Martensite transformation, start temperature

The Ms temperature, at which the diffusion-less martensitic transformation starts, depends on the alloy considered (its composition, etc.) it can be above or below room temperature. For the so-called austenitic steels Ms < < ambient temperature, whereas Ms > ambient temperature for the martensitic steels. 

Fig. 8.10. The displacive f.c.c. —> b.c.c. transformation in iron the volume of martensite produced is a function of temperature only, and does not depend on time. Note that the temperature at which martensite starts to form is labelled (martensite start) the temperature at which the martensite transformation finishes is labelled Mf (martensite finish).

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... 

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... 

A martensitic transformation occurs over a temperature range. The temperature at which the martensite first starts to form on cooling is called the Ms temperature. More martensite will form only if the temperature is lowered. The temperature at which the reaction is complete is called the Mf temperature. However, the concept of an Mf temperature may be more of a convenience than a reality because often there is no sharp completion of martensite formation. 

In most systems the martensitic reaction is geometrically reversible. On heating, the martensite will start to form the higher temperature phase at the As temperature and the reaction will be complete at an Af temperature, as illustrated in Figure 11.19. Martensite in the iron-carbon system is an exception. On heating, the iron-carbon martensite decomposes into iron carbide and ferrite before the As temperature is reached. Martensite can be induced to form at temperatures somewhat above the Ms by deformation. The highest temperature at which this can occur is called the Md temperature. Likewise, the reverse transformation can be induced by deformation at the Ad temperature somewhat below the As. The temperature at which the two phases are thermodynamically in equilibrium must lie between the Ad and Md temperatures. 

Experimental time-temperature-transformation (TXT) diagram for Ti-Mo. Xhe start and finish times of the isothermal precipitation reaction vary with temperature as a result of the temperature dependence of the nucleation and growth processes. Precipitation is complete, at any temperature, when the equilibrium fraction of a is established in accordance with the lever rule. Xhe solid horizontal line represents the athermal (or nonthermally activated) martensitic transformation that occurs when the p phase is quenched. 

It is well known that the martensitic transformation of Al-deficient NiAl (see Sec. 4.3.2) is thermoelastic and produces the shape memory effect. Consequently materials developments have been started which aim at applications as shape memory alloys (Furukawa et al., 1988 Kainuma et al., 1992b, c). The martensitic transformation temperature can be varied within a broad temperature range up to 900 °C, and thus the shape memory efftct can be produced at high temperatures which allows the development of high-temperature shape memory alloys. The problem of low room temperature ductility of NiAl has been overcome by alloying with a third element - in particular Fe - to produce a ductile second phase with an f.c.c. structure. 

The temperature at which the transformation to martensite takes place is found to be composition-dependent. Martensite starts to form when the temperature reaches about 700 °C for the lowest-carbon-content steels but not until a temperature of about 200 °C for austenite with a carbon content of 1.2 wt%. The temperature at which the martensite starts to form is usually labelled Mg, the martensite start temperature, and the temperature at which the transformation is complete is labelled M(. the martensite hnish temperature. 

C, this structure orders to form the B2 (CsCl) stracture (Rgure 8.13). If this latter phase is quenched (cooled rapidly) to room temperature the structure transforms via a martensitic transformation into a monoclinic B19 type. On cooling, the transformation starts at a temperature designated Ms, the martensite start temperature, and is complete by a temperature M, the martensite finish temperature. For the alloy NiTi, Ms is 60 °C and Mf is 52 °C. 

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. 

Notes R, As and Ms are starting temperatures for transformations to R-phase, austenite and martensite, respectively. AH is for the overall transformation between martensite and austenite. 

The shape-memory effect is a complex function of composition, martensite start temperature Mg, stress, strain, microstructure, texture, and aging treatment. It consists essentially of a reversible transformation strain and the associated macroscopic shape change. At low numbers of transformation cycles (e g., up to 100), the... 

Rg. 3.1-101 Concentration dependence of the martensite transformation temperatures. Ms - martensite start M( - martensite finish, i. e., austenite is transformed completely... 

Fig. 3.1-103 Martensitic transformation temperatures of Fe-rich Fe—Ni alloys. The reverse transformation is characterized by the Ag (austenite start) and Af (austenite finish) temperatures [1.82]...

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). 

The structures assumed by rapidly P-quenched binary Ti-TM sdloys are mapped in Fig. 5.1. Below a start temperature, the bcc structure begins a spontaneous allotropic transformation by means of a complicated shearing process to a structure known as martensite and designated a or a" depending upon whether the transformation product is hep or orthorhombic. When the distinction between a and a" is unimportant, the martensites are to be herein represented collectively by the notation a " Being of second order, the martensitic transformation is anticipated by a regime of structural fluctuations called difiuse (O phase. As represented in Fig. 5.1, the co phase, as... 

On cooling to an isothermal temperature below the martensite start (Mg) jwint of790 5 °C (1454 9 °F), first some a phase is formed above Mg and then the remaining, predominate portion of the p phase is transformed into a supersaturated hexagonal martensite (a"). Below Mg and above the martensite finish (Mf) temperature of 740 5 °C (1364 9 °F), there remains a residual P phase, which is probably transformed isothermally to a phase. The resulting structure for isothermal reaction is a + a", where the a" phase below 750 °C (1380 °F) decomposes discontinuously into a two-phase a + p structure and a metastable P phase enriched with p-stabilizing elements. 

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