Recycling process, microstructure and mechanical properties of titanium chips through equal channel angular pressing
Loading...
Citations
Altmetric:
Altmetric::
Download
Associated Records
Authors
File Licence
Editors
Date
2014
Source Title
Journal Title
Journal ISSN
Volume Title
Publisher
Melbourne Authors
Affiliation
Mechanical Engineering
Document Type
PhD thesis
Subject Code
Citation
Access Status
This item is currently not available from this repository
DOI
Access URL
Open Access URL
Open Access at PMC
Linked Resource URL
NHMRC Grant Code
ARC Grant Code
Description
© 2014 Dr Peng Luo
Abstract
It is recognized that the recycling of resources has become one of the critical problems for the sustainable development. For metallic wastes, a promising way to improve the recycling value is to innovate the traditional melting and casting by a solid-state process. With high strength and excellent biocompatibility, Ti (and alloys) is widely employed in aerospace industry and biomedical engineering, but much of the Ti may be lost to valueless chips by machining. In this research, chips of commercially pure Ti (ASTM grade 2) were successfully recycled into fully dense bulk material by equal channel angular pressing (ECAP) with ultrafine grain sizes below 1 μm, yield strength up to ~650 MPa, and an uniform elongation over ~10%. Moreover, it is demonstrated that less energy consumption is required due to the avoidance of melting.
By resolving the microstructure into high- and low-angle grain boundaries (HAGBs, with misorientation ≥ 15°, and LAGBs, < 15°), electron backscatter diffraction enables the parametric statistics of grain size and misorientation. It was observed that the grain refinement is more likely to occur after the recycling at 450°C than 590°C. Also, the substantial existence of LAGBs was revealed implying a prominent effect taken by LAGBs on mechanical property. Moreover, the surface oxide layer plays a significant role in consolidation. A key parameter, i.e., the specific surface area per unit weight, was used to analyze the consolidation effect dominated by the mechanisms of mechanical straining and interfacial diffusion. In addition, focused ion beam tomography under scanning electron microscopy is capable of conducting an in-situ observation of the oxide layer features by transmission electron microscopy. This is beneficial to the theoretical evaluation of the oxide dispersion contribution. Further, good quality of the recycled Ti manifests itself in comparative strengths to those observed in reference solid bar by compressive/tensile tests. Particularly, satisfactory ductility was obtained in the recycled Ti owing to excellent interfacial welding quality between chips.
A quantitative model was established for the recycled Ti to estimate the strain energy stored in HAGBs and LAGBs. This is of theoretical significance to understand the microstructure development since the formation of structure/substructure is attributable to various statuses of strain energy configuration that govern the dynamic restoration. Consequently, both dynamic recovery and continuous (in situ) dynamic recrystallization play indispensable roles in the microstructure evolution.
A paradigm of the Hall-Petch relationship was developed for Ti at various grain sizes (i.e. the circle equivalent diameters of HAGB domains). Also, this law is found to be valid in the form of LAGB intercept vs. experimental LAGB contribution. Complex strengthening mechanisms underlying the Hall-Petch relationships were then proposed on account of such contributory factors including HAGBs, LAGBs, interstitial solutes (e.g. oxygen, nitrogen and carbon) and oxides. The strengthening is primarily derived from grain refinement and interstitials. Simultaneously, LAGBs can make significant contribution. Nevertheless, the amount of oxides is very small and thus does not provide any significant strengthening through potential dispersion. A theoretical model for evaluating yield strength compares well with the experimental strength values.