Martensitic transitions in alloys

Martensitic transformations in alloys are essentially order-disorder displacive transitions that take place very rapidly, because atomic diffusion does not occur. The discussion of the formation of martensite in the Fe-C system, in Section 8.2.5, is an example. This transition is the transformation of a cubic phase containing excess carbon in interstitial sites into a tetragonal phase. As any one of three cubic axes can be elongated, three orientations of the martensite c axis can occur. This is a general feature of martensitic transformations and the different orientations that can arise are called variants or domains of the martensitic phase. These variants are simply twins (see Section 3.4.10). 

1090 °C, this structure orders to form the B2 (CsCl) stracture (Rgure 8.13). If this latter phase is quenched (cooled rapidly) to room temperature the structure transforms via a martensitic transformation into a monoclinic B19 type. On cooling, the transformation starts at a temperature designated Ms, the martensite start temperature, and is complete by a temperature M, the martensite finish temperature. For the alloy NiTi, Ms is 60 °C and Mf is 52 °C. 

The martensitic transformation is thus easily open to modification by heat treatment and alloying, which gives these microstructures great flexibility from the point of view of engineering design. 

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