Observations of fcc and hcp tantalum

Abstract

The metal tantalum has many varied uses including in microelectronics (especially in capacitors) as thin films, in medical applications as an implant material or for surgical instruments, in X-ray lithography for masks, and in high-temperature structural applications. Ta is particularly useful because it is relatively ductile, refractory in nature, and does not readily react with corrosive materials. The body-centered cubic (bcc) crystal structure of pure Ta, also known as the a-phase, is the most commonly observed, but Ta is also known to exist in two other allotropes, one tetragonal and the other (much less-wellknown) face-centered cubic (fcc). The tetragonal form (bTa) has been produced by various deposition techniques and often occurs mixed with the a-phase; the fcc phase has only previously been reported in thin films deposited by thermal evaporation. There have been other reports of 'bcc metals' such as V and Fe existing with an fcc crystal structure when the metal is deposited as a thin film. In the present study, fcc Ta with a = 0.43 nm has been observed using transmission electron microscopy in bulk samples of Ta that have been subjected to quasi-static tensile deformation that was so large as to cause fracture of the material. The fcc phase has a relatively small grain size but appears to be stable at room temperature. It is also shown that relatively large grains (10-20 nm in diameter) of Ta can also exist with an hcp structure with a = 0.304 nm and c = 0.494 nm.

Proposed mechanism of HCP → FCC phase transition in titianium through first principles calculation and experiments

“…The phase transition between hexagonal-close-packed (HCP) and face-centered-cubic (FCC) structures has been well-regarded as one of the most important solid-solid phase transitions, and has raised great research interests during the past decades 1 – 26 . The HCP-FCC phase transition could be induced by mechanical deformation or temperature change 5 – 23 , and has been extensively observed in a lot of metals and alloys such as Ti 5 – 8 , Hf 9 , Zr 10 , Co 11 – 13 , Si 4 , Ta 14 , Al 15 , Au 16 , LnH 17 , MgTi 18 , TiAl 19 , TiZr 20 , CoCrMo 21 , FeMn 22 , FeMnSi 23 etc. Especially, it is reported that the HCP-FCC phase transition could bring about the shape memory effect of several alloys 19 , 25 , and the appearance of the FCC structure can improve the ductility of the metals and alloys with the HCP structure 20 , 25 .…”

Effect of Vacuum Heat Treatment on the Microstructure of a Laser Powder-Bed Fusion-Fabricated NiTa Alloy

“…The SAED pattern shows the Ta precipitate to have a fcc crystal structure, 0.43 nm in the [111] direction. This is interesting, as the most common form of Ta, known as the α phase, has a bbc crystal structure, as reported by Janish et al [27], although the formation of fcc Ta structure in Ta thin films has also been reported by Denbigh et al [28]. Shen et al [29] pointed out that the small particles may exhibit lower potential energy and can become thermodynamically stable fcc crystal structures before melting.…”

Editorial: The August 2022 cover paper

“…The cover figure illustrates the potential for linking this type of modeling to EBSD and TEM texture studies. Indeed, TEM has revealed FCC and HCP grains (and even twinned FCC grains) existing in heavily deformed Ta [4], which is another reason for this writers interest in the selected paper.…”