Martensites crystallography

This chapter begins with a general consideration of the crystallographic features of martensitic transformations. The principles are general, and thus detailed descriptions of the crystal struaures and substructures for individual alloy systems such as Ni-Al versus Cu-Sn are avoided. A brief survey of shape-memory phenomena within the framework of martensite crystallography is presented this subject and the various martensite crystal structures are presented in detail in Chapter 26 by Schetky in Volume 2. Martensitic transformations and shape-memory phenomena are common to many... 

In 1964, two competing series of slender volumes appeared one, the Macmillan Series in Materials Science , came from Northwestern Morris Fine wrote a fine account of Phase Transformations in Comlen.ted Systems, accompanied by Marvin Wayman s Introduction to the Crystallography of Martensite Transformations and by Elementary Dislocation Theory, written by Johannes and Julia Weertman. The second series, edited at MIT by John Wulff, was entitled The Structure and Properties of Materials , and included slim volumes on Structure, Thermodynamics of Structure, Mechanical Behaviour and Electronic Properties. 

The crystallography of martensitic transformations has been widely analyzed [1-9]. We shall mainly follow Wayman s description [5] which is largely based on the original work of Wechsler et al. [1]. The famous f.c.c.—>b.c.t. (body-centered tetragonal) transformation in iron alloys is the basis for the hardening of steel and will be the focus of the discussion. 

The crystallography of the f.c.c.— b.c.t. martensitic transformation in the Fe-Ni-C system (with 22 wt. %Ni and 0.8 wt. %C) has been described in Section 24.2. In this system, the high-temperature f.c.c. solid-solution parent phase transforms upon cooling to a b.c.t. martensite rather than a b.c.c. martensite as in the Fe-Ni system. Furthermore, this transformation is achieved only if the f.c.c. parent phase is rapidly quenched. The difference in behavior is due to the presence of the carbon in the Fe-Ni-C alloy. In the Fe-Ni alloy, the b.c.c. martensite that forms as the temperature is lowered is the equilibrium state of the system. However, in the Fe-Ni-C alloy, the equilibrium state of the system in the low-temperature range is a two-phase mixture of a b.c.c. Fe-Ni-C solid solution and a C-rich carbide phase.5 This difference in behavior is due to a much lower solubility of C in the low-temperature b.c.c. Fe-Ni-C phase than in the high-temperature f.c.c. Fe-Ni-C phase. If the high-temperature... 

C.M. Wayman, Inrod.to Crystallography of Martensitic Transformation, Mcmillan, New York (1964). 

G.33 A. Kelly and G. W. Groves. Crystallography and Crystal Defects (Reading, Mass. Addison-Wesley, 1970). Careful analysis of the departure from perfect periodicity caused by specific defects in specific, and common, structures (BCC, FCC, HCP, etc.). Describes the crystallography of dislocations, point defects, twins, martensite, and crystal interfaces. 

Wayman C. M., Introduction to the Crystallography of Martensitic Transformations, Macmillan Company, New York New York, 1964. 

Shape-memory alloys show a thermoelastic martensitic transformation. This is a martensitic transformation, as described above, but which, in addition, must have only a small temperature hysteresis, some 10s of degrees at most, and mobile twin boundaries, that is, ones that move easily. Additionally, the transition must be crystallographi-cally reversible. The importance of these characteristics will be clear when the mechanism of the shape-memory effect is described. 

The interpretation of the behavior of Fe-Mn binary alloys containing 10 to 27% Mn requires an understanding of the behavior of the e-martensite phase. Despite recent informative research, particularly by Lysak and Nikolin ], the crystallography, formation kinetics, and mechanical behavior of the e-martensite phase are not fully resolved. The appearance of -martensite is associated with a low stacking fault energy of the austenite. Recent measurements (Fig. 3)... 

Joh] Johnson, K.A., Wayman, C.M., The Crystallography of the Austenite-Martensite Transformation in an Fe-Cr-C Alloy , Acta Crystallogr., 16(6), 480-485 (1963) (Crys. Strueture, Experimental, Theory, 16)... 

More on the t-Zr02 m-Zr02 Martensitic transformation in TZP ceramics is found in the work of Yin in his discussions on the thermodynamics, crystallography and kinetics of this transformation. The solid transformation in pure zirconia is ... 

In effect, in the above description one uses all aspects of the crystallography and PTMC to define martensite. 

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