Some recent papers of interest

March 15, 2010

[1] Comparing properties of substrate-constrained and freestanding epitaxial Ni–Mn–Ga films

Anja Backen et al

In order to use the magnetic shape memory alloy Ni–Mn–Ga with its high achievable strain of up to 10% for microactuators, freestanding epitaxial films are required. Here we show that these conditions can be fulfilled when using chromium as a sacrificial layer. The low misfit towards Ni–Mn–Ga enables epitaxial growth. Furthermore, Cr can afterwards be removed selectively by wet-chemical etching and during deposition no significant interdiffusion is observed. The structure, microstructure and magnetic properties of micrometer thick films are not affected by the etching process. Films are ferromagnetic at room temperature and we observe the coexistence of non-modulated (NM) and seven layered modulated (14 M) martensite. Tensile stress–strain measurement of a freestanding bridge reveals a broad strain plateau of 12% at a twinning stress of 25 MPa, indicating reorientation of NM variants.

[2] Kinetics of dislocations in pure Fe. Part I. In situ straining experiments at room temperature

D Caillard

In situ straining experiments have been carried out in pure Fe, in order to determine the geometry and the kinetics of dislocation glide at room temperature. Straight screw dislocations glide slowly in {1 1 0} elemental slip planes, at a velocity proportional to their length, whereas curved non-screw parts are highly mobile. The exact loop shape can yield the local stress as well as the difference of core energy between pure screw and near-screw orientations. The velocity–stress dependence of screws has been measured at the scale of a single dislocation source, and compared with macroscopic activation areas. The results are discussed in terms of the kink-pair mechanism.

[3] Kinetics of dislocations in pure Fe. Part II. In situ straining experiments at low temperature

D Caillard

In situ straining experiments have been carried out at low temperature in pure Fe, in order to study the change of mechanism occurring at around 250 K. The local stress necessary to move individual screw dislocations is in good agreement with the macroscopic yield stress at various temperatures. In the lower temperature range, straight screw segments have a jerky motion in {1 1 0} planes, at variance from the steady motion observed near room temperature. The distributions of waiting times in locked positions, and jump distances, the temperature variation of the average jump distance, and the stress/temperature variation of the macroscopic activation areas, are inconsistent with the kink-pair mechanism observed above 250 K. They have been interpreted in terms of a locking–unlocking mechanism, already proposed in hexagonal-closed-packed metals. Under such conditions, the change of mechanism at 250 K can account for the surprisingly low value of the flow stress extrapolated to 0 K (much lower than the theoretical Peierls stress).

[4] A micromechanical formulation for piezoelectric fiber composites with nonlinear and viscoelastic constituents

Anastasia H Muliana

This study presents a simplified micromechanical model to predict electromechanical behaviors of piezocomposites, having ferroelectric fibers and polymer matrix. A nonlinear electromechanical constitutive model is formulated for the ferroelectric fibers, i.e. PZT fibers. The nonlinearity is due to polarization switching in the PZT materials under high electric field and compression stress. Phenomenological models are used to represent stress–strain and polarization–electric field hysteresis responses during polarization switching. The nonlinear electromechanical constitutive relation is verified using hysteresis polarization and strain responses of PZT-51. Effective responses of piezocomposites at various fiber volume contents, generated using the simplified micromechanical model, are also compared with available experimental data. The effects of viscoelastic polymer matrix on the overall electromechanical hysteresis and creep behaviors of piezocomposites are also examined.

[5] An analytical model for constitutional supercooling-driven grain formation and grain size prediction

M Qian et al

Being able to predict the grain formation process and attendant grain size has been a central topic in solidification. Such an analytical model is presented for constitutional supercooling (CS)-driven grain formation with several simplifications. The model links the nucleation of new grains to the growth of a larger neighbouring grain. The average grain size (View the MathML source) is thus determined by two components: the minimum growth (rcs) necessary to establish sufficient CS (ΔTn) for nucleating new grains, and the spatial mean distance (View the MathML source) to the most potent available nucleants. Both spherical and planar growth fronts are considered, covering growth curvatures from small to infinite. Two distinct fundamental approaches are used, which result in identical descriptions of View the MathML source, where View the MathML source (D is the diffusion coefficient, v is the growth velocity, Q is the growth restriction factor). The model is compared with literature data produced under various conditions and demonstrated on aluminium alloys as an example.

[6] Surfaces, interfaces and phase transitions in Al–In monotectic alloys

I Kaban et al

Surface and bulk liquid phase transitions are measured by a unique method currently used to determine surface and interfacial tension of liquid alloys. Focusing on the Al–In system, the location of the liquid miscibility gap was determined from the critical to the monotectic temperatures. The surface tensions of nine liquid alloys, the interfacial tension between coexisting liquids and their densities were measured as a function of temperature. Implementing the bulk data extracted from the asymmetric miscibility gap into a sub-regular model reproduced the experimental surface and interfacial tensions. The wetting temperature was estimated to lie well below the monotectic temperature. The micrometer thickness of the In-rich films which wet the surface of the Al-rich liquid phase after solidification is suggested to be due to the growth of the equilibrium wetting film by diffusion from the Al-rich phase during cooling.

[7] On the characteristics of substructure development through dynamic recrystallization

Hossein Beladi et al

Substructure development in an austenitic Ni–30%Fe model alloy was investigated within a dynamic recrystallization (DRX) regime. The substructure characteristics of the deformed matrix and DRX grains were markedly different regardless of the grain size and orientation. The former largely displayed ‘organized’, banded subgrain arrangements with alternating misorientations, resulting from a limited number of active slip systems. In contrast, the substructure of DRX grains was generally more ‘random’ and exhibited complex subgrain/cell arrangements characterized by local accumulation of misorientations, suggesting multiple slip. The proposed mechanism of the unique substructure development within DRX grains suggests that the DRX nuclei, forming along pre-existing grain boundaries and triple points, essentially represent grain boundary regions, which experience multiple slip to preserve the compatibility with neighbouring deformed grains. This results in the formation of a complex cell/subgrain structure, which progressively extends as the grain boundary regions expand outwards during DRX growth.

[8] Effect of coalescence on threading dislocations in GaN films

Yun Liu and Jia Zhang

The effect of islands coalescence on threading dislocations (TDs) in GaN films (300 nm thick) grown on non-annealed and annealed sapphire substrates has been studied. Atomic force microscopy measurement shows the a-type TDs density first decreases and then increases during the coalescence process, while the densities of (a+c)- and c-type TDs decrease with increasing coalescence proportions. X-ray diffraction data indicates that lattice tilt of GaN films is greatly reduced by coalescence while the change of twist depends on coalescence proportions.

[9] Stress-enhanced growth rate of alumina scale formed on FeCrAlY alloy

X Zhao et al

The effect of stresses on the growth rate of alumina scale formed on FeCrAlY alloy is reported. It is found the compressive growth stress slows the outward diffusion of carbon in the oxide, resulting in enhanced O2- ion transport, thereby accelerating scale growth. The results are confirmed by the electrical measurements on the oxidised alloy that reveal a higher ionic conductivity can be correlated with a higher compressive stress. Possible mechanisms of this phenomenon are discussed.

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