[1] In situ study of nucleation and growth of the irregular α-Al/β-Al5FeSi eutectic by 3-D synchrotron X-ray microtomography

S Terzi et al

In order to better understand the formation of β-Al5FeSi intermetallic plates during solidification of Al–Si casting alloys, an Al–8% Si–4% Cu–0.8% Fe alloy has been studied by in situ microtomography using high-energy X-rays in the synchrotron. After formation of the aluminium dendrites, the β phase forms as an irregular eutectic together with eutectic α-Al. Only four plates were nucleated in the sample, and all nucleated in the very early stage of the eutectic reaction and subsequently developed into complex connected three-dimensional plates. The plates display very rapid lateral growth and slow thickening, which, together with the observation of imprints of dendrites and ridges in the plates, suggest a very weakly coupled eutectic.

[2] Deformation of hierarchically twinned martensite

P Muellner and A H King

Shape-memory alloys deform via the reorganization of a hierarchically twinned microstructure. Twin boundaries themselves present obstacles for twin boundary motion. In spite of a high density of obstacles, twinning stresses of Ni–Mn–Ga Heusler alloys are very low. Neither atomistic nor dislocation-based models account for such low yield stresses. Twinning mechanisms are studied here on a mesoscopic length scale making use of the disclination theory. In a first approach, a strictly periodic twin pattern containing periodic disclination walls with optimally screened stress fields is considered. Strict periodicity implies that the twin microstructure reorganizes homogeneously. In a second approach, a discontinuity of the fraction of secondary twins is introduced and modeled as a disclination dipole. The stress required for nucleation of this discontinuity is larger than the stress required for homogeneous reorganization. However, once the dipole is formed, it can move under a much smaller stress in agreement with experimental findings.

[3] New Interpretation of the Haasen Plot for Solute-strengthened Alloys

W A Curtin

The Haasen plot (inverse activation area 1/Δa versus offset flow stress σ-σs) for solute-strengthened alloys is usually assumed additive, 1/Δa=1/Δas+1/Δaf, with 1/Δafnot, vert, similarβ(σ-σs) due to forest interactions. Experiments often show a slope < β. Here, a model for the dislocation activation enthalpy is proposed that predicts a slope 1/(Δasσs) determined only by solute parameters Δas and σs and not directly connected to forest hardening. This parameter-free prediction agrees well with a wide range of experiments on Al-X alloys at T=78K.

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In situ neutron-diffraction study of internal strain evolution around a crack tip under variable-amplitude fatigue-loading conditions

S Y Lee et al

In situ neutron-diffraction measurements were performed to investigate the lattice-strain evolution around a fatigue crack under five different loading conditions (i.e., fatigued, tensile overloaded, compressive underloaded, tensile overloaded-compressive underloaded, and compressive underloaded-tensile overloaded) during fatigue crack growth. The results show that different crack-growth behaviors are closely related to the distinct strain distributions developed near the crack tip under the various loading conditions.

Title: On the uniqueness of measuring elastoplastic properties from indentation: The indistinguishable mystical materials

Authors: X Chen, N Ogasawara, M Zhao, and N Chiba

Source: Journal of Mechanics and Physics of Solids (Article in press)

Abstract:

Indentation is widely used to extract material elastoplastic properties from the measured force–displacement curves. One of the most well-established indentation techniques utilizes dual (or plural) sharp indenters (which have different apex angles) to deduce key parameters such as the elastic modulus, yield stress, and work-hardening exponent for materials that obey the power-law constitutive relationship. However, the uniqueness of such analysis is not yet systematically studied or challenged. Here we show the existence of “mystical materials”, which have distinct elastoplastic properties yet they yield almost identical indentation behaviors, even when the indenter angle is varied in a large range. These mystical materials are, therefore, indistinguishable by many existing indentation analyses unless extreme (and often impractical) indenter angles are used. Explicit procedures of deriving these mystical materials are established, and the general characteristics of the mystical materials are discussed. In many cases, for a given indenter angle range, a material would have infinite numbers of mystical siblings, and the existence maps of the mystical materials are also obtained. Furthermore, we propose two alternative techniques to effectively distinguish these mystical materials. The study in this paper addresses the important question of the uniqueness of indentation test, as well as providing useful guidelines to properly use the indentation technique to measure material elastoplastic properties.

Internet resources: The pointer at iMechanica