Fe-Ga atomic mechanisms of magnetostriction and stress induced martensitic transformations

February 6, 2011

[1] Atomic-scale modeling of nanostructure formation in Fe–Ga alloys with giant magnetostriction: Cascade ordering and decomposition

Boise et al

The Fe–Ga body-centered cubic (bcc) alloys within the 15–20 at.% Ga composition range have abnormally high magnetostriction. There is growing evidence that this effect is associated with the magnetic field-induced flip of tetragonal axes of nanoparticles of the ordered phase formed in this range. We studied structural transformations within this composition range at 550 °C by using computer modeling of the atomic-scale ordering and clustering in the atomic density field approximation. It is shown that the initial stage of equilibration of the compositionally homogeneous bcc solid solution with 19 at.% Ga results in bcc → B2 congruent ordering followed by a precipitation of Ga-rich B2 particles, which eventually transform to particles of the DO3 phase. At the composition 21 at.% Ga, the congruently ordered B2 phase undergoes further B2 → DO3 congruent ordering, which is followed by decomposition into an equilibrium mixture of the bcc and DO3 phases. An important result is that the phase transformations at 0.15 < c < 0.19 produce nanoparticles of transient B2 phase. We assume that the nanoprecipitates of the transient B2 phase undergo a diffusionless cubic → tetragonal transformation, forming the L10 phase during cooling to the room temperature, and that this involves a magnetic field-induced flipping of tetragonality of these nanoprecipitates which may be responsible for the giant magnetostriction.

[2] Effect of stress triaxiality and Lode angle on the kinetics of strain-induced austenite-to-martensite transformation

Beese and Mohr

The effect of the stress state on the transformation kinetics of stainless steel 301LN sheets at room temperature is investigated using newly developed experimental techniques for simple shear and large strain in-plane compression. In addition, uniaxial and equi-biaxial tension experiments are performed. Two-dimensional and stereo digital image correlation techniques are used to measure the surface strain fields. In situ magnetic permeability measurements are performed to monitor the evolution of martensite content throughout each experiment. The experimental results indicate that the martensitic transformation kinetics cannot be described solely by a monotonically increasing function of stress triaxiality: for instance, less martensite is developed under equi-biaxial tension than under uniaxial tension for the same increment in equivalent plastic strain. A stress-state-dependent transformation kinetics law is proposed that incorporates the effect of the Lode angle parameter in addition to the stress triaxiality. In the proposed model, the rate of martensite formation increases monotonically with the stress triaxiality and the Lode angle parameter. The comparison with the experimental data demonstrates that the proposed transformation kinetics law provides an accurate description of the evolution of the martensite content in stainless steel 301LN over a wide range of stress states.


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