Martensitic transformations hysteresis

Fig.3. Martensitic transformation hysteresis loops, second cycle, heating rate 30 K/min.

The transformation is beHeved to occur by a diffusionless shear process (83). It is often referred to as martensitic transformation, having a thermal hysteresis between the cooling and heating cycles. The transformation is dependent on particle size finer particles transforming at a lower temperature than... 

The formation of a heavily twinned material on cooling can be reversed by an increase in temperature, which causes the material to transform to the untwinned pre-martensite state. The transformation starts at a temperature, usually called As, the austenite start temperature, and is complete at a temperature Af, the austenite finish temperature (Figure 8.16). (These terms are related to the fact that the best-known martensitic transformation is that of austenite to martensite, in steels.) For the alloy NiTi, As is 71 °C, and Af is 77 °C. It is seen that Ms and Mf differ from As and Af. This is a hysteresis phenomenon, commonly found in solid-state transformations. 

Shape-memory alloys show a thermoelastic martensitic transformation. This is a martensitic transformation, as described above, but which, in addition, must have only a small temperature hysteresis, some 10s of degrees at most, and mobile twin boundaries, that is, ones that move easily. Additionally, the transition must be crystallographi-cally reversible. The importance of these characteristics will be clear when the mechanism of the shape-memory effect is described. 

One of the key features of zirconia lies in its polymorphism. Zirconia exhibits three polymorphs. The monoclinic phase is stable up to 1170°C where it transforms to the tetragonal phase, which is Itself stable up to 2370 C. Above this temperature, zirconia exists as a cubic Cap2 type phase. The reversible m- to t-Zr02 transformation is key to the use of zirconia in ceramics. First, it is reversible but occurs with a thermal hysteresis. Second, it is rapid and takes place by a diffusionless shear process similar to that of a martensitic transformation. Finally, it is dependent on particle size and occurs with a volume change (3 to 5%) [82]. 

Figure 5. Electrical resistance changes during cooling and heating Fe-Ni (70 30) and Au-Cd (52.5 47.5) alloys, showing the hysteresis of the martensitic transformation on cooling and the reverse transformation on heating, for nonthermoelastic and thermoelastic transformation, respectively (After Kaufman and Cohen, 1957)...

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. 

Fig. 1. Schematic of the hysteresis loop associated with a shape-memory alloy transformation, where M. and Afp correspond to the martensite start and finish temperatures, respectively, and and correspond to the start and finish of the reverse transformation of martensite, respectively. The physical property can be volume, length, electrical resistance, etc. On cooling the body-centered cubic (bcc) austenite (parent) transforms to an ordered B2 or E)02...

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. 

On the other hand, hysteresis of the temperature-induced structural phase transitions in nanostructures with first-order phase transitions reduce useful magnetocaloric effect to transform cycling between martensite (M) and austenite (A) phases under application. In addition, the size, surface and boundary effects on thermal hysteresis loops have been under consideration for the development of research on nanostructured materials. Experimental data indicate that nanostructured materials offer many interesting prospects for the magnetization data and for understanding of temperature-induced martensite/austenite phase transitions. 

The solid solution LaAg Ini- crystallizes with a CsCl type stmcture (Balster et al., 1975). This pseudobinary system undergoes a martensitic (displacive) crystal structure transition. At low temperature the X-ray powder patterns of polycrystalline samples show line splitting corresponding to a cubic-to-tetragonal transformation. The indium concentration 1 - x and thus the electron count per formula unit has a large influence on the transition temperature. This structural phase transition is revealed also in the temperature dependence of the electrical resistivity. For indium concentrations above 5% the curves show a pronounced hysteresis behavior. 

The thermal cycles and heat treatments required to obtain the various forms of Ce are discussed in ch. 4 sections 3.2 and 3.3. The hep form of Yb was reported by Kayser (1971) to be the stable form below 270 K. The room temperature fee form of Yb transforms martensitically to an hep form when cooled below 270 K. The transformation is reversible with about a 20 degree hysteresis of the start of transformation on cooling and heating. The transformation is especially sensitive to the presence of impurities. A new transformation reported by Hurd and Alderson (1973) was shown by Beaudry and Gschneidner (1974) to be due to the presence of hydrogen in their sample and not a new polymorphic phase of Yb. 

Forward and reverse transformation occur at different temperatures, resulting in a hysteresis as can be seen in Fig. 6.50. The start and end of the transformation from martensite to austenite are given by As (austenite start temperature) and At (austenite finish temperature). The reverse transformation takes place in the temperature interval from Ms to Mt (martensite start and finish temperatures). The shape of the hysteresis curve in Fig. 6.50 strongly depends on the thermomechanical treatment of the shape memory alloy (see also Sect. 6.4.1). 

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