Approach to the Theoretical Strength of TieNieCu Alloy Nanocrystals by Grain Boundary Design Alexandr M. Glezer 1,2,3)* , Nadezhda A. Shurygina 1,2) , Elena N. Blinova 1) , Inga E. Permyakova 1,2) , Sergey A. Firstov 4) 1) Bardin Central Research Institute for the Iron and Steel Industry, Vtoraya Baumanskaya ul. 9/23, Moscow 105005, Russia 2) Moscow State University of Instrumental Engineering and Information Science (MGUPI), Moscow 107996, Russia 3) National University of Science and Technology (MISIS), Moscow 119049, Russia 4) Frantsevich Institute of Materials Science Problems, National Academy of Sciences of Ukraine, ul. Krzhizhanovskogo 3, Kiev 03680, Ukraine [Manuscript received March 21, 2014, in revised form May 7, 2014, Available online 28 September 2014] The grain boundary design was used to introduce boride Ti 2 B and TiB 2 nanoparticles of 5 nm in size into grain boundaries of nanocrystalline Ti 50 Ni 25 Cu 25 alloy. As a result, the maximum normalized microhardness was increased by 20% and the theoretical limit of hardness is substantially approached. It is proposed that boride nanoparticles suppressed low-temperature grain-boundary sliding and, therefore, shifted the range of the anomalous behavior of HallePetch relation toward smaller sizes of the TieNieCu nanocrystals. KEY WORDS: Electron microscopy; Microhardness; Nanocrystal; Nanoparticle; Grain boundary sliding; Theoretical hardness 1. Introduction Grain boundaries (GBs) are very important structure elements determining strength properties of nanocrystalline materials [1] . When using engineering of GBs to affect GBs, one can control the physicomechanical properties of materials [2] . For example, the grain-boundary segregation of harmful impurities can cause embrittlement and a substantial decrease in the strength of ma- terials. In contrast, the formation of “useful” impurities, which increase the cohesion strength of GBs, can substantially increase the mechanical properties of materials [3] . It is well known that the grain-size dependence of the yield strength (hardness) of polycrystalline metals or alloys is described by the HallePetch relation [4] : s y  HV  ¼ s 0  HV 0  þ k y $D 1=2 (1) where s y is the yield strength, HV is the hardness, s 0 is the plastic flow stress and HV 0 is the hardness in a grain body, k y is the coefficient of proportionality characterizing the GB penetrability, and D is the average grain size. If Eq. (1) is assumed to be universal, it can be expected that any material consisting of a polycrystalline ensemble of grains can reach ultimate (theoretical) shear strength s y * [5] (theoretical hardness HV* [6] )(Fig. 1, curve 1) as D decreases into the nanometer range. However, many experimental researchers dealing with the mechanical properties of nanocrystalline mate- rials detected substantial deviations from Eq. (1) in the nano- meter grain size range [7,8] . Fig. 1 schematically shows the experimental behavior of s y , i.e., saturation (curve 2) or even decrease (curve 3) with decreasing D. This anomaly makes it impossible to achieve the near-ultimate strength of nanocrystals and, hence to use the physical strength margin of solids. Many researchers tried to explain the anomaly in the Halle Petch relation for nanocrystals [9e11] , and most proposed struc- tural models are based on dislocation concepts. According to literature [10] , the anomaly of the HallePetch relation and other specific features of the plastic deformation and fracture of nanocrystalline materials are caused by the change in the struc- tural mechanism of plastic flow from the classical dislocation mechanism to the mechanism of low-temperature grain-bound- ary sliding (LTGBS). According to the structural classification of nanocrystals with allowance for their deformation behavior [12] , the following three size groups exist: large, medium, and small nanocrystals. A grain body is the predominant structural element for large nanocrystals (grain size > 30 nm), and deformation proceeds via dislocation * Corresponding author. Prof., Ph.D.; Tel./Fax: þ7 495 777 93 50; E-mail address: a.glezer@mail.ru (A.M. Glezer). 1005-0302/$ e see front matter Copyright Ó 2014, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2014.09.006 Available online at ScienceDirect ScienceDirect J. Mater. Sci. Technol., 2015, 31(1), 91e96