Martensite finish temperature

In some cases, the carbon profile may not provide the necessary hardness or other properties. For example, if the carbon content is too high, quenching to room temperature may not produce all martensite at the surface because the high carbon content places the martensite finish temperature, Mj below room temperature. This results in the presence of soft retained austenite, and a low surface hardness. Conversion to martensite by subzero cooling to below the temperature can increase the hardness (Fig. 6) (12). 

Provided, of course, that we continue to cool the steel down to the martensite finish 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 two embrittling zones can be located in a continuous furnace cooling curve with respect to a critical temperature around 100 to 120°C. This temperature coincides with a -martensite finish temperature. 

A NiTiNOL shape memory metal alloy can exist in two different temperature-dependent crystal structures or phases called martensite (i.e., lower-temperature phase) and austenite (i.e., higher-temperature or parent phase). Several properties of the austenite and martensite phases are notably different. When martensite is heated, it begins to change into austenite. The temperature at which this phenomenon starts is called the austenite start temperature A). The temperature at which the phenomenon is complete is called the austenite finish temperature (A). When austenite is cooled, it begins to change into martensite. The temperature at which this phenomenon starts is called the martensite start temperature (M ). The temperature at which martensite is again completely reverted is called the martensite finish temperature (Mj). Composition and metallurgical treatments have dramatic impacts on the above transition temperatures. From the point of view of practical applications, NiTiNOL can have three different forms ... 

When a steel is cooled sufficiendy rapidly from the austenite region to a low (eg, 25°C) temperature, the austenite decomposes into a nonequilihrium phase not shown on the phase diagram. This phase, called martensite, is body-centered tetragonal. It is the hardest form of steel, and its formation is critical in hardening. To form martensite, the austenite must be cooled sufficiently rapidly to prevent the austenite from first decomposing to the softer stmeture of a mixture of ferrite and carbide. Martensite begins to form upon reaching a temperature called the martensite start, Af, and is completed at a lower temperature, the martensite finish, Mj, These temperatures depend on the carbon and alloy content of the particular steel. 

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...

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).

The schematic pseudobinary phase diagram shown in Fig. 7.1 can be used to understand the classification of titanium alloys. The diagram depicts the different phase fields on a plot of temperature versus the percent of p stabilizers added to a titanium alloy already containing some amount of a stabilizer. The upper solid curve is called the p-transus curve, while the lower solid curve is the a transus. The two dashed curves indicate the locus of the martensite start (Mj) and martensite finish (M temperatures as a function of composition. 

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. 

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). 

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. 

The fraction martensite increases as the temperature is lowered below the Ms to the Mf. On heating, the reversion starts at As and finishes at Af. 

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. 

The temperature at which the transformation from austenite to martensite begins (Ms), finishes (Mp), is 90% complete (Mgo), etc. 

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