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The alloying effects of Nb, Zr, and/or Sn on the phase stability and elastic properties of Ti are investigated by using a first-principles method. Our calculation results indicate that a carefully designed Ti–Nb–Zr–Sn system can be a good candidate for low modulus biomedical materials. We find that the well-known correlation between the e/a ratio and both elastic and phase stabilities for Ti alloyed with transition metal elements breaks down for the Ti–Sn alloy. [ABSTRACT FROM AUTHOR]

A theoretical investigation on tetradecker transition metal complexes of Cp−Fe−CB6−Fe−CB6−Fe−Cp (1) containing double planar hexacoordinate carbons and Cp−Fe−BB7−Fe−BB7−Fe−Cp (2) containing double planar heptcoordinate borons has been performed in this work at density functional theory level. CpFemonocations prove to effectively stabilize these unusual complexes, which are mainly maintained by effective d− coordination interactions between the partially filled Fe 3d orbitals and the delocalized molecular orbitals (MOs) of the four planar deckerlike ligands. The results obtained in these model computations expand the domain of ferrocene chemistry and could provide a new approach for synthesizing planar hyper-coordinate carbons and borons in transition metal complexes. [ABSTRACT FROM AUTHOR]

Quantifying the local lattice distortion (LLD) and exploring its correlation with material properties are of fundamental importance for the design of high entropy alloys (HEAs). In the present work, we expressed the LLD as the standard deviation of the bond lengths (SDBL). With which, we calculated the LLDs of the face-centered cubic (fcc) alloys, including the dilute and equiatomic binary TM-Co and TM-Ni (TM = transition metal) alloys and the equiatomic CrMnCoNiFe family HEAs by using a first-principles method. For the binary TM-Co and TM-Ni alloys with TM in the same period of the Chemical Element Periodic Table, the LLD and the number of valence electrons of TM exhibit roughly a parabolic relationship with a minimum in the middle of the period. For the HEAs involved in the present work, the composition-LLD-mechanical property relationship was constructed. It is found that the experimental yield strengths increase monotonically with the calculated LLDs, and to pursue a high LLD in the CoNi-based CrMnCoNiFe family alloys, one should increase Cr to an appropriate content and avoid introducing Fe. Furthermore, the influences of the atomic radius, electronegativity, and magnetism on the LLD were elucidated. The present work provides a comprehensive understanding of the LLD in both dilute and equiatomic FCC alloys. [Display omitted] • The LLD was expressed as the standard deviation of the bond length proposed based on the characteristics of FCC structure. • The composition-LLD-mechanical property relationship was constructed. • The influences of the atomic radius, electronegativity, and magnetism on the LLD were elucidated. • The electronegativity of the transition metals was quantified according to the first-principles calculations. • The valence electron concentration can be employed to roughly judge the magnetic state of Fe in HEAs. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the local asymmetrical distortion (measured as the dipole shape factor) induced by interstitial atoms (IAs: C, N, and O) in bcc-metals (V, Nb, and Ta) and the diffusion barriers of the IAs in the matrix are calculated by the use of a first-principles pseudopotential method. The obtained dipole shape factor and diffusion barrier are in reasonable agreement with the experimental values. The trend of the dipole shape factor and diffusion barrier with respect to the IAs and host metals are discussed in terms of the size difference between the IA and the interstice, the chemical and elastic interaction between the IA and the host metals. It is shown that both the size difference and the chemical interaction affect significantly the dipole shape factor, whereas the diffusion barrier is dominated by the chemical interaction. [Copyright &y& Elsevier]

