Low temperature aging of the freshly formed martensite in an Fe-Ni-C alloy
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INTRODUCTION
I T has been well substantiated that in certain steels freshly formed martensite exhibits anomalous axial ratio of tetragonality (c/a ratio) at low temperatures. In manganese ~ and rhenium 2 steels as well as in high carbon plain steels, 3 the c/a ratio of the freshly formed martensite is considerably lower than the value of the usual steel martensite with the corresponding carbon content. On the other hand, in nickeP and aluminum 5 steels, the c/a ratio is much higher than in the usual steels. When these freshly formed martensites are heated to room temperature, the c/a ratio changes approaching the value for the usual martensite, but does not reach it. These facts indicate that the usual martensite with a normal c/a ratio is likely to be in more or less tempered state and that freshly formed martensite at low temperatures should be taken as a product of truly diffusionless reaction, i.e., an ideal martensitic reaction product. Thus, the structure of fresh martensite and the associated carbon occupation sites are of great importance in elucidating the mechanism of martensitic transformation. On the basis of numerous investigations both in experiment and theory, several models have been put forward. 6-~3 Kurdjumov 6 suggested that the freshly formed martensite with a high c/a ratio has all the carbon atoms in the octahedral interstitial sites along c axis (Oc sites), while that with a low c/a ratio has the carbon atoms distributed over both Oc and Ob sites as a result of (011)M twins. 7,s When these martensites are heated to room temperature, the carbon atoms redistribute over On, Oh, and Oc sites to attain partially ordered state, which was assumed to be the state of the usual martensite. Theoretical consideration has been also given for this order-disorder transition. 8.9 Lysak and his co-workers ~~ and Fujita 12'~3suggested that the freshly formed martensite with a low c/a ratio has half the carbon atoms in the tetrahedral interstitial sites (T sites) and the remaining half in O,, sites. Different mechanisms to lead such carbon distribution have been proposed by these authors. On heating this martensite to room temperature, the M. HAYAKAWA, Assistant Professor, and M. OKA, Professor, are with the Department of Mechanical Engineering, Tottori University, Tottori, Japan. M. TANIGAMI, formerly Graduate Student, Tottori University, is now with Omron Tateishi Electronics Company, Kyoto, Japan. Manuscript submitted December 26, 1984. METALLURGICALTRANSACTIONS A
T-sited carbon atoms were considered to move into Oc sites approaching the state of the usual martensite. The high tetragonality in the freshly formed nickel and aluminum martensite was hypothesized by Lysak et al. ~4to be due to clusters present in these particular alloy systems in addition to the usual contribution by Oc-sited carbon atoms. A few alloy systems, e.g., Fe-Ni-A1 and Fe-Ni-Ti, 15-~8are known to result in tetragonal martensite due to such clusters; however, no evidence of such clusters present in an Fe-Ni-C alloy has been report
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