A new phase in an Fe-9.0AI-29.5Mn-1.2Si alloy
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(i 1 0 ) J / ( i 11)L.phasewith a deviation of 0.3 deg (011)d/( 131 )L-phasewith a deviation of 1.5 deg The L phase has never been observed in various Fe-A1-Mn, Fe-Mn-Si, Fe-A1-Si, and Mn-A1-Si alloy systems before.
I.
INTRODUCTION
R E C E N T L Y , we performed transmission electron microscopy studies on the phase transformation of an Fe-9.0A1-29.5Mn-1.2Si alloy. In the as-quenched condition, the microstructure of the alloy consisted of austenite (3/) and ferrite (r phases. No precipitates could be detected within the austenite phase; however, ex9 tremely fine D03 particles were formed within the ferrite matrix by a continuous ordering transition during quenching. When the as-quenched specimen was aged at temperatures ranging from 550 ~ to 950 ~ a complex phase transition (a + D03 + L phase) --> (a + D03 + A13 /3-Mn) --> (B2 + D03 § A13 /3-Mn) --> (B2 + A13 /3-Mn) --> (a + A13/3-Mn) --> (u + 3/) --* t~ occurred within the (a + D03) regions. A detailed examination concerning the complex phase transition of the present alloy upon aging is given in a separate paper, tll The purpose of this paper is to identify the crystal structure of the L phase and to determine the orientation relationship between the L phase and the ferrite matrix. It is worthwhile to note that the L phase has never been observed in Fe-A1-Mn, t2-91 Fe-Mn-Si, tl~ Fe-A1Si, tlS-Is] and Mn-A1-Si [~9-221 alloy systems before.
II.
cesses were performed at 550 ~ for various times in a salt bath. Specimens for electron microscopy were prepared by means of a double-jet electropolisher with an electrolyte of 60 pct ethanol, 30 pct acetic acid, and 10 pet perchloric acid. The polishing temperature was kept in the range from - 10 ~ to 10 ~ and the current density was kept in the range from 1.5 x 104 to 2.0 X 10a A / m 2. Electron microscopy was performed on a JEOL 2000FX scanning transmission electron microscope (STEM) operating at 200 kV. This microscope was equipped with an energy-dispersive X-ray spectrometer (EDS) for chemical microanalysis. In the present study, the elemental analysis of the precipitates was done in the STEM mode on thin films (not on the extracted precipitates); there is an error in the exact percentage of the elements in precipitates (mainly because of the matrix phase contribution to EDS analysis). However, the electron beam system on a 2000FX STEM-EDS can produce an electron spot 40 nm in size, which is much smaller than the size of the particles (120 to 180 nm) examined in the present work. Furthermore, it is well known that the dispersion of electrons in a thin foil is quite small. Therefore, the error in the data obtained by this method should not be very significant.
EXPERIMENTAL PROCEDURE III.
RESULTS AND DISCUSSION
The alloy, Fe-9.0 wt pct A1-29.5 wt pct Mn-l.2 wt pct Si, was prepared in an air induction furnace by using 99.5 pct iron, 99.7 pct aluminum, 99.9 pct manganese, and ferrosilicon. The melt was cast into a 30 • 50 • 200-mm steel mold, After being homogenized at 1250 ~ for 12 hours under a protective argon atmo
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