Transformational Plasticity in Bi203 37 6o0 -\ n I 200 1 1 1 1 1 1 0 20 40 60 80 I 0 0 I 2 0 140 160 180 200 MELTING TIME (HOURS) Fig. 5. Liquid nitrogen strength curves of pristine 0.5Liz0~0.5K20~2SiO2 glasses melted at 1040°C in a resistance furnace (0) bubbled with dry Ar and (A) under dry Ar.alkali carbonates and Si02 will be comprised initially of Li-rich and K-rich regions. With increasing melting time, these regions gradually interdiffuse, leading to a more homogeneous glass. Bubbling promotes more rapid homogenization of the melt than is possible by normal mixing by convection and diffusion. The catastrophic decrease in strength after extended melting of the glass bubbled with Ar is believed to be caused by Pt contamination from the crucible.Contamination of glass by colloidal Pt has been shown' to be more detrimental to glass strength than contamination by ionic Pt. Bubbling with O2 would inhibit the formation of colloidal Pt, but Ar bubbling would not; hence, the strength decreases.
ConclusionsMelting history affects glass strength even when different starting materials are used and when gases are bubbled through the melt, although the magnitude of the strength and the rate at which maximum strengths are reached may vary. In all cases, the dominating effect is the presence of microheterogeneities introduced by melting reactions, R contamination, or phase separation.Although stirring by the bubbling of gases through the melt decreases inhomogeneity arising from melting reactions, the nature of the gas ultimately determines the strength of the resulting glass. Oxygen diminishes contamination by colloidal Pt but, in the event of phase separation, diminishes the sizes of the separated phases, as discussed in Ref. 1.
ReferencesJames F. Sproull and Guy E. Rindone. "Effect of Melting History on the Mech-M. S. Maklad and N.Transformational plasticity associated with the monoclinic-tocubic phase transition at 730°C in Bi,O, was observed and characterized. This phenomenon is explained in terms of Greenwood and Johnson's model of internal stress-induced deformation, proceeding, in this instance, by a time-dependent, grain-size-sensitive creep mechanism, probably grainboundary sliding. Criteria are proposed for choosing other prospective transformationally plastic ceramics; they are met by Bi,WO,, which also exhibits extensive transformation plasticity.
Transformational superplasticity was studied in the compounds Bi,WO, and Bi,MoO,. The magnitudes of transitional strain are related to the (T,/T,,)'s of the phase transitions and are proportional to the externally applied stresses. Strain-rate sensitivities were similar, 0.85 and 0.86; however, the Bi,WO, exhibited a strain-axis intercept and the Bi,Mo06 a stress-axis intercept. The grain-size effect present in the Bi2WO6 supports an accommodated grain-boundary sliding mechanism for the superplastic deformation process.
Transformational superplasticity associated with the eutectoid reaction in the Bi203-Sm203 system was observed and characterized in hypoeutectoid and hypereutectoid compositions. Transformational strain varied linearly with applied stress and exhibited a stress-axis intercept, or threshold stress, that is related to the proeutectgid microconstituent. Results are explained quantitatively using the analysis by Greenwood and Johnson and a creep mechanism.
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