Detection of grain-boundary resistance to slip transfer using nanoindentation

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.

Time-of-flight atom probe measurements on Ni3Al and Co3W

In this study, a VG FIM100 was taken into operation, consisting of a field-ion microscope (FIM), a time-of-flight atom probe (TOFAP) and an imaging atom probe. A tungsten specimen was used to calibrate the conversion of flight times to m/n values. The resulting relative mass resolution of the TOFAP was calculated to be m/Δm≈500 FWHM. In time-of-flight measurements of homemade boron-doped Ni-rich Ni3Al, a so-called ladder diagram was constructed from the evaporation data of a <001> pole. This ladder diagram revealed a very high degree of ordering in the alternating pure Ni and mixed Ni/Al planes: only 0.4% of the detected Al atoms were located on pure Ni planes. The number of null pulses to start a new plane was found to be much higher for Ni/Al planes (5 × 10^2) than for Ni planes (1 × 10^2). Moreover, the ladder diagram showed that boron was uniformly distributed through the matrix with nearly all boron found on pure Ni-planes. The suggestion that boron preferentially settles on these planes would be supported by reports of a strong Ni–B bond, since on Ni planes, B atoms can be accommodated on octahedral interstitial sites surrounded by only Ni atoms. Finally, we performed time-of-flight measurements on Co3W. The ion species observed in these measurements included a wide variety of WNn2+ ions, with 0≤n≤9. Especially, the ions with odd n prevailed in this ion group.

Incipient plasticity during nanoindentation at grain boundaries in body-centered cubic metals

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.