Martensitic Transformation
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The most commonly encountered diffusionless transformation is the martensitic transformation (shown in Figure 3439) which, however, is only one subset of non-diffusional transformations. The martensitic transformation in steel represents the most economical example of diffusionless phase transformations but an increasing number of alternatives, such as shape memory alloys, are becoming important as well. Martensitic transformation has actually been observed in several materials and alloys, particularly in noble-metal-based alloys.

Diffusionless Phase Transformation & martensitic transformation

Figure 3439. Diffusionless Phase Transformation.

For instance, at room temperature and ambient pressure, Ti (titanium) has a hexagonal close-packed structure (called α-phase) with the lattice constants listed in Table 1721a. Its unit cell has two atoms at (1/3, 2/3, 1/4) and (2/3, 1/3, 3/4) and the space group number is 194 (P63/mmc). At room temperature and high pressure, it changes to the ω-phase [1,2] with the lattice constants listed in Table 1721b. Its unit cell has three atoms at (0, 0, 0), (1/3, 2/3, 1/2), (2/3, 1/3, 1/2) and the space group is P6/mmm. The α → ω transition in Titanium is a typical example of martensitic transformation.

 

 

[1] Jamieson J.C., Science, 1963, 140, 72; doi:10.1126/science.140.3562.72.
[2] Sikka S.K., Vohra Y.K., Chidaraman R., Prog. Mater. Sci., 1982, 27, 245.

 

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