Microstructural changes induced by Er and Zr additions to A356 alloy investigated by thermal analyses and STEM observations

This work is aimed at characterizing the changes induced in the microstructure of an Al-7Si-0.4Mg (A356) alloy when 0.3 wt% Er (alloy E3) and 0.3 wt% Er plusapproximately 0.5 wt% Zr (alloy EZ35) are added. The addition of Er and Zr caused the precipitation of Al3Er and Al3(Er, Zr) precipitates during solution heattreatment, as it was shown by isothermal DSC tests performed at the solution treatment temperature, microhardness measurements on samples subjected to differentsolution treatment time and STEM micrographs. The age hardening response of Er- and Zr-containing A356 is improved, thanks to the formation of further Al3Er andAl3(Er, Zr) precipitates during aging. Er- and Zr-containing precipitates were also identified inside eutectic Si and were identified as Al2Si2Er and Al2Si2(Er, Zr) for E3and EZ35, respectively. Finally, an Er-rich layer was identified at the interface between eutectic Si and Al after thermal treatment, possibly formed by Er atomsdiffusing along the Si twins and accumulating at the eutectic Si interface with the Al matrix.1. IntroductionA356 (Al7Si0.4 Mg) cast alloy is widely used in several industrialfields, especially where high specific mechanical properties and ex-cellent castability are required [1]. Its good mechanical properties de-rive from its significant age hardening response when subjected to aproper heat treatment [2–5]. The precipitation sequence and phaseevolution during aging of Al-Si-Mg cast alloys have been recently de-scribed as those of excess Si Al-Mg-Si wrought alloys [6]:00Al SSSMg/Si coclusters (GP zones)SiSiwhere SSS stands for supersaturated solid solution and GP for Guinier-Preston zone.A recent literature study demonstrated that, in the peak aged con-dition, the main strengthening precipitates in A356 are β′′ needles [6].As aging proceeds, the precipitates evolve, progressively enriching inMg [7], lose coherency with the matrix and coarsen. The natural con-sequence of the last two phenomena is the reduced pinning ability ofthe precipitates for dislocations, which implies the degradation in me-chanical properties typical of overaging.The addition of rare earths and transition metals in the chemicalcomposition of Al-Si-Mg alloys has proven to strengthen the material,thanks to the formation of additional precipitates in intra-dendriticregions [8–11]. In particular, some elements were found to formstrengthening precipitates at temperatures comparable to those usedduring solution heat treatment of Al alloys [12]. In this respect, in thelast years, several researches were focused on the effects of combinedadditions of Er and Zr in pure Al [13–17]. These elements can lead tothe formation of core-shell Al3(Er, Zr) L12precipitates, that are co-herent with the matrix up to a critical size and resistant to coarsening,maximizing the strengthening contribution [13–17]. Despite the posi-tive action of Er and Zr on pure Al, there is still lack of a comprehensivecharacterization of the effects of combined additions of those elementsto commercial Al-Si-Mg alloys.Scope of this work is to study the influence of Er and Zr additions onthe evolution of microstructural constituents, such as strengtheningprecipitates, during thermal treatment of a commercial Al7Si0.4 Mgalloy (A356), by means of Differential Scanning Calorimetry (DSC),microhardness tests and microstructural characterization with ScanningTransmission Electron Microscope (STEM). The development of in-novative Al alloys with improved mechanical properties could havebeneficial effects in many industrial fields, for example in the auto-motive industry, where optimized mechanical properties result inweight saving and, consequently, in reduction of fuel consumption.2. MaterialsandmethodsAl-15 wt% Er and Al-10 wt% Zr master alloys were added to acommercial A356 alloy, which was used as a reference, to obtain thechemical composition reported inTable 1; the chemical composition ofthe studied alloys was selected after preliminary studies performed bysome of the authors and reported in Refs. [10,11].https://doi.org/10.1016/j.matchar.2020.110117Received 4 June 2019; Received in revised form 31 December 2019; Accepted 2 January 2020⁎Corresponding author.E-mailaddress:marco1.colombo@polimi.it(M. Colombo).Materials Characterization 161 (2020) 110117Available online 20 January 20201044-5803/ © 2020 Elsevier Inc. All rights reserved.T