Nucleation barrier for phase transformations in nanosized crystals

On the basis of classical nucleation theory of martensitic transformation (MT), the effect of internal stress on autocatalytic nucleation of MT is quantitatively described. The complex stress field pictures outside a formed martensitic plate are first calculated through Eshelby's theory, and then the interaction energies between the martensitic plate and embryos with 24 possible variants can be further obtained. Based on the calculation of interaction energy, the effect of internal stress on autocatalytic nucleation of MT and morphological characteristics of martensite are explained. In this investigation, two kinds of alloys-non-thermoelastic materials (e.g., Fe-30Ni) and thermoelastic materials (e.g., Cu-46.6Zn)-are studied, and their difference in autocatalytic nucleation and morphological characteristics of martensite are indicated.

Section: Discussionmentioning

confidence: 99%

Effect of internal stress on autocatalytic nucleation of martensitic transformation

Meng

Rong

Hsu

2006

“…[24,25] Indeed, it is well established that the triggering of the martensitic transformation depends on the chemistry as well as on the local microstructural features that may (1) act as nucleation sites (deformation bands, dislocation distribution heterogeneities) or, on the contrary, (2) inhibit its formation by constraining effects (fine grain size). [26][27][28][29] The presence of this metastable homogeneous c layer formed by the HCPEB treatment at the top surface is very important for potential applications. Indeed, it has been clearly established that the redissolution of carbides or second-phase particles in the melted layer of HCPEB-treated samples brings a more homogeneous microstructure having enhanced corrosion resistance properties.…”

Section: Discussionmentioning

confidence: 99%

Texture and Microstructure at the Surface of an AISI D2 Steel Treated by High Current Pulsed Electron Beam

Zou

Grosdidier

Bolle

et al. 2007

Surface modifications were investigated for an AISI D2 steel after 25 pulses of high current pulsed electron beam (HCPEB) treatment. In particular, X-ray diffraction (XRD) was used to determine the thickness, nature of phase, and texture in the top surface melted layer. The mechanisms responsible for the structure and texture evolutions were further asserted by scanning electron microscopy (SEM) and electron backscattering diffraction (EBSD) results. Under the action of the beam, the large carbides present in the steel were melted and dissolved so that the top surface rapidly solidified as a 3-lm-thick layer of ultra-fine-grained metastable austenite without any martensite formation. The growth of the c phase led to an unusual <100> + <110> fiber solidification texture. In addition, R3-twin boundaries were observed to bridge, most often, near <110> domains. These features, observed here in a Fe supersaturated and highly undercooled melt, are discussed in light of recent findings on atypical growth in directionally solidified Al-based alloys. [40][41][42] Comparatively, the a phase present in the heataffected zone remained randomly oriented.

“…It is suggested by the theoretical analyses that the nucleation barrier of phase transformation in nanosized crystals is predominantly controlled by the grain size. [20] Due to the displacive character of martensitic transformation, the nucleation on small particle surfaces or grain boundaries is difficult, which leads to the decrease in M s .…”

Section: Orthorhombicmentioning

confidence: 99%

“…A decrease in M s has also been reported in other nanometer-sized particles. [20][21][22] The nucleation barrier of phase transformations in nanosized crystals has been calculated [20] based on the concepts of the surface stress associated with phase equilibrium and interface equilibrium, as well as the addition of shear energy required for martensitic transformation. It is suggested by the theoretical analyses that the nucleation barrier of phase transformation in nanosized crystals is predominantly controlled by the grain size.…”

Section: Orthorhombicmentioning

confidence: 99%

New Sequences of Phase Transition in Ni-Mn-Ga Ferromagnetic Shape Memory Nanoparticles

Liu

Nie

Wang

et al. 2007

In-situ high-energy X-ray diffraction was used for tracing the structural transitions in Ni 2 MnGa and Ni 47 Mn 25 Ga 22 Co 6 nanoparticles that were prepared by a ball-milling and subsequent annealing process. A decrease in martensitic transformation temperature was evidenced in all annealed nanoparticles. It was found that a three-layered modulated structure, usually observed as a metastable premartensitic phase in coarsegrained alloys, becomes a stable phase in the annealed Ni 2 MnGa nanoparticles up to 4 K.