Nanoindentation study of slip transfer phenomenon at grain boundaries
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Nanoindentation was undertaken near grain boundaries to increase understanding of their individual contributions to the material’s macroscopic mechanical properties. Prior work with nanoindentation in body-centered cubic (bcc) materials has shown that some grain boundaries produce a “pop-in” event, an excursion in the load– displacement curve. In the current work, grain boundary associated pop-in events were observed in a Fe–0.01 wt% C polycrystal (bcc), and this is characteristic of high resistance to intergranular slip transfer. Grain boundaries with greater misalignment of slip systems tended to exhibit greater resistance to slip transfer. Grain boundary associated pop-ins were not observed in pure copper (face-centered cubic) or interstitial free steel ~0.002 wt% C (bcc). Additionally, it was found that cold work of the Fe–0.01 wt% C polycrystal immediately prior to indentation completely suppressed grain boundary associated pop-in events. It is concluded that the grain boundary associated pop-in events are directly linked to interstitials pinning dislocations on or near the boundary. This links well with macroscopic Hall–Petch effect observations.
I. INTRODUCTION
Many materials are strengthened by the presence of grain boundaries so that higher strength results at smaller grain sizes. The Hall–Petch1,2 equation quantitatively describes the relationship [Eq. (1)] linking materials properties, such as the yield stress sy to the size of grains d. sy ¼ s0 þ kyd 1=2
;
ð1Þ
s0 is a material constant representing the stress required to cause dislocation motion within a grain and ky another material constant representing the grain boundary contribution to yield stress. The Hall–Petch relationship implies that grain boundaries, on average, have an inherent resistance to slip transfer. The grain boundary geometry should have a marked impact on the boundary’s resistance to slip transfer. In some cases, slip transfer can be due to direct transmission of dislocations across a boundary, such as with screw dislocations approaching a twin boundary, where both Burgers vector and line direction are identical across the boundary permitting the dislocation to transmit easily into the next grain (Fig. 1). More generally, direct transmission is accompanied by formation of Address all correspondence to these authors. a) e-mail: [email protected] c) e-mail: [email protected] b) Present address: IMMPETUS, Department of Engineering Materials, University of Sheffield, Sheffield S1 3JD, United Kingdom DOI: 10.1557/JMR.2009.0088 J. Mater. Res., Vol. 24, No. 3, Mar 2009
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residual dislocations on the grain boundary, which are required to conserve the overall Burgers vector. However, more often dislocations will pile up at the boundary until indirect slip transfer occurs as a raised level of local stresses cause activation of a dislocation source on the other side of the boundary. With either direct or indirect slip transfer, strengthening occurs
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