Materials Transactions Online

Materials Transactions, Vol.50 No.05 (2009) pp.1123-1127
© 2009 The Japan Institute of Metals

Effect of Nanocrystallization and Twinning on Hardness in Ni3Al Deformed by High-Pressure Torsion

Octav Ciuca1,2, Koichi Tsuchiya1,2, Yoshihiko Yokoyama3, Yoshikazu Todaka1 and Minoru Umemoto1

1Dept. of Production Systems Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan
2Innovative Materials Engineering Laboratory, National Institute for Materials Science, Tsukuba 305-0047, Japan
3Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Samples of Ni3Al intermetallic compound were subjected to deformation by high-pressure torsion (HPT). The plastically-deformed structure revealed a bimodal character: coarse grains, retaining a degree of long-range order, surrounded by regions of nanocrystalline, disordered grains. It was inferred that the grain refinement proceeds in an inhomogeneous manner throughout the sample. Grains as large as 100 nm in size were shown to contain only a low density of perfect dislocations, but a large density of nanotwins and stacking faults. These planar defects appeared to originate from the grain boundaries, suggesting that grain boundaries are active sources for Shockley-partial dislocations. Their formation is accompanied by a deviation of the microhardness dependence on grain size from the Hall-Petch behavior, potentially suggesting the activation of a deformation mechanism different from the one acting in coarse structures. The hardness saturates at a significantly larger grain size than in the case of nanostructured pure Ni.

(Received 2008/12/24; Accepted 2009/2/24; Published 2009/4/15)

Keywords: nanocrystalline materials, stacking faults, Shockley-partial dislocations, Hall-Petch effect

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