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

Abstract

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.

Nanoindentation study of corrosion-induced grain boundary degradation in a pipeline steel

“…Micro-hardness experiments indicate that segregation of solute atoms at the GBs results in either hardened or softened regions compared to the interior of the adjacent grains [11,12]. Nanoindentation has detected a significant increase in the intrinsic hardness of the material near the GBs due to dislocation pinning [13][14][15][16][17][18]. In situ nanoindentation measurements during electrochemical exposures of metals at cathodic potentials revealed near-surface softening due to absorbed hydrogen [19,20].…”

Mechanical Characterization of Mesoscale Interfaces Using Indentation Techniques

“…Moreover, GB pop-ins were not observed in interstitial-free steel or in fcc copper, but were observed in commercial steel with interstitials. Soer et al 62,67 studied grain boundaries in Fe-14%Si and Mo bicrystals and reported consistent GB pop-ins in Fe-14%Si, but not in Mo. Aifantis et al 65 also reported GB pop-ins in Fe-2.2%Si bicrystals.…”

“…Interestingly, the incipient pop-ins have been observed to be suppressed or mitigated in indentations conducted close to the grain boundary (compared to those conducted in the grain interior). 24,[60][61][62][63] These observations have suggested that the grain boundaries serve as potent sources of dislocations.…”

“…3b are referred as grain boundary (GB) pop-ins. 61,62,[64][65][66][67] These are attributed to slip transfer events at the grain boundary. These occur at significantly higher load levels (after the sample has experienced a significant amount of plasticity in the indentation zone).…”

“…In other studies, [61][62][63][68][69][70][71][72][73][74] researchers have used a Berkovich indenter to measure the local hardness in the grain boundary region as a function of distance from the grain boundary. In general, most of these measurements show an increase in hardness in the vicinity of the grain boundaries (up to 30%) compared to the grain interiors.…”