Relationship between austenite dislocation density introduced during thermal cycling and M s temperature in an Fe-17 wt
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THE thermal cycling of Fe-Mn–based alloys that exhibit a ␥ (fcc) } (hcp) martensitic transformation has been studied extensively to improve the understanding of shapememory effects (SMEs).[1,2,3] The studies showed a decrease in the martensite start temperature (Ms) of the alloys after thermal cycling. Tsuzaki et al.[1] reported that the austenite hardness of an Fe-25 wt pct Mn alloy, which was measured at 573 K, increased with the number of thermal cycles between 273 and 573 K due to the dislocations introduced into austenite during the reversible ␥ } martensitic transformation. This experimental result enabled them to explain, at least qualitatively, that the decreases in both the amount of martensite at room temperature and the Ms temperature with thermal cycling are attributed to the hardened austenite. Jin and Choi[2] also examined the change in austenite hardness at 623 K with the number of thermal cycles between 303 and 573 K in an Fe-17 wt pct Mn alloy. The objective of their study was to obtain a better understanding of the effect of thermal cycling on the SME in Fe-Mn binary alloys, which are mother alloys for Fe-Mn–based shape-memory alloys. They found that the austenite hardness at 623 K increased rapidly for up to five thermal cycles between 303 and 573 K and increased gradually with larger numbers of thermal cycles. This indicates that the dislocation density in austenite increased fastest during the first five thermal cycles. Tsuzaki et al.[1] and Jin and Choi[2] directly confirmed the dislocation forests in austenite after thermal cycling using a transmission electron microscope. Therefore, the decrease in Ms temperature during thermal cycling in Fe-Mn–based alloys has been explained qualitatively by the increase in shear stress for the ␥ → martensitic transformation, due YOUNG-KOOK LEE, Assistant Professor, is with the Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Korea. Contact e-mail: [email protected] Manuscript submitted November 10, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
to dislocations introduced into austenite during the reversible ␥ } martensitic transformation. The present study was performed to quantitatively investigate the changes in austenite dislocation density and Ms temperature with thermal cycling in an Fe-17 wt pct Mn alloy, based on a nucleation model of martensite reported by Olson and Cohen[4] and a thermodynamic model for a martensitic transformation reported by Ishida.[5] II. EXPERIMENTAL An Fe-17 wt pct Mn alloy was produced by melting pure electrolytic Fe and Mn in air using a high-frequency induction furnace. The alloy contained approximately 0.02 wt pct C and 0.015 wt pct N as impurities. The ingot of about 5 kg was homogenized in a protective Ar atmosphere at 1473 K for 24 hours and hot rolled to sheets with a 1.3 mm thickness. Specimens for dilatational measurements (5 ⫻ 1 ⫻ 10 mm) were prepared from the sheets and solution treated at 1323 K for 1 hour, followed by water quenching. After quenching, the austenit
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