Energy investigation of effects of O on mechanical properties of NiAl intermetallics.

We have investigated effects of O on mechanical properties of NiAl by calculating the cleavage energy (γ(C)) and the unstable stacking fault energy (γ(us)) using a first-principles method. O is shown to reduce γ(C)/γ(us) for the [001](110) and [100](001) slip systems, indicating that the presence of O should be associated with the ductility reduction of NiAl. Further, γ(C)/γ(us) of the NiAl-O system can be increased by Cr, suggesting the possibility to suppress the negative effect of O via alloying elements.

Bonding characteristics in NiAl intermetallics with O impurity: a first-principles computational tensile test.

We have performed a first-principles computational tensile test on NiAl intermetallics with O impurity along the [001] crystalline direction on the (110) plane to investigate the tensile strength and the bonding characteristics of the NiAl-O system. We show that the ideal tensile strength is largely reduced due to the presence of O impurity in comparison with pure NiAl. The investigations of the atomic configuration and bond-length evolution show that O prefers to bond with Al, forming an O-Al cluster finally with the break of O-Ni bonds. The O-Ni bonds are demonstrated to be weaker than the O-Al bonds, and the reduced tensile strength originates from such weaker O-Ni bonds. A void-like structure forms after the break of the O-Ni and some Ni-Al bonds. Such a void-like structure can act as the initial nucleation or the propagation path of the crack, and thus produce large effects on the mechanical properties of NiAl.

Formation of a coplanar O-Al bonding cluster: the effect of O impurity on a Σ = 5 NiAl grain boundary from first-principles.

The site occupancy, structure, and bonding properties of O in an NiAl grain boundary (GB) have been investigated by employing a first-principles total energy method based on density functional theory with the generalized gradient approximation and ultrasoft pseudopotential. The Σ5(310)/[001] tilt GB of NiAl has been chosen because (i) the Σ = 5 GB has been observed to be a higher fraction in NiAl experimentally, and (ii) the Σ5(310)/[001] is energetically favorable in comparison with the Σ5(210)/[001]. The NiAl GB is shown to favor the O segregation with a segregation energy of -1.75 eV, indicating that most of the O impurity will distribute in the NiAl GB thermodynamically. Moreover, O is shown to prefer occupying the interstitial sites rather than the substitutional sites in the GB according to the calculated formation energies. The O-Al bond is energetically favorable as compared with the O-Ni bond due to different electronegativity of Al and Ni in reference to O. Charge distribution and the density of states further indicate the intrinsic bonding properties of O-Al that contain obvious covalent characteristics. It is interesting to find that O is coplanar with the surrounding Al atoms in both interstitial and substitutional cases with lower formation energies, forming stronger coplanar O-Al bonding clusters. Such stronger bonding clusters in the GB can embrittle the NiAl intermetallics and thus are not beneficial to the plasticity of NiAl. Our results will provide a useful reference for improving the mechanical properties and for understanding the oxidation effect of the NiAl intermetallics.

Mapping and initial analysis of human subtelomeric sequence assemblies.

Physical mapping data were combined with public draft and finished sequences to derive subtelomeric sequence assemblies for each of the 41 genetically distinct human telomere regions. Sequence gaps that remain on the reference telomeres are generally small,well-defined,and for the most part,restricted to regions directly adjacent to the terminal (TTAGGG)n tract. Of the 20.66 Mb of subtelomeric DNA analyzed, 3.01 Mb are subtelomeric repeat sequences (Srpt),and an additional 2.11 Mb are segmental duplications. The subtelomeric sequence assemblies are enriched >25-fold in short,internal (TTAGGG)n-like sequences relative to the rest of the genome; a total of 114 (TTAGGG)n-like islands were found,55 within Srpt regions,35 within one-copy regions,11 at one-copy/Srpt or Srpt/segmental duplication boundaries,and 13 at the telomeric ends of assemblies. Transcripts were annotated in each assembly,noting their mapping coordinates relative to their respective telomere and whether they originate in duplicated DNA or single-copy DNA. A total of 697 transcripts were found in 15.53 Mb of one-copy DNA,76 transcripts in 2.11 Mb of segmentally duplicated DNA,and 168 transcripts in 3.01 Mb of Srpt sequence. This overall transcript density is similar (within approximately 10%) to that found genome-wide. Zinc finger-containing genes and olfactory receptor genes are duplicated within and between multiple telomere regions.