Ideal tensile and shear strength of a gum metal approximant:Ab initiodensity functional calculations

Abstract

The ideal tensile and shear strengths of binary β-phase Ti 3 Nb alloys have been investigated using ab initio density functional calculations. The binary alloy is considered as an approximant to the multifunctional Ti-Nb-Ta-Zr-O alloy known as "gum metal," which displays high strength, low elastic modulus, high yield strain, and very good ductility. This alloy has been reported to deform elastically until the stress approaches the ideal tensile strength. Our calculations have been performed for an optimized chemical decoration of the body-centered cubic (bcc) structure of the β phase. Previous work has demonstrated that this model yields elastic constants in very good agreement with those measured for gum metal specimens and leads to a reasonably accurate description of the martensitic transformations between the bcc β, the orthorhombic α and the hexagonal ω phases [Lazar et al., Phys. Rev. B 84, 054202 (2011)]. The simulations of the response to tensile and shear loading have been performed for large supercells which account also for the different orientations of the-Nb-Nb-chains characteristic for the β-phase structure relative to the direction of the applied load. The energy-strain and stress-strain curves are found to be very different from those reported for all bcc metals. Under uniaxial 100 loading we find an ideal tensile strength of 2.4 GPa, the upper limit to the tensile stress arising from a shear instability of the structure. Under uniaxial 110 load we calculate an ideal tensile strength of 2.2 or 2.8 GPa, depending on the orientation of the-Nb-Nb-chains relative to the loading direction. For a realistic multidomain structure the ideal strength is expected to correspond to the average of these values. An ideal strength of 2.6 GPa under 110 loading is roughly the same as under 100 load, despite a considerable anisotropy of the tensile moduli. For {211} 111 shear we calculate an ideal shear strength of 1.6 GPa, again as an average over different possible shearing directions relative to the Nb-Nb bonds. For the {110} 110 shear system we find a lower strength of 0.9 GPa. The structures reached at the stress maximum under 100 uniaxial tension and {211} 111 shear are identical, and since the maximal shear stress is much lower than the tensile stress, the alloy will fail by shear even under strictly uniaxial tension. The values of the ideal tensile and shear strengths are significantly low, even in comparison with those calculated for bcc V and Nb with very small shear moduli and approach the values reported for gum metal alloys.

Ab initio-predicted micro-mechanical performance of refractory high-entropy alloys

“…The resolved shear stress for the slip system at the instability is 3.5 GPa for ZrVTiNb, which is a good estimate of its ideal shear strength ( τ m ) for the same slip system without superimposed tension 29 . The estimated reduced shear strength, given by , , amounts to 0.1 in agreement with other bcc systems 15 31 . Owing to this particular failure behavior, the ZrVTiNb alloy is referred to as being intrinsically ductile , which represents the preferred failure mode over the intrinsically brittle one 14 .…”

Theory-Guided Materials Design of Multi-Phase Ti-Nb Alloys with Bone-Matching Elastic Properties

“…Employing our theory-guided materials design strategy, it will become possible to pre-select most promising Nb compositions by quantum-mechanical calculations and avoid casting and testing of large numbers of samples from the whole compositional range. This can be essential in both on-going and future development of Ti-Nb binary alloys [ 56 , 57 , 58 , 59 ], ternary materials [ 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 ] (including Ti-Al-V [ 68 , 69 , 70 ] and Ti-Nb-V [ 71 , 72 ]), as well as higher-order alloys [ 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 ] intended for bio-medical applications [ 83 , 84 , 85 , 86 , 87 , 88 ].…”

Influence of alloying elements Nb, Zr, Sn, and oxygen on structural stability and elastic properties of the Ti2448 alloy

“…First principles investigation on Ti 3 Nb shows that all phases are dynamically stable at low temperature [42]. The ideal tensile strength of binary β-Ti 3 Nb alloy is isotropic with a value around 2.5 GPa, but the ideal shear strength is anisotropic and varies from 0.9 to 1.5 GPa [43]. Both Ti2448 and Gum metal are based on the Ti-Nb-Zr system having bulk modulus around 80 GPa [44].…”