Slip Resistance of Ti-Based High-Temperature Shape Memory Alloys

Abstract

Titanium with Nb, Zr, and Ta alloying substitutions possesses high plastic slip resistance and high transformation strains upon bcc (b) to orthorhombic ða 00 Þ transformation. In the current study, we determine the critical resolved shear stress (CRSS) for slip in Ti alloyed for a wide composition range of Nb, Ta, and Zr. The CRSS is obtained with a proposed Peierls-Nabarro formalism incorporating the generalized stacking fault energy barrier profile for slip obtained from the first-principles Density Functional Theory (DFT) calculations. The CRSS for slip of the orthorhombic martensite increases from 80 to 280 MPa linearly with increasing unstable fault energy. The addition of tantalum is most effective in raising the energy barriers. We also demonstrate the composition dependence of the lattice parameters of both b and a 00 crystal structures as a function of Nb, Ta, and Zr additions showing agreement with experiments. Using the lattice constants, the transformation strain is determined as high as 11 % in the [011] pole and its magnitude increases mainly with Zr addition.

Critical Stresses for Twinning, Slip, and Transformation in Ti-Based Shape Memory Alloys

“…For complete details on lattice constant calculations, see Ref [10]. Table 1 represents the complete list of equilibrium lattice constants of Ti-Nb-Ta alloys considered in the present analysis.…”

“…These alloys can also be exploited in biomedical applications [7,8] removing the concerns with the use of nickel element in human body. Several experiments [1,2,6,9], and recent theoretical calculations [10] have shown that upon alloying Ti-alloys with Nb, Ta, and Zr, high transformation strains (11 %) and high-martensite slip resistance can be achieved in addition to the high transformation temperatures (over 400 K). There are four other parameters such as transformation stress, twinning stress, martensite slip stress, and austenite slip stress that govern the shape memory and superelastic behavior.…”

“…Overall, we investigate the aforesaid parameters (i, ii, and iii) in Ti-based alloys for 11 different compositions of Nb and Ta (in at.%) exhibiting the minimum and maximum transformation strains. The minimum and maximum transformation strains for different alloying compositions were obtained using lattice deformation theory in our previous work [10].…”

“…The advantage of Ta addition is that it increases the CRSS for martensite and austenite slip nucleation so that a large reversible deformation can be achieved without significant plastic strain accumulation. Recently, theoretical works [10] have established the transformation strains and the CRSS for slip in a large number of Ti-Nb-Ta alloys. The maximum transformation strain obtained theoretically is 6.5 % in Ti-6.25Nb-6.25Ta [10].…”

“…Recently, theoretical works [10] have established the transformation strains and the CRSS for slip in a large number of Ti-Nb-Ta alloys. The maximum transformation strain obtained theoretically is 6.5 % in Ti-6.25Nb-6.25Ta [10]. The lowest CRSS for martensite slip was 127 MPa in Ti-6.25Nb-6.25Ta and the maximum CRSS was 283 MPa in Ti-25Nb-25Ta, more than twice the increase in the CRSS upon alloying Ti with Nb and Ta.…”