Alloy partitioning in Ti-24Al-11 Nb by analytical electron microscopy
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were collected at an effective take-off angle of 35 deg, and the acceleration voltage employed was 300 kV. A beam diameter of approximately 350 A was used. A count rate of 2500 to 3000 CPS was found to be adequate to obtain a total number of counts of at least about 10,000 in all the peaks, thus maintaining a m a x i m u m 3o- error of -+6 pct. I4t K factors for A1, Ti (KAn-i) and Nb, Ti (KybTi) were determined by the procedure outlined by Cliff and Lorimer. tSl An alloy of composition Ti-15.1 pet Al-16 pct Nb (by weight) was used as a single-phase a2(Ti3A1) homogeneous standard for this purpose. X-ray peaks A1K~, T i K , , and NbK~ were used in the analysis. Correction Factors: Following Cliff and Lorimer, tS~ the K factors for various elements m a y be determined for a single-phase homogeneous standard from
K AB .
Ca IB . . . .
CB IA
KA "Ke
[1]
where CA and C8 are fractional concentrations of elements A and B, and IA and IB are X-ray peak intensities. The terms KA and KF arise out of absorption and fluorescence phenomena and are close to unity in most cases K. M U R A L E E D H A R A N and D. BANERJEE, Scientists, are with the Defence Metallurgical Research Laboratory, P.O., Kanchanbagh, Hyderabad-500 258, India. Manuscript submitted June 6, 1988. METALLURGICAL TRANSACTIONS A
in the thin film approximation. Nevertheless, these corrections require an assessment for the specific system under consideration. Table III shows percent errors in K A m and KNbTi , if the absorption correction (KA) is neglected, as a function of specimen thickness and the Nb content in a Ti-25A1 base composition. The correction factor, KA (expressed as a percent error in Table III), was determined from the standard equation given, for instance, in Reference 4. The error in K factor will directly reflect in the chemistry determination (Eq. [1]) and, hence, can be considered as an error in the chemistry itself. It can be seen clearly from Table III that: (1) A1K~ lines are strongly absorbed, and this can lead to errors in the microanalysis. Williams t41 describes an error of 3 pct in K factor as admissible, ~ in Table III, even for a specimen thickness of 500 A, errors in KAITi are greater than 3 pct. Hence, the thin film approximation will not be valid for A1 analysis. Further, the acquisition time for spectra will be very high for specimen thicknesses less than 5 0 0 , ~ to get at least about 10,000 counts in the NbK~ peak. (2) There is no significant absorption effect o n KNbTi up to a thickness of 1500 A or so. The fluorescence correction, KF, is neglected since the Ti-A1-Nb alloys under consideration do not have strongly fluorescing systems. A strongly fluorescing system should have one element with an excitation energy just less than that of the other's characteristic X-ray energy. Thickness Determination: It is clear from the earlier discussions that absorption corrections need to be applied in the microanalysis o f the Ti-A1-Nb system, as absorption of A1K~ lines within the specimen can lead to considerable errors when specime
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