The effect of ferroelastic domains and ultrahigh‐density dislocations on fracture toughness of high entropy niobates

This study investigates the fracture toughness and toughening mechanism of (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4, a high entropy material characterized by a high thermal expansion coefficient and low thermal conductivity. Our finding demonstrates a noteworthy 50% increase in fracture toughness for (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4 (∼3.09 MPa m0.5) compared to YNbO4 (∼2.11 MPa m0.5). The superior fracture toughness for (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4 can be attributed to the two key factors: relatively easy ferroelastic domain switching and the presence of ultrahigh‐density dislocations (∼7 × 108 mm−2). The enhanced ferroelastic toughening in (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4 is associated with its distinct characteristics, large atomic radius (RA), significant tetragonality (c/a ratio), and low monoclinic β angle. These factors contribute to larger spontaneous strain, larger coercive strain, and lower transformation strain, respectively. These properties facilitate the formation and switching of the ferroelastic domains. In addition, the interactions between the large strain field surrounding the dislocations and the cracks facilitate crack deflection and bridging. The combined effect of ferroelastic toughening and ultrahigh‐density dislocations ultimately leads to the significant improvement in the fracture toughness of (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4.