Phase martensitic

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. 

V. I. Levitas, A. V. Idesman, E. Stein. Finite element simulation of martensitic phase transitions in elastoplastic materials. Int J Solids Struct 55 855, 1998. 

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. ... 

V.V. Martynov, K. Enami, L.G. Khandros, S. Nenno and A.V. Tkachenko, Structure of martensitic phases... 

Figure 2. Thermal strain vs temperature curves for VsSi measured along [001] on heating (4.2-60K) and cooling (4.2-1.5K). Curve (a) is for an uniaxial stress (s 0.03o doo)) along [001] (b) and (c) are for biaxial stress applied along [100] and [010] with 0.5o (ioo> and o (ioo>, respectively. The x-ray data of Batterman and Barrett (reference 15) are also plotted for comparison. The insets show the directions of applied stresses and [in case of the curve (a)] the martensite-phase domains. (From reference 5)...

M. Liu, T.R. Finlayson, and T.F. Smith, Thermal expansion of VaSi with controlled martensite-phase morphology, Phys. Rev. 852 530 (1995). 

It has also been noticed that thicker corrosion films form on the martensite phase in cold worked steels than on the untransformed matrix, and thicker films can be more brittle and aid crack initiation . ... 

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... 

Heterophase Interfaces. In certain cases, sharp heterophase interfaces are able to move in military fashion by the glissile motion of line defects possessing dislocation character. Interfaces of this type occur in martensitic displacive transformations, which are described in Chapter 24. The interface between the parent phase and the newly formed martensitic phase is a semicoherent interface that has no long-range stress field. The array of interfacial dislocations can move in glissile fashion and shuffle atoms across the interface. This advancing interface will transform... 

Structures of the f.c.c. parent and b.c.t. martensite phases are shown in Fig. 24.3. The f.c.c. parent structure contains an incipient b.c.t. structure with a c/a ratio which is higher than that of the final transformed b.c.t. martensite. The final b.c.t. structure can be formed in a very simple way if the incipient b.c.t. cell in Fig. 24.3a is extended by factors of rji = 772 = 1.12 along x[ and x 2 and compressed by 773 = 0.80 along x 3 to produce the martensite cell in Fig. 24.36. This deformation, which converts the parent phase homogeneously into the martensite phase, is known in the crystallographic theory as the lattice deformation-1 Unfortunately,... 

In many cases, the martensite phase is internally twinned and is composed of two types of thin twin-related lamellae, as illustrated in Fig. 24.10. In such cases, the lattice-invariant shear is accomplished by twinning rather than by slip as has been assumed until now (see Fig. 24.106). The critical amount of shear required to produce the invariant habit plane is then obtained by adjusting the relative thicknesses of the two types of twin-related lamellae shown in Fig. 24.106. 

We now describe briefly martensitic transformations in three contrasting systems which illustrate some of the main features of this type of transformation and the range of behavior that is found [15]. The first is the In-Tl system, where the lattice deformation is relatively slight and the shape change is small. The second is the Fe-Ni system, where the lattice deformation and shape change are considerably larger. The third is the Fe-Ni-C system, where the martensitic phase that forms is metastable and undergoes a precipitation transformation if heated. 

The differential forms of the molar free energy for the parent and martensite phases are... 

This analysis shows that a compressive load decreases the molar free energy— and that a positive <5e yrt reduces the magnitude of the decrease for the martensite phase thereby resulting in an increased transformation temperature, consistent with Fig. 24.16. Further analysis shows that the observed shift in transformation temperature results from differences in the Young s moduli of the two phases (see Exercise 24.5). This result is consistent with LeChatelier s principle. 

Figure 24.16 Free energy of parent and martensite phases as a function of temperature, illustrating the effect of compressive uniaxial stress on martensite transformation...

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. 

Figure 24.21 shows a two-dimensional martensitic transformation in which a parent phase, P, is transformed into a martensitic phase, M, by a lattice deformation, B. Note that there is no invariant line in this two-dimensional transformation. Find a lattice-invariant deformation, S, and a rigid rotation, R, that together with the lattice deformation, B, produce an overall deformation given by... 

Figure 24.21 Two-dimensional transformation of parent phase, P, to martensitic phase, M, by the lattice deformation, B.

Figure 24.22 Production of an invariant line (habit line) along AB in a two-dimensional transformation of a parent phase, P, to a martensitic phase, M. The degree of matching of phases is indicated in (d) by shading shared sites in the interface.

The use of shape-memory alloys as actuators depends on their use in the plastic martensitic phase that has been constrained within the structural device. Shape-memory alloys (SMAs) can be divided into three functional groups one-way SMAs, tw o-vvav SMAs, and magnetically controlled SMAs. The magnetically controlled SMAs show great potential as actuator materials for smart structures because they could provide rapid strokes with large amplitudes under precise control. The most extensively used conventional shape-memory alloys are the nickel-titanium- and copper-based alloys (see Shape-Memory Alloys). 

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. 

The traverse might work acceptably well if the dynamics of the interface between the two phases is favorable systems with martensitic phase transitions may fall into this category Z. Nishiyama, Martensitic Transformations, Academic Press, New York, 1978. Note also that the special case in which the structural phase transition involves no change of symmetry can be handled within the standard multicanonical framework [55]. 

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