Martensitic transformations lattice deformation

Structures of the f.c.c. parent and b.c.t. martensite phases are shown in Fig. 24.3. The f.c.c. parent structure contains an incipient b.c.t. structure with a c/a ratio which is higher than that of the final transformed b.c.t. martensite. The final b.c.t. structure can be formed in a very simple way if the incipient b.c.t. cell in Fig. 24.3a is extended by factors of rji = 772 = 1.12 along x[ and x 2 and compressed by 773 = 0.80 along x 3 to produce the martensite cell in Fig. 24.36. This deformation, which converts the parent phase homogeneously into the martensite phase, is known in the crystallographic theory as the lattice deformation-1 Unfortunately,... 

These operations do not occur separately and in any particular sequence but are simply a convenient way to conceptualize the transformation as a series of operations, each of which can be analyzed separately, but which working together produce a martensitic structure containing an invariant plane. As such, they can be imagined to occur in any sequence. For purposes of analysis, it is convenient to imagine that the lattice-invariant deformation occurs first, followed by the lattice deformation, followed finally by the rigid-body rotation. We now show that a lattice-invariant shear by slip followed by the lattice deformation analyzed above can produce an undistorted plane. 

The input data for the model consist of the description of the lattice deformation and the choice of the slip system in the lattice-invariant shear. The model has successfully predicted the observed geometrical features of many martensitic transformations. The observed and calculated habit planes generally have high indices that result from the condition that they be macroscopically invariant. 

We now describe briefly martensitic transformations in three contrasting systems which illustrate some of the main features of this type of transformation and the range of behavior that is found [15]. The first is the In-Tl system, where the lattice deformation is relatively slight and the shape change is small. The second is the Fe-Ni system, where the lattice deformation and shape change are considerably larger. The third is the Fe-Ni-C system, where the martensitic phase that forms is metastable and undergoes a precipitation transformation if heated. 

Upon cooling, an In-Tl (19% Tl) alloy undergoes an f.c.c. solid solution —> f.c.t. solid solution martensitic transformation in which the lattice deformation is relatively slight, corresponding to... 

Figure 24.21 shows a two-dimensional martensitic transformation in which a parent phase, P, is transformed into a martensitic phase, M, by a lattice deformation, B. Note that there is no invariant line in this two-dimensional transformation. Find a lattice-invariant deformation, S, and a rigid rotation, R, that together with the lattice deformation, B, produce an overall deformation given by... 

Figure 24.21 Two-dimensional transformation of parent phase, P, to martensitic phase, M, by the lattice deformation, B.

Figure 4.5 Mechanism of martensitic transformation of a- to y-iron. (a) Shearing during which the body-centred cubic (b.c.c.) stacking order ABABA... is transformed to the face-centred cubic (f.c.c.) stacking order BACBA..., (b) shearing within the matrix lattice, (c) inner plastic deformation...

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