Martensites stress-induced

Note Cold working of cx" stabilizes it against reversion to p ex, hep martensite a", orthorhombic martensite stress induced by water queneb-ing or cold working (controversial as some investigators consider a" to fimm athermally) [Pg.602]

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. 

D. Schryvers and L.E. Tanner, Electron microscopy of stress-induced martensite and pretransition... 

V.V. Martynov and V.V. Kokorin, The crystal stmcture of thermally- and stress-induced martensites in... 

M. Liu, T.R. Finlayson, and T.F. Smith, Thermal expansion of martensitic In-Tl stress-induced micro structure and premartensite state, Phys. Rev. B 48 3009 (1993). 

Fig. 10.27. Schematic of biaxial loading apparatus used to examine stress-induced transformations and resulting microstructures in martensites (courtesy of R. James).

Figure 7 shows distributions of the plastic strain calculated for Type (B) and Type (C) specimens. As seen in Fig. 7, the theoretical distributions fit in with the measured distributions for low strain levels. However, the calculated values arc not in agreement with the experimental values at high strain levels. The lack of such coincidence at high strain levels is considered to be attributed to the contribution of strain associated with stress-induced martensite. 

Monl] Monzen, R., Kato, M., Stress-Induced Martensitic Transformation of Small Co-Fe Particles in a Cu Matrix , ISIJ International, 33(8), 898-902 (1993) (Abstract, Experimental, Morphology, 23) cited from abstract... 

Other transformations, such as ferroelastic transformation and twin formation in a system may also induce toughening effects. The former discussion on stress-induced transformation was Martensitic, involving both dilation and shear components of the transformation strain. Twin transformation typically only has a... 

R. P, Reed, G. J. Guntner, and R. L. Greeson, "On the Reduction in Te isile Strength and the Stress-Induced Martensitic Transformation of Some Austenitic Stainless Steels at very Low Temperatures, to be presented at the October, 1960 AIME in Philadelphia. 

RondeUi, G. and Vicentini, B., Evaluation by Electrocbemical Tests of the Passive Film Stability of Equiatomic Ni-Ti Alloy Also in the Presence of Stress-Induced Martensite, Journal of Biomedical Materials Research, Vol. 51, 2000, pp. 47-54. 

Initially the pseudo-elastic material is in its austenitic phase at room temperature. Initially the material in the austenitic phase deforms like a conventional material linear elastic under load. With increasing loads a stress-induced transformation of the austenitic to the martensitic phase is initiated at the pseudo-yield stress Rpe- This transformation is accompanied with large reversible strains at nearly constant stresses, resulting in a stress plateau shown in Fig. 6.53. At the end of the stress plateau the sample is completely transformed into martensite. Additional loading passing the upper stress plateau causes a conventional elastic and subsequently plastic deformation of the martensitic material. If the load is decreased within the plateau and the stress reaches the lower stress level a reverse transformation from martensite to austenite occurs. Since the strains are fully reversible the material and the sample respectively is completely recovered to its underformed shape. These strains are often called pseudo-elastic because the reversible deformation is caused by a reversible phase transformation and is not only due to a translation of atoms out of their former equilibrium position [74]. 

---