High-purity Zirconium under Niobium ion implantation: possibility of a dynamic precipitation?

Abstract

High-purity (6N purity) Zirconium was subjected to different Niobium ion (Nb⁺) fluences in a particle accelerator. Grazing incidence X-ray diffraction confirmed subsurface phase transformation. While an approximate scaling was noted between Nb⁺ fluence and quantum of phase transformation, the sample subjected to the highest ion irradiation also showed significant subsurface shear residual stresses (τ₁₃). Molecular dynamics simulations, considering momentum transfer, revealed a drop in τ₁₃ beyond a critical displacements per atom or Nb⁺ fluence. High-resolution cross-sectional transmission electron microscopy (HRXTEM) confirmed formation of bcc (body-centred cubic) β phase and also linked τ₁₃ with such transformation. HRXTEM revealed, at about 100 nm depth, presence of 10–15% β with 2–15 nm size. The β particles and the surrounding α also had significant microscopic shear strains. The dynamic nature of the Nb implantation is expected to create fluctuations in temperature, Nb concentration and relative lattice damage. Such fluctuation, on the other hand, is/was hypothesized to dynamically alter the critical nuclei size: a clear possibility of ‘dynamic precipitation’.

Keywords:

• ion damage
• Zirconium
• microstructure
• phase transformation
• residual stress
• molecular dynamics (MD)

Acknowledgements

The authors would like to acknowledge support from DST (Department of Science and Technology, India) for establishing the National Facility of Texture and OIM (Orientation Imaging Microscopy) at IIT Bombay. Financial Support from BRNS (Board of Research on Nuclear Sciences, India) is gratefully acknowledged. For simulations utilization of spacetime.iitb.ac.in cluster facility at IIT Bombay is gratefully acknowledged. Alexander Stukowski is greatly acknowledged for sharing source code of crystal analysis programs. Technical discussion with Prof. Roger Doherty is greatly appreciated.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1. Note that the rigid and stochastic regions are along two simulation walls each parallel to XZ and YZ (totalling to four side walls) and one bottom wall (along XY plane). The top wall of the simulation cell was not provided with these layers as the irradiating Nb⁺ atoms enter the simulation through this boundary only. Hence the effective vertical dimension of the simulation volume was 970 Å.

2. Note that XRD information on presence of Nb is indirect. The presence of β phase peaks in XRD is taken as the indication of existence of Nb in the present case.

3. The reason for applying the homogeneous nucleation was the TEM evidence of beta regions uniformly present across the sample without any apparent preference for any crystal defects.

Additional information

Funding

This work was supported by Board of Research in Nuclear Sciences.