-9 -30 -6 -1.8
Microstructures and Corrosion Behaviors
in an Fe-9Al-30Mn-6Cr-1.8C Alloy
-9
-30
-6
-1.8
Microstructures and Corrosion Behaviors
in an Fe-9Al-30Mn-6Cr-1.8C Alloy
Student C.L. Wang Advisor T.F. Liu C.G. Chao
A Thesis
Submitted to Department of Materials Science and Engineering College of Engineering
National Chiao Tung University in partial Fulfillment of the Requirements
for the Degree of Master
in
Materials Science and Engineering July 2010
I
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Microstructures and Corrosion Behaviors in an Fe-9Al-30Mn-6Cr-1.8C Alloy.
Student C. L. Wang Advisor Prof. T. F. Liu
Prof. C. G. Chao Department of Materials Science and Engineering
National Chiao Tung University
Abstract
The purpose of this study is to examine the microstructural developments of Fe-9 wt.%Al-30 wt.%Mn-1.8 wt.%C alloy (Alloy A(0 Cr)) and Fe-9 wt.%Al-30 wt.%Mn-6 wt.%Cr-1.8 wt.%C (Alloy B(6 Cr)) after being solution heat-treated (SHT) and then aged at 400 for various times by means of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition, the corrosion resistant properties of the as-quenched and aged alloys in 3.5% NaCl solution were also examined by electrochemical measurements and auger electron spectroscopy (AES).
In the as-quenched condition, the microstructure of both alloys A(0 Cr) and B(6 Cr) was single austenite (Ȗ) phase containing fine țƍ carbides, and no precipitates could be observed on grain boundaries. The fine țƍ carbides are (Fe,Mn)3AlC carbides with an Lƍ12 structure, which were formed within the Ȗ
condition, the corrosion resistance of the alloy B(6 Cr) was much better than that of the alloy A(0 Cr).
When the as-quenched alloy B(6 Cr) was aged at 400 for 3 h, the fine țƍ carbides grew significantly and no precipitates were formed on grain boundaries. Therefore, the corrosion resistant property of the alloy B(6 Cr) aged at 400 for 3h is similar to that of the alloy B(6 Cr) in the as-quenched condition. However,
when the aging time was increased to 24h, some coarse (Fe,Mn)3AlC carbides
(ț carbides) having an Lƍ12 structure started to appear on grain boundaries. With
increasing the aging time at 400 , the coarse ț carbides grew into adjacent
austenite grains through Ȗ țƍ ĺȖ0 (carbon-deficient austenite) ț carbide
reaction. The precipitation of ț carbides on the grain boundaries would deteriorate the pitting corrosion noticeably. After further prolonged the aging time, Cr-rich M7C3 carbides were also found to appear on grain boundaries. The
M7C3 carbide has a hexagonal close-packed (HCP) structure. Owing to the
heterogeneous precipitation of Cr-rich M7C3 on grain boundaries, the corrosion
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300nm 300nm
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300nm 450nm
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400nm 400nm
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