Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

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Questions

Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

  1. Why is a martensitic transformation often termed "displacive"?

    a Because the specimen is moved to a different location when it occurs
    b Because the free surface of the specimen becomes displaced when it occurs
    c Because certain atoms take up the locations previously occupied by other atoms when it occurs
    d Because all of the atoms within the transforming region are systematically displaced when it occurs

  2. Which of the following statements are correct concerning displacive (martensitic) and diffusional phase transformations?

    Yes No a Diffusional transformations always take place more easily than displacive transformations
    Yes No b The average velocity of the motion of individual atoms responsible for the transformation is higher for displacive transformations
    Yes No c A displacive transformation can be reversed, whereas a diffusional transformation cannot
    Yes No d Reversal of a displacive transformation may lead to recovery of the original specimen shape
    Yes No e Displacive transformations are in general more likely to occur at lower temperatures, whereas diffusional transformations are favoured at higher temperatures

  3. Martensitic transformations often exhibit hysteresis - for example, the temperature must be taken considerably above that at which the two phases have the same free energies during heating, in order for the transformation to go to completion, whereas it needs to be cooled well below that temperature in order for it to fully reverse. Which of the following explanations for this effect is correct?

    a Since martensitic transformations normally involve a shape change, and both phases are solid, elastic strain energy is created when a local region transforms in this way, requiring the thermodynamic driving force to be further increased (via a change in temperature) in order for the transformation to continue.
    b Because martensitic transformations occur very quickly, extra driving force is needed to provide the necessary kinetic energy for atomic motion
    c Martensitic phases are always metastable, so there is no well-defined temperature at which they are expected to transform
    d Formation of martensite phases always requires some twinning to occur at the same time, and this requires an extra driving force

  4. Unlike loading and unloading of a specimen to and from its conventional elastic limit, doing this to a superelastic material, to and from its superelastic limit, leads to energy being (permanently) absorbed within the specimen, despite the fact that the original specimen shape has been recovered. Assuming "ideal" superelastic behaviour, which of the following could happen to this energy?

    Yes No a Stored within the specimen in the form of a different proportion of the phases from that of the starting material
    Yes No b Stored within the specimen in the form of elastic strain energy in the regions within and around transformed phases
    Yes No c Stored within the specimen in the form of extra dislocations
    Yes No d Dissipated in the form of sound waves created when shear transformations occur
    Yes No e Dissipated in the form of heat

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

  1. A component, to be made from a NiTi Shape Memory Alloy, must be superelastic under service conditions. Thermal cycling, while monitoring the phases present, gave the plot below. Thermodynamic calculations indicate that the stress level to stimulate martensite formation rises with temperature at 1 MPa K−1. The flow stress (for dislocation glide) is 100 MPa at 20 °C and falls with increasing temperature at 0.3 MPa K−1. Calculate the maximum use T.

    Dependence of phase proportion on temperature during (unloaded) thermal cycling