Indentation strain burst phenomenon induced by grain boundaries in niobium
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H.W. Ngana) Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China (Received 8 February 2004; accepted 20 May 2004)
Using depth-sensing indentation, a pop-in phenomenon induced by grain boundaries, namely, a sudden indenter displacement jump when indented near a grain boundary segment, was observed in polycrystalline niobium. This grain-boundary type of pop-in occurs at a larger force than the initial elasto-plastic pop-in, which is observed with and without a grain boundary nearby. The experimental results show that this pop-in effect has a close relationship with the misorientation across the grain boundary. The occurrence of this pop-in phenomenon is rationalized in terms of slip transmission across the grain boundary.
I. INTRODUCTION
Numerous previous attempts have been made in the past to use subgranular microhardness indentation to probe the hardening behavior of grain boundaries in bicrystals or large-grained polycrystalline specimens.1–8 Whether hardness, which is defined as the indentation load divided by the area of the indent made, can be a sensitive enough parameter to represent the hardening potential of a grain boundary, however, is controversial. In many reports where an increase in the measured hardness was observed, the grain boundary hardening was attributed to segregation of impurity atoms. Examples of these include doped zone-refined metals such as Pb, Sn and Zn,1 tin segregation in alpha iron–tin alloy,2 calcium segregation in NaCl bicrystals,3 and in niobium bicrystals.4,5 In these experiments, proper heat treatment procedures were followed to achieve enough segregation of impurities at grain boundaries. In other works, the observed grain boundary hardening was attributed to the difficulty in slip transmission across grain boundaries. From macroscopic tensile tests on highpurity aluminium, Wyrzykowski and Grabski6 concluded that the Hall–Petch slope ky depends on the distribution function of the grain boundary diffusivity, indicating a dependence of ky on the grain boundary structure. Lee et al.7 observed larger grain boundary hardening effects from microhardness measurements in undoped Ni3Al
II. EXPERIMENTAL PROCEDURE
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0316 2478
http://journals.cambridge.org
than in boron-doped Ni3Al, and by referencing to bulk deformation of polycrystalline samples in which a higher Hall–Petch slope is observed in the undoped situation, these authors concluded that a link exists between the degree of grain boundary hardening and the ease of slip transmission across grain boundaries. However, in a recent study by Wo and Ngan,8 an increase in the measured hardness near grain boundaries was not observed in depth-sensing indentation (more commonly known as nanoindentation) experiments on undoped Ni3Al. Careful scanning electron microscopy (SEM) and atomic force microscopy (AFM) observation of the indents made near a grain boundary and far away from
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