Austenite state

Shape memory alloys work by viitue of their intrinsic switching between two crystalline states, i.e., martensite and austenite (Sherby et al., 2008). At lower temperatures, these alloys adopt the martensite state, which is relatively soft, plastic, and quite easy to shape at a certain higher temperature, they transform into the austenite state, which is a much harder material and not so easy to deform. Figure 1.1 illustrates the principles of shape memory effect in metal alloys. 

Magnetic materials based on the Fe—Co—V alloy system were the first ductile magnets. The intrinsic magnetic properties may be found in [3.6] while extensive magnetic materials data are treated in [3.56]. The optimum magnetic behavior is obtained for alloy compositions around Fe-55 wt% Co-10 wt% V. As the isothermal sections of the Fe—Co—V phase diagram Fig. 4.3-56a and Fig. 4.3-56b show, this alloy is mainly in the fee y-phase (austenite) state at 900 °C,... 

The properties of the shape memory alloy vary with its temperature. Above the transition temperature, the alloys crystallic structure takes on the austenitic state. Its structure is symmetric and the alloy shows a high elastic modulus. The martensitic crystalline structure will be more stable for thermodynamical reasons if the materials temperature drops below the transformation temperature. Martensite can evolve from austenitic crystals in various crystallographic directions and will form a twinned structure. Boundaries of twinned martensite can easily be moved for that reason SM elements can be deformed with quite low forces in the martensitic state. 

The almost-linear behavior between wire length and resistance can be explained by the fact that the actuator stroke is approximately proportional to the fraction of austenite and martensite in the alloy. Since the resistivity of martensite and austenite is different, the resistance of the SM actuator will vary according to the phase fractions only in the fully martensitic or austenitic state does the relationship between actuator stroke and resistance become non-linear. The stroke-resistance relation is independent of the ambient temperature (respectively, the t3rpe of surrounding medium or the t3rpe of convection) because it is affected only by the martensite fraction in the actuator material. However, a load force will induce a small amount of elastic strain, resulting in a shift of the stroke-resistance relation. 

The name given to the very hard and brittle constituent that is formed when steel is very rapidly cooled from the austenitic state. It is a ferrite, highly supersaturated with dissolved carbon. 

Segregation of bearings, with regard to residual austenite was performed with the aid of WIROTEST 202 and WIROTEST 12 finish. Selected rings with defined indications were subject to metalographic tests, in order to state whether residual austenite occurs, and then using the diffraction method, the percentage content of residual austenite. 

AH of the alloys Hsted in Tables 4 and 5 are austenitic, ie, fee. Apart from and soHd-solution strengthening, many alloys benefit from the presence of carbides, carbonitrides, and borides. Generally the cubic MC-type monocarbides, which tend to form in the melt, are large and widely spaced, and do not contribute to strengthening. However, the formation, distribution, and soHd-state reactions of carbides are very important because of their role... 

The austenite phase which can contain up to 1.7 wt% of carbon decomposes on cooling to yield a much more dilute solution of carbon in a-iroii (b.c.c), Fenite , together with cementite, again rather diaii the stable carbon phase, at temperatures below a solid state eutectoid at 1013 K (Figure 6.3). 

It is known that the common austenitic stainless steels have sufficient corrosion resistance in sulfuric acid of lower concentrations (<20%) and higher concentrations (>70%) below a critical temperature. If with higher concentrations of sulfuric acid (>90%) a temperature of 70°C is exceeded, depending on their composition, austenitic stainless steels can exhibit more or less pronounced corrosion phenomena in which the steels can fluctuate between the active and passive state [19]. 

All the stainless steels can be machined in the softened states, but they may present some problems unless the correct techniques are adopted. This is especially so with the austenitic grades where the extreme ductility minimises chip breaking and the work hardening may cause difficulties unless modest cuts are made. The free-cutting grades (those with high sulphur contents or selenium additions) are much easier to machine, but it must be remembered that they have somewhat reduced corrosion resistance, ductility and weldability compared to their normal counterparts. Detailed machining instructions are readily available from steel suppliers. 

Austenitic steels of the 304S15 type are normally heat treated at 1 050°C and cooled at a fairly rapid rate to remove the effects of cold or hot working, and in this state much of the carbon is in supersaturated solid solution. Reheating to temperatures below the solution treatment temperature leads to the formation of chromium-rich MjjCj precipitates predominantly at the grain boundaries with the production of chromium gradients and reduced corrosion resistance as is the case with the martensitic steels. Any attack is... 

For special applications special silicon-bearing austenitic steels are produced by a few manufacturers. Silicon contents are 4-5-3% and there is a corresponding increase in nickel and very low (<0-015%) carbon. In the welded state these are giving good service in nitric acid of over 90% strength up to 75°C. 

Phosphoric acid The austenitic grades are resistant to all strengths up to 80°C but are limited to 30-40% concentration at boiling point, the molybdenum-bearing types having the best corrosion resistance. Some test data for various types is shown in Fig. 3.28. Industrially, this acid is often encountered in an impure state with appreciable amounts of sulphuric and hydrofluoric acid present so that process testing is likely to be particularly necessary. The super-austenitic steels have enhanced resistance to phosphoric acid solutions. 

Potential-current density (E-i) curves, which have been determined for a number of the austenitic cast irons and also for the nickel-free ferritic irons, indicate that in general the austenitic cast irons show more favourable corrosion characteristics than the ferritic irons in both the active and passive states. 

In de-aerated 10sulphuric acid (Fig. 3.45) the active dissolution of the austenitic irons occurs at more noble potentials than that of the ferritic irons due to the ennobling effect of nickel in the matrix. This indicates that the austenitic irons should show lower rates of attack when corroding in the active state such as in dilute mineral acids. The current density maximum in the active region, i.e. the critical current density (/ ii) for the austenitic irons tends to decrease with increasing chromium and silicon content. Also the current densities in the passive region are lower for the austenitic irons... 

The austenitic grades, used mainly in the solution treated (softened) state, have low strength at ambient temperature but maintain strength at elevated temperatures much better than the martensitics and the ferritics. As can be seen from Figs 7.23 to 7.25, creep and rupture. strengths are far superior... 

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