the degradation due to damage is always coupled to other effects (strain hardening, grain shape change, texture development) causing an increase in the obtained hardness value for all of the sheet metals tested, thereby complete obscuring any degradation of the hardness due to damage. In the case of elastic modulus, all the sheet metals tend to pile-up upon indentation when they are severely deformed, leading to large systematic errors in the Oliver-Pharr methodology based modulus determination, whereas the elastic modulus is also intrinsically altered by the grain shape change and texture development seen for increasing deformation. Therefore, it can only be concluded that, contrary to the published results in the literature, neither the hardness degradation nor the elastic modulus degradation can be used as a precise probe for damage accumulation, at least when the indentation based methodology is carried out in the originally-proposed manner that is commonly used in the literature.

Abstract: Metallographic techniques are crucial tools for establishing the connection between observed mechanical behaviour of metals and the underlying mechanisms in their microstructures. In this work, we propose a methodology that minimizes deformation and/or modification of the microstructure during specimen preparation, and provides a 3D representation of the deformed micromorphology. This methodology involves opening up fractured (tensile test) specimens under the ductile-to-brittle transition temperature of metals to yield two parts in a brittle manner. The occurrence of brittle fracture is validated by analyzing the detailed, unaltered microstructure from SEM characterization and surface profilometry mapping of sets of two parts. It is found that this technique yields additional valuable information regarding the size and morphology of deformed grains or nucleation mechanisms of ductile damage. Experiments with a number of different steels show that the methodology can be applied to sheet of different formability.

Abstract: A physically-based multi-scale model for martensitic transformation induced plasticity is presented. At the fine scale, a model for one transforming martensitic variant is established based on the concept of a lamellae composed of a martensitic plate and an austenitic layer. Next, the behaviour of 24 potentially transforming variants is homogenized towards the behaviour of an austenitic grain. As a simple example, the model is applied to deformation and transformation of a single austenitic grain under different deformation modes.

Abstract: The past years have triggered considerable scientific efforts towards the predictive analysis of the reliability of solder connections in micro-electronics. Undoubtedly, the replacement of the classical Sn-Pb solder alloy by a lead-free alternative constitutes the main motivation for this. This paper concentrates on the theoretical, computational and experimental multi-scale analysis of the microstructure evolution and degradation of the conventional solder material Sn-Pb and its most promising lead-free alternative, a Sn-Ag-Cu (SAC) alloy. Special attention is given to the thermal anisotropy of bulk SAC and the interfacial fatigue failure of SAC interconnects.

Abstract: The deformation and failure behavior of an AM60 magnesium alloy was investigated using tensile test on circumferentially notched specimens with different notch radii. The strain and stress triaxiality corresponding to the failure point were evaluated using both analytical and finite element analyses. Combining with systematical observations of the fracture surfaces, it is concluded that deformation and failure of AM60 magnesium alloy are notch (constraint) sensitive. The failure mechanisms change from ductile tearing to quasi cleavage with the increase of constraint.