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The sequence of fracture processes in hydrogen-decrepitated Nd16-xDyxFe76B8 alloy
Abstract
The course of the fracture processes during the hydrogenation of the Nd16-xDyxFe76B8 alloy (x = 0 to 3) was studied. Positive volume changes of hydrogenated Nd-rich phase cause intergranular cracks between the grains of the Nd2Fe14B matrix phase. Hydrigen easily penetrates through those cracks into the alloy itself and the hydrogenation of the matrix phase followed by transgranular fracture starts as well.
After analyzing the phenomena and processes of hydrogen embrittlement of NdFeB permanent magnets, RF magnetron sputtering was used to fabricate Al thin films and then oxidized to form the Al/Al2O3 composite films on the magnets as the hydrogen resistance coatings. SEM and EDS were used to examine the morphology and composition respectively. Hydrogen resistance performance was tested by exposing the magnets in 10 MPa hydrogen gas at room temperature. The results show that the magnets with 8 μm Al/Al2O3 coatings can withstand hydrogen of 10 MPa for 65 min without being embrittled into powder. The samples with and without hydrogen resistance coatings have almost the same magnetic properties.
A review is given on the basic properties of 2:14:1 hard magnetic phases as well as technological procedures used for manufacturing R-Fe-B and R-Fe-C permanent magnets. First, we analyse the phase diagrams, crystal structure of hard magnetic phases and composition ranges in which these phases form solid solutions, as a result of various types of substitution. Then, the magnetic properties of and -based compounds are described. The routes used for manufacturing R-Fe-B and R-Fe-C permanent magnets are detailed. The magnets' microstructure as well as their coercivities are described in correlation with composition and manufacturing routes. The properties of nanostructure magnets are then presented. Finally, the stability of Nd-Fe-B and Nd-Fe-C magnets in various working conditions is analysed.
The nature of hydrogen decrepitation when applied to a cast Nd-Fe-B permanent magnet alloy has been studied by following the microstructural changes on polished surfaces of the material exposed to hydrogen at a pressure of 4 bar. The milling of the material decrepitated at a pressure of 10 bar has also been studied by determining the particle size as a function of milling time. The milled particles have also been examined by scanning electron microscopy.It is concluded that preferential swelling of the neodymium-rich phase occurs and that this gives rise to 1.(i) shear stresses leading to fracture at the interface between the particles and the matrix, and2.(ii) cracks in the iron-rich matrix phase which show 90° intersections characteristic of tensile stress relief.
In this article, the application of the hydrogen decrepitation (HD) process to the production of rare earth containing permanent magnets is discussed. It is shown that the HD process can be applied successfully to the production of SmCo5−, Sm2(Co, Fe, Cu, Zr)17− and Nd2Fe14B-type magnets. The particular HD conditions for each category of permanent magnet are described. The characteristics of the HD powder of all three alloy types are compared with those of the normal milled material and it is seen that significant advantages accrue from the use of the HD powder.