INTERFACE SCIENCE, 2, 137-146 (1994). (~ Kluwer Academic Publishers, Boston. Manufactured in The Netherlands. Mechanical Properties of Twist Grain Boundaries in Cu MIKI NOMURA AND JAMES B. ADAMS Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801 Received February 16, 1994; Revised March 22, 1994 Keywords: Grain boundaries, mechanical properties, computer simulation Abstract. In a previous paper we studied the Young's and shear modulus of a high-angle twist grain boundary (25) in Cu, using the EAM, and related it to the uniaxial strain derivatives of single crystals. In this paper, we discuss elastic properties of ten additional twist grain boundaries, from 8.8-43.6 ~ . The monolayer Young's modulus at each boundary was calculated and found to be 20-50% higher than the bulk value for all eleven boundaries for both csl and typel structures. The monolayer shear modulus at each boundary was calculated and found to be 93-98% lower than the bulk value for six grain boundaries with csl structure and found to decrease with increasing twist angle. The critical shear stress was also calculated for eleven boundaries with csl structure and found to roughly decrease with increasing twist angle. 1. Introduction The structure and mechanical properties of grain boundaries have been the subject of recent ex- perimental [1-7] and theoretical [8-20] interest. Both high-resolution transmission electron mi- croscopy (HR-TEM) and x-ray diffraction tech- niques are currently used to pinpoint the location of atoms at grain boundaries. Theoretical calcu- lations by Majid, Bristowe, and Balluffi [7] using the embedded-atom method (EAM) are in good agreement with experiment for 5 different twist boundaries (0 = 7.6 ~ to 0 = 36.9 ~ in Au. There have been numerous experimental and theoretical studies of the supermodulus effect in multilayer thin films [12, 13], and there is still substantial debate over the effect. However, for the simpler case of grain boundaries, there has been no experimental measurement of local elastic constants due to the difficulty of measur- ing them. The EAM and Lennard-Jones (L J) potentials have been applied by Wolf and co-workers to the study of the elastic properties of grain bound- aries, both in thin slabs and in superlattices [14- 21]. They found that several typical twist grain boundaries have a smaller shear modulus (G) parallel to the boundary. Kluge et al. calcu- lated local elastic constants at grain boundary using lattice-dynamics and finite-strain method and found that elastic constants are smaller at grain boundaries [14]. The EAM and LJ poten- tials yielded the same qualitative results [14-16]. Wolf and Kluge calculated the monolayer shear modulus of twist grain boundaries as a function of twist angle and found that it decreases with increasing twist angle [16]. Wolf and Lutsko studied tensile and shear elastic anomalies of su- perlattices of twist grain boundaries in metals [17-19]. Phillpot, Lutsko, and Wolf [20, 21] have also studied elastic constants of grain boundaries in silicon. In our previous work [22], the pressure deriva- tives and uniaxial strain derivatives of the elastic constants of copper were calculated using the embedded-atom method, and the results were in good agreement with experiment. Also, the elastic constants of a 235 twist grain boundary were calculated on a layer by layer basis, and the elastic behavior (both tensile and shear) in