Titanium as an endogenous grain-refining nucleus

Abstract

Several studies have confirmed the applicability of the inverse growth restriction theory for predicting grain size in titanium alloy systems. However, until now, no work has identified nuclei particles that could be used to refine the β-grain size of titanium alloys during solidification. This work investigated whether titanium powder can be used to nucleate β-grains during solidification. A novel technique was used to introduce titanium powder to a series of titanium alloys, which results in significant grain refinement with an order of magnitude increase in grain density. Electron back-scattered diffraction (EBSD) was used to prove that titanium substrates can epitaxially nucleate titanium during solidification, and although a number of other potential mechanisms were investigated, it was concluded that the titanium particles heterogeneously nucleate β-grains.

Keywords:

• grain refining
• titanium
• solidification

Acknowledgements

The authors would like to acknowledge the support of the CAST Cooperative Research Centre. CAST was established and is supported by the Australian Government's Cooperative Research Centres Programme. The authors also thank Dr. Suming Zhu from CAST for his technical assistance with EBSD and Mr Alex Buddery from CAST for his assistance with WDS.

Notes

Notes

1. The supplementary experiments used to investigate how additions of Ti powder reduce the grain size are referred to as Sup.A through to Sup.E.

2. This microstructure indicates that the sheet has undergone a transformation to the β-phase followed by subsequent cooling and transformation back to the α-phase. If this transformation had not occurred, the microstructure would still consist of equiaxed α-grains.

3. A very large sample size would reveal the true line of best fit for the population. In this case, the experimental regression fit is derived from a sample size of 10 where each sample is comprised of an average of three castings.