High resolution scanning auger microscopic investigation of intergranular fracture in As-quenched Fe-12mn
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INTRODUCTION
THE low
temperature mechanical properties of Fe-Mn alloys have been the subject of active research over the last several years. 1-4 This research is motivated by the potential economic benefit from replacing nickel by manganese in cryogenic structural steels and by the possibility that austenitic grades of Fe-Mn may offer particular advantages in structural applications at 4 K. The research in this laboratory 5-9has concentrated on the development of ferritic grades having manganese contents in the range 5 to 12 wt pct. One intriguing feature of the Fe-Mn system 5-8 is the anomalous change in its mechanical properties as the manganese content is increased from 8 to 12 wt pct. While the alloy yield strength increases monotonically over this composition range, the ductile-brittle transition temperature falls (Figure 1). At the same time the fracture mode below the ductile-brittle transition temperature changes from a typical transgranular cleavage at the 8 pct manganese level to a virtually complete intergranular failure in the 12 pct manganese alloy (Figure 2). The Fe-Mn system hence provides the only example known to us in which a simultaneous increase in strength and shift to an intergranular brittle fracture mode is associated with a decrease in the ductile-brittle transition temperature. The as-quenched microstructure of the Fe-Mn binary also changes dramatically over the 8 to 12 pct manganese range. At 8 pct manganese the alloy quenches into the dislocated lath martensite structure shown in Figure 3(a), while at 12 wt pct manganese the as-quenched structure is the irregular blocky martensite shown in Figure 3(b). The change in microstructure appears to be associated with the intrusion of the hexagonal e-martensite phase, which first appears at approximately 10 wt pct manganese, precedes the a' martensite in the transformation sequence, and, along with a
slight admixture of residual austenite phase, forms the block boundaries of the blocky martensite structure (Figure 4). 8 It is therefore natural to seek an interpretation of the anomalous change in the mechanical properties of the alloy in terms of the striking changes in its microstructure. Such an interpretation was proposed by Hwang and Morris 8 after a careful investigation of the microstructure and intergranular fracture mode in Fe-12Mn. They suggested that the lowering of the ductile-brittle transition temperature as the manganese content is raised from 8 to 12 wt pct is due to an Weight Percent Manganese
5o
METALLURGICAL TRANSACTIONS A
8
12
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-250 H.J. LEE, Staff Scientist, Lawrence Berkeley Laboratory, and J.W. MORRIS, Jr., Professor, Department of Materials Science, are both with the University of California, Berkeley, CA 94720. Manuscript submitted December 10, 1981.
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4-300 g Q
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Atomic Percent Manganese
Fig. 1--Change in the ductile-brittle transition te
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