Nonequilibrium grain-boundary segregation and ductile-brittle-ductile transition in Fe-Mn-Ni-Ti age-hardening alloy
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I.
INTRODUCTION
DURINGthe past several decades, impurities have been responsible for the brittleness of many materials. Temper embrittlementU-7] is representative of the brittleness in lowalloy steels and is caused by the segregation of impurities to the grain boundaries. Brittleness in coppel~8,9,~~ was due to the segregation of bismuth to the grain boundaries, which causes the decohesion of the grain boundaries. Tungsten was reported to be embrittled by impurities, u 1] Such studies were performed after the development of Auger electron spectroscopy (AES). Theoretical studies B243,141 were also performed on the grain-boundary segregation of impurities. On the other hand, manganese as an alloying element was reported to be the embrittler in a low-manganese alloy, t~s] and this was confirmed in other research.U2,~6] FeMn-Ni alloysB7,18,~91 were studied to investigate the possibility of partially replacing nickel with manganese in 18 pct nickel maraging steels. Feng et al. t2~ reported that an Fe-Mn-Ni alloy, hardened by face-centered-tetragonal (fct) Mn-Ni precipitates, was embrittled by manganese segregation to the grain boundaries during isothermal aging. Recently, Heo and Lee reported a ductile-brittle-ductile (DBD) transition in Fe-Mn-Ni alloys, [21,221 which is caused by manganese segregation to the grain boundaries and its desegregation into the matrix, and is deeply related to the precipitation reaction in the matrix. On the basis of a regular solution model, Heo t23,241analyzed the nonequilibrium grainboundary segregation behaviors of alloying elements in the Fe-Mn-Ni alloys. Additionally, Heo and Lee confirmed the existence of the DBD transition in Fe-Mn-Ni-W alloys.t2s] More recently, Heo t261also analyzed the brittlenesst271seen in an Fe-Ni-Ti alloy. The study revealed that the DBD transition also was seen in the Fe-Ni-Ti alloy, and the embrittler was titanium. It is the purpose of the present study to invesN.H. HEO, formerly Graduate Student, Department of Metallurgical Engineering, Seoul National Umversity, Seoul, 151-742 Korea, is Postdoctoral Associate, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted March 9, 1995. METALLURGICAL AND MATERIALSTRANSACTIONS A
tigate the relation between the ductile-brittle-ductile transition behavior in an Fe-Mn-Ni-Ti alloy and the grainboundary segregation of alloying elements.
II.
EXPERIMENTAL PROCEDURES
A. Specimen Preparation and Tests An Fe-Mn-Ni-Ti alloy of 12 kg was prepared in a 35 kV high-frequency vacuum induction furnace using electrolytic iron, electrolytic manganese, pure nickel balls, and a sponge type of titanium. The cast ingot was homogenized at 1200 ~ for 36 ks in an argon atmosphere, forged, and then hot-rolled to plates of 2-mm thickness. Specimens for tensile test were cut from these plates. Before final machining, all specimens were austenitized at 950 ~ for 3.6 ks, and water quenched. Chemical compositions in wt pct of the alloy were 8.4 Mn, 7.4 Ni, and 1.7 Ti,
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