Martensite crystal structure

This chapter begins with a general consideration of the crystallographic features of martensitic transformations. The principles are general, and thus detailed descriptions of the crystal struaures and substructures for individual alloy systems such as Ni-Al versus Cu-Sn are avoided. A brief survey of shape-memory phenomena within the framework of martensite crystallography is presented this subject and the various martensite crystal structures are presented in detail in Chapter 26 by Schetky in Volume 2. Martensitic transformations and shape-memory phenomena are common to many... 

Crystal structure Martensite (body centred cubic) austenite (face centred cubic)... 

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... 

Let us regard a binary A-B system that has been quenched sufficiently fast from the / -phase field into the two phase region (a + / ) (see, for example, Fig. 6-2). If the cooling did not change the state of order by activated atomic jumps, the crystal is now supersaturated with respect to component B. When further diffusional jumping is frozen, some crystals then undergo a diffusionless first-order phase transition, / ->/ , into a different crystal structure. This is called a martensitic transformation and the product of the transformation is martensite. 

However, important differences exist. Martensite and its parent phase are different phases possessing different crystal structures and densities, whereas a twin and its parent are of the same phase and differ only in their crystal orientation. The macroscopic shape changes induced by a martensitic transformation and twinning differ as shown in Fig. 24.1. In twinning, there is no volume change and the shape change (or deformation) consists of a shear parallel to the twin plane. This deformation is classified as an invariant plane strain since the twin plane is neither distorted nor rotated and is therefore an invariant plane of the deformation. 

This class of smart materials is the mechanical equivalent of electrostrictive and magnetostrictive materials. Elastorestrictive materials exhibit high hysteresis between strain and stress (14,15). This hysteresis can be caused by motion of ferroelastic domain walls. This behavior is more complicated and complex near a martensitic phase transformation. At this transformation, both crystal structural changes induced by mechanical stress and by domain wall motion occur. Martensitic shape memory alloys have broad, diffuse phase transformations and coexisting high and low temperature phases. The domain wall movements disappear with fully transformation to the high temperature austentic (paraelastic) phase. 

For Nitinol - at the transition Ms, atoms begins to shear uniformly throughout the crystal. As the temperature is lowered the atomic shear continues to increase. At temperature, Mf, the atoms shear to their maximum point and assume a new structure. Thus, between Ms and Mr temperature interval the crystal structure of Nitinol is undefined and belongs neither to austenite nor to martensite . Therefore, thermodynamically, it should be classified as the second-order transformation. This is illustrated in Fig. 3. Conventionally - above Ms temperature, the whole crystal assume a crystal structure identified as austenite . At Ms temperature, a new crystal structure of martensite begins to form through two-dimensional (planar) atomic shear. The two crystal structures of austenite and martensite therefore share an identical plane known as Invariant Plane. As the temperature is lowered, the two dimensional shear (or more correctly, shift ) continue to take place one plane at a time such that the Invariant Plane moves in the direction as to increase the volume of martensite at the expense of austenite . Ultimately, at Mt temperature the whole crystal becomes martensite . Since between Ms and Mf any given micro-volume of the crystal must belong to either the austenite or the martensite , the transformation is of the first-order thermodynamically. This case is pictorially illustrated in Fig. 4. 

It was based on these uniqueness and commonalities, my colleague and I submitted a paper entitled Crystal Structure and A Unique martensitic Transition of TiNi to a Journal concerned with metals and alloys for publication in 1965. But, the paper was rejected outright by two anonymous reviewers who could not accept our observation that the Nitinol transition was unique. Obviously the reviews contend that by accepting Nitinol transition being unique, may make all other martensitic transformations garden variety. This may upset the theory of martensitic transition formulated thus far. We then, submitted the paper to the Journal of Applied Physics and was accepted for publication and eventually appeared in print [10]. A few months after the appearance of this article, the editor of the very journal that rejected my paper, asked me to review two papers on Nitinol for the journal. Suddenly, I was an undisputed expert in Nitinol Up to this point I had not really start to apply covalent-bond concept but devoting more time in collecting experimental data [14,15], which may be important in support or non-support of covalent-bond concept. 

At Ms temperature TiNi initiates a uniform (inhomogeneous) distortion of its lattice — through a collective atomic shear movement. The lower the temperature, the greater the magnitude of shear movements. As a result, between Ms and Mr temperature the crystal structure is not definable. In sharp contrast, other known martensitic transformations initiate a nonuniform (heterogeneous) nucleation at Ms and thereafter the growth of martensite is achieved by shifting of a two dimensional plane known as invariant plane [28] at a time. Thus, between Ms and Mr temperature the crystal structure is that of austenite and/or martensite . 

The mechanics of the TiNi transition is, as shown by the X-ray, quite complex particularly when it comes to formation of twin or antiphase boundary. Although, personally I consider them as secondary importance toward the understanding of Nitinol transition itself, perhaps they should be mentioned to complete the picture. It has been known that the martensite (low-temperature phase) always has a crystal structure with lower symmetry than the austenite (high-temperature phase). In order to lower the free-... 

Assume that a hardened steel contains only two phases, martensite and austenite. The problem is to determine the composition of the mixture, when the two phases have the same composition but different crystal structure. The external standard method cannot be used, because it is usually impossible to obtain a reference sample of pure austenite, or of known austenite content, of the same chemical composition as the austenite in the unknown. Instead, we proceed as follows. In the basic intensity equation, Eq. (14-1), we put... 

Austenite has a mnch higher solubility for carbon than other forms of steel. Heating the steel to an anstenitizing temperatnre canses any carbides present to dissolve. Alloys capable of forming anstenite at high temperatnres, bnt that transform to other crystal structures at lower temperatures, are said to be hardenable by heat treatment. Martensitic steels are an example. Most carbon and low-alloy steels are hardenable by heat treatment. 

K. Zasimchuk, V.V. Kokorin, V.V. Martynov, A.V. Tkachenko and V.A. Chernenko, Crystal structure of martensite in Heusler alloy Ni2MnGa, Phys. Met. MetalL 69 104 (1990). 

V.V. Martynov and V.V. Kokorin, The crystal structure of thermaDy- and stress-induced martensites in... 

The change of crystal structure which occurs in some materials in which different crystal structures are stable over different temperature (and pressure) ranges. In ferritic steels, the most important transformation is from the high temperature form, austenite, to lower temperature transformation products, such as ferrite, pearlite, bainite, martensite and, in weld metals, acicular ferrite and ferrite with aligned second phase. 

Upon application of heat, the crystal structures of such alloys begin to change from the martensitic phase into the austenite phase (Ab) until transformation is complete at Ac. Similarly, during the cooling process, the austenite phase reverts back to the... 

One interesting alloy of titanium and nickel, called Nitinol, exhibits shape-memory properties. Below a particular temperature (the transformation temperature), the crystal structure of the alloy is such that it can be plastically deformed (martensitic). As the alloy is heated, the crystal structure alters to one that is more ordered and rigid (austenitic), and the deformed metal reverts to its original shape. This effect has been exploited in a number of devices, including a stent (a device used to hold open passageways such as arteries). The stent is placed inside a small-diameter catheter for insertion into the body, where it expands on being warmed to bod y temperature. 

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