Microstructure in Martensites

In the previous section, we showed one quantitative scheme for treating microstructure and its evolution. Earlier in the chapter we discussed the geometrical 

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Fig. 10.27. Schematic of biaxial loading apparatus used to examine stress-induced transformations and resulting microstructures in martensites (courtesy of R. James).

Theory of Martensitic Microstructure and the Shape-Memory Effect by Kaushik Bhattacharya, unpublished (1998) (a huge pity ) - available from author bhatta caltech.edu. Bhattacharya s article gives a complete and thorough discussion of the many ideas that have been brought to bear on the problem of microstructure in martensites. 

Bhattacharya K., Theory of Martensitic Micro structure and the Shape-Memory Effect (unpublished) (1998) - available from author bhatta caltech.edu Bhattacharya K., Wedge-like Microstructure in Martensites, Acta Metall. Mater. 39, 2431 (1991). Binder K., Ordering of the Face-Centered-Cubic Lattice with Nearest-Neighbor Interaction, Phys. Rev. Lett., 45, 811 (1980). 

THE MICROSTRUCTURE AND MARTENSITIC TRANSFORMATION IN A (POTENTIALLY) SHAPE MEMORY Ni-AI-Ti-B ALLOY... 

J.M. Ball and R.D. James. Theory for the microstructure of martensite and applications. In Proceedings of the International Conference on Martensitic Transformations, pages 65-76, Monterey, CA, 1993. Monterey Institute for Advanced Studies. 

There are a number of different examples within which it is possible to describe the kinematics of structural transformation. Perhaps the simplest such example is that of the transformation between a cubic parent phase and a transformed phase of lower symmetry such as a tetragonal structure. We note that we will return to precisely such structural transformations in the context of martensitic microstructures in chap. 10. If we make the simplifying assumption that the transformed axes correspond with those of the parent phase, then the deformation mapping is of the form... 

Our intention in the remainder of this section is to build up a picture of some of the various interfaces that are present in martensitic systems. Our approach will be to consider microstructural elements of increasing complexity, beginning first with the case of the simple austenite-martensite interface and culminating in the investigation of martensitic wedges within the host austenite. In all of these cases, the primary theoretical engine in our analysis will be the compatibility conditions and their outcome as typified by eqns (10.62) and (10.63). 

Figure 8.11 The microstructure of martensite regions in austenite. The arrows represent the direction of the elongated c axis the c axis is normal to the figure in the domains marked o...

Abe, R, T. Horiuchi, M. Taneike, and K. Sawada. 2004. Stabilization of martensitic microstructure in advanced 9% Cr steel dnring creep at high temperature. Mater. Set Eng. A 378 299-303. 

Fig. 16. SEM images of different microstructures of the modified layers on HS 6-5-2 steel surface a - dendritic microstmcture in the phase transformations zone, b - martensitic microstructure in the phase transformations zone.

As austenitic stainless steels are widely used, their fatigue and fatigue-relaxation behavior has been studied for more than three decades [106—112]. Their stress—strain behavior differs from that of martensitic steels. As mentioned concerning creep, the initial microstructures differ strongly and therefore hardening is observed in austenitic stainless steels, whereas softening is observed in martensitic steels. The evolution of the stress amplitude with the number of cycles is shown in Fig. 6.34(a) for various strain... 

Because their as-received condition microstructure differs strongly from that of tempered martensite-ferritic steels, the stress-strain behavior of austenitic stainless steels differs strongly from that of martensitic steels. During creep and cychc deformation with and without hold time, dislocation production and microstructure are observed, which lead to hardening instead of softening. As creep strain rates in martensitic steels are usually higher than in austenitic stainless steels, necking is... 

Some of the above discussed precursor phenomena are also observed prior to diffusion driven phase transformations. A typical example are the conventional EM tweed images obtained in the tetragonal parent phase in high Tc superconductors and other ceramics. In a recent survey by Putnis St e of such observations it was concluded that in these cases the tweed contrast resulted from underlying microstructures fomied by symmetry changes driven by cation ordering. These symmetry changes yield a fine patchwork of twin related domains which coarsen when the transfomiation proceeds. However, in view of the diffusion driven character of the latter examples, these cases should be clearly separated from those in the field of the martensites. 

A distinguishing feature of the soft-mode martensitic transformation is the anomalous pre-transformation bdiaviour that is observed in a range of physieal and microstructural... 

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