The effect of grain-boundary “shells” on the mechanical properties of an Al alloy

In this paper, the grain boundary (GB) wetting phase transition in aluminum-based 1163 alloy is studied. The transition from incomplete GB wetting to complete one occurs in the temperature range from 520 to 600°C. Below 520°C, the melt enriched in copper and magnesium forms separate lenticular droplets at the GBs. Above 520°C, the melt forms continuous layers along almost all boundaries between aluminum grains. In such a way, the melt forms a kind of liquid shell around each matrix grain. During quenching after annealing, the grain boundary “shells” solidify and consist mainly of the ternary intermetallic compound Al2CuMg. Previously, we observed that such GB “shells” of the second phase can significantly change, for example, the electrical or magnetic properties of a polycrystal. In this paper, we found for the first time that a change in the fraction of completely wetted GBs can also affect the mechanical properties of the material after quenching. In particular, in samples annealed below 520°C, when there are no completely wetted GBs, the conditional yield strength σ0.2 does not change. Above 520°C, as the fraction of completely wetted GBs increases, the σ0.2 value rapidly decreases by about one and a half times to 600°C. At the same time, the Young’s modulus, which is determined mainly by the bulk properties of aluminum grains, remains virtually unchanged with increasing temperature and increasing fraction of completely wetted GBs. Thus, the effect we have discovered provides materials scientists with a fundamentally new tool for purposeful modification of mechanical properties of various polycrystalline materials.

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