Microstructural investigation of Nd-rich phase in sintered Nd-Fe-B magnets through electron microscopy

The distribution, morphologies and structures of intergranular Nd-rich phase in sintered Nd-Fe-B magnets were studied through electron microscopy. Backscattered electron (BSE) imaging revealed that Nd-rich particles with various morphologies and sizes were randomly distributed at the grain boundaries and the triple junctions of the tetragonal Nd2Fe14B matrix. Through selected area electron diffraction (SAED) analysis under a systematic tilting condition, most intergranular Nd-rich phase particles, with sizes ranging from hundreds of nanometres to several micrometres, were identified as face-centred cubic (FCC) structure. Such particles possessed several approximate orientation relationships with their adjacent Nd2Fe14B matrix grains, such as (002)Nd2Fe14B//(200)FCC_Nd-rich [120]Nd2Fe14B//[001]FCC_Nd-rich, (002)Nd2Fe14B//( 22¯0)FCC_Nd-rich [110]Nd2Fe14B//[112]FCC_Nd-rich, as well as ( 011¯)Nd2Fe14B// ( 131¯)FCC_Nd-rich [111]Nd2Fe14B//[114]FCC_Nd-rich, which could be attributed to minimising interfacial energy. The combination of high-resolution electron microscopy with energy-dispersive X-ray spectroscopy revealed the internal inhomogeneous nature of Nd-rich phases. The large lattice distortion and nanoscale-ordered structures within a single Nd-rich grain were observed.

Graphical abstract(a) An HREM image of Nd-rich phase grain viewed along the [001] zone-axis; (b) The noise-filtered image; (c, d) Intensity profiles along the AB and CD lines, respectively, as designated in (b)Figure optionsDownload full-size imageDownload as PowerPoint slide