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Journal Articles

Interfacial segregation and fracture in Mg-based binary alloys; Experimental and first-principles perspective

Tsuru, Tomohito; Somekawa, Hidetoshi*; Chrzan, D. C.*

Acta Materialia, 151, p.78 - 86, 2018/06

 Times Cited Count:17 Percentile:6.25(Materials Science, Multidisciplinary)

We investigated the effect of solute elements on interfacial segregation and fracture in Mg alloys by experiments and first-principles density functional theory calculations in conjunction with interfacial fracture mechanics. Based on the assumption of brittle fracture in Mg alloys, the interfacial separation caused by segregated solutes in Mg can be efficiently described by the energy-based criterion of fracture, which is in good agreement with the fracture toughness obtained by experimental tests of Mg-M binary alloys. The electronic interaction, that is, the change in the electronic state between the interface and surface, mainly influences the ideal work of separation regardless of the type of interface. We found that IIIB ($$d^{1}$$) and IVB ($$d^{2}$$) solutes, such as Zr, show distinctive hybridization between the p band of Mg and the d band of the solute, which characterizes the strong fracture toughness of Zr-doped Mg alloys in both the calculations and experiments.

Journal Articles

Lattice softening in body-centered-cubic lithium-magnesium alloys

Winter, I. S.*; Tsuru, Tomohito; Chrzan, D. C.*

Physical Review Materials (Internet), 1(3), p.033606_1 - 033606_9, 2017/08

 Times Cited Count:0 Percentile:100(Materials Science, Multidisciplinary)

A first-principles investigation of the influence of lattice softening on lithium-magnesium alloys near the body-centered-cubic (bcc)/hexagonal close-packed (hcp) transition composition is presented. Results show that lithium-magnesium alloys display a softening of the shear modulus $$C_{11}-C_{12}$$, and an acoustic phonon branch between the $$Gamma$$ and $$N$$ high symmetry points, as the composition approaches the stability limit for the bcc phase. This softening is accompanied by an increase in the size of the dislocation core region. Ideal tensile strength calculations predict that ordered phases of lithium-magnesium alloys are intrinsically brittle. Methods to make the alloys more ductile are discussed, and the propensity for these alloys to display gum-metal-like behavior is assessed.

Journal Articles

Dislocations near elastic instability in high-pressure body-centered-cubic magnesium

Winter, I. S.*; Poschmann, M.*; Tsuru, Tomohito; Chrzan, D. C.*

Physical Review B, 95(6), p.064107_1 - 064107_9, 2017/02

 Times Cited Count:3 Percentile:69.03(Materials Science, Multidisciplinary)

At high pressure, Mg is expected to transform to the body centered cubic (BCC) phase. We use density functional theory to explore the structure of $$langle 111 rangle$$ type dislocation cores in BCC Mg as a function of pressure. As the pressure is reduced from the region of absolute stability for the BCC phase, the dislocation cores spread. When dislocation cores overlap the displacements of columns of atoms resemble the nanodisturbances observed in TiNb alloys known as Gum Metal. The ideal tensile strength of BCC Mg is also computed as a function of pressure. Despite its low shear modulus, BCC Mg is predicted to be intrinsically brittle at absolute zero.

Journal Articles

Effect of alloying on dislocation core; DFT study for improving mechanical properties

Tsuru, Tomohito; Chrzan, D. C.*

Dai-20-Kai Bunshi Doryokugaku Shimpojiumu Koen Rombunshu (USB Flash Drive), 3 Pages, 2015/05

Solution strengthening is a well-known approach to tailoring the mechanical properties of structural alloys. Ultimately, the properties of the dislocation/solute interaction are rooted in the electronic structure of the alloy. Accordingly, we compute the electronic structure associated with, and the energy barriers to dislocation cross-slip. Using the example of a-type screw dislocations in Mg, we compute accurately the Peierls barrier to prismatic plane slip and argue that some elements should interact strongly with the studied dislocation, and thereby decrease the dislocation slip anisotropy in the alloy.

