High friction stress and weak grain-refinement strengthening effect induced by severe lattice distortion in ZrTiNb medium entropy alloys

In this work, we have studied the grain-refinement strengthening effect of body-centered cubic Zr50Ti35Nb15 medium entropy alloy. The results suggest that the yield strength increases as grain size decreases from 530 μm to 0.51 μm, reaching the maximum value of 651 MPa, and still having decent ductility. Moreover, it is found that the alloys exhibit a high Hall–Petch intercept of 564 MPa and a low Hall–Petch slope, which means strong lattice friction stress and weak grain-refinement strengthening. The high lattice friction stress originates from the obstruction of dislocation motion caused by atomic size and modulus misfit. Dislocation interaction and multiplication in grain interior is difficult due to the high dislocation slip resistance, leading to a weak dislocation pile-up capability at grain boundary. Instead, dislocations nucleation and emission become the dominant deformation process since low shear modulus promotes dislocation source activation.