Determination of precipitate strengthening in Al-Cu alloys through micropillar compression: Experiments and multiscale simulations
September 29-October 4, 2019
Al-Cu alloys are efficiently strengthened by different types of precipitates: Guinier-Preston zones, θ'' (Al3Cu) and θ' (Al2Cu). The contribution of each type of precipitate to the strengthening of the alloy was determined by means of a high-throughput strategy based on micropillar compression. To this end, an Al-4 wt.% Cu alloy was manufactured by casting, following by several homogenization heat treatments at high temperature. The alloy was aged at 23ºC and 180ºC for different times to produce different precipitate structures . Micropillars were machined using a focus ion beam in grains oriented for single and multiple slip and compressed at ambient temperature. The critical resolved shear stress was determined as a function of the applied strain for micropillars with different sizes oriented for single slip to assess the size effect. It was found that the properties of the bulk crystals could be obtained by testing square micropillars with cross-section > 5 x 5 µm2. In addition, the precipitate type and spatial distribution as well as the mechanisms of dislocation/precipitate interaction were studied in the transmission electron microscope from lamella extracted from the deformed micropillars. It was found that Guinier-Preston zones and small θ'' precipitates (< 50 nm) were sheared by dislocations while dislocations formed Orowan loops around large θ' precipitates. Afterwards, the effect of latent hardening for the different types of precipitates was studied by compression of micropillars oriented for double slip (coplanar and non-coplanar) as well as for multiple slip.
In parallel, the critical resolved shear stress in the overaged Al-Cu alloys containing large θ' precipitates was simulated by means of dislocation dynamics simulations using the discrete-continuous method in combination with a fast Fourier transform solver to compute the mechanical fields . Simulations took into account the effect of precipitate shape, orientation and volume fraction as well the elastic mismatch between the matrix and the precipitate, the stress-free transformation strain around the precipitate and the dislocation character as well as dislocation cross-slip.
In addition, the results of the micropillar compression tests were used to calibrate the latent hardening parameters of a crystal plasticity model, so they can be used to predict the mechanical behavior of polycrystals by means of computational homogenization. Overall, the results of this investigation show how micropillar compression can be used as a high-throughput technique to obtain the bulk properties of precipitation-strengthened alloys as well as to validate the results of simulation strategies at lower length scales (dislocation dynamics) and to provide input information for simulations at larger length scales (computational homogenization of polycrystals).
 A. Rodríguez-Veiga, B. Bellón, I. Papadimitriou, G. Esteban-Manzanares, I. Sabirov, J. LLorca. A multidisciplinary approach to study precipitation kinetics and hardening in an Al-4wt.%Cu alloy. Journal of Alloys and Compounds, 757, 504-519, 2018.
 R. Santos-Güemes, G. Esteban-Manzanares, I. Papadimitriou, J. Segurado, L. Capolungo, J. LLorca. Discrete dislocation dynamics simulations of dislocation- θ’ precipitate interaction in Al-Cu alloys. Journal of the Mechanics and Physics of Solids, 118, 228-244, 2018.
Javier Llorca, Barbara Bellón, Rodrigo Santos-Güemes, and Sarra Haouala, "Determination of precipitate strengthening in Al-Cu alloys through micropillar compression: Experiments and multiscale simulations" in "Nanomechanical Testing in Materials Research and Development VII", Jon Molina-Aldareguia, IMDEA-Materials Institute, Spain Eds, ECI Symposium Series, (2019). https://dc.engconfintl.org/nanochemtest_vii/138