Inhomogeneous nano-mechanical properties in the multi-phase microstructure of long-term aged type 316 stainless steel
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K. Sawada and K. Kimura Materials Information Technology Station, National Institute for Materials Science, Tsukuba 305-0047, Japan (Received 23 October 2005; accepted 31 January 2006)
Microstructure-related local deformation behavior was evaluated using nanoindentation techniques for a type 316 austenitic stainless steel used in energy generation plants. The sample was aged for 39,332 h (4.5 years) at 700 °C. The microstructure included the phase precipitated at grain boundaries and in grain interiors. The nanohardnesses of the phase and the matrix in the aged and virgin samples were evaluated quantitatively. The hardness of the phase was found to be extremely high in the order of 17 GPa, which is much higher than the value of about 3.4 GPa for the matrix in the aged sample. The hardnesses of the phase at the grain boundary and the adjoining matrix were the same as those in the grain interior. Moreover, the hardness of the matrix of the aged sample was about 30% lower than that of the virgin sample while the Vickers hardness as a macroscopic strength of the aged sample was about 45% higher than that of the virgin one. The deformation and fracture behavior in a local region was discussed in terms of the inhomogeneous mechanical properties in the multi-phase microstructure.
I. INTRODUCTION
Macroscopic mechanical properties of structural materials are assembled by local deformation behavior in a multi-phase microstructure. For example, fracture phenomena are associated with the properties of each structure or phase such as grain boundary, second phase, and hetero-phase interface because these anomalous microstructures are preferential sites for crack initiation and propagation. However, the microstructural unit in the multi-phase material is generally in the order of a micron or smaller; therefore, it becomes difficult to evaluate the properties of each phase by classic methods. The only way to consider the deformation and fracture mechanisms in conventional techniques is through the microstructural consideration of the mechanical properties of component phases. Some precipitates of ceramics or intermetallic compounds are speculated to be very hard and the matrix phase to be relatively soft, but the quantitative hardness of each phase is unknown in most cases. A direct evaluation of the local strength of each phase and the revelation of its relation to the macroscopic properties are essential for a more exhaustive consideration. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0143 J. Mater. Res., Vol. 21, No. 5, May 2006
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The instrumented nanoindentation technique1,2 with a capability of in situ scanning-probe microscopy (SPM) can measure the nanoscale mechanical properties of a precise site in a microstructure with accuracy within a nanometer on a SPM image. Ohmura et al. have applied this technique to materials with a very fine microstructure of martensitic steel3–6 and ultrafine grained steel.
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