Martensitic microstructure

There are a number of different examples within which it is possible to describe the kinematics of structural transformation. Perhaps the simplest such example is that of the transformation between a cubic parent phase and a transformed phase of lower symmetry such as a tetragonal structure. We note that we will return to precisely such structural transformations in the context of martensitic microstructures in chap. 10. If we make the simplifying assumption that the transformed axes correspond with those of the parent phase, then the deformation mapping is of the form... 

The second modeling centerpiece considered in this chapter will be our discussion of martensitic microstructures. This work is offered on the basis of our... 

For the moment, our interest is more in what takes place between these interfaces. To illustrate the types of quantitative questions that can be addressed by the theory of fine-phase microstructures, we first elaborate further on the character of these microstructures. As seen above, the martensitic microstructure is characterized by the presence of different variants. However, in the presence of an applied stress, the relative proportion of the different variants can be changed. The experiments are carried out in a biaxial loading apparatus like that shown schematically in fig. 10.27. If we align our Cartesian axes with the two loading directions, then the state of stress to which the sample is subjected can be characterized by the scalars cti and <72 which are a measure of the magnitudes of the load in these two loading directions. 

As a result of the arguments set forth in this part of the chapter, we are now prepared for the job of evaluating the structure and energetics of martensitic microstructures. The fundamental idea is to construct arrangements of the different... 

Theory of Martensitic Microstructure and the Shape-Memory Effect by Kaushik Bhattacharya, unpublished (1998) (a huge pity ) - available from author bhatta caltech.edu. Bhattacharya s article gives a complete and thorough discussion of the many ideas that have been brought to bear on the problem of microstructure in martensites. 

A particular challenge for the continued vitality of the study of materials is to produce both falsifiable models and the reproducible data needed to renounce them. Within the context of the previous chapters, I note with particular enthusiasm the experiments using a biaxial testing machine to examine martensitic microstructures described in chap. 10. 

This equation reduces to Eh = -4.308 x 10 5T ln/n, i.e. Eh = -0.029 log/h at 20°C, where 0.98 bar hydrogen pressure would result from a hydrogen overvoltage of 29 mV. It is interesting to note that maximum embrittlement occurs here at a fugacity of 108 (i.e./fj = 104) compared with the much lower value of/h = 104 found for AISI 4340 steel by Scully and Moran,9 i.e. a martensitic microstructure is more readily embrittled by cathodic polarization than the duplex. 

Abe, R, T. Horiuchi, M. Taneike, and K. Sawada. 2004. Stabilization of martensitic microstructure in advanced 9% Cr steel dnring creep at high temperature. Mater. Set Eng. A 378 299-303. 

High-alloyed steels with a martensitic microstructure show machining results, which heavily depend on the workmaterial hardness and thus on the applied heat treatment. However, hardened and tempered martensitic stainless steels can be machined relatively well with suitable cutting parameters, tool materials, and coating systems, respectively. The dominant failure modes when using coated carbide tools for cutting hardened... 

Technically, it is almost impossible to harden pure iron in this way because the extreme cooling rates needed are very difficult to achieve. However, if carbon is added, it significantly reduces the required cooling rates, making the production of a martensitic microstructure feasible. 

Fig. 16. SEM images of different microstructures of the modified layers on HS 6-5-2 steel surface a - dendritic microstmcture in the phase transformations zone, b - martensitic microstructure in the phase transformations zone.

Martensitic microstructures based on orthorhombic phase (labeled a")... 

Alpha-Beta Alloys. T1-6A1-4V and other weakly beta stabilized alloys can be welded in the annealed condition or in the solution-treated and partially aged condition, with aging completed during postweld stress relieving. Strongly beta-stabilized alloys are embrittled by welding. The low weld ductility of most alpha-beta alloys is caused by phase transformations in the weld zone or in the HAZ which promote a martensitic microstructure. 

A. Abdollah-Zadeh, A. Salemi, and H. Assadi, "Mechanical behavior of CrMo steel with tempered martensite and ferrite-bainite-martensite microstructure". Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 483-84, 2008 pp. 325-328. 

Figore 10.21 Photomicrograph showing the martensitic microstructure. The needle-shape grains are the martensite phase, and the white regions are austenite that failed to transform during the rapid quench. 

The successful heat treating of steels to produce a predominantly martensitic microstructure throughout the cross section depends mainly on three factors (1) the composition of the alloy, (2) the type and character of the quenching medium, and (3) the size and shape of the specimen. The influence of each of these factors is now addressed. 

Heat Treatment For high-strength steels, the best combination of mechanical characteristics may be of Steels realized if a predominantly martensitic microstructure is developed over the entire cross section this is converted into tempered martensite during a tempering heat treatment. 

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