Martensite plates

In the case of Ni-Al the martensitic transformation occurs in a composition range between 62 and 67 at.% Ni where the excess Ni is accommodated randomly on the A1 sublattice. The resulting c/a ratio of the LIq structure is around 0.85, depending on composition. Below 63 at.% Ni the martensite structure has a (52) sequence of close packed planes (Zhdanov notation) which is currently denoted as 14M (formerly as 7R). At higher Ni contents this typical sequence is lost and the martensite plates are simply internally twinned without a specific periodicity. 

Figure 24.2 Macroscopic shape change produced when a martensite plate traverses a...

Microstructure can have a considerable effect on the susceptibihty of steels to hydrogen embrittlement [155]. Untempered martensite promotes environmental embrittlement, apparently due in large part to the brittle nature of the martensite plates [156, 157]. Grain refinement generally increases the resistance to cracking under a wide range of polarization and environmental conditions [158, 159]. 

Fig. 26. Microcracks running from non-metallic inclusions along hard martensitic plates in 25Mn-3Si-1.5Al-Nb-Ti steel, plastically deformed and immersed in 3.5wt% NaCl.

Figures 10 and 11 partially illustrate this phenomena, where individual martensite plates are shown. It can be noticed that these plates occur at preferential sites in definite orientation to others indicating an autocatalytic or chain type transformation. While some of these strained regions are activated by thermal fluctuations, other regions need additional free energy of activation. When warmed to room temperature some strained regions tend to relax however, the existing, untransformed regions are still in a more highly strained, unrelaxed position than they were in the previous cycle, since...

We now face a dilemma in the case of iron alloys or steels (or in general), the measured IPS is inconsistent with the correct structural change as given by the Bain strain on the other hand, the upsetting produced by the Bain strain is not an IPS. Incidentally, the invariant plane is the habit plane of the martensite plates, as shown in Figure 2. Modem crystallographic theories of martensite formation such as those of Bowles and Mackenzie (1954) (BM) and Wechsler et al. (1953) (WLR) rectify these apparent inconsistencies, so we will proceed to discuss these BM and WLR theories, which are fundamentally identical but differ in mathematical order. 

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