New observation of martensitic morphology and substructure using transmission electron microscopy
- PDF / 3,415,564 Bytes
- 9 Pages / 613 x 788.28 pts Page_size
- 20 Downloads / 198 Views
I.
INTRODUCTION
S I N C E dislocated and twinned martensite were discovered in the early 1960s, tl] electron microscopy has been employed as a means to discern the two major types of martensite in iron-base alloys. [2"3'4] Martensite in which internal twins are seen in the transmission electron microscope was called plate martensite, and martensite comprising a high density of tangled dislocations and essentially no fine twins was termed lath martensite. ]2-7j Numerous investigators t4"8-~~ have shown that there are only a few twins within lath martensite. Internal twins were not found in the lath martensite of Fe-V, Fe-W, Fe-Sn, Fe-Mn, and Fe-Mo alloys, t~~ More twins within lath martensite were observed in the 9Ni-0.24C and 9Ni7Co-0.24C t91 and 5Ni-3.8Cr-0.27C steels, t~4] Speich and Lesite t15] showed the relationship between the volume fraction of lath martensite and carbon contents for Fe-C alloys; the relative volume percentage of lath martensite was near 100 pct for 0.2 pct C steel and 75 to 90 pct for 0.4 to 0.6 pct C steels. The purpose of this article is to study the effect of tilting thin-foil specimens in the transmission electron microscope on martensitic images and the reliability of identifying twinned martensite by this technique. II.
EXPERIMENTAL PROCEDURE
The chemical compositions of the steels studied are given in Table I. Specimens 10 to 20 mm in diameter and 10-mm thick were quenched from 820 ~ 900 ~ 1000 ~ 1100 ~ 1200 ~ and 1300 ~ respectively. The austenitizing time was 10 minutes in all cases. In
YU HUA TAN, Professor, and SHU QIN HU, Graduate Student, are with the Department of Mechanical Engineering, Xiangtan University, Xiangtan, Hunan, People's Republic of China. DE CHANG ZENG, Doctoral Student, is with the Institute of Metal Research, Academia Sinica, Shenyang, People's Republic of China. XI CHUN DONG, Associate Professor, is with the Anshan Institute of Iron and Steel, Anshan, Liaoning, People's Republic of China. YUE HUI HE, Doctoral Student, is with the Central-South Institute of Mining and Metallurgy, Changsha, Hunan, People's Republic of China. Manuscript submitted September 19, 1990. METALLURGICAL TRANSACTIONS A
steel 20, the cooling medium depends on the austenitizing temperature; when the austenitizing temperature is 1000 ~ or above, water was used, whereas below this temperature, brine was used. All other steels were oil quenched from the austenitizing temperature. The martensitic morphology and substructure of these steels were studied in an S-570 scanning electron microscope, an H-800 transmission electron microscope, as well as in an optical microscope. Thin foils were prepared by grinding and mechanical polishing to 40/~m, followed by electrolytic polishing in a solution consisting of 10 to 20 pct perchloric acid and 80 to 90 pet alcohol at - 1 0 ~ in a CE-10 twin-jet polishing unit. Ill.
R E S U L T S AND ANALYSIS
A. Low-Carbon Steels
The microstructure of low-carbon steel 20 quenched from 980 ~ consisted of lath martensite with many intersecting packets. Each pac
Data Loading...
