Dislocation trapping of positrons
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requires that one avoid the intrusion of extraneous phases during thermal cycling. The fundamental reasons for this are still being sorted out, but the rough qualitative reasons seem straightforward: the success of the grain refinement treatment is based on control of the a-'/-a reversion cycle and this control is vitiated if the alloy contains a significant admixture of either e-martensite, which easily reverts to austenite at relatively low temperature, or retained austenite which does not retransform at all. It is difficult to be specific about the source of the intruding phases in their case, but the relatively high interstitial and residual contents in their alloy would certainly promote the intrusion of these phases, and this tendency would be aggrevated by the slower cooling rates they employed. To us, the most interesting and disturbing feature of the 8Mn work reported in the paper under discussion was the relative ineffectiveness of a final intercritical tempering in suppressing the ductile-brittle transition temperature, as compared with its benefit in Fe-Ni alloys of similar substructure. 3,4 As we briefly mentioned in the paper, an admixtue of E-martensite phase is found in the alloy after this low temperature heat treatment and was suspected as a potential source of a problem. That observation suggests the alloy design strategy of lowering the manganese content so as to further stabilize the a' martensite structure. This was done in recent work in this laboratory by Niikura and Morris 5 with very promising results; an excellent combination of strength and toughness at liquid nitrogen temperature was obtained with an Fe-5Mn-0.04C alloy in the 2B T condition. Further work on the 8Mn alloy done as a part of that investigation yielded additional evidence that the c-phase intrusion during tempering of the alloy inhibits the achievement of exceptional cryogenic structural properties. 1. S. K. Hwang and J. W. Morris, Jr.: Met. Trans. A, 1979, vol. 10A, p. 545. 2. S. Jin, J. W. Morris, Jr., and V. F. Zackay: Met. Trans. A, 1975, vol. 6A, p. 141. 3. S. Jin, S. K. Hwang, andJ. W. Morris, Jr.: Met. Trans. A, 1975, vol. 6A, p. 1721. 4. C. K. Syn, S. Jin, and J. W. Morris, Jr.: Met. Trans. A, 1976, vol. 7A, p. 1827. 5. M. Niikura and J. W. Morris, Jr.: Met. Trans. A, 1980, vol. l l A , p. 1531.
Dislocation Trapping of Positrons J. G. B Y R N E A N D M. L. J O H N S O N Earlier Johnson e t a l 1 showed that dislocations formed during the gradual plastic bending of oriented single crystals of Zn and Cu effectively trapped positrons causing the Doppler spectrum to sharpen as would be expected. This note is being written to clarify J. G. BYRNE is with the Department of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84112. M. L. JOHNSON is with Smith & Wesson, 2100 Roosevelt Ave., Springfield, MA 01101. Manuscript submitted February 6, 1980.
ISSN 0360-2133/80/0911-1627500.75/0 METALLURGICAL TRANSACTIONS A 9 1980 AMERICAN SOCIETY FOR METALS AND VOLUME 11A, SEPTEMBER 1980--1627 THE METALLURGIC
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