"Study on Deformation Microstructure of Hot Compressed Ni-30Fe Alloy"

超鉄鋼研究センター　冶金グループ　曹財榮（Jae-Young Cho）

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The key element in the method of heavy deformation of austenite in non-recrystallized region is to increase the density of localized non-homogeneous deformation defects within austenite grains. Since Charnock and Nutting reported the stacking fault energy across iron-nickel system, there have been a number of investigations into the deformation structures of austenite using austenitic Ni-30Fe alloy which has a stacking fault energy and deformation characteristics similar to those of low carbon austenite.

In general, the subdivision of grains during deformation takes place macroscopically with the formation of deformation bands and on a microscopic scale with the formation of cell blocks and cells. The deformation structures may differ considerably according to processing variables such as strain, strain rate and deformation temperature and material variables such as stacking fault energy and grain size.
Furthermore, considering the crystallographic nature of plastic deformation, the development of the deformation microstructure can be greatly affected by the original grain orientation relative to the imposed deformation conditions.

The investigation of identical grains before and after hot deformation made it possible to trace the evolution of microstructure deformed up to 50% at a strain rate of 1/s in a single pass at 1023K on a macroscopic scale using a deformation simulator (Gleeble 2000). In this study, the positions in the range of 0-100 micrometers from the center corresponding to the compressive strain rage of 1.4-1.65 were chosen for investigating the effect of grain orientation on the evolution of the deformation microstructure after hot compression test.

The localized non-homogeneous deformation bands occurred depending on the initial orientations of austenite grains. On the contrary, the near {110}<001> grains has undergone fairly homogeneous deformation even after a high strain of 1.65, which means that it is desirable to avoid the Goss orientation grains for grain refinement through austenite-to-ferrite transformation in the low carbon steel.

Further study will be carried out on the area fraction of deformation bands according to strain ratio and the critical strain value for the occurrence of deformation bands.

Reference:

1. D.W.Suh, S.Torizuka, A.Ohmori, T.Inoue and K.Nagai: ISIJ Int., 42 (2002), 432.
   
   
2. T.Inoue, S.Torizuka, K.Nagai, K.Tsuzaki and T.Ohashi: Meter. Sci. Tech., 17 (2001), 1580.