Martensite transformations

While quenched steel with carbon above 0,6%, the temperature of the end martensite transformation is below zero, thus the transformation of austenite into martensite is incomplete and this remaining cooled austenite is called residual austenite. 

Residual austenite is a steel structure which during cooling at martensite transformation temperature is not completely converted into martensite and remains unchanged at room temperature together with martensite. 

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... 

Precipitation Hardening. With the exception of ferritic steels, which can be hardened either by the martensitic transformation or by eutectoid decomposition, most heat-treatable alloys are of the precipitation-hardening type. During heat treatment of these alloys, a controlled dispersion of submicroscopic particles is formed in the microstmeture. The final properties depend on the manner in which particles are dispersed, and on particle size and stabiUty. Because precipitation-hardening alloys can retain strength at temperatures above those at which martensitic steels become unstable, these alloys become an important, in fact pre-eminent, class of high temperature materials. 

Fig. 2. The shape-memory process, where Tis temperature, (a) The cycle where the parent phase undergoes a self-accommodating martensite transformation on cooling to the 24 variants of martensite. No macroscopic shape change occurs. The variants coalesce under stress to a single martensite variant, resulting in deformation. Then, upon heating, they revert back to the original austenite crystallographic orientation, and reverse transformation, undergoing complete recovery to complete the cycle, (b) Shape deformation. Strain recovery is typically ca 7%.

Another property pecuHar to SMAs is the abiUty under certain conditions to exhibit superelastic behavior, also given the name linear superelasticity. This is distinguished from the pseudoelastic behavior, SIM. Many of the martensitic alloys, when deformed well beyond the point where the initial single coalesced martensite has formed, exhibit a stress-induced martensite-to-martensite transformation. In this mode of deformation, strain recovery occurs through the release of stress, not by a temperature-induced phase change, and recoverable strains in excess of 15% have been observed. This behavior has been exploited for medical devices. 

Martensitic Stainless Steels. The martensitic stainless steels have somewhat higher carbon contents than the ferritic grades for the equivalent chromium level and are therefore subject to the austenite—martensite transformation on heating and quenching. These steels can be hardened significantly. The higher carbon martensitic types, eg, 420 and 440, are typical cutiery compositions, whereas the lower carbon grades are used for special tools, dies, and machine parts and equipment subject to combined abrasion and mild corrosion. 

Fig. 8.7. The displacive f.c.c. —> b.c.c. transformation in iron. B.c.c. lenses nucleate at f.c.c. groin boundaries and grow almost instantaneously. The lenses stop growing when they hit the next grain boundary. Note that, when a new phase in any material is produced by o displacive transformation it is always referred to os "martensite". Displacive transformations ore often called "martensitic" transformations os o result.

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).

Martensite transformations are not limited just to metals. Some ceramics, like zirconia, have them and even the obscure system of (argon + 40 atom% nitrogen) forms martensite when it is cooled below 30 K. Helical protein crystals in some bacteria undergo a martensitic transformation and the shape change helps the bacteria to burrow into the skins of animals and people ... 

Fig. 11.10. Changes during the tempering of martensite. There is a large driving force trying to moke the martensite transform to the equilibrium phases of or + Fe3C. Increasing the temperature gives the atoms more thermal energy, allowing the transformation to take place.

In 1964, two competing series of slender volumes appeared one, the Macmillan Series in Materials Science , came from Northwestern Morris Fine wrote a fine account of Phase Transformations in Comlen.ted Systems, accompanied by Marvin Wayman s Introduction to the Crystallography of Martensite Transformations and by Elementary Dislocation Theory, written by Johannes and Julia Weertman. The second series, edited at MIT by John Wulff, was entitled The Structure and Properties of Materials , and included slim volumes on Structure, Thermodynamics of Structure, Mechanical Behaviour and Electronic Properties. 

Y. Wang, A. G. Khachaturyan. Three-dimensional field model and computer modeling of martensitic transformations. Acta Mater 45 759, 1997. 

THE MARTENSITIC TRANSFORMATION IN IRON-NICKEL ALLOYS A MOLECULAR DYNAMICS STUDY... 

Many metals and metallic alloys show martensitic transformations at temperatures below the melting point. Martensitic transformations are structural phase changes of first order which belong to the broader class of diffusion js solid-state phase transformations. These are structural transformations of the crystal lattice, which do not involve long-range atomic movements. A recent review of the properties and the classification of diffusionless transformations has been given by Delayed... 

At the moment it is not clear how far our first results concerning the role of defects are influenced by the vacancy concentration of 2%. We have used this rather high concentration in order to have a significant number of vacancies in our systems. It will be necessary to make further simulations using more moderate vacancy concentrations and probably other kinds of defects to get a real understanding of the role of defects in martensitic transformations. 

AB INITIO INVESTIGATIONS OF PHONON ANOMALIES AND MARTENSITIC TRANSFORMATIONS... 

In the case of Ni-Al the martensitic transformation occurs in a composition range between 62 and 67 at.% Ni where the excess Ni is accommodated randomly on the A1 sublattice. The resulting c/a ratio of the LIq structure is around 0.85, depending on composition. Below 63 at.% Ni the martensite structure has a (52) sequence of close packed planes (Zhdanov notation) which is currently denoted as 14M (formerly as 7R). At higher Ni contents this typical sequence is lost and the martensite plates are simply internally twinned without a specific periodicity. 

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