Evaluation of the effects of segregation on austenite grain boundary energy in Fe-C-X alloys
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I.
INTRODUCTION
IT generally has
been accepted that only two martensitic structures exist in the Ti-Nb alloy system. These structures, a ' and a", have been observed in several other titaniumbased transition element alloys] ~'21 The structure of a ' is hcp[t'3"41with lattice parameters identical to those of a-Ti. The a" structure is C-centered orthorhombic with a Cmcm space group. D'3-6] Hatt and Rivlin [6] and Pattanayak et al. L7] have reported the presence of a tetragonal phase in 25 and 21.7 at. pct Nb alloys, respectively; however, the identification of this phase as martensitic is uncertain. The a" structure may be viewed as being a transition from the hcp structure of a ' to the bcc structure of the/3 phase. The atom positions in a" are (0, 0, 0), (1/2, l / 2 , 0), (0, 1 2y, 1/2), and (1/2, 1/2 - 2y, 1/2) with y being approximately 0.2 in Ti-Nb alloys, t4'81 By varying b/a, c/a, and y, all three structures may be produced] 4~ For example, an ideal hcp structure is,obtained when y = 1/6, b/a = V'-3, c/a = V ~ . A bcc structure is obtained when y = 1/4, b/a = X/2, c/a = X/2-. A compilation of the existing lattice parameter data for the a ' and a " martensites is shown in Figures l(a), (b), and (c). These data indicate that the a'/a" "boundary" occurs at 7.2 at. pct Nb. A linear extrapolation of the data toward the bcc limit suggests that, at room temperature, the upper compositional boundary for a" should be 30.7 at. pct Nb. The data of Morniroli and Gantois t41for alloys of 25 to 28 at. pct Nb, however, do not follow this extrapolation. The Ms data for the Ti-Nb system are given in Figure 2. The calculation of Kaufman and Bernstein [9] of Tg ~" is also shown. Flower et al. [m] noted that since there were two martensites, there should exist two Ms curves, but the scatter in the data is too great to distinguish them. Extrapolation of the data indicates that alloys of 26, 30.5, and 32 at. pct Nb should have Ms temperatures of 273, 77, and 0 K, respectively. The upper compositional limit of a" as extrapolated
D.L. MOFFAT, formerly a Graduate Student in the Materials Science Program at the University of Wtsconsin-Madison, is a Research Associate in the Laboratory of Nuclear Studies, Comell University, Ithaca, NY 14853-5001. D.C. LARBALESTIER is a Professor in the Materials Science Program. the Department of Metallurgical Engineering, and the Applied Superconductivity Center at the University of Wisconsin-Madison, Madison, WI 53706 Manuscript submitted July 13, 1987. METALLURGICAL TRANSACTIONS A
from the lattice parameter data ( - 3 1 at. pct Nb) is in fair agreement with the composition having an estimated Ms of 0 K. However, the Ms extrapolation conflicts with the report that the 26.6 and 28.2 at. pct Nb alloys of Morniroli and Gantois were martensitic at room temperature. It is noteworthy that no experimental measurements of Ms below 200 ~ or for alloy compositions greater than 20 at. pct Nb have been reported. One reason given for this is that below 300 ~ the thermal arrest signal becomes too small to obse
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