Ffects of heat treatment on the magnetic properties of Nd-Fe-B based permanent magnets

Dependence of coercive force of (Nd,Dy)-Fe-B magnets on annealing time, temperature and the order of annealing step were investigated in detail. The magnets were prepared using two-powder blending method with addition of Dy-Al type alloy as a separate powder. The three schemes of heat treatment were used: HT1 - cooling at 925 to 20degreesC before annealing, HT2 - cooling at 925degreesC to annealing temperature and CHT - multiple step cycling heat treatment. It was determined that: (i) the time dependences of H. at different temperatures of HT1 are typical for ageing of solid solution. (ii) The alternation of HT at 925degreesC and at 550degreesC causes the reversible changes of H-ci. (iii) The annealing temperature dependences of H-ci corresponding to HT1 and HT2 schemes are great different. The analysis of these dependences revealed four phase transformations in microstructure of magnets in the range of 430 - 470degreesC, around 535, 590 and 640degreesC, to which the maximum of H-ci are corresponded. The relationship between annealing behavior of H-ci and hypothetic scheme of metastable Fe-Nd diagram was carried out, In accordance with this scheme below eutectic reaction at 640degreesC (Ldouble left arrowdouble right arrowA+Nd) the series of consecutive eutectoid decompositions (Adouble left arrowdouble right arrowF(i)+Nd, F(i)double left arrowdouble right arrowF(i+1)+Nd...) take place. It is assumed, that the origin of F-i phases occurs preferentially at the surface of neighboring Nd2Fe14B grains. The size and number of initial precipitates are determined by temperature of annealing. The separate large precipitates form "roughness" on the surfaces of Nd2Fe14B grains and decrease H-ci. At the contrary, continuous thin layers make "smoothing" better and increase H-ci. It is assumed also that a possible origin of "annealing hardening" and of high coercivity might be both the "smoothing" and higher concentration of Dy in F-i phases.