Martensitic phase transformation

This class of smart materials is the mechanical equivalent of electrostrictive and magnetostrictive materials. Elastorestrictive materials exhibit high hysteresis between strain and stress (14,15). This hysteresis can be caused by motion of ferroelastic domain walls. This behavior is more compHcated and complex near a martensitic phase transformation. At this transformation, both crystal stmctural changes iaduced by mechanical stress and by domain wall motion occur. Martensitic shape memory alloys have broad, diffuse phase transformations and coexisting high and low temperature phases. The domain wall movements disappear with fully transformation to the high temperature austentic (paraelastic) phase. 

In 1964, two competing series of slender volumes appeared one, the Macmillan Series in Materials Science , came from Northwestern Morris Fine wrote a fine account of Phase Transformations in Comlen.ted Systems, accompanied by Marvin Wayman s Introduction to the Crystallography of Martensite Transformations and by Elementary Dislocation Theory, written by Johannes and Julia Weertman. The second series, edited at MIT by John Wulff, was entitled The Structure and Properties of Materials , and included slim volumes on Structure, Thermodynamics of Structure, Mechanical Behaviour and Electronic Properties. 

Many metals and metallic alloys show martensitic transformations at temperatures below the melting point. Martensitic transformations are structural phase changes of first order which belong to the broader class of diffusion js solid-state phase transformations. These are structural transformations of the crystal lattice, which do not involve long-range atomic movements. A recent review of the properties and the classification of diffusionless transformations has been given by Delayed... 

Martensitic phase transformations are discussed for the last hundred years without loss of actuality. A concise definition of these structural phase transformations has been given by G.B. Olson stating that martensite is a diffusionless, lattice distortive, shear dominant transformation by nucleation and growth . In this work we present ab initio zero temperature calculations for two model systems, FeaNi and CuZn close in concentration to the martensitic region. Iron-nickel is a typical representative of the ferrous alloys with fee bet transition whereas the copper-zink alloy undergoes a transformation from the open to close packed structure. ... 

Some of the above discussed precursor phenomena are also observed prior to diffusion driven phase transformations. A typical example are the conventional EM tweed images obtained in the tetragonal parent phase in high Tc superconductors and other ceramics. In a recent survey by Putnis St e of such observations it was concluded that in these cases the tweed contrast resulted from underlying microstructures fomied by symmetry changes driven by cation ordering. These symmetry changes yield a fine patchwork of twin related domains which coarsen when the transfomiation proceeds. However, in view of the diffusion driven character of the latter examples, these cases should be clearly separated from those in the field of the martensites. 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

The structures and phase transformations observed in steels have been dealt with in some detail not only because of the great practical importance of steels, but also because reactions similar to those occurring in steels are also observed in many other alloy systems. In particular, diifusionless transformations (austenite -> martensite), continuous precipitation (austenite -> pearlite) and discontinuous precipitation (austenite -> bainite and tempering of martensite) are fairly common in other alloy systems. 

Aurantin, molecular formula and structure, 5 91t Ausimont, 7 641 Austempering, 23 287 Austenite, 23 272, 273, 275 decomposition of, 76 197-198 grain size of, 23 276-277 in hardening of steels, 76 196-197 phase transformations in, 23 277 transformation rates of, 23 282—283 Austenite phase, in martensite... 

Magnetic effects on metastable transformations. The underlying factor in all the above effects is the magnitude of the ratio (G" /G ° ) and especially its variation with temperature. It follows that there can also be a substantial effect on the driving force for phase transformations, including shear transformations. Thus the martensite start temperature, in most Fe alloys is dominated by the... 

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. 

The crystallography of the f.c.c.— b.c.t. martensitic transformation in the Fe-Ni-C system (with 22 wt. %Ni and 0.8 wt. %C) has been described in Section 24.2. In this system, the high-temperature f.c.c. solid-solution parent phase transforms upon cooling to a b.c.t. martensite rather than a b.c.c. martensite as in the Fe-Ni system. Furthermore, this transformation is achieved only if the f.c.c. parent phase is rapidly quenched. The difference in behavior is due to the presence of the carbon in the Fe-Ni-C alloy. In the Fe-Ni alloy, the b.c.c. martensite that forms as the temperature is lowered is the equilibrium state of the system. However, in the Fe-Ni-C alloy, the equilibrium state of the system in the low-temperature range is a two-phase mixture of a b.c.c. Fe-Ni-C solid solution and a C-rich carbide phase.5 This difference in behavior is due to a much lower solubility of C in the low-temperature b.c.c. Fe-Ni-C phase than in the high-temperature f.c.c. Fe-Ni-C phase. If the high-temperature... 

G.B. Olson and M. Cohen. Theory of martensitic nucleation A current assessment. In Proceedings of an International Conference on Solids Solid Phase Transformations, pages 1145-1164, Warrendale, PA, 1982. The Metallurgical Society of AIME. 

It has been stated that a martensitic phase transformation can be considered as the spontaneous plastic deformation of a crystalline solid in response to internal chemical forces [9], Give a critique of this statement. 

This class of smart materials is the mechanical equivalent of electrostrictive and magnetostrictive materials. Elastorestrictive materials exhibit high hysteresis between strain and stress. This hysteresis can be caused by motion of fenoelastic domain walls. Tins behavioi is mote complicated and complex near a martensitic phase transformation. 

Shape-memory alloys (e.g. Cu-Zn-Al, Fe-Ni-Al, Ti-Ni alloys) are already in use in biomedical applications such as cardiovascular stents, guidewires and orthodontic wires. The shape-memory effect of these materials is based on a martensitic phase transformation. Shape memory alloys, such as nickel-titanium, are used to provide increased protection against sources of (extreme) heat. A shape-memory alloy possesses different properties below and above the temperature at which it is activated. Below this temperature, the shape of the alloy is easily deformed due to its flexible structure. At the activation temperature, the alloy can be changed by applying a force, but the structure resists this deformation and returns back to its initial shape. The activation temperature is a function of the ratio of nickel to titanium in the alloy. In contrast with Ni-Ti, copper-zinc alloys are capable of a two-way activation, and therefore a reversible variation of the shape is possible, which is a necessary condition for protection purposes in textiles used to resist changeable weather conditions. 

Q.P. Sun (ed.) IUTAM Symposium on Mechanics of Martensitic Phase Transformation in... 

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