Precipitates and Grain Boundary Strength of an Fe-Mn-Ni Alloy
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Fe-Mn-Ni alloys show an attractive age-hardening behavior and their hardness values can reach those of the conventional 18Ni maraging steels.[1,2] Precipitation of h-MnNi intermetallic phase in the martensite (a¢) matrix was reported to be the origin of age hardening.[3,4] In spite of their excellent hardening properties, they experience serious grain boundary (GB) embrittlement even after a very short aging treatment. Premature intergranular brittle fracture occurs. GB strength recovers after prolonged aging. Many investigations were made to understand the cause of the GB embrittlement and de-embrittlement observed in Fe-Mn-Ni alloys during aging.[1,2,4–13] Squire et al.[8,9] suggested Mn segregation at prior austenite GBs (PAGBs) for the reason of embrittlement, and Heo and Lee[10] and Heo[11] observed co-segregation of Mn and Ni at GBs. Lee et al.[12] confirmed the presence of h-MnNi precipitates at PAGBs and lath boundaries and, based on the observation of precipitates on fracture facets, suggested that the decohesion of the interface of GB h-MnNi precipitates and a¢ matrix is the origin of the GB embrittlement.[1,7] Recently, Nedjad et al.[5,6] observed planar slip in the matrix due to the presence of precipitates in the peak-aged condition and proposed the inhomogeneous deformation in the precipitate-free
YOON-UK HEO, Research Assistant Professor, JAE HOON JANG, Graduate Student, and HU-CHUL LEE, Professor, are with theGraduate Institute for Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, Korea. Contact e-mail: [email protected] Manuscript submitted November 9, 2011. Article published online September 13, 2012 3940—VOLUME 43A, NOVEMBER 2012
zone (PFZ) at near GBs as the reason for GB embrittlement. h-MnNi phase is not an equilibrium phase at the aging temperature range of 623 K to 823 K (350 C to 550 C),[3,16,17] and the metastable h phase will transform to stable austenite (c) after prolonged aging or by aging at higher temperature. If the GB embrittlement was caused by the presence of h-MnNi precipitates at GBs, the GB strength will be recovered after the transformation of GB h precipitates to c. The tensile fracture strength of Fe-Mn-Ni alloys actually recovers with the aging time after peak aging treatment.[10] Gradual substitution of GB h precipitates to c is expected in this tempering range. This proposition can be tested by double heat treatment experiments. The main idea is that the alloys are peak hardened, that is, severely embrittled, by the first step of aging at lower temperature, and then the GB h precipitates converted to c phase by the second aging at higher temperature for a very short time to minimize the structural changes in the matrix or at PFZ. The effect of GB precipitates on the GB strength can be demonstrated by tensile tests. In this article, we attempted to demonstrate the importance of GB precipitates on the GB strength of Fe-Mn-Ni alloys. The GB h-MnNi precipitates were converted to austenite by a short second aging at higher temperature [793 K
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