CRYSTALLIZATION SEQUENCES AND MAGNETIC-PROPERTIES OF MELT-SPUN ND2FE14B-BASED NANOCOMPOSITES CONTAINING CO AND CR

The physical, chemical and microstructural characteristics of the two- phase nanocrystalline alloy Nd-2[Co-0.06(Fe1-xCrx)(0.94)](23.2)B-1.48 (0 less than or equal to x<0.9) made by melt-spinning methods were inv estigated to ascertain their effects on room-temperature magnetic prop erties, with the ultimate goal of understanding, controlling and optim izing the alloys' coercivity and exchange-enhanced remanence. Detailed investigations were made into the phase content, character and grain size of the alloys using synchrotron X-ray diffraction. The technical magnetic properties were studied with SQUID and VSM magnetometry. Diff erential thermal analysis was performed to monitor the crystallization sequences of the as-quenched alloys as a function of composition, as well as to identify the Curie temperatures of annealed alloys. Most an nealed samples consist of a phase isostructural with Nd2Fe14B (denoted Nd2M14B or 2-14-1) and a bce Fe-rich phase (denoted bcc-(FeCoCr)). Th e replacement of Fe by Cr (up to Cr content x similar to 0.03) causes an initial increase in the lattice parameters of the constituent phase s found in the annealed samples. Further Cr additions cause a sharp dr op in the lattice parameters. The calculated average grain sizes in th e annealed samples exhibit an abrupt increase for larger chromium cont ents, and the grain size of the Nd2M14B phase is larger than that of t he bcc-(FeCoCr) phase for all samples studied. The Curie temperatures of the 2-14-1- and bce Fe-type phases are found to decrease with incre asing Cr content. Thermal analysis and laboratory X-ray diffraction ex periments performed on selected annealed samples suggest that the enri chment of Fe in the nanocomposite alloy allows the system to access th e metastable non-stoichiometric Nd2M17-type phase prior to transformat ion to the 2-14-1 phase. Increased Cr content in the starting alloy st abilizes the amorphous-->Nd2M17-type (metastable) phase transition and the formation of the Nd2M14B phase, shifting them to higher temperatu res. The transformation behavior of the alloys as a function of Cr con tent ultimately controls the phase constitution, phase chemistry and t he grain size, which in turn control the room-temperature hysteretic m agnetic properties. (C) 1998 Elsevier Science S.A.