Elastic martensite

The transition temperatures which we find for the austenite - martensite transition in simulations without vacancies, are much to low. The introduction of vacancies lead to much more physical results, while the martensite - austenite transition is not affected by vacancies. From this we conclude that vacancies lower the energy barrier which the system has to overcome during the transition. The reason for this might be a weakening of long-range elastic couplings in the lattice. 

D.J. Gunton and G.A. Saunders, The Elastic Behaviour of In-Tl alloys in the vicinity of the martensitic transition, Solid State Commun. 14 865 (1974). 

Solids undergoing martensitic phase transformations are currently a subject of intense interest in mechanics. In spite of recent progress in understanding the absolute stability of elastic phases under applied loads, the presence of metastable configurations remains a major puzzle. In this overview we presented the simplest possible discussion of nucleation and growth phenomena in the framework of the dynamical theory of elastic rods. We argue that the resolution of an apparent nonuniqueness at the continuum level requires "dehomogenization" of the main system of equations and the detailed description of the processes at micro scale. 

If the SMA is sufficiently close to Tm, an imposed stress is sufficient to cause pressure-induced austenite —> martensite phase transitions in selected grains of the alloy, relieving the stress through pseudo-elastic deformation of the softer martensite grains. Similarly, if the original austenite-shaped alloy is brought below Tm to convert it to malleable martensite form, many deformations of macroscopic shape leave the martensitic atoms close to their... 

Such transformations have been extensively studied in quenched steels, but they can also be found in nonferrous alloys, ceramics, minerals, and polymers. They have been studied mainly for technical reasons, since the transformed material often has useful mechanical properties (hard, stiff, high damping (internal friction), shape memory). Martensitic transformations can occur at rather low temperature ( 100 K) where diffusional jumps of atoms are definitely frozen, but also at much higher temperature. Since they occur without transport of matter, they are not of central interest to solid state kinetics. However, in view of the crystallographic as well as the elastic and even plastic implications, diffusionless transformations may inform us about the principles involved in the structural part of heterogeneous solid state reactions, and for this reason we will discuss them. 

Let us assume that the martensite particle has grown in the form of an oblate spheroid, the volume of which is (4-n/3with c as the semi-thickness of the spheroid. The total Gibbs energy change for the formation of this particle includes a chemical, an elastic, and an interface energy term. It can be appropriately written as... 

Martensitic transformations involve a shape deformation that is an invariant-plane strain (simple shear plus a strain normal to the plane of shear). The elastic coherency-strain energy associated with the shape change is often minimized if the martensite forms as thin plates lying in the plane of shear. Such a morphology can be approximated by an oblate spheroid with semiaxes (r, r, c), with r c. The volume V and surface area S for an oblate spheroid are given by the relations... 

Let the uniaxial strain associated with the martensite transformation be <5e rt, and let E ,unJ and e eras um be the measured uniaxial strains in the martensite and parent phases, respectively. It is not necessary that e f unl = e aeas,unl. The elastic parts of the uniaxial strains in the two phases will be related through their respective elastic constants because the normal components of stress must be equal at the interface. 

The manufacturing of nickel-titanium rotary endodontic instruments, which involves machining of a wire blank into a variety of cross-sectional shapes that depend upon the particular product, is described in a recent review article [20], In that article it is stated that the nickel-titanium alloy for these instruments is in the superelastic condition, for which the alloy has the austenitic structure. This statement is highly plausible, because extensive reversible elastic strain (up to approximately 10% for uniaxial tension) could then occur in the instrument when the stress in the root canal reaches the level that causes transformation from austenite to martensite [21], The first published verification [22] of this superelastic condition was obtained by our research group from DSC experiments on nickel-titanium rotary instruments in the as-received condition. A subsequent study evaluated the rotary instruments after clinical use [23]. 

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