Martensitic transformations stabilization

Another important group of alloys involved in martensitic transformation is represented by several plutonium alloys. The martensitic nature, for instance, of the 6 to a transformation has been clearly established in Pu-Ga and Pu-Al alloys and a behaviour similar to that shown in Fig. 5.30 has been observed. For a systematic description of plutonium alloys, the stability of the different phases and their transformations see Hecker (2000). 

Bar] Baruj, A., Guillermet, A.F., Sade, M., The FCC/HCP Relative Phase Stability in the Fe-Mn-Co System - Martensitic Transformation Temperatures, Assessment of Gibbs Energies and Thermodynamic Calculation of TO Lines , J. Physique IV, 7(C5), 405-410 (1997) (Phase Relations, Thermodyn., Calculation, 18)... 

The zirconia toughening of various ceramics is of great interest and technological importance. It also undergoes Martensitic transformation. Figure 5.50 shows such a transformation in MgO-partially-stabilized Z1O2 [henceforth Mg-PSZ]. The Martensitic transformation upon cooling is a t -> m transition. 

In view of t ie mechanical property results obtained, it is conceivable that additional tensile tests of AISI 304 at 4 K might reveal an ultimate strength decline similar to that found in AM 350 and AISI 202 at 20 K. The relative stability of the AISI 304 (with regard to spontaneous martensitic transformation) may affect any tendency of the alloy to decline in ultimate strength at very low temperatures. 

Fe—Mn. Figure 3.1-104 shows the Fe—Mn phase equilibria indicating that Mn is stabilizing the fee y phase similar to Ni. It should be noted that quenching Fe-rich alloys from the y-phase field leads to two different martensitic transformations which may result in a bcc structure (a martensite) or an hep structure (s martensite). The transformation tenperatures are shown in Fig. 3.1-105. The martensitic transformation can also be induced by deformation. This property is exploited... 

The exceptional mechanical properties of zirconia are mainly based on the phenomenon of transformation toughening (this is described in detail below). The delayed martensitic transformation of the tetragonal high-temperature modification into the monoclinic low-temperature modification by adding stabilizing oxides leads to increased stress intensity factors JCic and also R-curve behavior. Hence, appropriately stabilized zirconia has been dubbed ceramic steel (Garvie et al, 1975). 

Stress-Induced Martensite. Martensitic transformation can also be induced in a quenched metastable P-phase by an externally appHed stress. The structxare of the stress-induced (or assisted) martensitic products has been reported to be fee (or fet), hep, and orthorhombic (distorted hep). However, previoiisly reported hexagonal (aO stress-induced martensites were orthorhombic (a") and that the misinterpretation arose fi t)m the overlap of a and a" reflections. The triggering stress to induce P-to-a" transformation is as low as 150 MPa (22 ksi). Hydrogen, a P-phase stabilizer, tends to increase the triggering stress. 

The influence of cold work on the stability of austenite at low temperature was considered. The austenite to martensite transformation is very strain-sensitive. Plastic deformation at room temperature usually induces martensite formation, while near the temperature elastic strain enhances the... 

There are more general problems of stability of materials and of phase transformations that are closely related to the tensile tests described above. Namely, the tensile test may be considered as a special case of so-called displacive phase transformation path. These paths are well known in studies of martensitic transformations. Such transformations play a major role in the theory of phase transitions. They proceed by means of cooperative displacements of atoms away from their lattice sites that alter crystal symmetry without changing the atomic order or composition. A microscopic understanding of the mechanisms of these transformations is vital since they occur prominently in many materials. 

It is partially stabilized zirconias (PSZ) that have justified the resounding article ( Ceramic steel ) published in 1975 by Garvie et al. [GAR 75], The title suggests that a ceramic can exhibit the high mechanical performances associated with steel, but also that toughening mechanisms recall those used by steel manufacturers. The t- m transformation of zirconia is a martensitic transformation, in analogy with the transformation used to obtain martensite in tempered steels, and the role of microstructural parameters inZr02 is similar to what is observed in metals. 

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