Ideal strength of bcc molybdenum and niobium
- LBNL Library
The behavior of bcc Mo and Nb under large strain was investigated using the ab initio pseudopotential density-functional method. We calculated the ideal shear strength of the {l_brace}211{r_brace}<111> slip systems and the ideal tensile strength in the <100> direction, which are believed to provide the minimum shear and tensile strengths. As either material is sheared in either of the two systems, it evolves toward a stress-free tetragonal structure that defines a saddle point in the strain-energy surface. The inflection point on the path to this tetragonal ''saddle-point'' structure sets the ideal shear strength. When either material is strained in tension along <100>, it initially follows the tetragonal, ''Bain,'' path toward a stress-free fcc structure. However, before the strained crystal reaches fcc, its symmetry changes from tetragonal to orthorhombic; on continued strain, it evolves toward the same tetragonal saddle point that it reached in shear. In Mo, the symmetry break occurs after the point of maximum tensile stress has been passed, so the ideal strength is associated with the fcc shear. In Mo, the symmetry break occurs after the point of maximum tensile stress has been passed, so the ideal strength is associated with the fcc extremum as in W. However, a Nb crystal strained in <100> becomes orthorhombic at tensile stress below the ideal strength. The ideal tensile strength of Nb is associated with the tetragonal saddle point and is caused by failure in shear rather than tension. In dimensionless form, the ideal shear and tensile strengths of Mo (tau* = tau-sub m/G111 = 0.12 sigma* = sigma-sub m/E100 = 0.078) are essentially identical to those previously calculated for W. Nb is anomalous. Its dimensionless shear strength is unusually high, tau* = 0.15, even though the saddle-point structure that causes it is similar to that in Mo and W, while its dimensionless tensile strength, sigma* = 0.079, is almost the same as that of Mo and W, even though the saddle-point structure is quite different.
- Research Organization:
- Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA (US)
- Sponsoring Organization:
- USDOE Director, Office of Science. Office of Basic Energy Studies. Materials Science and Engineering Division; National Science Foundation (US)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 805162
- Report Number(s):
- LBNL--51645; B& R KC0202030
- Journal Information:
- Physical Review B, Journal Name: Physical Review B Journal Issue: 9 Vol. 66
- Country of Publication:
- United States
- Language:
- English
Similar Records
The ideal strength of tungsten
Ideal strength of diamond, Si, and Ge