Materials Science and Engineering A 426 (2006) 187–193 Inﬂuence of barium addition on the microstructure and the rheological behaviour of partially solidiﬁed Al–Cu alloys A. Fallet a , G. Chichignoud b , C.L. Martin a,∗ , M. Su´ ery a , Ph. Jarry c a Institut National Polytechnique de Grenoble, Laboratoire GPM2, UMR CNRS 5010, ENSPG, BP 46, 38402 Saint Martin d’H` eres Cedex, France b Institut National Polytechnique de Grenoble, Laboratoire LTPCM, UMR CNRS 5614, ENSEEG, BP 75, 38402 Saint Martin d’H` eres Cedex, France c ALCAN CRV, Unit´ e de Recherches Fonderie, 725 rue Aristide Berg` es, 38341 Voreppe Cedex, France Received 24 October 2005; received in revised form 27 March 2006; accepted 31 March 2006 Abstract The effect of a barium addition to Al–Cu alloys on the morphology of liquid ﬁlms in the mushy zone has been studied. It is shown that barium improves wetting of the solid phase by the liquid and thus, delays coalescence of the grains. In addition, tensile and shear tests have been carried out during solidiﬁcation to determine the inﬂuence of Ba on the mechanical behaviour of the mush. It is observed that the fracture stress in tension is affected by the presence of Ba at the very end of solidiﬁcation whereas an effect of Ba is not detectable in shear. It is proposed that this is due to delayed coalescence for Ba-treated alloys for which liquid ﬁlms embrittle the alloy in tension up to very high solid fractions. © 2006 Elsevier B.V. All rights reserved. Keywords: Solidiﬁcation; Aluminium alloys; Mechanical properties testing; Wetting 1. Introduction Hot tearing is a severe defect widely encountered in industrial casting processes of aluminium alloys. It occurs during solidiﬁ- cation when the alloy is in a mushy state where both solid and liquid phases coexist. Towards the end of solidiﬁcation solid dendrites start to coalesce and solid permeability drops dramat- ically. Thus, the liquid phase that remains in the mush cannot accommodate the thermal strain, the solidiﬁcation shrinkage and the possible casting strains experienced by the ingot. This can lead to the formation of pores that may degenerate into hot tears. Many studies on hot tearing phenomena have been performed [1–4] and several hot tearing criteria have been suggested [5–8]. The rheological behaviour of the mushy zone plays a great role in the hot tearing susceptibility of the alloy. Constitutive equa- tions have been proposed to model the mechanical behaviour of semi-solid alloys accounting for the effect of the stress state [9–13]. They are generally based on the viscoplastic behaviour of the solid alloy extended into the semi-solid state by taking into account the presence of liquid ﬁlms. This is done by intro- ducing either an internal cohesion variable by Ludwig et al. [9] ∗ Corresponding author. Tel.: +33 4 76826337; fax: +33 4 76826382. E-mail address: christophe.martin@inpg.fr (C.L. Martin). and Martin et al. [10] or the fraction of grain boundary area cov- ered with liquid by Van Haaften et al. [11]. These models agree that, at high solid fractions, the presence of thin liquid ﬁlms that wet the dendrites has a great inﬂuence on the rheology. The presence of such liquid ﬁlms towards the end of solidiﬁcation depends on the wetting properties of the solid phase and more speciﬁcally on the solid–liquid interface energy γ sl . A decrease of γ sl enables more efﬁcient wetting of the solid phase by the liquid and thus, leads to the embrittlement of the mush. Gullo et al. [14] have studied the inﬂuence of barium as an additional element on thixoforming of Al–Mg–Si alloys. They concluded that barium decreases γ sl . The aim of this study is ﬁrstly to verify the surface active role of barium for Al–Cu alloys in the semi-solid state and secondly to determine the consequences of the addition of barium for the mechanical behaviour of the mushy zone through tensile and shear tests carried out during solidiﬁcation at small strains (<0.3) and slow strain rates (<10 -3 s -1 ). 2. Materials An alloy for the study was cast in Alcan CRV solidiﬁca- tion laboratory. The base metal was 99.99% Al in order to eliminate possible inﬂuence of other elements on the results. It was alloyed with 4.26 wt.% Cu and 350 ppm Ti, grain reﬁned 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.03.102