Magnetocaloric effect, excess vacancy annihilation, equilibrium shapes and triple junctions

March 22, 2011

[1] Systematic study of the microstructure, entropy change and adiabatic temperature change in optimized La–Fe–Si alloys

J Liu et al

A systematic study of the microstructure and magnetocaloric effect in LaFe11.8Si1.2 and LaFe11.6Si1.4 alloys over a large range of annealing temperatures and times has been carried out. With the aim of obtaining the pure 1:13 phase and maximum magnetic entropy change the annealing temperature was optimized at 1373 K for LaFe11.8Si1.2 and 1323 K for LaFe11.6Si1.4. We found a unique morphology of eutectoid-type lamellae, which is suggested to be an intermediate phase upon formation of the 1:13 phase. Adiabatic temperature change ΔTad measurements were employed to directly assess the magnetocaloric effect. By application of a magnetic field of 1.9 T large ΔTad values of 7.3 K and 7.0 K in the vicinity of the transition temperatures were found for LaFe11.8Si1.2 and LaFe11.6Si1.4, respectively, after optimized annealing. By considering the partial irreversibility of magnetostructural transition the influence of thermal and magnetic hysteresis on magnetic entropy change and ΔTad is also discussed.
Highlights

► Optimized annealing temperature and time in La-Fe-Si. ► Lamellar structure as an intermediate phase ► Large adiabatic temperature change of about 7 K for La-Fe-Si.

[2] Modeling of excess vacancy annihilation at different types of sinks

F D Fischer et al

The equilibrium site fraction of vacancies increases with temperature and, thus, annealing and rapid quenching may lead to states with a significant vacancy supersaturation. Excess vacancies can then gradually annihilate at available sinks represented by jogs at dislocations, by grain boundaries or free surfaces. Significant supersaturation by vacancies may also lead to the nucleation and growth of Frank loops acting as additional sinks. Three models corresponding to three different annihilation mechanisms are developed in this paper. They refer to annihilation of excess vacancies at jogs at dislocation with a constant density, at homogeneously distributed Frank loops with a constant density and at grain boundaries. The simulations based on the models are performed for individual annihilation mechanisms under isothermal and non-isothermal conditions as well as for simultaneous annihilation of vacancies at Frank loops and dislocation jogs and grain boundaries using different cooling conditions.
Highlights

► The kinetics of generation and annihilation of vacancies is modeled. ► Dislocation jogs, grain boundaries and Frank loops act as sources and sinks. ► The kinetics is determined by the densities of considered objects. ► The vacancy concentration kinetics reflects the thermo-mechanical treatment.

[3] The equilibrium crystal shape of nickel

H Meltzman et al

The crystal shape of Ni particles, dewetted in the solid state on sapphire substrates, was examined as a function of the partial pressure of oxygen (P(O2)) and iron content using scanning and transmission electron microscopy. The chemical composition of the surface was characterized by atom-probe tomography. Unlike other face-centered cubic (fcc) equilibrium crystal shapes, the Ni crystals containing little or no impurities exhibited a faceted shape, indicating large surface anisotropy. In http://dx.doi.org/10.1016/j.actamat.2011.02.021addition to the {1 1 1}, {1 0 0} and {1 1 0} facets, which are usually present in the equilibrium crystal shape of fcc metals, high-index facets were identified such as {1 3 5} and {1 3 8} at low P(O2), and {0 1 2} and {0 1 3} at higher P(O2). The presence of iron altered the crystal shape into a truncated sphere with only facets parallel to denser planes. The issue of particle equilibration is discussed specifically for the case of solid-state dewetting.
Highlights

► The ECS of pure Ni is completely facetted with both dense and high-index planes. ► The partial pressure of oxygen has a significant effect on the surface anisotropy. ► The addition of Fe decreased the anisotropy and de-stabilized high-index planes. ► During solid dewetting nucleation barriers prevent equilibration of the top facet.

[4] Triple junction effects in solids

B Zhao et al

The grain boundary–free surface triple line tension and grain boundary triple line tension were investigated in copper using a recently introduced novel approach. The effect of triple line tension on grain growth, Zener drag and Gibbs–Thompson relation was studied. The results showed that the triple line tension has a considerable effect on grain growth, particle–boundary interactions and void shape, especially for nanocrystalline materials.
Highlights

► The effect of triple junctions on a variety of metallurgical phenomena was investigated. ► For copper the groove root triple line energy was determined as 1.5 E-8 J/m. ► For copper the grain boundary triple line energy was determined as 6 E-9 J/m ► In nanocrystalline materials the triple line energy contributes to the driving force for grain growth. The triple line energy prevents a grain boundary to wet nanoscopic particles and voids.

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