Solid Solution Strengthening of the Body-Centered Cubic Phase in V-9Si-6.5B-xCr Alloys

Vanadium-silicide-based alloys are promising candidates for high-temperature applications, e.g., for use in turbines. Recently, we have observed that the addition of Cr to the V_ss-V₃Si-V₅SiB₂ ternary eutectic alloy, which might be beneficial for the oxidation resistance, replaced V up to 30 at.% without changing the ternary eutectic microstructure. In addition, the high-temperature compressive strength increased significantly [1], likely caused by solid solution strengthening of the body-centered cubic (bcc) matrix phase due to the Cr addition.<br/>To better understand the strengthening mechanisms and the effect of the Si and Cr additions, we performed nanoindentation tests in the bcc phase of arc-melted and annealed V-9Si-6.5B-xCr (x = 0, 5, 10, 20, 30 at.%) alloys to determine the hardness. A detailed microstructural analysis was conducted to determine the composition of the phases. Furthermore, the strengthening effect was evaluated using the Maresca-Curtin model of screw dislocation strengthening [2]. The measured variation in hardness showed a significant dependence on the bcc phase composition. Si increased the hardness regardless of the Cr content. Similarly, regardless of the Si content, as the Cr content increased from about 7 at.%, the hardness increased and reached its limit at around 30 at.% Cr, then increased slowly until 40 at.% Cr at least.<br/>Our findings can be well described by the Maresca-Curtin model. Assuming that the solid solution hardening contribution of Si in V is the same as in Cr, we can predict the yield stress of the bcc phase composed of V, Si and Cr elements as a function of strain rate, temperature and composition. This good agreement suggests that both Si and Cr contribute to the solid solution strengthening in a way that is superimposable on each other.<br/><b>References</b><br/>[1] G. Hasemann, L. Thielemann, W. Yang, S. Ida, R. Schwaiger, K. Yoshimi, M. Krüger, Influence of Cr additions on the microstructure and compression behavior of ternary eutectic V-Si-B alloys, submitted.<br/>[2] F. Maresca, W.A. Curtin, Theory of screw dislocation strengthening in random BCC alloys from dilute to “High-Entropy” alloys, Acta Materialia 182 (2020) 144–162. https://doi.org/10.1016/j.actamat.2019.10.007.