Effect of Zr substitution on microstructure and magnetic properties of YDy-based R ₂Fe ₁₄B magnets (R=Y+Dy+Nd)

The effect of Zr substitution on the microstructure and magnetic properties in [Nd0.5 (YDy)0.25] 2.2-x Zrx Co1.5 Fe12.5 B (x=0-0.7) ribbons melt spun at a wheel speed of 10 ms has been systematically studied. For as-spun Zr-free ribbon, a coercivity Hcj of 15.5 kOe is obtained but the demagnetization curve exhibits a large shoulder, resulting in a very low maximum energy product (BH)max (3 MGOe). With increasing Zr content (x), Hcj first decreases and then increases. When x=0.4, both Hcj and (BH)max reach their optimized values of 10.6 kOe and 9.6 MGOe, respectively. The Hcj and (BH)max for the sample at 200°C are 5 kOe and 4.3 MGOe, respectively. Transmission electron microscopy characterizations show that the average grain size is 200, 65, and 50 nm for x=0, 0.4, and 0.7, respectively, indicating that the substitution of Zr can effectively inhibit grain growth. However, an excessive substitution of Zr results in the appearance of the 2:17 phase and thus the reduction of magnetic properties.