Strain-induced martensitic transformation

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]. 

The martensitic transformation is responsible for the high work hardening capacity of most of the austenitic stainless steels. Consequently, in these steels a wide range of mechanical properties can be obtained through a combination of martensitic transformation and normal strain hardening. For this reason the relative amounts of strain-induced martensite in each alloy are reported. 

Strain-induced martensite has been found to increase with decreasing temperature and increasing plastic strain in austenitic stainless steels [1]. Consequently, the reported low-temperature martensite suppression may be predictable in terms of the existing theory since the corresponding plastic strain also decreased substantially. However, there is also the possibility that the martensitic transformation is suppressed at very low temperatures in these alloys independently of the strain. 

Stress induced martensitic transformation and twinning alone are not responsible for large strain deformation. 

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. 

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. 

Formation of the martensitic phase and strain-induced transformation of austenite to martensite increases the susceptibility of 304 stainless steel to HE [182,183]. In the presence of hydrogen, more transformed martensite is formed, increasing the ductile loss of the material [184]. Caskey [185] observed that large grain sizes deteriorate the FS of austenitic stainless steel due to the presence of hydrogen. Tsay et al. [186] evaluated the... 

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... 

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. 

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... 

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]. 

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