Oxygen diffusion in grain boundaries: a ToF-SIMS investigation on hot-rolled steel sheets

Abstract

A study of grain boundary diffusion of oxygen in hot-rolled steel sheets is carried out by means of time-of-flight secondary-ion-mass-spectrometry (ToF-SIMS). This involves polishing of the sample surface prior to the oxygen exposure. A nickel layer deposited after exposure ensures a homogeneous extraction field for ToF-SIMS measurements at the Ni-steel interface. The sample is bevelled at an angle of 11.5° to spread up the diffusion pathway by a factor of 5. The oxygen distribution is then acquired via ToF-SIMS in imaging mode from which diffusion parameters are calculated according to the Whipple-Le Claire's approach.

Accurate and Precise Measurement of Oxygen Isotopic Fractions and Diffusion Profiles by Selective Attenuation of Secondary Ions (SASI)

“…We note that the most accurate and precise 18 O isotopic fraction is determined when high SASI is applied to the 16 O and the 18 O is not attenuated (A3), as the segment with low attenuation for 16 O (A2) still shows a slightly increased apparent 18 O fraction due to slight saturation of the detector (as observed for the corrected 16 O signal in comparison with A3 in Figure 2). Figure 3 also shows the apparent 18 O isotopic fractions determined using other primary ion source alignment modes typically applied to reduce the PI currents and avoid dead-time effects. The values for B1 and B2 were obtained using the conventional "burst mode", as described by De Souza et al 31 For B1, all eight bursts were used to determine C′ x ; this leads to a higher apparent value due to some saturation of the detector by the arrival of the first burst of 16 O ions (this is evident in the mass spectrum as the subsequent seven pulses were lower in intensity than the first one).…”

“…41,42 This will reduce significantly the analysis time to obtain a complete picture of all the diffusion and segregation processes taking place during the annealing and exchange. Figure 5 shows the 18 Figure 5b) is due to weaker MO − signals and poorer statistics when using lower PI current for burst BA mode. This effect is most significant for the SrO − profiles.…”

“…It would be possible to mitigate the saturation of 16 O which leads to the inaccurate value for C1 by further reducing the PI current (note we used a pulsed current of 0.068 pA@50 μs, which is at the low end of the range 0.03−0.05 pA @100 μs specified by Holzlechner et al). 32 However, decreasing the PI current further would decrease the precision of the measurement of 18 O SI intensity, adversely affecting the precision of the measured C1. Both BA and CBA modes avoid dead-time effects in the detector by decreasing the PI intensity, which enables the detection of the species with high ion yields and/or high concentration, such as 16 O, by globally decreasing the intensity of the spectrum.…”

“…Figure 2 illustrates the effects of different SASI levels applied in HCBM mode for the analysis of 16 O and 18 O isotopes in a LSM20 ceramic sample with natural isotopic abundance. The high PI current used in HCBM assures maximum beam stability while the SASI levels were changed during the analysis for each oxygen isotope.…”

“…After the equilibration annealing, the sample was quenched, and the tube was evacuated and refilled with 18 O enriched oxygen (1 bar, 18 O isotopic fraction 95%). The 18 O exchange was also performed at 1000°C for a total time of 30 min. The 18 O isotopic fraction in the gas phase was analyzed by leaking a small volume of the exchange tube atmosphere to a residual gas analyzer (M-070QA-TDF, Canon Anelva, Japan).…”