Characteristics of lath martensite: Part I. crystallographic and substructural features
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I,
INTRODUCTION
THEmartensite
structure formed in low carbon and dilute alloy steels, and in Fe-Ni alloys containing less than about 28 pct Ni I consists of packets of lath shaped ferrite units having a habit plane close to {lll}f.* In contrast to plate
*Subscripts f and b refer to fcc austenite and bcc (or bct) martensite, respectively.
martensite in ferrous alloys, the crystallography and the formation mechanism of lath martensite is poorly understood. 2 This is in large part due to the lack of accuracy in experimental data on crystallographic and substructural features of lath martensite. Usually electron microscopy is used for experimental investigations. Habit plane analysis using more accurate optical microscopy methods is difficult because of the small size of the laths. 3 Because the Ms temperature of alloys transforming to lath martensite is usually quite high, the small amounts of austenite retained at room temperature are not detected by X-ray diffraction, and orientation relationships are usually studied using the less accurate electron diffraction technique. In early studies on lath martensite 4,s it was found that only a few surface traces existed within a single austenite grain. This led to the conclusion that the habit plane is {lll}f. Subsequent investigations 3'6-8 have shown that the habit plane is irrational, although close to {11 l}f, but uncertainty still exists as to the exact location of the habit plane. Marder and Krauss 6 performed optical two surface analysis on lath B. E J. SANDVIK and C.M. WAYMAN, respectively, are Research Associate and Professor in the Department of Metallurgy and Mining Engineering, University of lllinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted April 30, 1982.
METALLURGICALTRANSACTIONSA
martensite in Fe-C alloys, and reported a habit plane close to {557}f, but the number of laths investigated was small. Schoen et al 7 made a similar investigation on Fe-Ni-C alloys, and on an Fe-Ni alloy with a Ni concentration gradient. The habit plane was found to lie within 12 deg of {lll}f. In a recent investigation Wakasa and Wayman 3 used an Fe-20Ni-5Mn alloy, which contained small amounts of martensite in an austenite matrix, and reported a mean habit plane about 4.5 deg from {lll}f. Another recent investigation on an Fe-l.5Mn-0.1 C alloy by Davenport 8 reported a habit plane close to {232}e or {154}b. In these investigations the austenite orientation was determined by means of (prior) austenite annealing twin traces, and the exact variant of the austenite-martensite orientation relationship associated with a particular habit plane was not determined. Several habit plane analyses performed using electron microscopy have also been reported. 9,1~ These were performed using single surface trace analysis, and cannot be considered very accurate. The planes reported range from {011}b to {123}b, i.e., a spread of about 20 deg. Investigations on the orientation relationship between adjacent laths within a packet of lath martensite and on the austenite-martens
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