PHASE-FORMATION IN MELT-SPUN ND-FE-MO-TI ALLOYS

We report for the first time the range of ThMn12-type crystal structur e formation in melt-spun Nd-Fe-Mo-Ti ribbons. These materials, which c ombine the melt-spun microstructure with the favorable intrinsic magne tic properties of the NdFe12-x(Mo,Ti)(x) phase, are important as precu rsors to the formation of hard magnetic materials by nitrogen absorpti on. Alloys with compositions Nd1.15Fe10+xMo2-2xTix (0 less than or equ al to x less than or equal to 1), which grade the composition smoothly between the two end point alloys Nd1.15Fe10Mo2 and Nd1.15Fe11Ti, were melt spun at quench wheel velocities 5 m/s less than or equal to v(s) less than or equal to 30 m/s. From x-ray diffraction patterns we cons truct a phase formation diagram as a function of Ti content x and whee l speed upsilon(s). We find that the desired ThMn12 structure is obtai ned only in ribbons quenched at low upsilon(s). At higher speeds, the ribbons quench instead into the disordered TbCu7-type crystal structur e. Further, the ThMn12 structure is considerably less stable at the Ti -rich end of the composition range than at the Mo-rich end: for Nd1.15 Fe10Mo2, the ThMn12 structure is obtained for upsilon(s) less than or equal to 17.5 m/s, whereas in Nd1.15Fe11Ti only ribbons quenched at up silon(s)=5 m/s are of the ThMn12 type. Heat treatments of Nd1.15Fe11Ti ribbons melt spun at 30 m/s confirm the relatively difficult formatio n of the ThMn12 structure type; annealing temperatures in excess of 10 00 degrees C are required to form the ThMn12 crystal structure.