Martensite deformation induced

It is well known that paramagnetic phase in austenitic stainless steels, such as an Fe-18Cr-8Ni, transforms into ferromagnetic a martensite phase by plastic deformation at low temperature [4,5]. Since the amount of the deformation-induced martensite increases at a larger... 

If the relation between the amount of the deformation-induced martensite and the amount of plastic deformation be known, it would be easy to design the profile of the saturation magnetization by changing the local strain. A simple model to evaluate the distributions of strain and saturation magnetization is obtained in order to clarify the results mentioned in the present study. 

Z. Mei, J.W. Morris, Influence of deformation-induced martensite on fetigue crack propagation in 304-type steels, MetaU. Trans. A 21A (1990) 3137—3152. 

Fig. 30. A simultaneous saxs/waxs experiment on an oriented sample which requires both area detectors for the saxs and waxs region. An HDPE sample is stretched. The deformation induces a martensitic phase transformation as evidenced by the occurrence of extra reflections in the waxs region (top row of diagrams, see arrow). Void scattering is found in the saxs patterns (bottom row of diagrams). For a further explanation one is referred to the text and to References 122 and 123.

It has long been recognized worldwide that also a cold work process particularly severe can contribute to corrosion in austenitic stainless steels by sensitizing the material [15-19]. Grinding must be completely rejected, in particular when high speed without lubricant is used. The sensitization process is due to two simultaneous actions. First, any heavy cold-working may leave a system of traction residual stresses (see Sect. 3.4 and Fig. 3.11) that favor stress corrosion. Secondly, plastic deformations induced by cold working may transform part of the metastable austenite into brittle martensite. 

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. 

The typical for Ni-Al alloys 7R and 3R (LIq) structures of martensite were formed in the investigated alloy. The thermally induced martensite in the homogenised sample was mainly of 7R type (Fig.2b). The LIq martensitic structure was mainly observed in samples after the hot deformation. 

If the SMA is sufficiently close to Tm, an imposed stress is sufficient to cause pressure-induced austenite —> martensite phase transitions in selected grains of the alloy, relieving the stress through pseudo-elastic deformation of the softer martensite grains. Similarly, if the original austenite-shaped alloy is brought below Tm to convert it to malleable martensite form, many deformations of macroscopic shape leave the martensitic atoms close to their... 

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. 

In most systems the martensitic reaction is geometrically reversible. On heating, the martensite will start to form the higher temperature phase at the As temperature and the reaction will be complete at an Af temperature, as illustrated in Figure 11.19. Martensite in the iron-carbon system is an exception. On heating, the iron-carbon martensite decomposes into iron carbide and ferrite before the As temperature is reached. Martensite can be induced to form at temperatures somewhat above the Ms by deformation. The highest temperature at which this can occur is called the Md temperature. Likewise, the reverse transformation can be induced by deformation at the Ad temperature somewhat below the As. The temperature at which the two phases are thermodynamically in equilibrium must lie between the Ad and Md temperatures. 

These features suggest the following picture for the deformation of this type of steel at very low temperature the stress produces a deformation due to the motion of dislocations (effect visible by electron microscopy). These motions induce a transitory phase e in the lattice, and this e phase rapidly evolves toward the a phase. The martensitic a phase is therefore only a secondary phenomenon bound to the dislocation motion and a simple function of the strain associated with this motion. 

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]. 

IMPACT. Figure 6 illustrates the impact strength and relative amounts of strain-induced martensite of AISI 202, AM 350, and USS Tenelon. An impact transition was found in USS Tenelon, the transition temperature range being 130 to 180 K. A striking illustration of this impact transition was provided by the fact that specimens tested in impact at 195 K evidenced widespread plastic deformation and yet specimens tested at 135 K failed in a brittle manner (Fig. 10). Impact specimens of AISI 202 tested at 195 K and USS Tenelon tested at 195 , 174 , 163 , and 152 K did not evidence complete fracture. However, the unbroken area... 

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... 

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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