Multi-Step Internal Nitriding of Molybdenum-Zirconium Alloys

In order to overcome the problem of recrystallization embrittlement of molybdenum, internally nitrided dilute Mo-Zr alloys with a heavily deformed microstructure near their surface were prepared by a novel method involving three-step nitriding at 1000 to 1600 degrees C in N-2 gas. The grain structure was investigated after each nitriding step, and the mechanical properties of the final specimens were then evaluated. The first nitriding step was carried out below the recrystallization temperature for the alloys, and resulted in a uniform dispersion of ultrafine Zr-nitride precipitates within the unrecrystallized molybdenum matrix. After the second and third nitriding steps, these precipitates grew into disk-shaped particles with a diameter of 20 to 50 nm, and acted as pinning sites for grain boundary migration in the matrix, thus maintaining its deformed microstructure. The recrystallization temperature for specimens subjected to three-step nitriding was increased to above 1700 degrees C. This is 100 degrees C higher than that for a Mo-Ti alloy subjected to almost the same multi-step internal nitriding procedure. A nitrided Mo-1.0 wt%Zr alloy exhibited high ductility even at about - 40 degrees C, and its yield strength at 1500 degrees C was about 2 times higher than that of a nitrided Mo-0.5 wt%Ti alloy.