A Mossbauer study of dilute tin-lead alloys
Abstract
Mossbauer Spectroscopy has been performed using the $\sp{119}$Sn resonance on a variety of low concentration tin in lead samples, ranging from 1.3 at.% to 18.5 at.% tin in lead. We observe three basic resonances, which we identify as dispersed tin in lead (I), precipitated $\beta$ (white) tin in lead (II), and precipitated $\alpha$ (gray) tin in lead (III), where the latter is only observed below 286K. Contrary to earlier published results that reported an anomalous fall-off in the elastic fraction for dispersed tin in lead, we find only the typical temperature dependence. The Mossbauer temperature is found to be 104K for the dispersed state, as found from a Debye model analysis based on the transmission integral. Both this Mossbauer temperature and the isomer shift relative to a CaSnO$\sb3$ source agree with earlier published values within experimental error. For the precipitated state (II), it is found that in the initial stages of precipitation the isomer shift of the resonance is close to that of $\beta$-Sn, while the Mossbauer temperature is nearly that of the disperse state (I). As the sample is annealed or cold-worked, the precipitates grow and both the isomer shift and the characteristic Mossbauer temperature approach that of $\beta$-Sn, 140K. The precipitation-dissolution reaction is traced using a sample containing 18.5 at.% tin in lead by annealing at progressively higher temperatures and following changes in the isomer shift. Dissolution is found to begin as low as 360(5)K. A significant result that has not been reported by earlier investigators of this system is the observation using Mossbauer spectroscopy of the $\alpha$ phase of tin (III). Furthermore, the formation of this phase took place without the usual long incubation period required when a lead host matrix is not used. Moreover, a technique for curvefitting such Mossbauer spectra based on an analytic expansion of the transmission integral is demonstrated that isolates each component in the unresolved resonance. The errors inherent in using Lorentzian curvefitting techniques are demonstrated and the envelope of applicability of Lorentzian data fits is discussed. A correction to Lorentzian analysis is also demonstrated to give accurate Mossbauer temperatures if the thickness number is less than 5. We find that the Mossbauer Effect technique can be used for assessing the phase diagram in systems such as the Sn-Pb alloy, and our results indicate that the accepted phase diagram may need refinement.
Degree
Ph.D.
Advisors
Mullen, Purdue University.
Subject Area
Condensation|Materials science
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