Abstract
Predicting the mechanical strength and ductility of transition metal alloys remains challenging because these depend on a variety of intrinsic and extrinsic factors. In this talk I will discuss two recent examples to illustrate how first-principles calculations can be used to yield new insights into both types of factors, and provide strategies for alloy design. We will consider first the ideal tensile strength and corresponding intrinsic ductility for group 5 and 6 metal-based refractory alloys forming with body-centered cubic (bcc) structures. Group 6 metals, such as Mo and W, are intrinsically brittle, and this limits their applications. Our first-principles calculations show that alloying group 6 transition metals with group 4 or 5 transition metals can transform these materials into ones that display intrinsically ductile behavior. Remarkably, this transformation in ductility can be understood as an electron filling effect with the intrinsically ductile response the manifestation of a Jahn-Teller distortion. The second example will be devoted to hexagonal-close-packed (hcp) Ti, where it is known that very small concentrations of O interstitials can induce significant strengthening, but with the detrimental effect of lowering toughness. To provide new insights into this long-studied problem, we performed first-principles calculations to analyze the interactions between interstitial atoms and screw dislocation cores in hcp-Ti. Both indirect studies of generalized stacking fault energy and direct investigate of dislocation core show that oxygen and other large interstitials are strongly repelled by the screw dislocation core. This repulsion would forces these interstitials to “diffuse” from their original interstitial site during dislocation slip. The origin of this repulsion is the low symmetry structure of hcp lattice: in the hcp-Ti screw dislocation core, the interstitial volume is largely reduced. The results are shown to be in good agreement with experimental observations derived from in-situ small-scale mechanical tests.
References:
1. L Qi, DC Chrzan, “Tuning Ideal Tensile Strengths and Intrinsic Ductility of bcc Refractory Alloys,” Physical review letters, 112 (11), 115503 (2014).
2. Q Yu, L Qi, T Tsuru, R Traylor, D Rugg, JW Morris, M Asta, DC Chrzan, A Minor, “Origin of dramatic oxygen solute strengthening effect in titanium,” Science, 347 (6222), 635-639 (2015)
3. L Qi, T Tsuru, M Asta, DC Chrzan, “Interactions between Oxygen Interstitial and -Type Screw Dislocations in alpha-Titanium” (arXiv:1502.05129 [cond-mat.mtrl-sci])

CV_Liang_Qi.pdf