Modeling Precipitation Hardening and Yield Strength in Cast Al-Si-Mg-Mn Alloys

An integrated precipitation and strengthening model, incorporating the effect of precipitate morphology on precipitation kinetics and yield strength, is developed based on a modified Kampmann–Wagner numerical (KWN) framework with a precipitate shape factor. The optimized model was used to predict the yield strength of Al-Si-Mg-Mn casting alloys produced by vacuum high pressure die casting at various aged (T6) conditions. The solid solution strengthening contribution of Mn, which is a common alloying element to avoid die soldering, was included in the model to increase the prediction accuracy. The experimental results and simulations show good agreement and the model is capable of reliably predicting yield strength of aluminum die castings after T6 heat treatment, providing a useful tool to tailor heat treatment for a variety of applications.

Assessment of hardening due to non-coherent precipitates in tungsten-rhenium alloys at the atomic scale

In metallurgical applications, precipitation strengthening is of great technological importance to engineer materials with the required strength. While precipitation hardening is essential for many applications involving operation at elevated temperatures, its subsequent embrittlement can be a showstopper for the overall performance of a component. In the nuclear industry, irradiation-induced/enhanced precipitation and the resulting embrittlement often limit the lifetime of components. In fusion applications, tungsten (W) based alloys are known to harden and embrittle as a result of irradiation-assisted transmutation to rhenium (Re) and its subsequent precipitation into non-coherent precipitates. Hence, a fundamental understanding of the interaction of dislocations with non-coherent precipitates is of great interest. In the present work, the interaction of dislocations with non-coherent Re-rich σ, χ and hcp phase precipitates embedded in a bcc W matrix is assessed. Large-scale atomistic simulations are performed to clarify the interaction mechanisms and derive the obstacle strength of the precipitates in the quasi-static limit. Thereby the impact of precipitate shape, size, interspacing and composition is assessed. Based on those results, an analytical model to predict precipitation hardening of σ, χ and hcp phase particles in bcc W is proposed and compared to available experimental data from mechanical tests on irradiated materials.

Estimation of material parameters based on precipitate shape: efficient identification of low-error region with Gaussian process modeling

In this study, an efficient method for estimating material parameters based on the experimental data of precipitate shape is proposed. First, a computational model that predicts the energetically favorable shape of precipitate when a d-dimensional material parameter (x) is given is developed. Second, the discrepancy (y) between the precipitate shape obtained through the experiment and that predicted using the computational model is calculated. Third, the Gaussian process (GP) is used to model the relation between x and y. Finally, for identifying the “low-error region (LER)” in the material parameter space where y is less than a threshold, we introduce an adaptive sampling strategy, wherein the estimated GP model suggests the subsequent candidate x to be sampled/calculated. To evaluate the effectiveness of the proposed method, we apply it to the estimation of interface energy and lattice mismatch between MgZn2 ($${{\rm{\beta }}}_{1}^{\text{'}}$$ β 1 ' ) and α-Mg phases in an Mg-based alloy. The result shows that the number of computational calculations of the precipitate shape required for the LER estimation is significantly decreased by using the proposed method.

Microstructure of Al-Cu, Al-Zn, Al-Ag-Zn, and Al-Zn-Mg Alloys

The precipitation phenomena and their connection with the microstructure of several Al alloys (Al-Cu, Al-Zn, Al-Ag-Zn, Al-Zn-Mg) are described with respect to the concentration and applied thermal treatment. The alloys were rapidly quenched or slowly cooled from a temperature higher than the solid solution temperature to room temperature. Both quenched-aged and slowly cooled alloys were heated from room temperature to the solid solution state and cooled back to room temperature, and their microstructure and precipitation phenomena were followed in situ by X-ray powder diffraction, e.g., anisotropy of thermal expansion, phase transitions, thermal hysteresis in phase transitions, change of precipitate shape, partial or complete dissolution of precipitates in the matrix, and formation of solid solution. It has been shown that the microstructure strongly depends on the previous thermal history of the alloys.

Modeling Particle Distances of Coherent Prolate- and Oblate-Shaped Precipitates in bcc Systems

In a wide range of materials, precipitation hardening is the key for optimizing properties such as strength or creep performance. In order to model strengthening effects with physically based concepts, precipitate kinetic simulations have to be linked to micromechanical models. Part of this link is the precipitate distance distribution in the glide planes of dislocations. Recently, a new model for the calculation of distance distributions has been introduced, which is specially designed for arbitrary size distributions and, thus, capable of handling more realistic microstructures when compared to classical approaches. Up to now, this model has been restricted to spherical precipitates. In this work, the model is advanced to account for all kinds of spheroids, that is, ellipsoids with rotational symmetry. Any prolate, oblate or globular precipitate shape can be represented by a specific shape factor, or aspect ratio, and an effective radius. The result is represented in the form of a multiplicative factor for particle distances depending on the aspect ratio only, and can be expressed as a single explicit formula. It is shown, that prolate shape is most effective for minimizing particle distances in glide planes, followed by oblate shape and finally spheres. Since numerous precipitate types feature needle-or platelike shapes, the present model offers a wide field of applications.