Relationship between yield point phenomena and the nanoindentation pop-in behavior of steel
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Chang-Seok Oh Advanced Materials Research and Implementation Center, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea
Kyooyoung Lee Technical Research Laboratories, POSCO, Kwangyang, Jeonnam 545-090, Republic of Korea
Easo P. George Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Heung Nam Hana) Department of Materials Science and Engineering and Center for Iron and Steel Research, RIAM, Seoul National University, Kwanak-gu, Seoul 151-744, Republic of Korea (Received 4 April 2011; accepted 8 June 2011)
Pop-ins on nanoindentation load–displacement curves of a ferritic steel were correlated with yield drops on its tensile stress–strain curves. To investigate the relationship between these two phenomena, nanoindentation and tensile tests were performed on annealed specimens, prestrained specimens, and specimens aged for various times after prestraining. Clear nanoindentation pop-ins were observed on annealed specimens, which disappeared when specimens were indented right after the prestrain, but reappeared to varying degrees after strain aging. Yield drops in tensile tests showed similar disappearance and appearance, indicating that the two phenomena, at the nano- and macroscale, respectively, are closely related and influenced by dislocation locking by solutes (Cottrell atmospheres).
I. INTRODUCTION
Recently, nanoindentation has been used to probe small-scale mechanical properties, which are relevant to a wide range of materials and applications. The response of a material to nanoindentation is usually presented in the form of a load–displacement curve. When metallic materials undergo irreversible permanent deformation, discrete physical events, such as dislocation nucleation,1–18 dislocation source activation,14,15 phase transformation,16,19–30 and mechanically induced twinning,31,32 can be detected as discontinuities on nanoindentation load–displacement curves. Among these, we consider here just the onset of plasticity resulting from dislocation nucleation or dislocation source activation, which can produce geometrical softening accompanied by a sudden displacement excursion in the very early stage of mechanical contact during load-controlled nanoindentation, an event referred to in the literature as a “pop-in.” Geometrical softening can also occur as a result of yield point phenomena manifested as yield drops a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.208 J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012
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followed by propagation of Lüders strain in the early part of the tensile stress–strain curves of body-centered cubic metals. A mechanism for this was first proposed by Cottrell33 wherein dislocations that are trapped by solute atmospheres break free simultaneously at the upper yield point and multiply, causing instantaneous softening and a sharp yield drop. Then, when the load is removed after plastic deformation and the specimen al
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