The microstructural characterisation of Nd-Fe-B alloys. I: Light element microanalysis

Abstract

Analyses of oxygen and boron with spatial resolutions of a few μm have been carried out using wavelength dispersive X-ray spectrometry (WDX) in a SEM on an as-cast Nd 15Fe 77B 8 alloy (annealed at 1100°C for 1 h, followed by furnace cooling) and a laboratory produced [hydrogen decrepitation (HD) route] Nd 16Fe 76B 8 sintered magnet. The optimum experimental parameters including standards, accelerating voltage and analysing crystal were determined. Excellent apparent concentration reproducibility has been achieved. Area/peak factors (APFs) for boron K for the Nd 2Fe 14B matrix phase and Nd (1+ϵ)Fe 4B 4(ϵ∼0.1) boron-rich phase have been determined with a Pb stearate crystal on a JEOL 840A SEM relative to a pure boron standard. The integral intensity ratios were corrected using both the commercial LINK on-line ZAF-4 (FRAME-C) programme and the COR-2 programme. Probably the best accuracy of boron measurements has been achieved by use of APFs and the COR-2 programme (equipped with Heinrich's mass absorption coefficients). In contrast, the boron results (pure boron standard) from the on-line ZAF-4 programme are much higher than the reported stoichiometric concentration of the matrix and the boron-rich phase, although this has been improved by the use of an FeB standard. The mass absorption coefficient for oxygen absorbed by Nd derived from pure Nd 2O 3 powder using the convergent beam technique (for thickness determination) and ultra-thin window EDX on a TEM is reasonably close to Heinrich's and Henke's values. The results show that the triple junction regions and the Nd 2Fe 14B-Nd 2Fe 14B grain boundaries contain the Nd-rich phase with Nd: ∼90 at%, Fe: ∼1 at%, oxygen: 8–9 at% and boron: <1 at%. Auger spectroscopy results show that the oxygen present in the matrix phase and in the boron-rich phase exists only in the surface layer. By contrast the oxygen reacted with the Nd-rich boundary phase to form a bulk oxide.