Hall-Petch equation in a hypoeutectic Al-Si cast alloy: grain size vs. secondary dendrite arm spacing

The Al-Si cast alloy family is widely used in the production of complex castings for various applications and known for its very good castability and high strength-to-weight ratio. However, early cracking under tensile loading is sometimes a limiting factor. Among other parameters, it is yet controversial whether grain boundaries are dominant strengthening factor in cast alloys, instead of dendrite/eutectic boundaries. This study presents the effect of secondary dendrite arm spacing (SDAS) and grain size on crack initiation and propagation of Al-Si cast alloys under tensile loading. The Al-10Si (wt.%) alloy with modified Si morphology was cast using inoculants (Al-5Ti-B master alloy) under different cooling rates to obtain a range of grain sizes (from below 138 µm to above 300 µm) and SDAS (6, 15 and 35 µm). Conventional tensile test as well as in-situ tensile test in a scanning electron microscope, equipped with an electron backscatter diffraction (EBSD) was carried out to understand the deformation mechanisms of the alloy. Observation of slip bands within the dendrites showed that in modified Si structure, the interdendritic (eutectic) area takes more portion of the strain during plastic deformation. Besides, only a few cracks were initiated at the grain boundaries; they were mostly initiated from dendrite/eutectic interface. All cracks propagated trans-granularly. Hall-Petch calculations also showed a strong relationship between SDAS and flow stress of the cast alloy. Although statistically correct, there was no physically meaningful relationship between the grain size and the flow stress. Nevertheless, formation of identical slip bands in each grain could be an evidence for the marginal effect of the grain size on the overall strength development of the alloy. Consequently, among other effects, the combinational dominant effect of SDAS and modest effect of grain size shall be considered for modification of the Hall-Petch equation for precise prediction of mechanical properties of cast alloys.