Solid Solution Ta Research Articles

Shock wave propagation and spall failure of nanocrystalline Cu/Ta alloys: Effect of Ta in solid-solution

The role of the concentration and distribution of the Ta solute in a solid solution in the shock response and spall failure of a bulk nanocrystalline Cu (nc-Cu) system is investigated using large scale molecular dynamics (MD) simulations. The nanocrystalline Cu/Ta (nc-Cu/Ta) microstructures comprise a 16 nm grain size Cu matrix with distributions of 3.0%, 6.3%, and 10.0% Ta atoms either along the grain boundary or randomly in the Cu matrix. The shock response is investigated by identifying the modifications in the dynamic evolution of defect structures (dislocation nucleation and interactions), as well as the nucleation and evolution of voids. The MD simulations reveal the complex role of Ta in altering the spall behavior of the nc-Cu system. The presence of Ta is observed to improve the spall strengths of the nc-Cu system, and the strengthening behavior is achieved by limiting the capability to nucleate dislocations during shock compression and under tensile pressures. The highest values for spall strength are observed for a random distribution of Ta at a concentration of 6.3% Ta. While the increase in strength for a 16 nm grain sized nc-Cu system is reported to be ∼8%, the role of the Ta solute is observed to be more pronounced at a larger grain size of 30 nm with a ∼15% increase in the spall strength for randomly distributed Ta with a concentration of 6.3%. The links between the concentration and the distribution of Ta in the solid solution and the evolution of microstructures under shock loading conditions are discussed.

Intermetallic NiAlTa alloys with strengthening Laves phase for high-temperature applications. I. Basic properties

Various Ta-containing NiAl-base alloys with the Laves phase TaNiAl, with C14 structure, as strengthening second phase were prepared by ingot metallurgy. They were studied with respect to the deformation behaviour at ambient and high temperatures — including elastic deformation and creep — as a function of alloy composition and microstructure. NiAl dissolves up to 0.2 at% Ta in solid solution. NiAl with up to 3 at% Ta forms precipitate particles of Laves phase primarily on grain boundaries, whereas the Laves phase covers the grain boundaries completely to form a continuous skeleton for higher Ta contents. The observed strengthening of NiAl by Ta is a result of both solid-solution strengthening and second-phase strengthening. The latter effect increases with increasing fraction of Laves phase and is accompanied by an increasing brittle-to-ductile transition temperature. The oxidation behaviour was checked. These alloys are promising for applications at temperatures above those of the Ni-base superalloys.

Solidification Processing Routes to High Aspect Ratio Reinforcements in γ-TiAl

AbstractAdditions of ∼5%B1 and ∼9%Ta to binary γ-TiAl result in the formation of a monoboride isomorphous with TiB but containing Ta in solid solution. This boride exhibits strongly anisotropic growth from the melt, producing rod-like primary phases which are of potential interest for creep strengthening of the γ matrix. Fibrous borides with aspect ratios larger than ∼20 and volume fractions of ∼0.12 have been produced in arc-melted Ti-48AI-9Ta-4.3B alloys. Creep testing at 1255 K indicates that these “in-situ” composites have much higher creep strength than γ-TiAl, but the contribution of the reinforcements is relatively small compared with the solute-strengthening of the matrix.

Directional Solidification of Unstabilized HfO2‐Ta

Aligned eutectic composites of unstabilized ZrO/sub 2/ and Ta can be prepared crack-free even though the matrix has been completely transformed into the monoclinic lattice, i.e. the transformation-induced volume change does not result in the excessive cracking which is usually observed. Composites with Y/sub 2/O/sub 3/-stabilized ZrO/sub 2/, however, crack extensively and exhibit inhomogeneous eutectic structures. Similar results are obtained with directionally solidified, stabilized ZrO/sub 2/--W and stabilized HfO/sub 2/--W. The fact that ZrO/sub 2/ and HfO/sub 2/ have modifications with identical crystal structures suggested that compositions of unstabilized HfO/sub 2/ with Ta would also exhibit an improved solidification behavior compared with mixtures containing Y/sub 2/O/sub 3/-stabilized HfO/sub 2/. Eutectic areas of HfO/sub 2/--Ta sintered powder rods exhibited almost no cracking. There are two possible explanations for the fact that the composites with unstabilized ZrO/sub 2/ and HfO/sub 2/ survive the tetragonal..-->..monoclinic transformation without cracking: (1) the matrix is prestressed by the Ta fibers when expanding at the lattice tranformation; or (2) since the composites are sintered in vacuum, ZrO/sub 2/ or HfO/sub 2/ are substoichiometric. This condition, together with Ta in solid solution, may cause the monoclinic lattice cell to exhibit a density similar to that of the tetragonalmore » form. Both hypotheses, however, must be carefully investigated. (JRD)« less