Materials Science

Approaching the Ideal

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Science  13 Apr 2007:
Vol. 316, Issue 5822, pp. 174-175
DOI: 10.1126/science.316.5822.174d

Frenkel predicted 80 years ago that the ideal strength of a metal should be 1/5 of its shear modulus, but in most metals the actual strength ratio is closer to 1/1000 because of the motion of dislocations at much lower stresses. Li et al. use computational methods in an effort to understand the behavior of a family of body-centered cubic (bcc) Ti-Nb-based alloys known as Gum Metals. These alloys have the unusual property of sustaining very large elastic deformations before yielding, as well as substantial plastic deformation before failing. The authors argue that for this behavior to occur, the ideal strength must be below a stress at which the material would deform by ordinary dislocations, and that the material must always fail by shear rather than cleavage fracture. Using ab initio calculations to determine the elastic properties of related Ti-V alloys, they find that at a ratio of valence electrons to atoms close to the Gum Metal value, the bcc lattice becomes unstable; thus, the Gum Metals intrinsically have a low ideal strength and tend to fail in shear even when pulled in tension. Further, at values close to this transition, it is possible to introduce sufficient obstacles for dislocation motion through the addition of extra alloy elements without complete loss of ideal strength. The authors believe that similar computations could identify useful alloys that exist close to this edge of bcc stability. — MSL

Phys. Rev. Lett. 98, 105503 (2007).

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