Tensile deformation mechanism of directionally solidified Nb-Si based ultrahigh-temperature alloy at 1250-1500 °C

With the development of Nb-Si based ultrahigh-temperature alloys, understanding their tensile deformation has become necessary for their practical applications. This study examined the high-temperature tensile deformation mechanisms of an optimized directionally solidified (DS) Nb-22Ti-15Si-4Cr-4Zr-3Al alloy, which exhibits excellent strength retention up to 1500 °C. It reveals the multiple factors in the DS lamellar-rod-like γ-Nb₅Si₃/Nb_SS eutectic structure that contribute to the outstanding high-temperature strength. The <0001>-oriented γ-Nb₅Si₃ exhibits high deformation resistance, with a predominant slip mode of {10 1‾ 0}<0001>, along with {112‾ 2}<112‾ 3> slip (1250 °C and 1400 °C) or potential {10 1‾ 0}<112‾ 0> slip (1500 °C). Simultaneously, Nb_SS is strengthened through the widespread formation of 1/2<111> dislocation networks and immovable <001> dislocations during multiple-slip. With increasing temperature, multiple-slip in Nb_SS becomes more apparent, and the strengthening effect of precipitated secondary γ-Nb₅Si₃ becomes significant at 1500 °C. Moreover, the γ-Nb₅Si₃/Nb_SS interfaces maintain a strong bond up to 1500 °C, effectively blocking or pinning dislocations by forming three-dimensional networks in the DS structure. These phase interfaces also impede the propagation of cracks and force them to deflect or bridge, thereby benefiting the high-temperature tensile properties. Furthermore, the initiation of microcracks, as well as changes in the failure process at elevated temperatures, were also discussed.