Journal Articles

Effect of solute atoms on dislocation motion in Mg; An electronic structure perspective

Tsuru, Tomohito; Chrzan, D. C.*

Scientific Reports (Internet), 5, p.8793_1 - 8793_8, 2015/03

 Times Cited Count:41 Percentile:12.97(Multidisciplinary Sciences)

Much of the world's future economy hedges on the ability to improve the efficiency of mechanical machines without sacrificing their performance. This requires the development of lighter and stronger structural alloys. Magnesium alloys are very promising for structural applications, but they suffer from a fatal flaw. The mobility of the dislocations is determined by the bonding within the alloy, and this bonding is best modeled using approaches rooted firmly in quantum mechanics. Here, we compute the electronic structure of a screw dislocation gliding on a prismatic plane within Mg as it passes over its Peierls barrier. We show that the ductility of Mg is increased because some substitutional additions strongly bind to and stabilize a compact core structure for screw dislocations that are formally able to glide on both the basal and prismatic planes.

Journal Articles

Origin of dramatic oxygen solute strengthening effect in titanium

Yu, Q.*; Qi, L.*; Tsuru, Tomohito; Traylor, R.*; Rugg, D.*; Morris, J. W. Jr.*; Asta, M.*; Chrzan, D. C.*; Minor, A. M.*

Science, 347(6222), p.635 - 639, 2015/02

 Times Cited Count:106 Percentile:2.85(Multidisciplinary Sciences)

Given that solute atoms interact weakly with the long-range elastic fields of screw dislocations, it has long been accepted that solution hardening is only marginally effective in materials with mobile screw dislocations. This accepted wisdom has recently been questioned by first-principles calculations suggesting that solutes may interact much more strongly with the screw dislocation core. We report here the results of a combined experimental and computational study undertaken to elucidate the profound hardening effect of oxygen in pure hexagonally-close-packed structured $$alpha$$-Ti. High resolution and in situ transmission electron microscopy nanomechanical characterization establish that the strengthening is due to the strong interaction between oxygen and the core of screw dislocations that mainly glide on prismatic planes. First-principles calculations of the screw dislocation core reveal a simple crystallographic source for the oxygen-dislocation interaction that is consistent with experimental observations. The distortion of the interstitial sites at the dislocation core creates a very strong but short-range repulsion for oxygen atoms. These mechanisms effectively pin the dislocation near the oxygen interstitial. These results establish a highly effective mechanism for strengthening by interstitial solutes that, contrary to prior understanding, may be significant in many structural alloys.

Oral presentation

Computational study on anisotropic plasticity caused by crystal structure and microstructure

Tsuru, Tomohito; Aoyagi, Yoshiteru*; Shimokawa, Tomotsugu*; Yamaguchi, Masatake; Itakura, Mitsuhiro; Kaburaki, Hideo; Kaji, Yoshiyuki; Chrzan, D. C.*

no journal, , 

Magnesium alloys, one of the lightest metal alloys among metals in practical use, have great potential for next generation of structural materials. Ultrafine-grained metals have been desired to improve strength without allowing. However these materials have crucial disadvantages in practical use in that the elongation-to-failure is relatively low due to the strong anisotropy in plastic deformation of the hexagonal crystal. In this study we investigate the origin of plastic anisotropy caused by crystal structure and microstructure by computational approach.

Oral presentation

Anomalous strengthening mechanism in HCP Ti

Tsuru, Tomohito; Chrzan, D. C.*; Rodney, D.*

no journal, , 

The properties of the dislocation/solute interaction are rooted in the electronic structure of the alloy. We explored the origin of anomalous locking mechanism in Ti using first-principles calculations. Stable configurations of a screw dislocation were investigated by stacking fault energy on various slip planes and direct calculations of dislocation in pure Ti. Unlike Zr, a screw dislocation is most stable when they dissociate into pyramidal plane. The Peierls stress for this locking configuration to move on prismatic plane is surprisingly about 800 MPa, which means that Ti has a special strengthening mechanism on its own without any solid solution.

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