Characterization of the α'-Martensite phase and its decomposition in Ti-6Al-4V additively manufactured by selective laser melting

Cho, J 2018, Characterization of the α'-Martensite phase and its decomposition in Ti-6Al-4V additively manufactured by selective laser melting, Doctor of Philosophy (PhD), Engineering, RMIT University.


Document type: Thesis
Collection: Theses

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Title Characterization of the α'-Martensite phase and its decomposition in Ti-6Al-4V additively manufactured by selective laser melting
Author(s) Cho, J
Year 2018
Abstract This thesis has focused on characterising the α′-martensite phase in Ti-6Al-4V additively manufactured by selective laser melting (SLM) and understanding its decomposition behaviours under different conditions. The α'-martensite phase is a predominant phase in SLM-fabricated Ti-6Al-4V alloy under typical SLM processing conditions. It can be categorized into four types, namely primary, secondary, tertiary and quaternary, in accordance with their size and location in prior-β grains. Some α'-martensite phases are heavily twinned while other α'-martensite phases contain very few twins. It is shown that SLM-fabricated Ti-6Al-4V alloy that has smaller prior-β grain sizes and a larger amount of twinned martensite plates exhibit better tensile strength (1034 MPa) and ductility (11.1 %) than other SLM-fabricated Ti-6Al-4V.

The slice-and-view technique with focused ion beam / scanning electron microscope (FIB/SEM) dual beam system has been used to further investigate the three dimensional (3D) morphology of the α'-martensite phase. Although different morphological features of the α'-martensite phase have been observed in 2D micrographs, it is confirmed that the 3D morphology of the α'-martensite phase in SLM-fabricated Ti-6Al-4V is all plate- or patch-like, irrespective of their size. The α'-martensite phase forms in sequence from primary to quaternary martensite in a manner of gradually filling the space of the retained β-phase between the martensite plates that are formed earlier than those to form.

To acquire a profound understanding of the in-situ decomposition of the α′-2 martensite phase during the SLM process, SLM-fabricated Ti-6Al-4V and water-quenched mill-annealed (MA) Ti-6Al-4V specimens were annealed under a variety of conditions.
Twinned α′-laths in SLM-fabricated Ti-6Al-4V are still visible after being annealed at 410 °C for 2 h and at 600 °C for 0.5 h but they disappeared at higher annealing temperatures or longer annealing times. The β-phase began to be observed in both SLM-fabricated Ti-6Al-4V and water-quenched mill-annealed (MA) Ti-6Al-4V when annealed at temperatures higher than 600°C for longer than 2 h. The morphology of the β-phase changed from discrete particle-like to thin fibre-like with increasing annealing temperature above 600 °C for a fixed annealing time of 2 h.

The thickness of the α-phase decomposed from the α′-phase in SLM-fabricated and water-quenched MA Ti-6Al-4V grew with increasing annealing time and/or temperature. The growth rate of the α-lath thickness in water-quenched MA Ti-6Al-4V was faster than that in SLM-fabricated Ti-6Al-4V with increasing annealing temperature for a fixed annealing time of 2 h. In contrast, their growth rates are similar when annealed at 800 °C with respect to different times. The aspect ratio of the α-phase in both alloys gradually decreased with increasing annealing temperature and time. The α′-phase was retained in both alloys when annealed at temperatures below or at 600 °C. For example, no significant change in the aspect ratio and thickness of the α'-phase in both SLM-fabricated and water-quenched MA Ti-6Al4V was observed after annealing at 600 °C for various annealing times. The α′-phase gradually decomposed into the α-phase and -phase at temperatures above 600 °C. However, the α-phase in the SLM-fabricated Ti-6Al-4V showed lower aspect ratios than that in waterquenched MA Ti-6Al-4V after annealing at temperatures from 700 °C to 930 °C for a constant hold of 2 h or at 800 °C for 0.5 h to 20 h. The different growth rates and changes in 3 aspect ratio of the α-phase in both alloys are attributed to different microstructures such as the size of the α′-phase and prior-β grains in those alloys before annealing.

High energy synchrotron X-ray diffraction was employed to investigate the decomposition of the α′-martensite phase in SLM-fabricated and water-quenched MA Ti-6Al4V. The α′-martensite phase and a small amount of β phase were found in both alloys. No α″-martensite and ω-phase were found during in-situ heating. However, the α2-phase (Ti3Al) appeared at temperatures higher than 500 °C in both alloys. With increasing temperature, the micro-strain level applied to the HCP lattice (α- or α′-phase) was reduced while the c/a ratio of the lattice was increased. The volume fraction of the β-phase kept increasing at temperatures higher than 700 °C. The lattice volume of the α- or α′-phase in water-quenched MA Ti-6Al-4V gradually increased while that in SLM-fabricated Ti-6Al-4V fluctuated. The twinned α′-phase in the SLM-fabricated Ti-6Al-4V showed different decomposition
behaviours relative to the α′-phase in water-quenched MA Ti-6Al-4V.

Scanning electron microscopy and micro X-ray diffraction (μ-XRD) were performed to investigate microstructural inhomogeneity in SLM-fabricated Ti-6Al-4V cylindrical specimens of 12 mm in diameter and 20 mm in height. μ-XRD (beam size: 500 μm) was applied to 40 uniformly spaced regions at different build heights, assisted with microscopic examination. The thickness of α- or α′-phase gradually decreased from the bottom to the top of the SLM-fabricated Ti-6Al-4V specimen. The distribution of lattice
parameters for each phase was mapped out based on a detailed analysis of the μ-XRD twodimensional (2D) diffraction data (ring patterns). The α-phase and β-phase were observed in the bottom half of the specimen while the region above the mid-height is comprised of predominant α'-martensite with the top 3.5-mm thick region being fully martensitic. In 4 relation to this, the elastic modulus was found to decrease almost linearly from the midheight to the top. It was further shown that μ-XRD 2D ring patterns offered an improved approach to characterising the β-phase in SLM Ti-6Al-4V.

The research findings made from this project contribute to an improved understanding of the α′-martensite phase in SLM-fabricated Ti-6Al-4V.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Metals and Alloy Materials
Keyword(s) Additive manufacturing
Selective laser melting
Laser powder bed fusion
Ti-6Al-4V
Decomposition of martensite
3D microstructure
In-situ observation by synchrotron X-ray source
microstructure
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