The mechanical response to nanoindentation near grain boundaries has been investigated in an Fe-14%Si bicrystal with a general grain boundary and two Mo bicrystals with symmetric tilt boundaries. In particular, the indentations performed on the Fe-14%Si show that as the grain boundary is approached, in addition to the occurrence of a first plateau in the load versus depth nanoindentation curve, which indicates grain interior yielding, a second plateau is observed, which is believed to indicate dislocation transfer across the boundary. It is noted that the hardness at the onset of these yield excursions increases as the distance of the tip to the boundary decreases, providing thus a new type of size effects, which can be obtained through nanoindentation. The energy released during an excursion compares well to the calculated interaction energy of the piled-up dislocations. Hall-Petch slope values calculated from the excursions are consistent with macroscopically determined properties, suggesting that the Hall-Petch slope may be used to predict whether slip transmission occurs during indentation. No slip transmission was observed in the Mo bicrystals; however, the staircase yielding commonly found during initial loading was suppressed in the proximity of a grain boundary due to preferential dislocation nucleation at the boundary. An estimate for the nucleation shear stress at the boundary was obtained from the measured interaction range.

In this paper, it is shown that the occurrence of dislocation pileups across grain boundaries, as well as subsequent emission to the adjacent grains, is captured theoretically by gradient plasticity and confirmed experimentally by nanoindentation. From a theoretical point of view, this is accomplished (within a deformation theory framework applicable to continued loading) by accounting for a specific interfacial term in the overall potential of the material, in terms of which its response, taken to conform to strain gradient plasticity, is defined. The main features that result from the addition of this interfacial term are (i) significant size effects of Hall-Petch type in the overall stress-strain response of polycrystals and (ii) the determination of an analytical expression for the stress corresponding to the onset of dislocation transfer across interfaces. From an experimental point of view, the effective stress at which dislocation transfer takes place across an interface can be obtained from nanoindentations performed in close proximity to an Fe-2.2 wt.% Si grain boundary, since they exhibit a distinct strain burst that is related to the presence of the boundary. It is possible, therefore, to fit the theoretically determined analytical expression for the interfacial yield stress to the experimental data. From this fit, first estimates are obtained for key material parameters, namely the interfacial term and the internal length, that are required for the theoretical formulation. Dislocation mechanics are employed to provide physical insight of these parameters.

Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. AbstractUsing in situ nanoindentation in a transmission electron microscope (TEM) the indentation-induced plasticity in ultrafinegrained Al and Al-Mg thin films has been studied, together with conventional quantitative ex situ nanoindentations. Extensive grain boundary motion has been observed in pure Al, whereas Mg solutes effectively pin high-angle grain boundaries in the Al-Mg alloy films. The proposed mechanism for this pinning is a change in the atomic structure of the boundaries, possibly aided by solute drag on extrinsic grain boundary dislocations. The mobility of low-angle boundaries is not affected by the presence of Mg. Based on the direct observations of incipient plasticity in Al and Al-Mg, it was concluded that solute drag accounts for the absence of discrete strain bursts in indentation of Al-Mg.

Nanoindentation measurements near a high-angle grain boundary in a Fe-14%Si bicrystal showed dislocation pile-up and transmission across the boundary. The latter is observed as a characteristic displacement jump, from which the Hall -Petch slope can be calculated as a measure for the slip transmission properties of the boundary. D

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