Deformation mechanisms martensitic transformations

A) As the material is cooled it undergoes a martensitic transformation. By transforming to equal amounts of two variants, the macroscopic shape is retained. (B) Deformation occurs by movement of variant boundaries so the more favorably oriented variant grows at the expense of the other. Reprinted with permission of Cambridge University Press from W. F. Hosford, Mechanical Behavior of Materials (New York Cambridge Univ. Press, 2005). 

Figure 4.5 Mechanism of martensitic transformation of a- to y-iron. (a) Shearing during which the body-centred cubic (b.c.c.) stacking order ABABA... is transformed to the face-centred cubic (f.c.c.) stacking order BACBA..., (b) shearing within the matrix lattice, (c) inner plastic deformation...

The crystalline phase follows a few independent slip systems in which classical crystal plasticity theories cannot be utilized to model them [93-95]. Similar to the metallic crystalline phases, inelastic deformation in crystalline polymeric systems follows three different mechanisms (a) crystallographic slip, (b) twining, and (c) Martensite transformations [96]. All these mechanisms leave the crystallographic axis inextensible and provide less than five independent... 

Most of the austenitic stainless steels are known to undergo a strain-induced martensitic transformation [1], Aless well-known fact is that certain commercial grades of AISI 304 and AISI 304L also undergo spontaneous transformation upon quenching to 76°K [2]. This report will be confined to the mechanical properties of alloys that undergo strain-induced transformation only. The strain-induced martensitic transformation is dependent on the temperature of deformation and the nature of the applied stress. A treatment of one theory of strain-induced martensitic transformation may be found in the work of Patel and Cohen [3]. 

Besides dislocation movement, there are other mechanisms of plastic deformation. These are the martensitic transformation we already discussed, diffusion creep at high temperatures (to be covered in chapter 11), and finally the so-called twinning. Mechanical twinning usually contributes only slightly to plastic deformation and is in general more difficult to activate than dislocation movement. Therefore, it will be discussed only briefly. 

R-phase, which serves as an intermediate phase to facilitate the transformation between martensite and austenite. Formation of the R-phase is reported to arise from the presence of dislocations and precipitates [11]. A substantial dislocation density is expected in the vwought nickel-titanium endodontic instruments and orthodontic wires, which are subjected to extensive mechanical deformation during manufacturing processes [12], Microstructural precipitates are a consequence of the inevitable deviation of the nickel-titanium alloy composition from the equi-atomic NiTi composition [13,14],... 

Figure 3, which is a replot of the data of Fig. 1, was included to focus attention on the deformational behavior of these steels as measured by elongation. At 70 F, these steels deform by localized necking and extensive reduction of area, followed by fracture in the necked area (see Fig. 10). At -320 and -423°F, these same steels deform by a much more uniform elongation and reduction of area over the entire reduced section (see Fig. 10). This condition is most pronounced in the 62 cold-worked samples. At low temperature, the increase in strength in the reduced section due to plastic strain and associated martensite formation more than offsets the increase in stress due to the reduction in cross-sectional area (i.e., necking down) hence, the sample elongates over the entire reduced section prior to failure. At -423 F this elongation frequently occurs in a discontinuous manner, accompanied by audible clicks, serrations in the stress—strain curve, and striations in the sample, whose appearance is not unlike Luder s bands. The cross section of such a striation is shown in Fig. 12. These striations have been observed in other alloys by other investigators, and have been variously attributed to catastrophic twinning, thermal instability, and the burst-type formation of dislocations [1]. In this material another possibility exists, namely, the formation of martensite. This transformation is known to occur by an instantaneous shear mechanism and yields a volume increase which could account for the serrated stress—strain curve [5]. These effects demonstrate again that the...

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