AB INITIO CALCULA TIONS OF THEORETICAL TENSILE STRENGTH IN MET ALS AND INTERMET ALLICS

The paper gives an account of applications of quantum-mechanical (first-principles) electronic structure calculations to the problem of theoretical tensile strength in metals and intermetallics. First, we briefly describe the way of simulating the tensile test and the electronic structure calculational method. Our approach is then illustrated on calculations of theoretical tensile strength in W, NiAl, and transition metal disilicides MoSi2 and WSi2 with C1b structure. In W and NiAl, we consider uniaxial tension along [001] and [111] directions. Although tungsten is elastically nearly isotropic (C44 ≈ C0), theoretical tensile strength exhibits a marked anisotropy (σth[001] = 29GPa, σth[111] = 40GPa). Calculated results compare favorably with experimental value of 24.7 ± 3.6 GPa obtained for tungsten whiskers grown along the [110] direction. Similarly, in NiAl, the 'hard' orientation [001] differs very significantly from the [111] orientation (σth[001] = 46GPa, σth[111] = 25GPa). This anisotropy is explained in terms of higher-symmetry structures present or absent on the calculated deformation paths. In disilicides, the theoretical strength is calculated for [001] loading (σth[001] = 37GPa and 38GPa for MoSi2 and WSi2, respectively). The role of relaxation of internal structure parameter is discussed and changes in bonding conditions during the tensile test are analyzed.