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 investigated to ascertain their effects on room-temperature magnetic properties, with the ultimate goal of understanding, controlling and optimizing 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. Differential 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 annealed samples consist of a phase isostructural with Nd2Fe14B (denoted Nd2M14B or 2-14-1) and a bce Fe-rich phase (denoted bcc-(FeCoCr)). The 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 phases found in the annealed samples. Further Cr additions cause a sharp drop in the lattice parameters. The calculated average grain sizes in the annealed samples exhibit an abrupt increase for larger chromium contents, and the grain size of the Nd2M14B phase is larger than that of the 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 increasing Cr content. Thermal analysis and laboratory X-ray diffraction experiments performed on selected annealed samples suggest that the enrichment of Fe in the nanocomposite alloy allows the system to access the metastable non-stoichiometric Nd2M17-type phase prior to transformation to the 2-14-1 phase. Increased Cr content in the starting alloy stabilizes the amorphous-->Nd2M17-type (metastable) phase transition and the formation of the Nd2M14B phase, shifting them to higher temperatures. The transformation behavior of the alloys as a function of Cr content ultimately controls the phase constitution, phase chemistry and the grain size, which in turn control the room-temperature hysteretic magnetic properties. (C) 1998 Elsevier Science S.A.