Volume Fraction Of Stress-induced Martensite Research Articles

Influence of Microstructure on the Performance of Nitinol: A Computational Analysis

One material that has found particular favor for use in biomedical endovascular stents is the near equi-atomic NiTi alloy, Nitinol. One remarkable trait exhibited by this superelastic material is the improvement of its fatigue performance with increasing mean strain (Ref 1, 2). Clarification into this phenomenon still remains incomplete in the literature. This study proposes a microstructural explanation for this unique macroscopic behavior; it is hypothesized stress-induced martensite (SIM) will stabilize with increasing strain which, in turn, leads to the observed increase in fatigue life. Finite element analysis (FEA) is employed to investigate the behavior of a “v-strut” stent subcomponent under various strain levels. The volume fraction of SIM is analyzed to identify its potential influence on macroscopic response and, ultimately, fatigue behavior. In addition, a computational investigation is performed on the effect of crystallographic texture on macroscopic response. Granular transformational behavior is analyzed using FEA models with realistic and idealized grain structures, specifically evaluating the effect of individual grain orientations on the stress-induced martensite transformation.

0.2C-1.5Si-1.2Mn鋼のTRIP効果におよぼすひずみ速度の影響

Effect of strain rate on tensile properties and stress-induced martensitic transformation behavior in a 0.2C-1.5Si-1.2Mn (0.2C TRIP) steel was investigated at strain rates between 3.3×10–6 s–1 and 103 s–1. The 0.2% proof stress and tensile strength increased and uniform elongation decreased with an increase in strain rate in the 0.2C TRIP steel. At low strain rates below 10–4 s–1, the 0.2C TRIP steel was obtained good uniform elongation. In the strain-rate dependence on stress-induced martensitic transformation behavior, the volume fraction of stress-induced martensite decreased at strain rates higher than about 10–2 s–1 due to the temperature rise caused by adiabatic deformation. The difference of stress-induced transformation behavior between the 0.2C and 0.4C TRIP steels seems to be associated with the stress partitioning to retained austenite. Furthermore, the stress partitioning is affected by the volume fraction of not only retained austenite but also ferrite and bainite.

Role of stress-induced martensitic transformation in TRIP effect of metastable austenitic steels

Abstract The effect of the Ni equivalent (Ni eq . ) on the transformation-induced plasticity (TRIP) effect in metastable austenitic steels was investigated to clarify the conditions of stress-induced martensitic transformation behavior for maximum uniform elongation. Results of static tensile tests showed that the tensile strength increased with decreasing Ni eq . and that uniform elongation reached a maximum value at the Ni eq . of 23.7%. The volume fraction of martensite at the same true strain increased as the Ni eq . decreased. The conditions under which the stress-induced transformation resulted in the maximum uniform elongation due to the TRIP effect in the metastable austenitic steels were summarized in terms of the volume fraction of stress-induced martensite and the rate of transformation, which were almost coincident with previous results.

Influence of Strain Rate on TRIP Effect in SUS301L Metastable Austenite Steel

In order to investigate the effect of strain rate on TRIP effect in metastable austenitic steels, tensile deformation behavior of a JIS-SUS301L steel was examined at wide range of strain rates between 103/s and 10−3/s, in addition of the comparison with data of stable austenitic steel (JIS-SUS310S). Concerning of the SUS301L steel, the 0.2% proof stress increased and the uniform elongation decreased with increasing of strain rate. The tensile strength decreased with an increase in strain rate between 10−3/s and 101/s and then increased again at strain rate more than 101/s. The significant difference between the strain rate dependences on tensile strength and uniform elongation of SUS301L and SUS310S was clarified especially at strain rates between 10−3/s and 100/s. As far as the effect of strain rate on stress-induced martensitic transformation behaviors is concerned, the volume fraction of stress-induced martensite at the same true strain decreased with increasing of strain rate. These stress-induced transformation behaviors were associated with the tensile strength and uniform elongation of SUS301L steel between 10−3/s and 100/s. The conditions of stress-induced martensitic transformation in order to obtain better uniform elongation at a high strain rate of 100/s were discussed by the calculations using the Weng secant method.

Tensile Deformation Behaviors of Metastable Austenitic Stainless Steels Studied by Neutron Diffraction

Tensile deformation behaviors of three austenitic stainless steels, JIS-SUS310S, 304 and 301L, were studied by static tensile tests and in situ neutron diffraction. In the mechanical properties obtained by the static tensile tests, the 304 and 301L steels showed better balance of tensile strength and uniform elongation than the 310S one because of TRIP effect. The angular dispersion neutron diffractions with a wavelength of 0.16 or 0.182 nm were performed during stepwise tensile testing by using a neutron diffractometer for residual stress analysis (RESA) at the Japan Atomic Energy Agency. The lattice plane strain, stress-induced martensite volume fraction, dislocation density and so on were estimated by the profile analysis as a function of applied stress. The change in lattice plane spacing for austenite indicated four deformation stages. In the comparison of lattice plane strain among the tested steels, a phase stress caused by the stress-induced martensite seems to overlap the intergranular stress of austenite phase. Judging from the results of profile analysis, the strain partitioning of austenite phase in metastable austenitic steels became larger with increasing of the volume fraction of stress-induced martensite during tensile deformation.

Martensite/particle interactions and the shape memory effect in an Fe–Mn–Si-based alloy

The effect of carbide precipitates with a size range of 30–300 nm on the austenite to martensite transformation has been studied. Such particles are known to enhance shape memory, and it was the aim of this work to clarify how the particles exert a favourable effect on shape memory. Differential scanning calorimetry revealed that the presence of particles increases the amount of thermally induced martensite. X-ray diffraction showed that the presence of particles increases the amount of stress-induced martensite also. Surface-relief produced on a pre-polished surface by bending deformation showed that the particle-containing samples exhibited a more complex and highly tilted surface-relief indicative of the formation of a larger volume fraction of martensite. The reversion characteristics of the particle-containing and solution-treated samples were similar: both showed co-reversion of multiple variants of martensite within the same volume of microstructure. However, a higher volume fraction of martensite reverted for the particle-containing sample on recovery annealing. The increased density of nucleation sites for martensite formation and a higher volume fraction of stress-induced martensite for a given strain are therefore considered to be the main contributions of relatively coarse carbide particles to the improvement of shape memory performance.

Field-induced strain under load in Ni–Mn–Ga magnetic shape memory materials

Single-crystal Ni2MnGa shows a nearly 0.2% strain under a magnetic field of 8 kOe at −8 °C. Polycrystalline samples have been prepared near stoichiometry to study the composition dependence of the magnetic and elastic properties. A narrow band of compositions has been found having a range of Curie and martensitic transformation temperatures, Tc and T0, extending to above room temperature. The compressive stress–strain characteristics in variable transverse field were studied in samples selected to have T0 just below room temperature. Stress-induced martensite was observed as expected and the magnetic field was applied under fixed load for various stresses. A transverse field of 3200 Oe caused the sample to strain under load doing work that increased up to 1.3 J/kg with increasing volume fraction of stress-induced martensite.