Diffusion of dopant in rutile is the fundamental process that determines the performance of many devices in which rutile is used. The diffusion behavior is known to be highly sample-dependent, but the reasons for this are less well understood. Here, rutile is studied by using first-principles calculations, in order to unravel the microscopic origins of the diverse diffusion behaviors for different doping elements. Anomalous diffusion behavior in the open channel along the [001] direction is found: larger atoms including Sc and Zr have lower energy barrier for diffusion via interstitial mechanism, apparently contradicting their known slow diffusion rate. To resolve this, we present an alternate model for the overall diffusion rate of the large-size dopants in rutile, showing that parallel to the [001] channel, it is limited by the formation of the interstitial states, whereas in the direction perpendicular to [001], it proceeds via a kick-out mechanism. By contrast, Co and Ni prefer to stay in the interstitial site of rutile, and have conventional diffusion with a very small migration barrier in the [001] channel. This leads to highly anisotropic and fast diffusion. The diffusion mechanisms found in the present study can explain the diffusion data measured by experiments. [ABSTRACT FROM AUTHOR]

[Display omitted] • X-Al interaction energy is proposed as an index for screening alloying element X in high temperature Ti alloys. • Effects of temperature and off-center occupation of alloying atom on interaction energy are considered. • Mo, Tc, Ru, W, Re, and Os are identifed to benifit both strength and thermal stability of high temperature Ti alloys. Increasing the solubility of Al and inhibiting the growth of Ti 3 Al precipitates in high temperature titanium alloy improve both the thermal strength and thermal stability of the alloy. In principle, this may be achieved by adding some other alloying atoms which attract Al so as to serve as traps for Al in the alloy. In the present work, the interaction energies between Al and alloying atoms X with X covering all the transition metal (TM) and noble metal (NM) elements in the Chemical Elemental Period Table (CEPT) are calculated by using a first principles method in order to screen the traps for Al. The effects of temperature (taking Mo-Al interaction as an example) as well as the newly found off-center site-occupation of alloying atoms (Acta Mater. 197, 2020, 91) on the interaction energies are considered. We show that the interactions between Al and the TM alloying atoms early in the CEPT are weak. The middle TM alloying atoms attract but the late TM and NM ones repel Al. At finite temperature, the lattice vibration enhances whereas the configurational entropy weakens the Mo-Al attraction. Both effects become stronger with increasing temperature. The physical origins underlying the X-Al interactions are revealed by analyzing the electronic density of states and bonding charge density. Alloying atoms such as Mo, Tc, Ru, W, Re, and Os are identified to benefit the strength and thermal stability of high temperature titanium alloys. This work is helpful to the rational design of high temperature titanium alloys. [ABSTRACT FROM AUTHOR]

Introducing high symmetric cubic disordered β or ordered β 0 phases by alloying β -stabilizers improves the strength and high-temperature deformability of intermetallic γ -TiAl. However, the β phase is prone to decomposition of lower symmetric hexagonal phases such as ω 0 and ω ' ' , which are extremely brittle and harmful to the ductility of the alloy. Therefore, understanding the effect of the β -stabilizers on the phase stability of TiAl is crucial for the rational design of TiAl based alloys. In the present work, the equilibrium lattice structures, bulk moduli, and heats of formation of β , β 0 , ω ' ' , and ω 0 of TiAl with some typical β stabilizers V, Nb, Ta, Cr, Mo, W, and Mn were calculated by using a first-principles method, with which the relative phase stability were discussed. We showed that the phase stability sequence is β < β 0 < ω 0 < ω ' ' for Ti 4 Al 3 Nb and Ti 4 Al 3 Ta, β < ω 0 < β 0 < ω ' ' for Ti 4 Al 3 V, Ti 4 Al 3 Mo and Ti 4 Al 3 W, and β < ω 0 < ω ' ' < β 0 for Ti 4 Al 3 Cr and Ti 4 Al 3 Mn. The β -stabilizing effect of the alloying element X increases from Nb/Ta to V/Mo/W to Cr/Mn. The relative stability of the various phases is roughly in accordance with the trends of the unit volume and bulk modulus against the phases with a few exceptions. The phase diagrams of the alloys associated with these phases were estimated by comparing the free energies against the temperature of various phases, evaluated with the calculated unit volumes and bulk moduli. • Phase stability of TiAl alloying with transition metal elements is determined. • Phase stability is roughly in accordance with the unit volume and bulk modulus. • Strategy for the composition design of TiAl based alloys is presented. [ABSTRACT FROM AUTHOR]