Phase transformation kinetics under stress, solid state dendrites, Potts model for 3D grain growth, and, microstructure and magnetic properties

March 29, 2009

[1] Austenite–ferrite transformation kinetics under uniaxial compressive stress in Fe–2.96 at.% Ni alloy

Y C Liu et al

The effect of an applied constant uniaxial compressive stress on the kinetics of the austenite (γ) → ferrite (α) massive transformation in the substitutional Fe–2.96 at.% Ni alloy upon isochronal cooling has been studied by differential dilatometry. All imposed stress levels are below the yield stress of austenite and ferrite in the temperature range of the transformation. An increase in compressive stress results in a small but significant increase of the onset temperature of the γ → α transformation and a decrease of the overall transformation time. A phase transformation model, involving site saturation, interface-controlled growth and incorporation of an appropriate impingement correction, has been employed to extract the interface-migration velocity of the γ/α interface. The interface-migration velocity for the γ → α transformation is approximately constant at fixed uniaxial compressive stress and increases with increasing applied uniaxial compressive stress. Furthermore, the value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the γ/α volume misfit depends on the transformed fraction and decreases significantly as the applied uniaxial compressive stress increases. An understanding of the observed effects is obtained, recognizing the constraints imposed on the phase transformation due to the applied stress.

[2] Competition between surface energy and elastic anisotropies in the growth of coherent solid-state dendrites

M Greenwood et al

A new phase-field model of microstructural evolution is presented that includes the effects of elastic strain energy. The model’s thin interface behavior is investigated by mapping it onto a recent model developed by Echebarria et al. [Echebarria B, Folch R, Karma A, Plapp M. Phys Rev E 2004;70:061604]. Exploiting this thin interface analysis, the growth of solid-state dendrites are simulated with diffuse interfaces and the phase-field and mechanical equilibrium equations are solved in real space on an adaptive mesh. A morphological competition between surface energy anisotropy and elastic anisotropy is examined. Two dimensional simulations are reported that show that solid-state dendritic structures undergo a transition from a surface-dominated [Meiron DI. Phys Rev A 1986;33:2704] growth direction to an elastically driven [Steinbach I, Apel M. Phys D – Nonlinear Phenomena 2006;217:153] growth direction due to changes in the elastic anisotropy, the surface anisotropy and the supersaturation. Using the curvature and strain corrections to the equilibrium interfacial composition and linear stability theory for isotropic precipitates as calculated by Mullins and Sekerka, the dominant growth morphology is predicted.

[3] Implementation of exact grain-boundary geometry into a 3-D Monte-Carlo (Potts) model for microstructure evolution

O M Ivasishin et al

A three-dimensional Monte-Carlo (Potts) model was modified to incorporate the effect of grain-boundary inclination on boundary mobility. For this purpose, a straightforward geometric construction was developed to determine the local orientation of the grain-boundary plane. The combined effects of grain-boundary plane and misorientation on the effective grain-boundary mobility were incorporated into the Monte-Carlo code using the definition of the tilt–twist component. The modified code was validated by simulating grain growth in microstructures comprising equiaxed or elongated grains as well as the static recrystallization of a microstructure of deformed (elongated) grains.

[4] Effects of quenching speeds on microstructure and magnetic properties of novel SmCo6.9Hf0.1(CNTs)0.05 melt-spun ribbons

J-B Sun et al

By adding carbon nanotubes (CNTs) to SmCo6.9Hf0.1, novel SmCo6.9Hf0.1(CNTs)0.05 as-cast alloy has been prepared, which consists of Sm(Co,Hf)7 as the main phase, a small amount of SmCo5 and a particle-like grain boundary phase Hf(CNTs). SmCo6.9Hf0.1(CNTs)0.05 ribbons melt-spun at speeds of 10–50 m s−1 have a single TbCu7-type structure. Increasing the quenching speed can result in a decrease in ribbon thickness and grain boundary width. Meanwhile, the grain size tends to be smaller and the grain boundary phase tends to be more dispersed. A new Sm(Co,Hf)7(CNTs)x boundary phase may be formed in SmCo6.9Hf0.1(CNTs)0.05 ribbons. Increasing the quenching speed can also enhance coercivity, remanence and remanence ratio. The ribbons melt-spun at a speed of 50 m s−1 display the best magnetic properties: Hci = 18.781 kOe, Ms2T = 76.87 emu g−1, Mr = 66.79 emu g−1 and Mr/Ms2T = 0.87.

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