| Project Team | Institution | Role | ||
| Assoc Professor Matthew Barnett | Deakin University | Chief Investigator | ||
| Dr Colleen Bettles | Monash University | Senior Research Fellow | ||
| Mr Daniel Curtis | Monash University | Research Assistant | ||
| Mr Gregory Guo | The University of New South Wales | Postgraduate Student | ||
| Professor Peter Hodgson | Deakin University | Chief Investigator | ||
| Mr Edward Lui | The University of Melbourne | Postgraduate Student | ||
| Mr Alexandre Monvoisin | Monash University | Intern | ||
| Professor Barry Muddle | Monash University | Chief Investigator | ||
| Dr Hoi Pang Ng | Monash University | Research Fellow | ||
| Dr Ananthi Sankaran | Deakin University | Research Fellow | ||
| Ms Melanie Semblanet | Monash University | Intern | ||
| Dr Xiaolin Wu | The University of Melbourne | Research Fellow | ||
| Assoc Professor Kenong Xia | The University of Melbourne | Chief Investigator | ||
| Dr Wei Xu | The University of Melbourne | Research Fellow | ||
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Executive Summary
New insights into the nucleation modes and thermal stability of metastable phases in binary Ti-V and ternary Ti-V-Cu alloys have been obtained. Heterogeneous nucleation of β′-phase on dislocation structures has been characterized and this is expected to have a crucial influence on the control of the α-phase microstructure during thermomechanical processing. The effect of Cu addition on the early-stage α-phase nucleation in solute-rich Ti-V alloys has been studied. Several potent nucleation sites for a, and their associated interactions, have been identified, contributing to an enhanced understanding of the development of α-phase microstructures.
New insights into the nucleation modes and thermal stability of metastable phases in binary Ti-V and ternary Ti-V-Cu alloys have been obtained. Heterogeneous nucleation of β′-phase on dislocation structures has been characterized and this is expected to have a crucial influence on the control of the α-phase microstructure during thermomechanical processing. The effect of Cu addition on the early-stage α-phase nucleation in solute-rich Ti-V alloys has been studied. Several potent nucleation sites for a, and their associated interactions, have been identified, contributing to an enhanced understanding of the development of α-phase microstructures.
Project Aims/Targets
The primary activity of project A6 is focused on the understanding of fundamental mechanisms behind the nucleation of α-phase in metastable β-Ti alloys, including the influence of transient ω-phase and the b¢-phase (a product of β-phase separation reaction) on this nucleation. It will also investigate the effects of heat treatment cycles and thermo-mechanical processing on the nucleation mechanisms and kinetics. The research effort of this project aims to provide a scientific basis for the development of Ti-alloys with enhanced properties, such as the deep-hardenability sought after for thick-sectioned titanium components.
The primary activity of project A6 is focused on the understanding of fundamental mechanisms behind the nucleation of α-phase in metastable β-Ti alloys, including the influence of transient ω-phase and the b¢-phase (a product of β-phase separation reaction) on this nucleation. It will also investigate the effects of heat treatment cycles and thermo-mechanical processing on the nucleation mechanisms and kinetics. The research effort of this project aims to provide a scientific basis for the development of Ti-alloys with enhanced properties, such as the deep-hardenability sought after for thick-sectioned titanium components.
Project Progress: Technical Details and Research Outcomes
Effect of Heating Rate on Age-Hardening Response
Pervious studies indicated that ternary Ti-V-Cu alloys may exhibit properties suitable for deep-hardening applications. While generic laboratory heat treatments are commonly performed with salt baths, controlled heating experiments were conducted on the Ti-V-Cu alloys using an infra-red furnace (Deakin), in order to study the influence of heating rate on the age-hardening response of the alloys. Precise temperature ramp rates ranging from 10°C/min to 1000°C/min were applied. The findings revealed that the initial hardening characteristics of β-Ti alloys are highly sensitive to the rate of heating (see Figure A18). Such a phenomenon has been correlated to the number density and thermal stability of incipient metastable phases/structures evolved in these alloys under different heating conditions. The thermal stability of ω-phase, in particular, is found to have substantial impact on the morphology of equilibrium α-phase.
