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M: Metallphysik
M 4: Symposium Unterkühlte Metallschmelzen IV
M 4.6: Fachvortrag
Monday, March 26, 2001, 18:30–18:45, S8
Growth kinetics and microstructure morphology of Nd-Fe-B-Ti-C alloys in dependence on cooling parameters and composition — •Octavian Filip1, Regina Hermann2, Ingrid Bächer2, and Ludwig Schultz3 — 1Institute of Solid State and Materials Research (IFW) Dresden, 01069 Dresden, Helmholtzstrasse 20 — 2IFW Dresden — 3IFW) Dresden
The formation of dendritic α-Fe and grain growth during heat treatment is a crucial problem during industrial production of high performance magnets. The growth kinetics and the microstructure of stoichiometric Nd-Fe-B alloys with Ti and C additions (0.5 to 3 at%) were investigated using the electromagnetic levitation and the melt spinning techniques. In order to study nucleation and growth phenomena, low cooling rates (1 to 60 K/s during levitation) as well as high cooling rates (about 104 to 106 K/s) were applied. It was ascertained from in-situ-temperature-time-measurements that the growth velocity of the hard magnetic Nd2Fe14B-phase is drastically reduced by Ti and C additions. The formation of properitectic γ-Fe is diminished but can not be avoided during slow cooling. Higher cooling rates which appear during melt spinning lead to complete suppression of the α-Fe phase. Additionally, a noticeable grain refinement is obvious compared to the composition without additions. It could be shown that the Ti and C additions, which form stable TiC at about 200 K above the liquidus temperature, act as nucleants for the formation of hard magnetic Nd2Fe14B grains if the cooling rate exceeds a critical value.