Matrix strength evaluation of ultra-fine grained steel by nanoindentation
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N. Tsuji and N. Kamikawa Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan (Received 4 June 2003; accepted 20 October 2003)
Nanoindentation was performed to evaluate the matrix strength of the ultra-fine grained steel produced by accumulative roll-bonding and subsequently annealed. The nanohardness, associated with the matrix strength, decreases with increasing annealing temperature. Because the matrix strength corresponds to the first term 0 of the Hall–Petch relation, this result suggests that the 0 might not be constant for these steels between various grain sizes. Therefore, it is suggested that the change of the macroscopic strength during annealing is dominated by not only the grain coarsening leading to a reduction of grain refinement strengthening, but also the softening of the matrix strength.
I. INTRODUCTION
Ultra-fine grained (UFG) metals are expected to demonstrate exceptional mechanical properties such as high tensile strength and toughness. Recently, methods for producing UFG bulk metals by the severe plastic deformation (SPD) process1,2 have shown high strength in conventional tensile tests.3 The source of the high strength of the UFG materials is widely believed to be due to grain refinement strengthening, which is described through the Hall–Petch relation as = 0 + kd −1 Ⲑ 2
,
(1)
where is flow stress, 0 is the overall resistance of the crystal lattice to the dislocation movement, k is the locking parameter, and d is the grain diameter. The first term 0 corresponds to the matrix strength that is mainly dominated by strengthening factors such as solidsolution hardening, precipitation hardening, and dislocation interactions. The second term is associated with the contribution of grain boundaries. Although it has been reported that the flow stress of UFG materials roughly holds the Hall–Petch relationship,3,4 the detailed contribution from each strengthening mechanism is still unclear. For example, the UFG microstructures fabricated by SPD must include dislocations, and nonequilibrium structures have been reported at grain boundaries in the SPD materials.5 Additionally, dissolution of second phases might occur during SPD.6 All of these factors can contribute to the net strength of the UFG metals. For further interpretation of the strength mechanisms of UFG J. Mater. Res., Vol. 19, No. 1, Jan 2004
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metals, it is important to understand the contribution of each term individually. It has been shown by Ohmura et al.7–9 that the nanoindentation technique is a useful way to measure the matrix strength of the martensite phase, which has a very fine submicrometer structure, without the influence of the high-angle grain boundaries. The purpose of the current study is to reveal the matrix strength of the UFG ferritic steel produced by accumulative roll-bonding (ARB) using the nanoindentation technique. We examined the annealing temperature dependence of the ma
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