Pervious studies indicated that ternary Ti-V-Cu alloys may exhibit properties suitable for deep-hardening applications. While generic laboratory heat treatments are commonly performed with salt baths, controlled heating experiments were conducted on the Ti-V-Cu alloys using an infra-red furnace (Deakin), in order to study the influence of heating rate on the age-hardening response of the alloys. Precise temperature ramp rates ranging from 10°C/min to 1000°C/min were applied. The findings revealed that the initial hardening characteristics of β-Ti alloys are highly sensitive to the rate of heating (see Figure A18). Such a phenomenon has been correlated to the number density and thermal stability of incipient metastable phases/structures evolved in these alloys under different heating conditions. The thermal stability of ω-phase, in particular, is found to have substantial impact on the morphology of equilibrium α-phase.
isothermal ageing temperature (500°C).
Formation of β′-phase on Dislocations
Metastable β′-phase has long been reported in the literature to be a potent nucleation site for α, but the actual nucleation mechanisms and the crystal structure of β′ remain unclear. Considered as a product of the β-phase separation reaction (β-->′β+β′), b¢ has recently attracted the interest of the project team for its ability to nucleate on dislocations. Figure A19 (a) shows an array of β′ precipitates heterogeneously nucleated on dislocations in a binary Ti-25V alloy. The variant selection of the β′ phase appears to be determined by the type/character of the associated dislocations. The practical significance of this is related to being able to control the density and distribution of β′-phase through the dislocation structures generated by thermomechanical processing , and in turn control the preferred nucleation of a-precipitates. The corresponding microstructure of the Ti-25V alloy after continued ageing is shown in Figure A19 (b).
The crystallography of β′-phase is under investigation in Monash. The study of thermo-mechanical processing in relation to the nucleation of α-phase in β-Ti alloys will represent a major collaborative effort among Deakin, Monash and The University of Melbourne in this project.
Metastable β′-phase has long been reported in the literature to be a potent nucleation site for α, but the actual nucleation mechanisms and the crystal structure of β′ remain unclear. Considered as a product of the β-phase separation reaction (β-->′β+β′), b¢ has recently attracted the interest of the project team for its ability to nucleate on dislocations. Figure A19 (a) shows an array of β′ precipitates heterogeneously nucleated on dislocations in a binary Ti-25V alloy. The variant selection of the β′ phase appears to be determined by the type/character of the associated dislocations. The practical significance of this is related to being able to control the density and distribution of β′-phase through the dislocation structures generated by thermomechanical processing , and in turn control the preferred nucleation of a-precipitates. The corresponding microstructure of the Ti-25V alloy after continued ageing is shown in Figure A19 (b).
The crystallography of β′-phase is under investigation in Monash. The study of thermo-mechanical processing in relation to the nucleation of α-phase in β-Ti alloys will represent a major collaborative effort among Deakin, Monash and The University of Melbourne in this project.
The heterogeneous nucleation of α-phase has primarily been related to metastable phases such as w and β′, dislocations and grain boundaries. The influence of solute-clustering, which plays a vital role in the nucleation of strengthening precipitates in aluminium alloys (e.g. θ′ in Al-Cu alloys), has rarely been considered in β-Ti alloys. This project has initiated a high-resolution electron microcopy study of the nucleation mode of α-phase in Ti-V alloys with a ternary alloy addition of Cu. Solute partitioning of heavy atoms has been found to occur in the presumably early-stage nucleants of α-phase. The finding inspires a new area of study on the heterogeneous nucleation mechanism of α-phase.
Future Activity Plan
Crystallographic investigations of the metastable phases in β-Ti alloys will continue and further STEM characterization will be pursued with an aberration-correct TEM (FEI Titan) in Monash. The research findings on the evolution of β′-phase and the microstructural development of α-phase under the influence of Cu were presented in the PTM2010 conference in France. Sizable β-Ti alloy ingots are being sourced from outside Australia. Thermomechanical processing (high-temperature rolling and forging) of the Ti alloys will be actively carried out in collaboration with Deakin and The University of Melbourne.
Crystallographic investigations of the metastable phases in β-Ti alloys will continue and further STEM characterization will be pursued with an aberration-correct TEM (FEI Titan) in Monash. The research findings on the evolution of β′-phase and the microstructural development of α-phase under the influence of Cu were presented in the PTM2010 conference in France. Sizable β-Ti alloy ingots are being sourced from outside Australia. Thermomechanical processing (high-temperature rolling and forging) of the Ti alloys will be actively carried out in collaboration with Deakin and The University of Melbourne.